HAFLER DH500 HIGH END AUDIOPHILE UPGRADE KIT TOSHIBA JFET CASCODE TOPOLOGY

$265.0 Buy It Now or Best Offer
free,30-Day Returns

Seller Store fantasiaaudio
(13591) 100.0%,

Location: Blackwood, New Jersey
Ships to: US,
Item: 283993213513

Restocking Fee:No
Return shipping will be paid by:Buyer
All returns accepted:Returns Accepted
Item must be returned within:30 Days
Refund will be given as:Money Back
Country/Region of Manufacture:United States
Type:Power Amplifier
HAFLER:HAFLER

NOTE: INSTRUCTIONS AND PICTORIALS WILL BE DELIVERED VIA A USB FLASH DRIVE. THIS FORMAT INCLUDES COLOR PICTURES AND THE ABILITY TO ZOOM AND PRINT. IF YOU DO NOT HAVE ACCESS TO A COMPUTER OR PRINTER AND REQUIRE A HARD COPY OF THE INSTRUCTIONS, PLEASE CONTACT ME VIA EBAY EMAIL BEFORE PURCHASING FOR ADDITIONAL PRICING AND SHIPPING CHARGES. HAFLER DH-500 ULTIMATE UPGRADE KIT. THIS UPGRADE ADDRESSES MANY ISSUES PERTAINING TO AUDIO AMPLIFIER DESIGN PRACTICE. THIS SIMPLE UNIQUE JFET CASCODE DESIGN OFFERS CLEAR AUDIBLE IMPROVEMENTS AND INCREASED POWER OUTPUT. INCLUDES GOLD PLATED COPPER RCA INPUTS, LARGE GOLD PLATED OUTPUT BINDING POSTS CAPABLE OF ACCEPTING .154 INCH DIAMETER WIRE WITH SPECIAL INSULATING BUSHINGS TO FIT 1/4 INCH CHASSIS HOLES , TWO PC-1 DRIVER BOARDS WITH ALL COMPONETS, ASSOCIATED WIRING AND PARTS, AND DETAILED INSTUCTIONS INCLUDING COLOR PICTURES AND PICTORIALS ON A USB FLASH DRIVE. THIS PACKAGE IS DESIGNED FOR INTERMEDIATE TO EXPERIENCED KITBUILDERS. YOU MUST HAVE GOOD SOLDERING PRACTICE SKILLS AND BE ABLE TO FOLLOW INSTRUCTIONS. YOU MUST BE ABLE TO READ COLOR BANDS ON 1% RESISTORS (MATERIALS SUPPLIED) AND INSTALL AND SOLDER SMALL PARTS ON PC BOARDS. YOUR TOOLS SHOULD INCLUDE THE FOLLOWING: A 40 TO 60 WATT SOLDERING IRON WITH 3/16 OR 1/4 TIP WHICH CAN REACH 600 DEGREES F. ROSIN CORE SOLDER (SUPPLIED) A DAMP SPONGE OR CLOTH TO WIPE THE TIP OF THE IRON A WIRE STRIPPING TOOL FOR REMOVING INSULATION FROM 16GA TO 24GA WIRE A MEDIUM-BLADE SCREWDRIVER NEEDLE-NOSE PLIERS SMALL DIAGONAL OR SIDE CUTTING PLIERS A 1/4 SPIN-TITE NUT DRIVER PLIERS, NUT DRIVERS, OR SOCKET SET FOR INSTALLING INPUT AND OUTPUT JACKS YOU WILL ALSO NEED BOTH AN AMMETER CAPABLE OF RESOLVING 250MA DC AND A DC VOLTMETER CAPABLE OF RESOLVING 10MV DC. A VARIAC WOULD ALSO BE A PLUS BUT NOT REQUIRED. *OPTIONAL 18000MFD, 22000MFD, 24000MFD, AND EVEN 39000MFD NIPPON MAIN FILTER CAPACITORS ARE AVAILABLE TO PURCHASE AT MY EBAY STORE. OTHER PARTS INCLUDING INRUSH LIMITERS, 35 AMP 800 VOLT DIODE BLOCKS, POWER SWITCHES, AND FUSE HOLDERS ARE ALSO AVAILABLE IN STORE. POWER OUTPUT, DISTORTION, AND NOISE SPECIFICATIONS OF ORIGINAL DESIGN GUARANTEED TO BE EXCEEDED. SEE PICTURES OF TYPICAL DH-500 MEASUREMENTS. AS WITH ANY HIGH POWER APPLICATIONS, QUA-CO LLC ASSUMES NO RESPONSIBILITY FOR IMPROPER ASSEMBLEY, FUSING OR INSTALLATION CAUSING AMP OR SPEAKER FAILURE. IF YOU ARE INTERESTED IN HAVING THIS UPGRADE INSTALLED PROFESSIONALLY BY A QUALIFIED ENGINEERING SERVICE, PLEASE EMAIL ME FOR CONTACT INFORMATION. Being intrigued with audio since 1973 has given much insight on the topic of amplification and proper design practice. My interest in audio continues to be both my hobby and my passion. Employment at both Dynaco as a technician and Hafler Company as a project engineer was not only enjoyable but allowed me to work and converse with some very accomplished audio engineers. I continue to work on many audio projects, but will discuss my views on power amplification here. So what is it that allows a power amp to excel in its ability to reproduce music? Why does Brand A power amplifier sound better than Brand B power amplifier? Let us assume both have similar power, distortion, noise, gain, slew rate, etc. specifications. Brand A is proclaimed to be an exceptional amp. It certainly looks better cosmetically. Does that mean it sounds better? It costs $1,000 more than Brand B. Does that mean it is better? We read or hear about terms like triode sound, no coloration, more natural, less fatiguing, cleaner, neutral, transparent, precise imaging, superb stage, etc., but what does it all mean? We cannot measure these terms quantitatively with standard test equipment. We can make standard measurements as set forth by The Institute of High Fidelity but that is as far as it goes. Double blind listening tests are the only way to proceed at this point, revealing the truth about your $500 amp or your $1,500 amp. Of course all testing of this nature is now deemed subjective, but a trained ear will fairly consistently pick the superior amp. So what can we surmise at this point? Why does Brand A amp own the preferred sound? We can agree that it is surely not because it looks better or costs more. So what is it? What is the solution? I have concluded after years of listening tests, circuit analysis, and endless bench testing, the answer is not as elusive or fleeting as I once felt but really what one would expect as a now seasoned engineer. 1. Simple circuitry – This means not pushing the audio signal through more semiconductors and capacitors than needed. We are dealing with non-linear components and need not compromise the audio essence with more ingredients than needed. This will only lead to unnecessary phase shift, distortion, and loss of clarity. 2. Good design practice – This is where many amplifier designs fall short. The first problem area overlooked is PC board layout. It is crucial that 2 layers or multi layer PC boards have minimal or no track crossover from layer to layer. This will tend to inject capacitance into unwanted areas and cause instability. In turn, this leads to the addition of more capacitors to stabilize circuitry, effectively robbing bandwidth and introducing additional phase shift. The amp design is now flawed by poor PC board layout. Amps that have multiple small value pF caps or caps tacked on to the foil side would by suspect of this flaw. The second problem area and one that will corrupt the sound of even the best amp designs is proper grounding technique. Input, output, and power supple ground must follow the star ground scenario. For some reason, many manufacturers still to this day do not seem to get the importance or take grounding serious enough. Ground is ground. Period! It drives me crazy when I see resistors in the ground path to balance out ground loop situations or daisy-chaining unrelated grounds to save 12 inches of wire. These acts will muddle the audio content and ruin what we are trying to accomplish. If you have any of these situations, correct them or save your money and go buy a $159 receiver. When evaluating a design, look at the B+ and B- rails in relation to power supply and signal grounds. Then, try to visualize the circuit backwards where the grounds now become the B+ or B- and the B+ and B- become the grounds. This look can be very revealing. Would you insert current limiting on power supply rails and expect music to sound dynamic and accurate. I already have a problem with fuse resistance in the rails skewing dynamic capacity and incorporate bypassing measures when required but that is a topic for a different discussion. 3. Quality parts – Audio engineers are often limited to the final costs requirements of a new design. Unsurprisingly, the first parts to be downgraded to save on cost are capacitors (except for their own unit of course). This is a poor choice. Even under the double blind listening test, most audiophiles can consistently pick out the difference between quality polypropylenes and inexpensive metal films. Semiconductors should also be of good quality with low noise and distortion characteristics. Now all that is needed is a properly sized power transformer and main power supply filtering capacitors along with some desirable bypassing. Pure amplification does not have to be elusive. Listen to simplicity and enjoy. PC-1 DESIGN GOAL REALIZATION A. Precise PC board layout with no track crossover. B. First stage single ended Jfet/bipolar cascode differential amplifier with CCS regulation. Jfets incorporated are genuine Toshiba 2SK170 or 2SK117 devices with IDSS closely matched. If you know anything about these Jfets, I need not say anything else. High input impedance, high gain, tube like presence, and eerie low noise in listening tests makes it a no brainer. Cascode configuration reduces Miller effect, drastically increasing bandwidth and lowering distortion. This all important first stage configuration eliminates the need for massive gain and feedback used in many amps to cover up distortions and design flaws. C. Second stage also uses SE bipolar cascode amplifier with CCS regulation. Again, Miller effect is reduced and bandwidth is not compromised. D. Simple bias control and emitter follower circuit to drive output stage is added and all that is needed to complete the design goal. Clipping now occurs at 335 watts into 8 ohms instead of the 270 watts from original PC-10 or PC-19. E. Polypropylene capacitors throughout with all HE electrolytic caps bypassed with polypropylenes. F. Vital star grounding realized with pictorial showing proper wiring arrangement. SPECIFICATIONS Signal to Noise Ratio, unweighted: Exceeds 100 db referred to 250 watts into 8 ohms 103 dB Typical Typical THD at 200 watts into 8 ohms: 20 Hz: 0.0025% 1 kz: 0.0025% 20 kHz: 0.008% Input Impedance: 33,000 ohms. Input Sensitivity: 2.03 volts rms for 250 watts into 8 ohms. Frequency Response into 8 ohms: – 3 db, 4.8 Hz to 225 kHz at 1 watt. Semiconductor Complement per board: 9 transistors, 2 Jfets, 5 diodes, 4 zener diodes FEEDBACK: Item as advertised. Well packed. Very fast shipping. Great transaction! Excellent service and support Fast shipping. Amp adjusted and tested. This is an an outstandiing product. I listened to the JFET cascode kit in the DH-200 today. Holy cow !!! I compared it against an original DH-200 using one channel from the original design and one channel from another DH-200 amp with your new driver boards with the preamp in mono and adjusting the balance back and forth. All I can say is WOW ! I didn’t expect to hear that much of a difference. Stereo Imaging is better. switching both channels to the new amp you can close your eyes and place some sounds in a very, very specific front/back left/right space. There is a sense of ease and the music seems to have more depth and space. It’s hard to believe how good the DCM TF350 speakers sound with this new amp. They are pretty good speakers but nothing that exotic. The lows are quite a bit better as well maybe some of which is due to the original amp having original power supply caps. The original amp is one that you repaired for me and has a capacitor upgrade kit so I know it’s working correctly and is a good comparison. D***, now I won’t want to use the old amps until I upgrade them. I just wanted to let you know I was able to install the kit and have been listening for a few days. I could go on and on but it was probably the best $ I have spent in a long time on audio gear. I have a pair of very inefficient electrostatics. The 500 has never had trouble driving them but now they really sing. Thanks again for bringing affordable products to us Hafler fans. Wow it gives me goose bumps its amazing performance. Not only did I install your cards but added 4 Nichicon 33,000uf power caps with soft recovery rectifiers along with dual mono torodial transformers. This jumped several levels in performance believe I’ll have this amp until I die. Ed, Ran some music through it last night. First impression: WOW! VERY NICE! I’m feeding it with an Adcom GTP-500II preamp and have it connected to my Infinity Kappa 7.1 series 2 speakers. It was articulate, clean, quiet, dynamic. The sound stage was wide and deep and made my speakers fade into the wall. Vocals were silky and never strained or harsh. The detail was incredible even with a very complex mix. The highs were clean and effortless and the bass was deep, rich and tight. Ed I installed your FET boards one and a half years ago. My 500 sounds better than ever. It just keeps getting better. Thanks for building such a great product at a fair price. What a slap of my face when I heard the first tracks compared to the DH-220 ! None of the personal tweaks I had done (recap, transformer etc…) made a significant sound change. But your design is absolutely amazing, I feel like dancing again listening to my tracks ! Ed‘s design and your assembly of these boards have put together a remarkable product. The best thing I did was listen to you when you said I should replace the Power Supplies, when all four of these amps were on and running at normal volume they would draw about 5 or 6 Amps when looking at my power conditioners meter, now 3 amps if even. As for sound quality they are remarkable, I am running a pair of Dahlquist DQ-10’s with the two matching Subwoofers and the Dahlquist DQ-LP1 Electronic Crossover, these speakers are known for their remarkable reproduction of sound, and I thought they were good before, but now, it is unbelievable. I am hearing instruments in songs that I have played for years and never heard. I went to the Hafler line, because after a lot of searching and reviewing to find the closest sound to a vacuum tube amp, in a solid state, all roads lead to Hafler, I have never look back, now I have four brand new amps that could very easily go up against amps costing $5,000 dollars or more. So thank you for an excellent job. Anyone out there thinking their Halfer amp is shot or too old, contact Jim and bring new life to your system, if you really appreciate high quality sound, and want a high end system do this upgrade, you won’t be sorry. As you know, I’ve been developing speakers and assembling systems for my clients for over 20 years. I’ve dealt with many brands of amplifiers, preamplifiers, DAC, cables, etc. The best sonic balance (not the depth or wideness of the soundstage, just the sonic balance – the closest to a live performance) I have found in any amplifier at any price is in the original DH-500 (I’ve used amplifiers worth tens of thousands of dollars). I might also add that I’m a musician and that I’ve been going to live shows (pop. Jazz, classical, electronic, world music) for more than 40 years. I won’t bother you with describing how this or that musical piece sounded in comparison to what I’ve heard before, but I will tell you that not only did your amplifier reproduce the original Hafler frequency balance, but somehow improved it to my total astonishment. But this is not all. I’ve heard many amplifiers with great depth and wideness of soundstage (I have to mention that these criteria come second to the frequency balance for me), but I have to say I’ve simply never heard anything close to what your new DH-500 can produce in these regards. It is simply incredible. To give you just an example, it is possible to hear the first violin in a violin concerto clearly closer than the second and third violins and to hear all the other instruments of the orchestra just as they should sound vis-à-vis the others, exactly at their place sonically and physically. The same is true for jazz, rock, pop, electronic (I simply never heard such beautiful deep bass frequencies, so detached from the rest of the sounds). Jim and Ed, you know how important it is to us crazy sound and music lovers when we can improve our sound by even a fraction. With your amplifier, I’ve gone from the seventh sky to the 8th one, which I didn’t know existed! So thank you so very much. Pierre Murray President ArtVibes Audio P.S.; If I may make a suggestion, you should sell this amplifier for at least $7,000. I don’t believe it is possible to find a comparable amplifier under $15,000, which is the level I stopped at in my searches (even dual monos and mono blocks). My favorite one is the Pass Lab 100.8, and I feel this new DH-500 is in the same class, even though I prefer the Hafler to the PLab. I’d like to give you a heart-felt thanks for what you did for my DH500. It’s taken some time to fully appreciate… Right out of the box I noticed that the amp made my JBL 4412’s sing, a strange word but that sums it up. Back in it’s youth the DH500 sounded great, and I have no way to do a side by side between 1988 and 2018, but this made me very happy. Unfortunately this exacerbated problems I was having with poor room acoustics, and lead to a major re-organization of my audio space- a big, time-consuming project. That finished, I brought my parametric EQ into my preamp EPL along with a new 31 band Graphic EQ and mostly fixed the problem. I’ve spent many hours at this point sitting on the couch at the focal point of the 4412’s, listening very carefully. I’ve concluded that the ”imaging” of the soundstage in the reborn DH500 is almost certainly better than it has ever been. I say this because I’m hearing things I’ve never heard before on CD’s that I know well. Specifically, I can hear little tweaks on the board by the sound engineer changing the location of the musicians within both 3D image-space and sonic layers of frequency space ever so slightly to great musical effect. Of course this is both a blessing and a curse – some recording engineers have the gift and some don’t. It’s a whole new dimension to the music. I hate to admit this, but I was a bit hesitant to ship my amp off to a complete stranger. I am grateful that you took the time to get me on board. You, my friend, and the engineer that designed the driver boards, are the real deal, true afficionados pushing the audiophile envelope to new heights. OH MY GOODNESS!! Is it working? Is? It? Working? IS IT WORKING????? Never in my life have I heard anything like this! Audio nirvana! My Polk RTi A9’s have always been super thirsty, which is why I bi-amp them—till now it’s been the only way to get decent bass. Their sensitivity is garbage. Well, once I finally had time to hook up the amp, I fired up a test song from one of my favorite playlists. I had only intended to verify the amp was working, but an hour later, I realized that the speakers had disappeared and I had just sat there enjoying songs I’ve heard a hundred times like they were brand new. Expansive soundstage—it was just me and the music. Bass was deep and tight, not boomy or loose in any way, but still powerful and commanding. The mids, my god the mids! They stood out on their own with a clarity and smoothness that was astonishing. The highs were so clean and airy—I had not even a HINT of listening fatigue after the hour session—in fact, I had to tear myself away from the music to get some work done. Another awesome thing was that the music sounded fantastic at all levels—even quiet, it still had powerful presence and compelling clarity. Before I quit though, I did an A-B with my Adcom 555SE (a not inconsequential $1400 amp), and a heavily modded Yaquin tube amp I had nearby. The Adcom had great mids, but nowhere NEAR the commanding presence and incredible bass of the QC600. It sounded shrill, hollow and empty, flat and disappointing. (It actually sounded great, but after the QC…well now I am officially SPOILED!!) The tube amp was nice and warm, but just didn’t have the power that the Polks needed. It too, while sounding really great, was greatly disappointing after hearing the QC600. This amp has been one of the most expensive investments I’ve ever made, because now my other amps are all worthless, hahaha! But seriously, NEVER in all my years of audio have I gotten so much value from my investment. This was worth the wait and the money 100x over. Thank you so, so much for sharing your gift with the world, and with me. This IS an important detail: my super thirsty RTi A9 Polk’s have ALWAYS needed to be bi-amped to really get them to open up and sound full and big—especially on the low end. To wit, I had them wired with an Adcom GFA-555SE for the Bass/low end, and either an Adcom 555SE (special boutique upgrade akin to your QC-600, custom driver board, silver solder matched components, etc., etc.) or a Yaquin 100B Tube amp with VERY expensive and exotic tubes rolled into it for the top end/mids/highs. I alternated with solid state and tubes so that I could use the SS amp for movies and theater and the tubes for music just to help conserve tube lifespan. I have bee driving the whole thing with a Krell Foundation 4k preamp, using their automatic room correction. Anyway, when used in this combination, the amps really light up the Polk’s, and the sound is phenomenal, especially the solid state/tube combo. The speakers easily handle around 200-300 watts RMS, and really need that extra power they get from being bi-amped for the bass to appear at all. With this setup, there is no need for a separate subwoofer—these babies shake the house! Now, on to the most important part: with the QC-600, I get the same level of bass and sound, except clearer, cleaner, wider, more open, faster, more detailed, and if you can believe it—even more powerful—FROM A SINGLE AMP. That’s right—the QC600 effortlessly outperforms two other expensive amplifiers COMBINED—both a pure SS and a tube/SS setup. I was able to A/B every possible combination directly, and I can say with absolute certainty that by the end of my experimenting, I was convinced. I literally pulled all the old amps from my cabinet right then and there and replaced them—PERMANENTLY. The results are just that good. And they’re only getting better as the amp burns in and opens up more and more every day. Thanks once again—I wanted to add this extra bit so you can use it in your ads should you choose. I think it’s important for people like me who are naturally skeptical to understand that if they are even a little hesitant about doing this upgrade—DON’T BE. It is the single best investment I have ever made in my long audio career in terms of sheer value. It’s not an exaggeration or hyperbole of any kind to say that this upgraded amplifier can hold its own against amps priced far higher. So to anyone who may read this: ”JUMP. Make the leap. You.. WILL.. Not.. Regret.. It. Audio Nirvana is closer than you think!” Hi, Jim: I’ve been listening to my new QC-600, and I have to say all the reviews I’ve read are right on and then some. My Klipsch Heresy speakers sound different than before the upgrade: bass is deeper, highs and mids more out front, instruments more separated, more air around everything. It’s extraordinary.Great job on the amp, Jim. Ed, your design on the PC boards is fantastic. Very happy with the sound. Truly the finest my music has ever sounded.Jim, your communication while I was waiting for my turn in your queue, while you worked on my amp, and follow-up with photos and documentation chronicling the upgrade were above and beyond my expectations. I feel very lucky to have found you guys on ebay. Hi Jim,I hope all is well with you and your family. Now that I’ve listened to my upgraded Hafler 500, I just wanted to let you know that I’m super happy with it. It’s more than I imagined! The thing that really sets it apart for me is the way it drives my 1.7 Maggie’s. I was previously using a Hafler 220, and while the sound was good, I noticed that some recordings were lacking in bass. With the upgraded 500 driving them, that issue is long gone. The 500 drives them wonderfully, and there is plenty of bass!I will likely send you a Hafler 220 next. 😊 Thank you Jim, I’m very appreciative!!Rick Hi Jim,I unpacked and plugged in the amp today and it does sound fantastic! Incredibly crisp and clear, with easy power.After more listening:I’ve really been enjoying the QC-280. It sounds great with all kinds of music. It’s nuanced and precise playing The Nutcracker (my 2-year-old daughter’s current favorite), with easy separation of instruments and a wide sound stage. It handles the dynamic range very well, with barely any loss of detail in the quiet parts. The controlled power of the guitar riff on Dire Straits’ ”Money for Nothing” was enough for my Infinity CS-3008’s to knock a picture off the wall–with the volume at 1:00. Ella Fitzgerald and Count Basie sound like they’re playing ”Honeysuckle Rose” right in the room with you. I wish I could say more, but I’m more of a music lover than an audiophile. Joe Hi, Jim: I’ve been listening to my new QC-600, and I have to say all the reviews I’ve read are right on and then some. My Klipsch Heresy speakers sound different than before the upgrade: bass is deeper, highs and mids more out front, instruments more separated, more air around everything. It’s extraordinary. Great job on the amp, Jim. Ed, your design on the PC boards is fantastic. Very happy with the sound. Truly the finest my music has ever sounded. Jim, your communication while I was waiting for my turn in your queue, while you worked on my amp, and follow-up with photos and documentation chronicling the upgrade were above and beyond my expectations. I feel very lucky to have found you guys on eBay. Jim,It sounds fantastic! The day my reworked amp arrived, I spent the entire evening listening to a wide variety of music and loved every minute of it – straight out of the box. No fiddling with the controls on my preamp, no acclimating my ears to the sound – just sonic bliss. To describe this upgraded amp as ”musical” falls short. It lends liveliness and presence to songs that I’ve heard a million times previously but had never ‘experienced’ them like this before. I love it! You can listen to music for hours with this amp, without the fatigue that was an a facet of the character of the original DH-220.Its also important that I mention the communication by the seller was top notch every step of the way. I got daily updates via email on the progress of the upgrade and prompt replies to my email queries before, during, and after the sale. If I owned another Hafler amp, I wouldn’t hesitate to upgrade it.-Jon G. If you own a vintage Hafler amplifier do not hesitate to get the full PC1 upgrade Jim is offering. I’ll break this down into two parts. First is the work and Jim himself. At every step of the upgrade Jim kept me informed including photos of my amplifier as he transformed it. Every picture showed me that this man is meticulous and conscientious in the work he does. Craftsmanship is rare these days and Jim is truly a craftsman. Now how does the amplifier sound. Is this hyperbole or the real deal? To remove any variables I kept my audio system the same as it was, the only difference is I connected my new QC280 amplifier Jim built to the pre-out on my NAD C368 BluOs and reconnected my KEF XQ10’s to the new amp leaving my KEF 400b subwoofer on the sub out. My source material was CD’s ripped with Exact Audio Copy and 32/192 kHz streaming through the BluOs. This amplifier was a revelation! The soundstage was massive, both wide and deep and truly holographic in its’ imaging. Dynamics were crisp and instantaneous. I put on Steve Reich’s Music for 18 Musicians and every instrument was clearly defined and organic in timbre. Next up was Miles Davis In a Silent Way. Again, from Miles’s trumpet to Shorter’s sax to DeJohnette’s brush work on drums and cymbals and McLaughlin’s guitar runs all seemed to be in the room with me. I stayed up until dawn going through every genre, classical, jazz, rock and roll, Afro-Pop, and all of it was like I was truly hearing it for the first time. This amplifier upgrade is the best value I have found in a very long time and Jim himself is a gem. I am currently building myself a pair of CSS 1TD speakers and I can’t wait to hook them up to this amazing amp.Yours in the love of music and high fidelity, Jeff Young Jim,I have had a chance to put the amp through a wide range of my musical tastes and can only come to one noticeable difference – EVERYTHING!!!I have been able to listen for a long period of time at a high level without any listening ‘fatigue’. I am hearing things on recordings I have been listening to for may years that I never heard before. Especially subtle differences as a song I’ve known for many years now hearing there is some reverb and delay in the recording I never heard. I can clearly hear the differences in how recordings are produced, some very minimally (clean instruments and vocals) and others that sound ‘over produced‘. My wife who is not an audiophile says everything sounds so much more clear with defined presence. I hear complete silence through the speakers where before I would hear a hum at high levels. I thought my speakers were past their prime and ready to replace, but now its like I’m listening to new speakers. I installed a few more cooling fans in the cabinet that my stereo components are in because before your upgrade the amp fan would usually kick on to the high speed. Now the cabinet temperature is at around 10 -15 º cooler and I have yet to hear the fan go to high, and can’t hear it on low because it is so quiet.I have had this amp for over 40 years and about 10 years ago began looking for someone who could diagnose and replace/upgrade what it needed so I wouldn’t have to part with it. It basically has been idle for that time until I came across your ad on eBay. My only regret is not packing the amp as you suggested by double boxing. I believe I had enough cushioning but another box around it with more insulation would have protected it on the way home. The dent it suffered is hidden by the cabinet, so the fact that it works flawlessly is what I care about. The wait time to send the amp and get it back was well worth it. Your attention to detail and meticulous workmanship is hard to find today. It really shows how much you care and love what you do. You have definitely made a big impact on my love and appreciation for a good sound system. I can’t thank you enough. I would recommend you to anyone looking to upgrade a Hafler amp needing some TLC. P.S. The first album I played was Lyle Lovett ‘Step inside this House‘ double album all the way through. It was a good first test because the album although recorded in 1998 is one of my favorite quality recordings of great musicians and well produced. Turn it up loud for ‘Rolin By’. Another favorite is Dire Straits ‘Brothers in Arms’, I was blown away listening to the remastered recording.Thank you.Stephen Grzeszkiewicz Hi Jim, I hope all is well with you and your family. I just wanted to let you know that I finally finished the PC-1 upgrades to my DH-200. It took a while. I only was able to work on it a few hours here and there. On Tuesday I did the final adjustments. The bias pots only need @ 1.5 turns to get a very steady 225ma. The dc-offset pots only needed a nudge and I got -00.3mv and -00.5mv and called it a day. So now was the time for sound testing. I hooked up the QC-280 to my MC DH-101 (never heard since I got it back from upgrading), added my Gamma 20 Tuner and my 30-year-old Philips cd player. On Saturday my wife, using a 1990s Technics receiver and a 1990s Technics cd player was playing some of her cd’s one being Broadway songs. The speakers were about 5 feet apart, on the floor, and were sounding the best I have ever heard them (this being before I heard Ed’s PC-1 boards). I was very happy with the sound to the point that my wife asked if I wanted to hear one of my cd’s and I responded “no, I am very happy just sitting here and listening to the sound coming out of the speakers”. The sound seemed as if you were in the audience of the show. So, on Tuesday, thru the Philips cd player and the upgraded Hafler, I played the Beatles ‘Love’ discs and Eric Clapton ‘Unplugged’. On Wednesday, I played ‘The Essentials of Billy Joel’. The song “The River of Dreams” was unbelievable. I had to listen a second time the sound was so fantastic. On Thursday, I listened to Steely Dan ‘Aja’, a cd of big band songs, and Brian Seltzer ‘Dirty Boogie’. As you know, the sound is phenomenal. It is more than I ever imagined. It is everything the testimonials say. The guitar strings, piano notes, the drums, the horns, the stand-up bass, the chimes, cymbals, the strings, the back-up singers you never heard before, the singers stretching out the notes, etc. You hear everything. I thought the system I bought 40 years ago was a nice sound. But I think the PC-1 upgrade lets thru twice the sound the original Hafler produced. I could not be happier. – Danny Danny’s ‘Follow-up’ Comments: I just wanted to let you know what I have been experiencing. And I would assume it will be things you have heard many, many times. As you may remember, I have some big, old KLH’s for speakers. These QuaCo / Hafler amps are great. As I have said before, its more than I could have ever imagined. This week I have been listening to it for a couple of hours a night. After an hour and half or so, the amp starts sounding very good. The problem is, it is now getting late and I should be going to bed, but I can’t. Last night about 11:00, The Blues Brothers song, “Sweet Home Chicago” came on. OMG, the musical sound you heard! The soundstage / imaging was great. I just closed my eyes and listened in amazement. I could not go to bed yet, I just had to listen to a few more songs. It was great. A few months ago, I never would have thought that from a 30-year-old cd player I got for $100 and 20-year-old bargain basement speakers for less than $200, that they would produce this kind of sound. I cannot imagine what the sound will be like out of a good pair of speakers. My thought is – once the amp gets about 100 – 125 hours of play time, it will be very close to its ultimate sound. Hopefully, by then my ears will be accustom to the much bigger soundstage and I will be more prepared to audition better speakers so that I can hear what the amp is truly able to give. When the time comes, I will start with the speaker companies you have suggested. In hindsight, I am so happy that in one of your earlier emails you commented that you would gladly just refurbish my DH–200, but if and when I wanted, you could take the amp to a whole new level. And I know it took me awhile to realize that the additional cost was minimal (maybe $100 or so), but I am very glad you made the comment and planted the seed in my head. Thank you so much. – Danny Cisco’s ’follow-up’ comments: This IS an important detail: my super thirsty RTi A9 Polk’s have ALWAYS needed to be bi-amped to really get them to open up and sound full and big—especially on the low end. To wit, I had them wired with an Adcom GFA-555SE for the Bass/low end, and either an Adcom 555SE (special boutique upgrade akin to your QC-600, custom driver board, silver solder matched components, etc., etc.) or a Yaquin 100B Tube amp with VERY expensive and exotic tubes rolled into it for the top end/mids/highs. I alternated with solid state and tubes so that I could use the SS amp for movies and theater and the tubes for music just to help conserve tube lifespan. I have bee driving the whole thing with a Krell Foundation 4k preamp, using their automatic room correction. Anyway, when used in this combination, the amps really light up the Polk’s, and the sound is phenomenal, especially the solid state/tube combo. The speakers easily handle around 200-300 watts RMS, and really need that extra power they get from being bi-amped for the bass to appear at all. With this setup, there is no need for a separate subwoofer—these babies shake the house! Now, on to the most important part: with the QC-600, I get the same level of bass and sound, except clearer, cleaner, wider, more open, faster, more detailed, and if you can believe it—even more powerful—FROM A SINGLE AMP. That’s right—the QC600 effortlessly outperforms two other expensive amplifiers COMBINED—both a pure SS and a tube/SS setup. I was able to A/B every possible combination directly, and I can say with absolute certainty that by the end of my experimenting, I was convinced. I literally pulled all the old amps from my cabinet right then and there and replaced them—PERMANENTLY. The results are just that good. And they’re only getting better as the amp burns in and opens up more and more every day. Thanks once again—I wanted to add this extra bit so you can use it in your ads should you choose. I think it’s important for people like me who are naturally skeptical to understand that if they are even a little hesitant about doing this upgrade—DON’T BE. It is the single best investment I have ever made in my long audio career in terms of sheer value. It’s not an exaggeration or hyperbole of any kind to say that this upgraded amplifier can hold its own against amps priced far higher. So to anyone who may read this: ”JUMP. Make the leap. You.. WILL.. Not.. Regret.. It. Audio Nirvana is closer than you think!” Thanks again Jim, and I WILL be back to use your amazing services again very soon! Sincerely,Cisco OH MY GOODNESS!!Is it working? Is? It? Working? IS IT WORKING?????Never in my life have I heard anything like this! Audio nirvana! My Polk RTi A9’s have always been super thirsty, which is why I bi-amp them—till now it’s been the only way to get decent bass. Their sensitivity is garbage. Well, once I finally had time to hook up the amp, I fired up a test song from one of my favorite playlists. I had only intended to verify the amp was working, but an hour later, I realized that the speakers had disappeared and I had just sat there enjoying songs I’ve heard a hundred times like they were brand new. Expansive soundstage—it was just me and the music.Bass was deep and tight, not boomy or loose in any way, but still powerful and commanding. The mids, my god the mids! They stood out on their own with a clarity and smoothness that was astonishing. The highs were so clean and airy—I had not even a HINT of listening fatigue after the hour session—in fact, I had to tear myself away from the music to get some work done. Another awesome thing was that the music sounded fantastic at all levels—even quiet, it still had powerful presence and compelling clarity.Before I quit though, I did an A-B with my Adcom 555SE (a not inconsequential $1400 amp), and a heavily modded Yaquin tube amp I had nearby. The Adcom had great mids, but nowhere NEAR the commanding presence and incredible bass of the QC600. It sounded shrill, hollow and empty, flat and disappointing. (It actually sounded great, but after the QC…well now I am officially SPOILED!!) The tube amp was nice and warm, but just didn’t have the power that the Polks needed. It too, while sounding really great, was greatly disappointing after hearing the QC600.This amp has been one of the most expensive investments I’ve ever made, because now my other amps are all worthless, hahaha! But seriously, NEVER in all my years of audio have I gotten so much value from my investment. This was worth the wait and the money 100x over. Thank you so, so much for sharing your gift with the world, and with me. I can tell you with absolute certainty that I will be back for more in the future! It was an absolute joy working with you. I will remember in the future also your advice about double-boxing, it was an eye-opener just how close I came to having something bad happen to this absolute treasure!With the quality I see inside this amp, I would expect it to play faithfully for maybe 20-30 years with no problems. I’d love to pass it on to my son someday :)Thank you, Jim. I wish you and yours a wonderful year, filled with happiness, health, laughter, family, smiles, friends, and MUSIC.Sincerely,Cisco Hi Jim: I bought my Hafler in the late 1970’s along with a Luxman tube pre-amp. At the time this was a big step up in affordable quality. Eventually I put my gear in storage, which ended up being for 25 years. I decided to resurrect this equipment last January. I happened upon your eBay offering and after giving you a call, decided to move forward with it. I was expecting maybe marginal improvement in performance but I was looking to at least to have an evaluation and replace any aged parts. During this time I also re-tubed my preamp and cleaned it up in anticipation of the audition of 21st century electronics. Your communication was excellent during this process. You provided very affordable recommendations. Just a note, anybody out there, if Jim makes a recommendation for parts – don’t go cheap. Follow his direction. When I received the amp, I hooked up the Luxman, and I was off to the races. The initial impression was the surfacing of music in the tracks that were never clearly audible before. Separation in stereo was markedly improved. The new N.O.S. stock tubes in the pre-amp brought a sound stage to this amplifier that was nothing short of profound. THIS AMPLIFIER IS MADE FOR TUBES. This prompted me to look in the marketplace for new technology in a tube pre-amp. I settled upon a PrimaLuna Dialogue pre-amp. Outstanding performance and build quality. After rolling some N.O.S. tubes, this turned out to be superior to the Luxman. Now I’m hearing vocal back tracks in Jan and Dean that I never heard before. The Doobie Brothers tracks I thought were out-takes because there is so much more music that I never heard before. Jim’s efforts are one of the great open secrets in audio. This quality in sound is the biggest bargain I’ve run across. Thanks again Jim Hi Jim,I can tell you that the original 500 you reconstructed sounds incredible! (more 3D than my other older one, but with the same perfect frequency balance). In fact, the 3D enhanced 500, due to the increased depth of the sound, provides more space between sounds and instruments. I will have you do the same reconstruction to my other 500 at a later date.Kind Regards,Pierre Jim,Thank you for the work on my DH-200, now QC-280. I still can’t fathom the difference that you’ve built into this amp, It’s too much (in a good way)! I’ve only played the amp for an hour or so and am floored by the bass response. I could go on, but you know what you built for me, so I won’t. Suffice to say, I am one very happy customer 🙂 Update: Thank you again for the upgrade work on my ’QC-280’, it’s so clear. And seems to appear more transparent and more dynamic every day (maybe the speakers are finally getting run in too? who knows!). I’m having to weed out the ’not so well produced’ sources, not necessarily a bad thing. This amp is deep (as in, way down the audiophile rabbit hole!!). Regards, Mark. Hi Jim,I wanted to send you an update on how the Hafler 500 (QC 600) has been performing. It’s been a couple of months since the upgrade and I am really enjoying the amp. The Solstice speakers are moderately efficient so it doesn’t take much form the QC 600 to shake the house. What I have really noticed is the increase in sound stage, the clarity of the individual instruments, especially with Hi-Res sources. Bass response is fast and punchy way different than the Adcom amp I used to own. It has been a blast to listen to my collection of music and hear parts of the recordings that I was not able to discern before. The amp runs super quiet thanks to your work getting the fan to run correctly. I was really impressed with how fast you responded to questions I had and the level of detail to questions and to the repair process itself. I wanted to let you know how pleased I am with the process and the end result. Your attention to detail is tremendous and your fees are really reasonable!Best Wishes,Dave B. Hi Jim,Remember me? You did a total rebuild on my DH-500 amp just about a year ago. I happened to be going through my old email drafts & saw one to you that I didn’t send. So I thought I’d contact you & let you know about my amp. It’s still working great! Here’s my old unsent email: ”The Hafler sounds great! I could hear the difference from what it was before & now immediately. The sound is super clear. The highs are much, much better, they probably stand out most but the overall sound is just tremendous! The whole system sounds great!I also definitely noticed the increased power. I am very happy with the way it preforms now. Wow, last night, I listened to it for a couple hrs. When I listened to stereo (just the Hafler) not 7.1 multi-channel, I could especially hear the increased clarity & improvement in sound very easily. I got it last Friday from FedEx. It survived shipment well. Excellent re-packaging by you, thank you! On Sunday, I took apart the body to put the face plate & handles. The quality of your workmanship inside is just awesome! …the pictures you sent me don’t do it justice! Everything you did is very clean & neat.,, very well done. I know quality when I see it! I’m really happy with everything & glad I had you do the work.” Thank you for all the high-quality work & care you put forth on this project for me. It was well worth it & much appreciated! Wishing you the best Jim, take care! Sincerely, Bob Hey Jim, been meaning to send you a line or two with my thoughts about the ”new” DH200/QC280. 😁I’ve been giving it some very serious listening sessions since discovering I’d wired the right speaker out of phase. 😝 In a way I’m glad I messed up, I can be confident I’m listening objectively and not hearing what I want to hear.So what I’m hearing is nothing short of spectacular! The combination of the Hafler and Infinity’s is incredible. As you pointed out these speakers have the reputation of being amp-killers. Paired with an amp that took measures to prevent speaker-killing and I think we have a thing of beauty. The amp has the balls to push the speakers and they have the balls to take all it can dish out. I haven’t come close to pushing either but enjoy the pure, clean power while listening ’comfortably loud’. I have no doubt the sound will be just as pure at ear-splitting levels as well.Like you said, some music is better produced than others and I find myself cringing a tad from time to time, but when you hit a well produced song it is absolutely thrilling. I listen to just the Infinity’s on 2 channel most of the time, but occasionally I’ll kick it into full 9 channel wall of sound and it’s really something.I’m totally happy that I went ahead and had you do the full mod on my amp. It was totally worth every cent not only to have an absolutely top notch power-amp, but the nostalgia factor of having the core of my 30 plus year old hi-fi system back can’t be measured. I don’t think a modern 2 channel amp can come close to the old Hafler’s for the price of this upgrade. This is as close to the warm sound of a tube amp as anybody can possibly hope to come.I look forward all week to my weekend listening sessions.Thanks Jim!Theo Hi Jim,I wanted to give a few impressions after having the amplifier on for over 200 hours. I want to first state that I have mainly used tube amps in my main system (Marantz 8B, Eico HF 81 Sam Kim mod, Dynaco St 70, Dynaco ST 35 power supply upgraded and others). With that said, I have a bias and a love for the midrange, especially vocals and strings, a good and properly working tube amp can bring. My favorite, the ST35 is only 16 or 17 watts, but the EL 84 tube is fast in the transients and has airy highs and a good foundation for bass. I have used my Hafler XL280 in my main system and my 2nd system uses an old NAD 3020 amplifier which had been serviced recently. My impressions will now focus only on the QC-280 with only a few comparisons to the other amps.Associated Equipment: conrad johnson PV5 (Bill Thalmann mods) pre-amp; Sound Dynamics RTS 9 speakers, Linn Lingo turntable with Dynavector 10X4 cartridge, Thorens TD 147 turntable with Audio Technica AT33 mono cartridge, Marantz SA8001 CD player and a Yamaha T85 tuner.Break in period observations: the amplifier did not ever get hot to touch throughout the burn in period. I could place my hand on the top and touch the cooling fins without any problems. A 40 watt light bulb gets hotter to touch than the amp. When the amp was first on for an hour and listening to the tuner, I could tell the sound was clean, dynamic and in control with orchestral music. The floor noise was clean. As time passed, at 50 hours I could hear a change in the soundstage getting wider and going outside the speakers with music I was familiar with. At 100 hours, using the same music, I felt the soundstage reached its potential and I could get a better sense of the front to back depth. I could easily hear the space of the recording venue. Likewise, it took the bass the same amount of time to sound its best in terms of detail and solidarity. I have found this with my tube amps when they haven’t been played for long periods of time. Yes, IMHO, break in is important before judging an amp. Impressions: after 150 hours of break in, I found this amp to be dynamic, able to resolve musical details and was in control of the music at all listening levels. Complex passages and the many instrumental soloists playing at the same time found in Decca’s Gerhard ”The Plague” by Antal Doriti or London’s ”Messiaen” by Antal Dorati proved to be no problem for this amp. Individual instruments were easily heard and no smearing of the instruments was evident. It was easy to hear a soloist in a choir as the choir harmonized in the background (evident in the soundtrack ”Glory” with the Harlem Boy’s choir). You can hear cello soloists breathing and in the case of Glenn Gould, hear his humming as he played the piano (sometimes annoyingly). Microdynamics are good when going from a soft to loud passages. Musical lines are easy to hear. The only area I found not as pleasing as my trusty ST 35 tube amp was getting the nth degree of musicality in the midrange. This is not a criticism, but maybe a bias. What I mean is that on unamplified music like Yo Yo Ma playing his cello or a violinist like Heifetz, I can hear and in a physical sense the bowing of the strings as they play. On piano music or hearing drum cymbals, the decay of the notes is more pleasing to my ear and realistic on a tube amp. The QC-280 gets to 90% of what I sense and enjoy. Is it more correct?, I don’t know. But with the bass, there is no comparison. This amp brings a solidarity in the bass foundation that I have never heard. There is detail and a ”correctness” of how it presents itself. It is my speaker that is the limitation as it only goes down to 35 Hz. Drum whacks are realistic and quick. No boomy bass here. My last point is that regardless of the listening level, this amplifier controls the music, whether it a singer and a guitar or a 100 piece orchestra. Usually when you play music loud (which I seldom do), it just breaks up and gets distorted. Not this amp.In closing, this amp has the ’WOW’ factor. What I mean is when you are playing the stereo system and listening from a different room and either I or a friend says ”Wow, that really sounds good”, the amp has done its job. Jim, a job well done and a true bargain for having done this under $1,000 to boot. This isn’t my Hafler XL280. Thanks (and I’m sorry I babbled on too much.)John H. Jim,I got my amplifier a few nights ago and today I was able to listen to it… MAAANN!! I can’t believe what I’m hearing!!! I’m using the Klipsch Chorus 2’s with full Crites upgrades, (and also have Klipsch Forte II’s and a Cerwin Vega VS15’s). Completely changed the sound of my speakers! I can’t believe what my speakers were capable of. The sound is like wide open and CLEAR!! I can crank it up louder than I did before since the bass is now not boomy, it is powerful and it hits just right! And it doesn’t even distort… the mids and highs sound like the person is right there singing in my living room. I feel like I want to turn it up more, but after a couple songs I had to turn it down to give my ears a little break. But at any volume, this thing really makes a difference on what is is playing… Now the music is alive!!Thank you, Jim, for making our amplifiers sound the way they should!! Now I feel like my amplifier is capable of sounding the same or even better than some expensive amplifiers I’ve been testing before. Didn’t know this Hafler DH500 would be this good with your ultimate upgrades!!Thank you very much for everything. God bless you,CarlosPS.: If I ever need another amplifier, I will, with no doubt, buy another one and send it your way to get the same upgrades! Jim,Got my amp yesterday and was listening to it for a while. I won’t go into all the things I heard in detail. Just want to say WOW!After waiting for about 3 years to go ahead and do this. I imagined it was A BIT over-hyped. Well, let me say that I could NOT have imagined what I am hearing. I know some people are able to articulate what they are hearing. I have No words. Just emotions. Wow! My wife came in the room after I was listening for the first 1/2 hr. Asked what I thought. My heart was beating so fast she said ”You better not have a heart attack!” I was awe struck. Full of Emotion. You’re right. Listening to a simple old Stones song. ( I’ve probably heard 900 times ). It was like I’ve never really heard all the sound on that recording. Thanks to you and Ed (AND Bill from NJ. for putting this all together). Bill is as passionate as you and Ed about his Hafler work. He was thrilled and will be doing the same upgrade to his amp (I was lucky to get this back from him!). One thing I Am bummed out about, now my other amps will probably fall short 😩. Looks like there will be even Less getting done around the house now!We will talk again.Thanks,Brian O. I received my amp on Friday, 30th of November. Unpacked the amp, looked it over, installed it into my stereo. I now am ready to hear my QC-600 amp. I have a passive crossover for my stereo. The four M&K S-100B speakers (two a side) are powered by the QC-600. The M&K S-100B speakers are 4 ohm each and hocked together in series for the amp to power into 8 ohm load. There are two MX-200 sub-woofers (220 watts each), using the M&K passive crossover so the crossover of 100hz for the QC-600 and sub-woofers. After a quick balance with the EQ and noise generator I was set. I played a few rock n roll songs, some chamber music, some horn jazz, and some opera. Several years ago, when studios were going digital, old analog equipment was at a good price. It was great getting a reel to reel to play copies of the master mixes I have of many groups. It sounds fantastic. Dynamics, and clarity are wonderful. I now enjoy the next level of perfection at home. I also have an DH-220 that you converted to QC-280 that I thought was incredible, and it is. This is now my entertainment/movie room amp, while the QC-600 is in my music library/listen room amp. Both are great, but, the QC-600 is an edge better. I do not know if it really is, or, just in my head. Little joke because where are your ears? Jim, your work is of a highly respectable level of accomplishment and the product more than lives up to it’s description. I need to thank you and if anyone is wondering if they want your service, no question needs to be asked about quality of the product, craftsmanship, or your attitude to deal with. Gary K. Jim, I’m writing to let you know that my new preamp reveals a lot of difference with the Hafler upgrades. It’s an Apt Holman that has new capacitors courtesy of another fine Ebay seller. So in addition to testifying to your excellent service and all your help with my Hafler rebuild, I can now talk about the great sound – because I hear it. Your build of the PC-1 boards, combined with new filter capacitors from Qua-Co, made a major musical improvement in the sound of my Hafler DH-200, which dates from the very first year of that amp’s production. To gauge the difference, I made recordings of several favorite pieces before and after the PC-1 upgrade: Bob Marley’s Coming in from the Cold on vinyl, Bill Evans’ My Romance at the Village Vanguard on Spotify’s high quality setting and a track from the Heath Brothers’ Family Jazz CD with both an extended bass melody line and a wonderful piano solo. All are well recorded. I used Audacity and a Blue Snowball USB mic. The difference, in a word, was definition – more of it. On low frequencies, the upgrade produced sweeter, clearer, crisper sound and tempered the lush, resonant and boomy qualities that existed before. All the richness remained, but the accuracy and detail improved and the sound became more spacious, less cluttered. Piano notes sound like wound steel strings vibrating and are more clearly distinguishable from notes in the same octave played on the upright bass. The middle- and upper-range tones gained similar clarity and precision. Like dead grass was raked from the lawn. The differences also are visible in the track waveforms in Audacity. These subtle changes are audible with careful listening, but they also build up when playing music casually in the background. After even a few minutes, I find myself saying, simply, ”Wow. That *really* sounds good.” I have been an audio enthusiast for 40+ years. This amp is fantastic. Now, about those speakers….I am sure a lot more detail will appear when those get upgraded next. Your clear instructions, illustrations and responses to questions allowed me to install and correctly bias with confidence. It was a fun project with great results. Thanks, Jim! Al Hello Jim:The amp arrived yesterday about 7 pm without mishap. I waited about 30 minutes to let it warm up before I played a cd. Needless to say, I was in a state of expectation as to what I was going to hear after reading all the glowing reviews. To reiterate what speakers I am using, they are the Polk SDA-SRS 1.2TL, their flagship model, 12 speakers in each and weighing 180 pounds each. Their frequency response is 20 to 20,000Hz. So the music I was hearing with them before the upgrade was impressive. Settling down in my listening chair, I started a cd. My initial impression was not that startling. The piece that I was playing did not sound much different than when played through the Hafler SE240 that I was using as an interim amp. That initial impression was soon to change as I played another cd, and another, and so on until the realization came that the sound from symphony orchestras and from electronic music had no harshness or distortion in certain passages, where there was before the upgrade. The cymbals, chimes and bells were crystal clear. The deep bass notes could be felt in your body without any aural harshness. In fact, some pieces that I had previously marked as good but not worth listening to before the upgrade took on new life and were a pleasure to listen to. I listened to one piece after another until it became late and it was time to retire. So, to summarize, the impression of what I was hearing with this upgrade was a richness, or fullness in sound with no perceivable distortion or flatness, an improvement in stereo separation, with improved clarity at the far points, as well as the ability to hear the sound of instruments in the background move to the forefront with presence and clarity where I could barely hear them before. My listening pleasure has just been taken to a new level. I don’t know if what I now hear can be improved on to any extent, since I now think I have reached perfection. I would recommend this upgrade without any hesitation to anyone wanting to enhance their listening pleasure. You won’t be disappointed. Thanks Jim, for the great and passionate work you do to enhance the listening pleasure of music lovers and to Ed for the brilliant QC upgrade. The updates and photos of the work as you were performing it also kept me involved in the process. Thanks again for opening a new adventure in listening pleasure.Best regards, Kurt Jim, The amp arrived Wednesday with no damage. Unboxed and plugged it in last night and took it for a test drive. Unbelievable! This is THE sound that I thought I could never afford. I bought the Klipsch speakers a few years ago and was not overly impressed with them. They had deep bass but lacked punch (and the highs were a little harsh). That was apparently an amp issue and not a speaker issue. My apologies to Klipsch (I know there are plenty of Klipsch critics out there). I can now feel the snare drum in my chest. I could hear the change in the high hat sound as the drum stick hit different parts of it. And I thought the ”let the amp warm up” thing was questionable. Nope, I could absolutely hear the difference 15 and 30 mins into it. I’m a Rock guy so last night I took it through Led Zep III, Iron Maiden, Mad Season, vintage Heart, Type O Neg, Skynyrd, and Fleetwood Mac – all vinyl. I now realize that the remastered Led Zep III could be better. Jim, thanks for the attention to every detail and the superior workmanship. I am also so thankful that my fuse blew last fall or else I don’t know if I would have pulled the trigger on this upgrade. I saw your upgrade offer on Ebay a while ago but just did not think it was worth the hassle of the shipping and waiting….HA! Thank God for that fuse blowing. -Dave Jim, I’d like to give you a heart-felt thanks for what you did for my DH500. It’s taken some time to fully appreciate… Right out of the box I noticed that the amp made my JBL 4412’s sing, a strange word but that sums it up. Back in it’s youth the DH500 sounded great, and I have no way to do a side by side between 1988 and 2018, but this made me very happy. Unfortunately this exacerbated problems I was having with poor room acoustics and lead to a major re-organization of my audio space- a big, time-consuming project. That finished, I brought my parametric EQ into my preamp EPL along with a new 31 band Graphic EQ and mostly fixed the problem. I’ve spent many hours at this point sitting on the couch at the focal point of the 4412’s, listening very carefully. I’ve concluded that the ”imaging” of the soundstage in the ’reborn’ DH500 is almost certainly better than it has ever been. I say this because I’m hearing things I’ve never heard before on CD’s that I know well. Specifically, I can hear little tweaks on the board by the sound engineer changing the location of the musicians within both 3D image-space and sonic layers of frequency space ever so slightly to great musical effect. Of course this is both a blessing and a curse – some recording engineers have the gift and some don’t. It’s a whole new dimension to the music. I hate to admit this, but I was a bit hesitant to ship my amp off to a complete stranger. I am grateful that you took the time to get me on board. You, my friend, and the engineer that designed the driver boards, are the real deal, true afficionados pushing the audiophile envelope to new heights. Thanks and God Bless you and yours, Jerome S. Here’s what a recent customer had to say about Ed’s PC-1 design and my installation work on his 2 Mono 500’s: Dear Jim and Ed, I wanted to let both of you know how ecstatic I am with the sound and build quality of my two bridged Hafler DH500s. I am having the same feelings of wonder and awe I got 35 odd years ago when I first bought assembled and listened to them.I’ve tried several different amps over the years including Carver, Proton, Adcom, Nakamichi, and a few others, but nothing came close to the power and clarity of my Hafler’s. They’ve been used exclusively with a pair of Polk rta12b speakers bought at the same time, and until recently, the only speakers I ever needed.While Jim was rebuilding my amps, I had the good fortune to acquire a nice set of Infinity rs2b’s. An order of magnitude better than the Polk’s, they also quickly showed every flaw of any piece of equipment attached to them. The Nakamichi Stasis 7 did not sound good on them, a Proton AA-1150 sounded more musical and less harsh. I could tell that it just wasn’t powerful enough, the infinities are serious power hogs.So yesterday when my amps returned home from their visit with Jim, I spent the next 12 hours listening. Suddenly, all of the drivers in my speakers came alive, the emims got the power they needed to keep vocals and percussion separate and distinct. Where I once thought I might need a subwoofer, the speakers now extended way down to their bottom end, you could hear bass notes fade clearly as the strings vibrations lessened. Spatial imaging took on outstanding depth as well as breadth.Like I said, it felt like the first time all over again.Kudos to you for the board design Ed, and you’ve got a real treasure in Jim. He went above and beyond to make everything perfect, I’m really surprised he hadn’t changed his phone number.Jim, I can’t say enough about how great it was to do business with you! Thanks for answering ALL my questions and helping me with your advice. Kind regards,A very satisfied customer,Russ Hunsberger Jim, My Hafler’s P505’s are exceptional! Between you and Ed at QUA-CO, I could not be happier.Ed‘s design and your assembly of these boards have put together a remarkable product. The best thing I did was listen to you when you said I should replace the Power Supplies, when all four of these amps were on and running at normal volume they would draw about 5 or 6 Amps when looking at my power conditioners meter, now 3 amps if even. As for sound quality they are remarkable. I am running a pair of Dahlquist DQ-10’s with the two matching Sub woofers and the Dahlquist DQ-LP1 Electronic Crossover. These speakers are known for their remarkable reproduction of sound, and I thought they were good before, but now, it is unbelievable. I am hearing instruments in songs that I have played for years and never heard. I went to the Hafler line, because after a lot of searching and reviewing to find the closest sound to a vacuum tube amp, in a solid state, all roads lead to Hafler, I have never looked back. Now I have four brand new amps that could very easily go up against amps costing $5,000 dollars or more. So thank you for an excellent job! Anyone out there thinking their Hafler amp is shot or too old, contact Jim and bring new life to your system, if you really appreciate high quality sound, and want a high end system do this upgrade, you won’t be sorry. I love music and you and

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