MODIFICATIONS
ART USB Dual Pre
Behringer B2 Pro
CME UF8
Denon DP-61F
Dynaco ST-70
Hafler DH-110
Hafler XL-280





  Klipsch Promedia 2.1
Klipsch KG1
Mogami/Canare RCAs
Scarlett 2i2
Tannoy 603







HAFLER DH-500
UPGRADES and MODIFICATIONS






  • The mod begins


    I have always been a fan of Hafler equipment. My first amplifier was a DH220 kit I bought in 1984...and in the early 1990s I was lucky enough to work for an audio salon that carried the entire line. Among the items I bought back then was a XL280 and one of the last DH110 preamps off the assembly line. But because I was living in an apartment I never bought one of Hafler's higher wattage amps new when I had the chance.

    Having modified my other Hafler equipment over the years I decided that a great project would be to resurrect an old DH500. Online I found a cheap one...it had seen better days and looked like it had seen some serious band use...one power supply cap had been replaced and one Hitachi mosfet had been replaced with a non-matched version. It looked like it had been dropped down a flight of stairs...but it powered right up and played ok.





    It was filthy. The fan was clogged with dust, which covered everything.
    The wiring was messy, it looked like a kit version.



    Much cleaner after it was blown out. The boards evoke a nice 1980s feel.

    CIRCUIT MODS
    Originally I planned on simply cleaning up the unit and adding some Musical Concepts boards. I had used their mods on several pieces of equipment in the past...and found them to be far superior to the original circuits. The photo below is in the process of rewiring and adding a pair of new 39,000uf caps with the stock boards. But after putting everything back together I found that the old replacement mosfet someone had installed was causing bias problems and wouldn't adjust properly. Since replacement Hitachi mosfets are long gone...I wondered what to do.

    Luckily Exicon makes high power mosfets intended for audio amplifiers which are drop-ins for the DH500/XL600...the EC10N20 and EC10P20. I bought a matched set from Musical Concepts and put them in myself with new mosfet sockets and a pair of new PA-3 boards.

    I also sourced a new Oslo high current power switch and completely removed the DC-offset relay circuit. At this point pretty much the only original items left were the transformer and fan assembly.

    BELOW: After upgrades: new Exicon mosfets, PA-3s with Blackgates, and pseudo dual-mono power supply with four 39,000uf 100V caps. The cap wiring was tricky. The sound? The Exicons are very detailed and clean, and the overall sound was dynamic and 3D, with monstrous bass impact. I was happy.

    But in 2014 I got the upgrade fever again and decided I didn't like the rats-nest of wiring in the power supply. So I replaced it with a pair of Musical Concepts PS-200 boards and eight Cornell Dubilier 12,000uf 100V caps (96,000uf). IXYS 600V rectifiers were added along with a set of MC PA-4 'Standard' boards. These use a transparent PETP input cap of overseas origins. Dale and Takman resistors as well as Silmic and Nichicon FGs round things out (below).

    You'll notice that there are no polypropylene bypasses on the PA-4 board. Bypasses are a decades old trick that made sense on slow electrolytics, but things are different today...as the quality of electrolytics really has come a long way in just the last few years. Current wisdom is that when improperly done, bypasses can add an unnaturally bright leading edge to transients...obscuring the natural decay of a note. A good analogy is the jaggedness caused by the sharpness filter in Photoshop. The new power supply boards shown below eliminate the stray inductance caused by the old wiring. By design there are no bypasses on the power supply....the IXYS circuit is so efficient its not necessary.

    The amp sounded excellent at this point. Spacious and dynamic, with a pure and accurate soundstage. Transients were now ultra clean without hyper-detail. At first I thought the sound a bit polite compared to the older PA-3s with Blackgates and power supply with multiple bypasss. But after a week or so of break-in the sound developed a dynamic, natural soundstage with absolutely no glare or edge even at stellar volumes. I realized the PA-3s had a brighter edge in my system that seemed detailed...but was actually edgy. In case you wondered I'm using vintage modified Klipsch Fortes IIs with 98db efficiency.

    WIRING: a frequently overlooked upgrade is the power switch wiring. People will replace nearly everything in a DH500 yet leave the skimpy 30 year old 18g wire in place which can create a current bottleneck. An interesting view is rather than imagine an amp as the last electrical item in a chain, that a foot of internal wire doesn't matter...its actually the opposite. The first thing the amp "sees" on its route out to the wall socket is its internal wiring. And rather than sitting at the end of a 110V wire its actually sitting between the rails. So as a first step I replaced the power cord with a heavy gauge 3-prong but clipped its ground wire to prevent any ground loops. I then replaced all the power switch wiring with quality stranded 16g and changed the ancient terminal strip to a Molex "euro-style" version. The original fuse holder near the power switch looked grungy so I replaced it as well.

    Little noticed is a twisted-pair that runs from the terminal strip to the the bottom of the fan assembly. If you flip the fan over you'll see it passes through the inrush limiter (thyrister) and two thermal breakers. This means one rail of 110V entering the amp loops under the fan module and back out before even getting to the transformer. As you would expect I replaced that with 16g as well. I swear these wiring changes added significant clarity and solidity to the sound at higher volumes.

    FAN: I removed the fan assembly by taking out the two rear housing screws...the motor spindle came apart easily. I then cleaned the shaft and lubed it with some very light CRC precision oil. When tightening everything back up I spun the fan by hand to align the shaft with the bronze bearing. The result was a whisper quiet fan.


    As a final touch I sourced a new late-version DH-500 faceplate which Rockford used to replace the olive-colored/sheet metal version around 1990. I also sourced some new XL600 handles which after measuring and drilling made for a great pseudo-XL600 front panel. This let me use this in a rack with two XL280s to create a matched look.

    COMMENT: Amplifiers in the 1970s-1990s were Class A/B or Class A designs using the concepts of high-current and continuous watts...with the expectation of driving difficult speaker loads at high volumes without running out of steam. Because of their robust designs, even in stock form Hafler 500s were routinely used by bands and nightclubs due to their clarity and seemingly endless power reserves.

    However, things changed in the late 1990s as retailers began shifting their store stock to mini-systems and 'surround sound in a box' designs. While the use of separates or high-dollar receivers remained obvious to the audiophile market, the general public wanted cheap & simple solutions that were easy to hookup. The industry responded and while separates still exist today they inhabit a high-end niche...the more mainstream consumer products are Bluetooth and streaming-based using highly efficient, smaller, and somewhat magical Class-D amplifiers (look up the amps by SURE Electronics).

    Myself? I find the idea of using separates and a modernized 30-year old amp to supply 255+ continuous, musically transparent and bulletproof watts very gratifying.

     

     

     


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