ART USB Dual Pre
Behringer B2 Pro
Denon DP-61F
Dynaco ST-70
Hafler DH-110
Hafler DH-500

  Klipsch KG1
Klipsch Promedia 2.1
Mogami/Canare RCAs
Nakamichi CM-300
Scarlett 2i2
Tannoy 603
Vox Pathfinder 15


If you're a Hafler fan you know that Hafler's legendary power amplifiers began with the DH-200 amplifier released in the late 1970s. When the CD format appeared in 1983 and music changed from analog to digital...listeners were astounded at the new amount of detail in recordings. Taking quick advantage of a better source, audiophiles discovered that the signal path in amps and preamps responded well to parts updates like polypropylene capacitors, film bypasses on the electrolytics, and by enlarging the power supply. Due to their unique kit-form and affordability Hafler equipment was perfectly positioned to be a guinea pig for swapping parts. The result was that Hafler became largely responsible for the modding craze of the 1980s.

All this attention didn't escape Hafler's notice and around 1988 the XL-280 was released with several of the popular mods. Electrolytics were bypassed, polypropylene capacitors and metal film resistors were used, and the power supply was a pseudo dual-mono/dual-voltage power supply with a two 8900uf/80V caps per channel.

Most interesting was that the XL-280 also came with a small "Excelinear" trimpot on each board that allowed the user to adjust the feedback loop to "null" out distortion. The setting was different for each pair of speakers and was made using a special "XL-10" device supplied to the dealers.


The XL-10 was a small metal box with two knobs, a single RCA input and two speaker jacks. The nulling process was very simple: once hooked up properly any music played sounded like small whisper out of one tweeter. The other speaker played at normal volume and to make the process easier it helped to lay it face down on the carpet. You then pressed your ear to the upright speaker and adjusted the null by using small factory-supplied screwdriver through a hole in the top cover of the amp.

While slowly rotating the trimpot with your ear to the tweeter, treble sounds would go from a subtle tss, tss sound to a mellower shh, shh and back to a tss, tss. By fiddling you could find one position that sounded slightly quieter...that was the null point. You then did the same test for the other channel. The result is that the amp was optimized for your speaker load and distortion was said to be reduced up to 70db. Whether this adjustment produced any audible benefit was open to debate at the time. Stereophile magazine gave the amp a Class C rating.

The downside of the Excelinear adjustment was that every change of speakers required a new nulling. If you were friends with the local Hafler dealer you could drive down and borrow their XL-10. But few customers knew about this device and I suspect in the mid 1990s when Hafler/Rockford stopped selling to the consumer market that dealers simply threw the XL-10 in the trash or gave them away. The dealer I worked for in the 1990s wouldn't give me theirs 'just in case' an old customer wanted to borrow it. Whatever happened to it I'll never know.

But realizing I couldn't have it, I simply opened up the XL-10 at work and sketched the interior. It contained a coarse and fine trimpot and a couple of resistors, simple. I bought a small test box from Radio Shack, some speaker jacks, the parts, and built my own. It worked perfectly and I used it for years. I wanted to include a photo here but both it and my schematic have gone missing. Keep in mind that this box only worked with the Hafler XL-280/XL-600 amps. If you're still using the original boards no need to stress about yours....simply put the trimpot setting in the center on both channels and enjoy.

The XL-280 is a great platform to use for mods because of it's six MOSFETS per channel (compared to four on the DH-200/220), 145 continuous watts per channel, and 'dual mono' power supply. One downside is that the original 30 year old boards lack a DC offset adjustment meaning it can't be adjusted if transistors are out of spec. As you can tell from my other Hafler pages I'm a big fan of Musical Concepts mods. So if in doubt I suggest upgrading the original boards to modern versions.

Below is a stock XL-280 with four Musical Concepts 12,000uf 80v caps replacing the 8900uf stock versions. I upgraded the board electrolytics and changed the film caps to other versions. Excellent sound quality, very punchy and 3D.


For my home theater I decided I wanted a true mono center-channel amp. I had spent quite a bit of time building a modified DH-500 for my front speakers (see link at top) it made perfect sense to use an 'identical' design for the center channel.

My first idea was to run a stock XL-280 the amp in factory mono mode by flipping the switch on the back. But I worried about the accuracy of summing two channels to combine the waveforms...especially if the original boards were old and out of spec. The other option was to only use one channel...but that felt like cheating by using half of a stereo amp. I decided it made more sense to have the entire power supply and transformer feed one channel...very robust.

So I bought a Dayton Audio APA150 150W mono amplifier as a test bed. This was a nice little amp and I liked its performance but my plan was to use its case to create a Hafler clone. I gutted it, cut and relocated the heat sink to the rear, and drilled holes for the Exicon MOSFETS. I shoe-horned in the original transformer and added some 12,000uf 80v caps and a single PA3-D board. The results were better than I expected.


The photo below with the cover off speaks for itself. It powered up perfectly, biased OK, no DC offset issues. The rear of the case retained the factory 120mm computer fan for cooling. I used this in my home theater for a couple of months. But...I found that it became very hot in use. The small heat sink coupled with a large transformer and little air flow was worrisome. So despite its success sonically...I decided to try another route.



I decided the best solution was to use a XL-280 chassis for my mono design. Not only would it match my current rack of Hafler amps, but its design was already intended to cool the Hafler circuitry. So I acquired one online with the sculpted faceplate and handles.

I retained the transformer but got rid of the rest of the power supply...the four large capacitors and related board. I installed a Musical Concepts PS-100 with IXYS 600V rectifier and wired it for a single channel using the 63V tap from the transformer. I had considered blue Cornell Dublier caps like the ones in my DH-500 but found they were too tall for the XL-280 chassis.

I replaced the old MOSFETS with new Exicon versions so they would match the Exicons in my DH-500. The other channel was left completely disconnected (if I get sentimental for the old Hitachi MOSFETS I can always switch everything to that channel). I replaced the Hafler board with a single Musical Concepts PA3-D. I made some updates to the original parts.

- Higher voltage electrolytics are Blackgate FGs 100uf/100v.
- The other electrolytics are Elna Silmics.
- An exotic PETP input cap is used with .01uf Wima FKS 2 bypass per board spec.
- The other .1 bypasses were changed to K73-11 PETP versions.
- The new power supply capacitors are 15,000uf 80v Panasonic TSUPs with an IXYS 600V soft-recovery rectifier. No worries about headroom...I think 30,000uf is just fine for a single channel.

The result is a dynamic and crystal clear true-mono center channel amp.

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All mods are illustrative only, perform at your own risk.
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