If you're a Hafler fan you know that Hafler's legendary power amplifiers began with the DH-200 released in 1979. A few years later in 1983 when the CD digital format appeared hobbyists discovered that amps and preamps responded well to upgrades like polypropylene capacitors, film bypasses on electrolytics, and enlarging the power supply. Due to their kit form and affordability Hafler products were perfectly positioned to test these concepts. The result was that Hafler was largely responsible for energizing the modding craze of the 1980s that continues today.

All this attention to upgrades didn't escape Hafler's notice and around 1988 the XL280 was released including several of the popular mods. Its 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. Stereophile magazine gave the amp a Class C rating. When Rockford took over Hafler in 1987 it added optional sculpted faceplates to its amplifiers which upgraded the styling of the Xk-280. Uniquely, the XL280 came with a tiny screwdriver to let you adjust the "Excelinear" feedback loop through the top panel which is explained below.

THE XL-10 DEVICE
A tiny trimpot on the board for each channel lets you adjust the feedback loop value. This allows you to "optimize" the amp for your speakers and measureable distortion was said to be reduced up to 70db. Whether this adjustment produced any audible benefit was open to debate at the time. To make the adustment a small box called the XL-10 was given by Hafler to its dealers. About the size of a paperback book, it had two potentiometer knobs, a single RCA input and two speaker jacks. When attached to the XL280 music would play like a small whisper out of one speaker while the other speaker played at normal volume. You then pressed your ear to the quieter tweeter and using the screwdriver through a hole in the top cover of the amp made an adjustment.

While slowly rotating the trimpot treble sounds would go from a subtle tss, tss sound to a quieter 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 downside of this adjustment was that every different speaker pair 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 when the XL280 was discontinued just in case an old customer wanted to borrow it. Whatever happened to it I'll never know.

However, I opened up my store's XL-10 and sketched the interior. It contained a coarse and fine trimpot and a couple of resistors, simple. So 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 for my XL280. I wanted to include a photo here but both it and my schematic have gone missing. Keep in mind that this adjustment tool only worked with the Hafler XL280/XL600 amps. If you're still using its original boards no need to stress about yours, simply put the trimpot setting in the center on both channels and enjoy.

BASIC MODS
I think the XL280 is a great platform to use for mods because of it's six MOSFETS per channel compared to four on the DH-200/220. With 145 continuous watts per channel and 'dual mono' power supply its a a compact and robust amp. One downside is that the original 35+ 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 photo from about 2005 with four Musical Concepts 12,000uf 80v caps replacing the 8900uf stock versions. I had upgraded the stock board electrolytics and film caps.

Later I upgraded the boards to Musical Concepts PA-4 versions. The power supply became a PS200 with an IXYS 600V rectifier. I use this amp as a rear channel amp in my 5.1 system. Clean and tidy.

XL280 MONO PROJECT
In the early 2000s I was constructing a new home theater and decided I wanted a mono center-channel amp. I could have used a stock XL-280 in mono mode by flipping the factory 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. So I decided it made more sense to have the entire power supply and transformer feed one channel in true mono fashion.

For my project I bought a Dayton Audio APA150 150W mono amplifier as a test bed. I gutted it, relocated the heat sink and drilled holes for the MOSFETS. I shoehorned in an XL280 transformer and added some 12,000uf 80v caps and a Musical Concepts PA3-D board. The results were better than I expected.

It powered up perfectly, biased OK, no DC offset issues. The rear of the case retained a factory 120mm computer fan for cooling. I used this in my home theater for a couple of months but I found that it became hot in use due to the large transformer and poor airflow. So despite its success sonically I decided to try another route.

XL280 TRUE MONOBLOCK
I decided to use a XL280 for my next mono design and acquired one with a sculpted faceplate and handles. The original MOSFETS were fine but I replaced them with new Exicon versions and a Musical Concepts PA6s. I also installed a Musical Concepts PS-100 power supply with IXYS 600V rectifier and wired it for a single channel using the 63V tap from the transformer. Panasonic TUP 15,000uf 80V caps gave me 30,000uf for a single channel(!) I have since replaced the input wiring to match the stereo XL280 above.

The results were better than I expected. Used on my center speaker intelligibility and clarity was pristine. Dynamics were more powerful and clean and high volumes are clear as a bell. Along with my DH-500 on the front channels and XL280 with PA-4s on the rears it creates a very robust and satisfying listening 5.1 system. Great stuff.