Modifying the Hafler DH-500 amplifier

Hafler DH-500 is one of the best built power amplifiers with extremely high quality sound rivaling the best available today power amplifiers. Just two numbers to think about:
Power is 255 Wpc in 8 ohm, and typical THD at 1 kHz - 0.002%;

Click on the image to enlarge


Following the discussion in:
I've decided to modify my recently acquired Hafler DH-500.

Original specs looked quite impressive, as well as resume of Mr. Hafler himself, so despite the multiple posts, I assumed that there is nothing can be done to dramatically improve the sound of this amp, without rebuilding the whole amplifier from the scratch (even popular Musical Concepts modification only declare the improvement in sound quality but does not publish actual, "improved" over original parameters). Especially, I don't see any reasons to temper protection circuitry circuits.

But some small modifications that add reliability, do minor improvement of the sound, and address some usability issues were worth doing.

My modifications of the Hafler DH-500:

1. To prevent the power switch fails I upgraded it from stock 15A to 20A rated switch.

2. I replaced original RCA jacks with high quality gold plated RCA jacks.

3. Input wiring was replaced with shielded coaxial cable. That one gave immediate result. Original wiring was quiet only with shorted inputs. Without any cables connected amp produced a noticeable buzz (hum). After modification amp became quiet with open inputs also.

I had a question: what kind of cable to use? In reality, it does not matter. I used some junk wire from my box (E35614 AWM 1533 VW-15C ). Almost any coax wire would work, with exception of the cheap wire that came with chinise headphones.
Make sure that:
- copper is not corroded (beter be shiny). If you have silver coated it'd be even better.
- if unknown, insulation material between innner wire and woven copper shield is not less than 1 mm thick, so the diameter of the insulation is greater than 3 mm, or 1/8 inch.
- material of the insulation is not critical, something like teflon is preferable from the technological point of view (easy to deal with when you solder it).

4. Gold plated brass binding posts were installed for the speaker outputs. Bigger holes in the upgraded posts made connection of the 14 GA speaker wire much easier.

5. Power cord upgrade included installation of 20-amp gold plated IEC adapter. This permits usage of commercial or DIY power cords, which have IEC adapters on the component end. I don't believe that different power cords "sounds differently", but if particular installation permits, I prefer the shortest and thickest (14 GA is a reasonable grade) cord possible.

6. Ground connect/lift switch was added for the ground power wire to minimize possible audio hum in the large audio and video systems like mine caused by a "ground loop". Raising the Ground Lift switch to its up position puts a 10 Ohm resistor in series between the power ground and the chassis. The chassis is still grounded.

7. Bias was set to 350 mA of idle current. DC offset in both channels is less than 10 mV.




Amp before

Amp after