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:
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.
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.
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.