[simoon]

I have upgraded the old out of tolerance electrolytic caps on the driver boards with new, audio grade caps on 3 Hafler DH-500s. The integrity of the original circuit is intact. I just replaced old parts with the same value of new caps.

The first one I did sounds perfect. I believe, the amp sounds better than new.

I then did another identical one for a friend. He's also impressed with the results. Again, no problems.

The third one I did has a slight buzz in the rt channel. It is not a hum. It hasn't altered the sound quality of the channel. The volume of the buzz does not go up with system volume. I've been over it several times. All the soldering is solid, and the parts are the same as the other 2.

Anyone have any ideas?

[AVnerdguy]

DO you have a scope? Check the ripple on the big power supply caps. If that's good check the bias current at the output transistors. Have seen it off and will induce a low level hum. Changing out old caps may change the voltages a bit. If the outputs are switching at the wrong time it could cause that issue.

[avahifi]

Check your wiring layout carefully too.  The wires running to the fan can create noise if not routed exactly per the instructions, as can the AC wiring to the thermal sensors under the heatsink assembly.

Frank Van Alstine

[simoon]

I would assume that if it is the ripple on the PS caps, the buzz would be in both channels.

I haven't checked the bias yet, that was on my list.

I did check the DC offset though, almost dead at zero V.

[simoon]

Check your wiring layout carefully too.  The wires running to the fan can create noise if not routed exactly per the instructions, as can the AC wiring to the thermal sensors under the heatsink assembly.

Frank Van Alstine

Now that sounds like a good possibility. I did have to manipulate the wires quite a bit to get the heatsink raised up and reinstalled.

I am betting that you've nailed the problem. But if anyone would, the money would be on you!