There were really four tweaks that solved the hum problem:
- Use of the choke
- Zener diode tweak
- Replacing the heater supply schottky rectifiers with general purpose diodes.
- Installing snubber caps on the diodes in #3 above.
The last two were the latest and solved the hum issue from all reports.
We're now shipping with a V1.02 PCB that Dsavitsk designed. Changes are as follows:
- Trace moved under the PCB that was impinging on one of the standoffs
- Allowance for zener tweak with normal through-hole mounting.
- SMD pads on the bottom of the PCB for snubber caps on the heater rectifier (general purpose diodes).
Wow, thanks for the great information. A few more questions if you don't mind. How many of these tweaks are included in a complete kit (I'm pretty sure I read the Zener diode and a Hammond choke are part of it)? Regarding the scope of the project, aside from the SMD pads for the snubber caps, is the rest of the project through hole?
This seems like an exciting project for me, but I may need to tackle a smaller one or two (maybe a CMoy or Mini3) before I take it on, just to feel a little more comfortable.
All of the tweaks are included in the kit. The new PCB recognizes this by incorporating all of them. I'm not sure why there would be a question of doing otherwise if it makes the amp better, but maybe making that strong statement will make it clear.
The SMD for the snubber caps are trivial. You can still solder leaded axial capacitors across the diode leads instead, if you wish - making it 100% through-hole. If you haven't soldered parallel leaded parts before, trust me when I say the SMD capacitors are much simpler. They're large pads, so 1206 SMD will easily work - or smaller, if you desire. I'll spec some parts shortly and update the BOM when I get a chance. For now, I've been including the leaded snubber caps in the kits.