Buzzing power amp (Sherwood AM-8500) [UPDATE: Now measuring]

[pjug]

That way a lot of obvious voodoo bullshit lies; and - as far as I can tell - very little that's convincingly not bullshit...

You don't think the mechanical hum is likely to be the transformer? A DC blocker may help or it or may not. I have similar hum in one of my amps and a DC blocker did not help. Others have said that a DC blocker fixed this problem.

Edit: maybe you will still think this is all BS but here you go anyway
https://www.audioholics.com/amplifier-reviews/monoprice-monolith-7-channel-amplifier-review
If you have dimmers in your house or the issue known as DC offset then the Monolith’s massive transformers may start to hum. They did in my case when the lights on my microwave were on and set to low. To be clear, this isn’t a design problem with the Monolith. To fix the humming, I plugged the Monolith into Emotiva’s CM2 AC line restoration and common mode filter system. This two-outlet receptacle is specially designed to eliminate DC offset issues without limiting current or dynamics.

 

[Gphoton]

I had a SHERWOOD S6040 CP MOSFET DUAL MONO MOSFET AMPLIFIER 100 WPC back in the 1980's. It was nice sounding, I sold it to my brother in law. I used to sell Sherwood. They used discrete transistors when much of less expensive audio was moving towards the ICs of the day. It sounded okay on my Kef 303's. It did run HOT though, seriously. Egg frying hot. Which meant it had a high bias on the output transistors which is probably why it sounded good. I would suggest the following options: 1) if it is a labor of love or an interesting learning experience, fix it, with the caveat that there is danger. You can buy a single channel FNIRSI small yellow oscilloscope for around $70 if you shop around. With a 10x probe it can go to 800v. If you don't know what that means then don't start. Replacing the caps (most likely the problem) won't be that cheap. OR 2) buy or kit build a TDA3255 amp from the TI eval board or 3e audio, which may cost similarly when labor is included and sound better.

 

[trl]

Interesting myself in finding the root-cause too. The sound was pretty similar with a relay that is switching very fast too, although I understand the trafnsformer was the noisy one.

 

[Nebbermind]

After 25 yrs, the e-caps are probably drying up. I believe those huge reservoir caps in the rectifier circuit should be near end of life.
Another possibility dry solder joints due to aging

 

[Deleted member 48726]

There is a Sherwood AM7040 for sale in my area. I wonder if the OP's issue was resolved or not?

 

[Kaktus_Kontrafaktus]

There is a Sherwood AM7040 for sale in my area. I wonder if the OP's issue was resolved or not?

It wasn't but simply because I didn't bother to try. It doesn't really bother me in practise, not enough to get the unit to the next proper specialist and go without it for a couple of weeks, anyway.
The hum coming from the transformer should be easy to check for, it's clearly audible and independent from the audio signal.

 

[Kaktus_Kontrafaktus]

Did you resolve the issue ?
Just curious.

I decided that I could live with it since having it fixed seemed like too much effort/cost back then.

Using another (possible defective?) DAC, I discovered some new weird behaviour: https://www.audiosciencereview.com/...-dx3-pro-quiet-mains-hum-on-rca-output.54054/

I'd be interested in your take.