Hi CS,
ok, did that, transformer about 1m/3 feet away. any other transformer inside the housing disconnected. Power for mute in/out relays from lab supply.
Hum is still there. But, when I switch the mains transformer off, mute is off (as in speakers running of the power supplys caps while they are discharging) the hum is still there.
Hmm, interesting, but not unusual.
Can you confirm this:
Configuration
(1) There is no mains connected to the amplifier whatsoever.
(2) The amplifier is in no magnetic fields: near to cables carrying mains, TV, fan etc
(3) The amplifier is on a non magnetic electrically insulating material.
(4) The mains torodial transformer is outside the amplifier case 1m away and is powered by the mains (230V 50Hz)
Symptoms
(1) With the above configuration the hum can be heard from both left and right speakers
(2) When the mains supply is completely removed from the transformer (live neutral and earth) the hum is still present from both speakers while the reservoir capacitors still have a voltage across them.
Now remember, the crossover sits over /on top of the power supply. When I disconnect the cable from the xover to monoblock in, there is small white noise, cable back on, buzz is back.
Can you also confirm this. With the configuration above, if you disconnect the two cables connecting the xover to the monoblocks, the hum stops and all you can hear is some background noise. Is that background noise normal and acceptable to you?
So next step is replace foil caps parallel to rectifier diodes in power supply with ceramics, as that seems to add some resistance and it acts as a snubber. But still, without transformer, why is it still oscillating? Crossover is not powered.
With monoblocks in mute, it draws 1.6 or so W, monoblocks on 12W. The transformer does not get louder wirh more load.
diodes are these
**broken link removed**
thread on bypass resistors on diodes
https://www.diyaudio.com/forums/power-supplies/133916-bypass-capacitors-diodes.html
I hope you do not mind me saying this, but it would be best to avoid considering any solutions at this stage until the root cause of the problem is established.
But just to say that from the symptoms, the fault is unlikely to be connected with the diodes or suppressor capacitors (to suppress diode commutation noise) across the diodes .
It is unwise to use ceramic capacitors in an audio amplifier, except possibly as suppressors across the rectifier diodes, but even so, polypropylene film capacitors would be a better choice. Ceramic capacitors not only generate distortion but their capacitance varies with voltage, temperature, and age. Ceramic capacitors can also act as electrical/sound transducers. These shortcomings are outweighed by positive characteristics for other applications.
Looks like no building work for me today- it's mild but pouring with rain.
spec
PS: I see no expense has been spared on components- those film capacitors must have cost a buck or two.