Preamp hum - any idea to minimize it? (Schematics added)

[Salar]

Hello,
I have a slight, always prominent hum on my loudspeakers with this class a amp. The hum is independent from the volume and sounds a bit like a ground loop.

Schematic

First I thought, it w a s a ground loop, but it seems to be a weakness of the design.
Opened the earth connector at the AC plugs for testing, this had no influence on the hum.
I played a bit with the resistors circled in red and green on the schematics. (red connects to the chassis, green bridges the ground of the preamp) For example, removing the connection to the chassis minimizes the hum a bit, but hum will raise when turning up the volume.
Bridging the ground of the preamp with a cable and removing the resistor/cap pair of 4.7 ohm and 47nF raises the hum, even when volume knob is zero
All those tests were done with all sources disconnected.
Even unplugging the internal cable which leads from the volume knob to the inputs of the amp (all inputs are high level, no phono) will minimize the hum a bit further.

Again, I have to put my ears to the speaker to notice the hum.
But when my 20 years old Musical Fidelity B1 cheapo does not produce any hum at all, this amp, I guess even four times as expensive than the B1, should not produce any hum as well.
BTW, also hiss is more prominent than with the B1.
Any ideas about design flaws or maybe any parts to swap?
I changed the electrolytics for the Preamps power suppl, tonight I will change the Electrolytics of the power Amps. I don´t think, that this will help a lot...
All the best,
Salar

[lineup]

My first thought was that this amplifier was old and had bad eletrolyts in power supply.
But if you have good caps it is something else.

First: hum, 100 or 120 Hz, comes always from transformer + power supply.

This hum is carried by wires into amplifier.
Often because the Grounding wiring and decoupling is not done optimal.
Wost effect is when there is 100 Hz reaching the input transistors.

I see closest to the JFET input gate is one 1 Mohm resistor connected to ground.
This 20 x 50k(pot).
I usually feel alright if having 5x (~220k) even if the general rule is to try to have input bias resistor 10x (470k).

From one schematic is difficult to say if an amplifier have risc of humming.
Wiring scheme is a bit better, because it tells more how grounding is done physically.


Heve you tried to measure the hum at for example the 60 volt lines?

[Salar]

Hallo lineup,
thanks for the kind reply.
Physically, the grounding was done as follows:
Close to the volume pot/line inputs, 10 Ohm/100 nF were soldered in for grounding the Preamp circuit to the cabinet.
Also in this area, the pcb ground traces for both channels were cut and brigded with 4.7 ohm and 47nF- by the manufacturer I guess.

This produced an audible hum.

I did close the gap in the groundtrace of the preamp-pcb and cut a new gap closer to the star ground, where I did put the 4.7 ohm /47nF.

The Amp has four boards:
One fro the preamp, one for the preamp´s power supply, two boards for each power amp.

The center of the star ground for the whole amp is on the preamp pcb.
From there, one wire gores to the ground of the preamp´s power supply board.
Two wires go to the left and right amplifier boards Electrolytics an rectifier, from there to the loudspeaker negative terminal.

And, as mentioned before, two traces for the preamps ground.
They are cut open before they reach the star ground and bridged with 4.7 ohm/47nF.
I checked this gap, with With 0 ohm resistace, in the preamp´s ground trace, (Like this, the board´s layout was originally produced), the amp would even hum more.

I mentioned the 10 Ohm/100 nF connection from the preamp to the chassis.
Like it was done before , almost thewhole ground trace of the pcb was between the star ground and 10 Ohm/100 nF connection to the chassis.
I removed 10 Ohm/100 nF completely from the preamp board and made a new connection to the chassis on the preamp´s powersupply board.
There were even markings for this connection on the powersupplay board, but it was not equipped. I guess, because the chassis is painted and you need to scratch the paint away for proper contact to the chassis. Where the 10 Ohm/100 nF connection was before, the chassis was not painted, so it could be soldered to it.

Those mods, moving the 4,7ohm/47nF bridge closer to the star ground and moving the 10 ohm/100nF ground to chassis connection to the powersupply board
minimized the hum a bit.

BTW, the 10ohm/100nF connection to ground is described in the schematics as 10 ohm/47nF.

I do not understand, what you mean with the pot.
Replace 1 Mohm with 470k?

With my loudspeakers, I already reach unbeareable loudness with the volume knob turned up one third.
Would 470 ohm lower the overall volume as a side effect and therefor also lower hum?
All the best,
Salar