Amplifier 50 Hz hum

Thread Starter

K04

Joined Jun 17, 2022
9
I'm trying to detect the source of the problem and hope you can help me to where should i look for it.

So when I'm raising the volume on the amplifier the hum gets more quiet then if its 0 volume.(Without any input, just the speaker connected)

It's not ground loop probably. Already checked.
 

Thread Starter

K04

Joined Jun 17, 2022
9
Can you post the make and model of the amplifier and a link to the schematic diagram if you have it?
It's an older home made one. I'm trying renovate it, so I don't have it sadly. I can send you the circuit of the amplifier, but the problem is not there in my guess.
Trying to make it alive first, so it's not that pretty, yet.
I ordered some new capacitors for the transformator (they were like 30+ years old, but not used much) but I think it's not the problem.
Just let me know if you guys have a guess, but nothing for sure.
Maybe someone met with a problem like this where the hum is still there with min volume.

Also I've found a ground loop, it's a bit better now but still there.
 

MrChips

Joined Oct 2, 2009
26,530
It's an older home made one. I'm trying renovate it, so I don't have it sadly. I can send you the circuit of the amplifier...
Please send or post your circuit diagram.
It would appear that the problem is related to the impedance presented at the volume control.
 

AnalogKid

Joined Aug 1, 2013
10,098
If the hum volume is constant with increasing signal volume, that usually means the hum is entering the circuit after the volume control. Often, the power to the power amplifier section is not regulated; the circuit relies on the power stage input common mode rejection. Or, if the amp has +/- power rails, that the hum present on one rail is inverted WRT the other rail, at least partly cancelling out.

Schematic, photos, a block diagram of the signal path - anything will help.

ak
 

Thread Starter

K04

Joined Jun 17, 2022
9
If the hum volume is constant with increasing signal volume, that usually means the hum is entering the circuit after the volume control. Often, the power to the power amplifier section is not regulated; the circuit relies on the power stage input common mode rejection. Or, if the amp has +/- power rails, that the hum present on one rail is inverted WRT the other rail, at least partly cancelling out.

Schematic, photos, a block diagram of the signal path - anything will help.

ak
Its a +/- power rail
 

AnalogKid

Joined Aug 1, 2013
10,098
The output coupling capacitor is only 22 uF. With an 8 ohm speaker, that is a highpass filter of 900 Hz. For way better bass performance and less phase shift into a 4 ohm speaker at 20 Hz, a better value is 3900 uF.

Any idea where the schematic came from? It looks like it might be part of an instruction manual for a pre-fab module.

Power supply schematic?

Here is the current datasheet:
https://www.st.com/resource/en/datasheet/tda7294.pdf

ak
 

Thread Starter

K04

Joined Jun 17, 2022
9
It is a pre fab module. The original one wasn't working.
The Power supply is the original so I don't have any schematic.
 

Alec_t

Joined Sep 17, 2013
12,904
Are you using screened wiring from the signal source to the amp input?
If so, is the screen grounded at both ends or just one?
 

Thread Starter

K04

Joined Jun 17, 2022
9
Are you using screened wiring from the signal source to the amp input?
If so, is the screen grounded at both ends or just one?
There was screened wires originally, but i replaced it with casual copper wires without screen.
 

Thread Starter

K04

Joined Jun 17, 2022
9
They were very old and the way it was wired...probably easiest to tell that all of the inputs was connected somehow but tbh i don't know how that happened. It' was a mess. Now only one input is connected, directly to the AMP.
 
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