Vintage amp - 50Hz Harmonics in the noise

[alexk0il]

Hi,

I have a ~50yo vintage amp/receiver that gives me sound I am happy with. All is good, except for a humming that I hear between the tracks if I put the amp to higher volumes. It's obviously getting worse with headphones, but even at 1.5 m distance to my speakers I can still hear it.

According to my phone app the humming is at 50Hz harmonics, to be precise at100Hz, 150Hz, 350Hz, 450Hz, etc. See the brighter vertical lines in the attached spectrum diagram below.

I believe I can hear a similar humming if I put my ear close to where the transformer is, didn't open the case though yet.

I upgraded most all the large caps about 4 years ago, the amp comes with a 2 pin EU plug. BTW, the radio receiver part is broken, I somehow managed to damage it during the recap process.

Anyway, I attached the amp's ground connector to the heat radiator, it helped a bit. Tried switching off other electrical aplliances that might be connected to the same power rail in the house, no difference.

Time to replace or is there anything I can try to reduce this noise/humming?

Screenshot_2023-04-29-12-58-23-449-edit_org.intoorbit.spectrum.jpg

[Macca]

most likely a ground loop, and the transformer hum is coincidental.

Often there's no easy solutions to a ground loop. I'd probably replace it with something more modern if I were you.

[alexk0il]

Watched a really long youtube video that was chasing ground loops in a vintage amp. Turned out that the case/motherboard screws got loose, causing ground loops. So...

I opened the amp, did a bit of cable management inside the case, tightened a quite a few loose screws. This reduced the hum a lot.

Then I did a cable management inside the cupboard. Found a long hdmi cable going to TV on one side and disconnected on another. Removed it, and to my surprise the humming went down by another level. Perhaps it behaved as an amplifying antenna inducing additional noise to my poor amp.

And finally, I accidentally found that both the background hiss (inaudible with speakers though) and the humming almost disappeared when I crank the volume all the way to max. Not sure why, both of these noises go up all the way to 15 o'clock and almost disappear quick when I turn the knob to 18 o'clock.

I'm curious, is this expected or have I discovered an Nobel Prize level anomaly in my G-class amp?

Anyway, the amp is usable again, I might keep it for a while. If the noise doesn't cone back I might try adding a power switch activated by sound outputs from my pre-amp.

[Macca]

Then I did a cable management inside the cupboard. Found a long hdmi cable going to TV on one side and disconnected on another. Removed it, and to my surprise the humming went down by another level. Perhaps it behaved as an amplifying antenna inducing additional noise to my poor amp.

Yes could be, I've encountered similar effect. Cable management is worth the effort sometimes.

[alexk0il]

Yes could be, I've encountered similar effect. Cable management is worth the effort sometimes.

This is easy to explain. But noise going down when volume goes above certain level is counterintuitive. I'm puzzled....

[Firebottle]

It's likely to be a change of impedance within the amp between stages.

[alexk0il]

Had another round of cable management in my cupboard, main goal was to further reduce the interference.:

1. Moved power supplies of all other units in the cupboard to a different socket.
2. Created space between the amp and other units in the cupboard to reduce
3. Moved all irrelevant wires away from the amp
4. Finally, I arranged the slack of the long wires to create a twisted couple, then rolled the into a smaller loops

The amp is whisper quiet at max volume now, virtually no hiss, no hum. Wow, never had such a clean sound from the amp. Super happy.

What's strange about my amp is that the top cover is painted both inside and outside, it doesn't have any contact with the grounded base part of the amp. I am thinking of shaving the paint under the screws that connect it to the base, this should effectively ground it. Would it be a good idea or am I missing something big time?