robbie010

Amplifier Hum - Whats realistic?

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As jazid says it is probably a bit more of an in depth fault and eventually you may need more specialised help to find the fault. But humming is usually down to grounding in a power amp. 

Most 50 watt amps are designed for speakers in the 80/90 dB range not 100+ speakers 

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Super Wammer

Why do the capacitors near the volume control have masking tape on them ?

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3 hours ago, robbie010 said:

I can’t see any pots in there....

D7A57D15-C0E4-485B-86EB-06EA8CAD31BE.jpeg

I can only add a couple of small points to what others have said, all of which I agree with. Basically, a push-pull design like that, based on circuit boards and with a reasonably conventional power supply, has many soruces of noise. It would be minimal with the sort of speakers it would normally be used with.

Without quite a lot of measuring, it's hard to ascertain the source, but if it's all but inaudible with 'normal' speakers, it's not a major ground loop or the like. At a guess, I'd put it down to a combination of PCB layout, internal layout and some power supply noise. For the first and last of those, there's not much you can do without fairly serious modification. For the internal layout, at a guess I'd say the ribbon cable is just input switching and the actual signal cable from the input boards is the black wires that in the picture run from around the volume control and sort of snake underneath the board in the centre. You could try re-routing those a bit to take them away from the mains transformer a bit more and get them out from under the board in the centre assuming they aren't connected to it (could be, if it's killing the input with a timer on initial switch-on). If you feel like a bit of relatively simple metalwork, I'd also suggest some sort of metal screen between the mains transformer and the volume control. Similarly, the line-level feed from the volume control to the bottom board in the picture looks as though it runs very close to the mains transformer, so a small piece of metal somewhere there wouldn't do any harm.

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Super Wammer
On 27/03/2020 at 13:31, rabski said:

I can only add a couple of small points to what others have said, all of which I agree with. Basically, a push-pull design like that, based on circuit boards and with a reasonably conventional power supply, has many soruces of noise. It would be minimal with the sort of speakers it would normally be used with.

Without quite a lot of measuring, it's hard to ascertain the source, but if it's all but inaudible with 'normal' speakers, it's not a major ground loop or the like. At a guess, I'd put it down to a combination of PCB layout, internal layout and some power supply noise. For the first and last of those, there's not much you can do without fairly serious modification. For the internal layout, at a guess I'd say the ribbon cable is just input switching and the actual signal cable from the input boards is the black wires that in the picture run from around the volume control and sort of snake underneath the board in the centre. You could try re-routing those a bit to take them away from the mains transformer a bit more and get them out from under the board in the centre assuming they aren't connected to it (could be, if it's killing the input with a timer on initial switch-on). If you feel like a bit of relatively simple metalwork, I'd also suggest some sort of metal screen between the mains transformer and the volume control. Similarly, the line-level feed from the volume control to the bottom board in the picture looks as though it runs very close to the mains transformer, so a small piece of metal somewhere there wouldn't do any harm.

I pulled the tubes this morning and did a little tidying of the internal signal cables, you were right. The signal cables were touching the large transformer at the front and picking up noise. 

I used some silver foil tape to shield them and move them away from the transformers as best I could. There was definite improvement in the noise floor! 👍🏻

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I once traced a hum to a 2mm bare wire at the input socket of an amp. Had to make 'flying leads' to rectify.

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Posted (edited)

Putting the volume control that near a mains transformer is asking for trouble.

There,s some copper foil around the alps pot probably in an attempt to reduce noise pickup from the mains transformer. Unfortunately copper nor aluminimu foil will work well trying to sheild against magnetic interference. You need a high permeability material like mu metal instead which you can get in adhesive backed foil form. 

In a similar case I used an aluminium "wall" between a mains transformer and i/p wiring to which I stuck mu metal foil. Using an oscilloscope probe as a "sniffer" you could easily see the dramatic reduction in pick up behind the "wall". 

Edited by zeta4
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