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Tripmaster 24th August 2010 01:25 PM

Transformer Screen & Signal Grounding Issues

I'm still trying to locate the source of my hum. I thought it might be worth fitting an aluminium screen around my F5 transformer. I'm not expecting it to cure the hum, but it might be beneficial.

I propose to bend a strip of 1mm thick, 10cm high aluminium around the perimeter. Do you think this will provide a good EMI screen?

Suggestions welcome! :)

Melon Head 24th August 2010 02:11 PM

I have been thinking about this for a long time and have not been able to find any specific answers.
I know they use iron to screen magnetic hum fields but I have not seen anyone mention using aluminium to do the same job.
I know aluminium is good for screening electric fields but is it good for magnetic fields?

Hopefully Nelson or someone else can put us straight here.

jackinnj 24th August 2010 02:21 PM

Aluminum won't help -- but you can use a "bimetallic" can -- like a soup can which has some effectiveness in shielding.

Some transformers have 2 screens -- one on the primary, another on the secondary, and a lead attached to the "can". The lead on the can is attached to the chassis where the green wire (in the states) is affixed. See the Cinemag website.

Other things you should look to -- detach the power supply from the amplifier -- use a pair of 2.2K 3W bleeder resistors on the positive and negative rails and see how much ripple you have -- could be a bum filter capacitor. In addition, I've seen some designs which use too big a snubber capacitor across the diodes, or across the transformer secondary -- just puts too much strain on the filter caps, sometimes you can have a faulty PEM (power entry module) which leaks a.c. to the chassis.

If you approach the problem stepwise, the solution should be near at hand.

Melon Head 24th August 2010 02:26 PM

Yeah. So it looks like it has to be iron, or a tin can (iron with tin coating).

Tripmaster 24th August 2010 02:29 PM

Thanks guys. Back to the drawing board...

jcx 24th August 2010 03:05 PM

look at the noise with soundcard and fft sw?

mains frequency fundamental "hum" is likely magnetic field coupling

2nd and higher harmonic series noise may indicate ps currents flowing in common impedance with the signal gnd - indicating a need to change wiring topology - adding shielding won't help

Rodeodave 24th August 2010 05:53 PM

2 Attachment(s)
Could we see a picture of your amp's insides please?

I recently had mains-hum issues with a gainclone. The wire running from the PSU to the PCB were almost touching the toroid, resulting in hum in the left channel. I wrapped the toroid in copper sheets I once bought in an art supply store. This primitive screen, once earth-grounded via the bolt/chassis cured the problem. Just make 100% sure the shield doesn't form a turn around the core...

Tripmaster 24th August 2010 08:52 PM my guest.

Its been a pulled about a bit whilst trying to locate the source of the hum.

Rodeodave 24th August 2010 09:17 PM

Nice pic, will meditate over it once I'm sober enough. Is there any additional info other than that there's hum? 50Hz?

Melon Head 24th August 2010 09:23 PM

Is the hum coming from the channel that is closest to the ac mains wires (blue and black wires) or is it coming from the other channel?

I don't expect this to help but I would probably twist those blue and black wires together.

I also see those red and black secondary transformer wires going up near the amp channel and crossing wires.
I would shorten those wires or whatever you need to do to route them straight down to the bridge rectifier.

You could also try rotating the transformer.

It might come down to a process of elimination.

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