Transformer Screen & Signal Grounding Issues

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Hi

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! :)
 
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.
 
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.
 
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
 
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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...
 

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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.
 
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.


Hello again :)

Its coming from both channels, and is at the same volume.

I'll whip the transformer out tomorrow and see if it helps.

Keep them coming!
 
Ok you shouldn't have issues because of the speakers.

I would definitely try turning the transformer 180 degress with secondaries coming out near the front of the case right next to the bridge rectifiers.

If you can pick up some galvanised iron sheet (or a tin can) it might be worth making your own magnetic shield to put around the transformer.
 
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...
I'll whip the transformer out tomorrow and see if it helps.
...

Good thinking, take the transformer out of the amp, as far away from signal carrying wires as possible. That's how I diagnosed the problem with my Gainclone.

Twist all AC carrying wires (mains, secondary,...).

I can't really tell from the picture, where do your circuit grounds meet, and are the connected to earth ground? If yes, through a thermistor hopefully?

Are all RCAs and speaker terminals insulated from the case?

I too would suggest to turn the transformer 180°. It looks like you're using a power socket/fuse/switch combo, that's fine, but move the screw terminal block to the other end of the chassis, running twisted wires from the switch. Will be very neat once the xformer is turned.

Also, move the bridges to where the 2-port screw terminals are currently, this will result in shorter wires going to the PSU capacitors, which is highly desirable. Possibly you can ditch the secondaries' extensions and run them directly into the bridges. I don't see why not.

In general keep all AC carrying wires as far from any audio circuitry as possible.

And don't give up, hum is almost always curable.
 
Aluminium actually can provide a magnetic shield. Through lossy magnetic shielding
"Lossy magnetic shielding depends on the eddy-current losses that occur within highly conductive materials (i.e., copper, aluminum, iron, steel, silicon-iron, etc.). When a conductive material is subjected to a time-varying (60 hertz) magnetic field, currents are induced within the material that flow in closed circular paths - perpendicular to the inducing field. According to Lenz's Law, these eddy-currents oppose the changes in the inducing field, so the magnetic fields produced by the circulating eddy- currents attempt to cancel the larger external inducing magnetic fields near the conductive surface, thereby generating a shielding effect."

But it is only about 5-6dB according to John Swenson in a bottlehead post

Hammond Chokes w/ Quickie - Cheap and Worthwhile!
 
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