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Transformer hum options

Bert Beukema

Bert Beukema

Well-Known Member
Hi all,

I restored a Denon PMA701 that was completely butchered. It took me over a year as a side project but it is finally nearing the end. There is one thing that is bothering me though: transformer humming over speakers (noise).

One of the things that was butchered in this dual mono amplifier was the tertiary transformer feeding the pre-amp, protection and lamp. It was a tiny transformer and it was gone. I've implemented 2 transformers (one for the pre-amp at 24v) and another for the rest (at 6v). The 24v transformer is an isolated transformer in an epoxy case in the position of the original transformer. This is close to the power switch. I've secured it off the frame with pole brackets.

Thing is, it is generating an audible humming noise over the speakers. If I move the transformer out of the enclosure of the case the humming stops. It is a new transformer. I believe this might have something to do with being in close vicinity to the power switch (and powerline). That being sad, I don't really know what to do about it.

Tried my test transformer which is not isolated and it hummed three times as hard. Tried a metal shield between the transformer and power switch innard, no effect. The only thing that actually helps is moving the transformer out of the case.

Anyone have any ideas or should I just accept this?

Regards,

Bert
 
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egellings said:
Have you tried rotating the xfmr a bit to see if there is a hum-null position?
Click to expand...
What he said. Try and rotate it 90°, mount on its side, etc. I have seen some preamps with the transformer mounted outside on the back of the chassis before. Also how long are the wire runs from the transformer? Twisting the wires from the transformers to where ever they connect.
 
Hey there,

There is only a very small space where I can fit the transformer unfortunately. I tried mounting it upside down (no effect). Moving it sideways doesn't help either.
Up to now the only thing to get it dead silent is to move the transformer out of the case. That will stop the humming.
Wires are pretty short because i've reworked the powerline bus; that was butchered as well.
But I did not try twisting the wires, let's see what that will do.

Thanks!

Bert
 
What I did was redo the wiring harnass for the mains. The cables are thick isolated, twisted and isolated again.
This reduced the hum by around 50%. It is passable now, you can hear it directly in front of the speakers. About 1 meter away and it is gone.
There was nothing else I could do in that very small space unfortunately.
There used to be a bespoke transformer there but the idiot before me busted it.

I do think it is somewhat odd because all of the new components are shielded.
Might have something to do with the power switch but that one is also a special bespoke unit.
Perhaps I can add some AC line capacitors over the input / output connections.

Bert
 
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