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Transformers in integrated amplifier


Jun 9, 2021
While hunting for the source of the hum coming from my Cambridge Audio 840A I noticed that it might not come from the big toroidal transformer in the middle of the amp, but rather from a small one that's mounted to the back plate of the amp. Some investigation revealed that this may be the transformer for the pre-amp.

Although it seems strange (at least to me) that the smaller one is the noisiest, it definitely seems so. I was a bit cautious putting my ear too close to the amp with the top cover removed and powered on though.

This raises a few questions:
Could it be that the smaller one has a (mechanical) hum, while the bigger one is quiet?
If it's the small one, what would be the best way to get it quiet?

From the service manual I got the following details for the small transformer (the one that seems to hum):
Toroid 230V/50Hz 7.5VAC750MA 17VAC*2 TO7405B STB
The big one is:
Toroid 230V/50Hz 36.5VACx4 5.5A TI-053044C


Major Contributor
Feb 6, 2020
Hum is a mechanical effect. Try shock mounting the xfmr, using a rubber or foam pad to mechanically isolate it from its mounting surface.
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