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PostPosted: Wed Oct 28, 2015 5:53 pm
by rockdrigo
Hi guys,

I have a problem:

When I inject pure tones in my ST70 with the speakers replaced with 8 ohm power resistors, I can clearly hear the tones ¨transduced¨by some part of the amplifier. I tried grabbing all possible ¨transducers¨ (transformer bells, tubes, metal sheets) but the sound keeps coming. I can also clearly listen to music (not only tones). Both channels independently and/or together.

Any hint?



Re: Microphonics?

PostPosted: Wed Oct 28, 2015 10:31 pm
by Geek
The output iron is "singing". This is normal for an un-potted transformer and is of no operating concern.

Re: Microphonics?

PostPosted: Thu Oct 29, 2015 4:47 am
by rockdrigo
Hi, indeed, the iron is probably singing but I doubt that it does not have any operational effect since the mechanical vibration certainly soaks energy that should be delivered to the speaker. I have built three other ST-70 without this problem.