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I'm building a microphone preamplifier. I was having problems with hum (intermittent ground hum when touching the case) and decided to try adding an audio isolation transformer (600 ohm : 600 ohm) before its output. The circuit is powered by AA batteries.

I wired this up and then tried it out plugged into a guitar amplifier, an amplifier that I'd been using before (with success), to test the circuit before I added the transformer.

The amplifier blew its fuse after a couple of seconds of loud low buzz/hum. This was a surprise, because as I understand it a 1-to-1 transformer doesn't change the voltage or current, and for a fuse to blow the current would have to go over the fuse's limit.

What about the signal did the amplifier not like? What could have gone wrong?

To be clear, The preamp was working without the transformer (other than the occasional ground hum. which I've now worked around with some electric tape). The only thing that changed was putting in the transformer right before the output jack. the transformer can't be wired up backwards because it's 1:1. right? so that would seem to eliminate some error in connecting the transformer.

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    \$\begingroup\$ Perhaps the transformer was wired incorrectly. Or not. Insufficient information. \$\endgroup\$
    – DrMoishe Pippik
    Commented Jun 20 at 2:25
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    \$\begingroup\$ How is your preamp powered (a self-contained battery perhaps?). In some cases the connecting cable carries both DC and the AC(audio) signal, eliminating the need for batteries. In that case, a 1:1 transformer shorts out the DC source. \$\endgroup\$
    – glen_geek
    Commented Jun 20 at 2:56
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    \$\begingroup\$ @steev To figure it out I was asking you to provide more info. We don't still know if you made some incorrect connections to explain what happened. We don't know how your preamp was powered either. Maybe your preamp saw hum and drove the hum through transformer to guitar amp. We have no clue what your preamp looks like. In order to replicate your experiment, we need to know what was connected and how. \$\endgroup\$
    – Justme
    Commented Jun 20 at 3:53
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    \$\begingroup\$ Are you sure, that the preamplifier can drive this transformer? The datasheet on page 8 says, that the load should be > 5 kohm. \$\endgroup\$
    – Jens
    Commented Jun 20 at 18:38
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    \$\begingroup\$ The SSM2167 amplifier IC has 40dB of gain. If the mic wires were loose, it received mains hum and amplified it to levels that damage the guitar amp, which also has a huge gain. Only unamplified things or things with low enough output voltage ability should be connected into guitar amps that are intended to take in only low enough voltages. I am not saying it did happen but do verify yourself - the audio isolation transformer may have nothing to do with it, or it might have been incorrectly connected. There is not enough info to try buying what you bought and connecting them the way you did. \$\endgroup\$
    – Justme
    Commented Jun 20 at 21:56

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