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Strange 150hz tone behaving like ground loop (sort of)
Old 1 week ago
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Strange 150hz tone behaving like ground loop (sort of)

Hey everybody, I had a strange problem last show I could use some input on.

We rolled into a new club and our recon had us already aware their power was not optimal. We set up our PA system and Eq'd it, got the canned music going and it sounded great. We set up the instruments and Mics. Everything still sounding great, then we started setting up the Marshall, and it's very elaborate analog pedal board.

As soon as the guitar player turned up the volume, we could hear a noticeable 150hz hum (much higher than a normal ground loop). If there were no gain intensive pedals on, the tone was very manageable, but as soon as a compressor was hit or a distortion box, the tone(hum) could get as loud as the music at a low volume.

It was at this moment we realized the entire PA was producing the same tone, only very quietly, and it did not raise with volume. We tried different guitars, plugging into the Marshall directly, and even using a 15 watt solid state backup amp. We moved around the building and even went into the basement trying different circuits, and could still produce this 150hz tone.

Here's where it got even stranger; we had a generator delivered, and plugged the Marshall directly into the generator running from outside the building. The tone still persisted. We decided to power through the night, none the less and we discovered that it was a manageable problem when the whole band was playing.

I had never run into anything quite like this, and I don't want to state any conclusions here as to not cloud responses. Besides, I really don't have a clue.
I would appreciate any analysis you could venture here.
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