| First try to figure out if it a buzz, hum, hiss, or distortion.
A hum is usually 60 or 120Hz, a lower frequency. A buzz is usually a higher irritating frequency. I hiss is a smoother noise.
It can be that although the noise is not picked up by your amp, the headphone circuit in the amp can be picking it up. There may be a problem in the amp's headphone circuit making it more sensitive to picking up external noise. If you can eliminate the external noise, it might resolve your problem.
There are some simple tests that you can perform. Try a different set oh headphones. Try the amp on a different power circuit (one that has a different circuit breaker on the line), if the noise goes it could be a wiring problem in the apartment. In general, overhead fluorescent lights, a light dimmer, an aquarium pump, or even a fridge motor can all cause noise in an amp. Eliminate sources like this and see if they help. If there is a nut on the headphone jack on the amp chassis, make sure it is tight.
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Official Ampeg Portaflex Club #89
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