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Super Noisy Phono Pre--Help?

I have a Fosgate Signature Pre. I feed it with a Lyra MC cartridge mounted on an SME V arm. Until last week, all was bliss. Then the preamp started producing a random popping sound and a motor sound like there was poor electric motor plugged into the same outlet. I thought it was a tube, but swapping out tubes didn't change it. I took it to my local service department. They cleaned and resoldered some of the connections. Now the popping sound is gone, but the constant motor sound persists. The motor sound goes away if I disconnect my turntable. I can influence the sound by moving my interconnects around. I cannot change it by relocating my preamp. It seems like it might be some sort of interference, but I can't understand why it has shown up all the sudden after a couple of years of perfect sound. Suggestions, please. Could there be something wrong with my interconnects or my cartridge?
jmmorford

Showing 2 responses by lewm

lewm
12,567 posts
 
Al, I had/have an issue with my Aesthetix Janus full function preamplifier that may be very like the one described by the OP. When the tt is "on", I heard a sound on the speakers that was very like that of a motor, the turntable motor. The sound was only present when the tt motor was running and immediately ceased when I shut it off, and only when the Janus was set to "phono". Extensive investigation suggested that the tt and the Janus were very well isolated acoustically from one another, so I tend to rule out the idea that a microphonic tube was picking up actual motor vibrations. Further, the tubes were not demonstrably microphonic in the first place. Switching outlets and grounding vs ungrounding the tt also had no positive effect. Finally, I faced the possibility that the coupling capacitors in the Janus, which I had force-fit onto the PCBs using longish leads, might be picking up motor noise "through the air". I then replaced those caps with smaller ones that allowed the use of much shorter leads, and this cured the problem. But it was very odd; I would not have thought that EM radiations from the motor could manifest as "the sound of a motor". I wonder whether something similar is going on here.
lewm
12,567 posts
 
Dear Ralph, I neglected to mention that the phenomenon I encountered was not present when either of two other entirely different preamplifiers were substituted for the Janus. Further, the noise was present when the tonearm/cartridge was not in use; all one had to do was turn on the tt motor to produce the noise, in both channels. Waving the arm wand back and forth across the surface of the platter also had no effect, good or bad. And finally, as noted, the problem was cured by reducing the lengths of the leads on the coupling capacitors (or cannot rule out that there was a weird defect in one or more of the coupling capacitors themselves, since I replaced the couplers when I arranged for shorter lead lengths). The situation you describe, would, I expect, result in hum or other mixed frequency noise. What I heard was the actual sound of a motor running.
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