Need some help with tracking down hum.
Jul 11, 2005 at 7:32 AM Thread Starter Post #1 of 4

JimmyDean

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I have a problem with the amp im using as my headphone amp. Its (I think) an early 70s electro voice amplifier. Its small, and is usually used for speakers. I have found, however, that it sounds exceptional with headphones. For some reason, I just love listening to this thing instead of my fairly crappy JVC RX-552's headphone output (although it drives my speakers fine). I compare the 2 like this. The JVC sounds really "refined", almost mid scooped in a way. The electro voice sounds full, and its much more fun to listen to it. There is a persistant humming, however. It sounds like it might bt 60hz, which makes me think ground problems (this amp only has a 2 prong plug). There right channel used to be bunk and only buzz, but I fixed it by taking the whole thing apart. Found a broken solder joint and blown fuse. Id really like to fix this humming, itd make this amp really shine. Im thinking this. On the back, the diagram says this one screw is "system ground". Im thinking I should mod this to a 3 prong plug, and ground the unit. Would this help the hum?
 
Jul 11, 2005 at 2:19 PM Post #2 of 4

Sewer Guy

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I don't think the grond will do any good. I have a lot of experience with tube amps. More than likely one of the filter capacitors is going bad. The unit is old
& the electrolyte dries out.
The other prime suspect would be a front end tube; however I would replace the filter cap's first.
 
Jul 11, 2005 at 2:41 PM Post #3 of 4

Budgie

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I agree with Sewer Guy. I would replace the power supply filter caps. Electrolytics caps dry out slowly, and have a typical life of about twenty years or so, usually. The ideal would be to replace all the electrolytics in the amp, but the power supply caps are a likely culprit for hum. It would not hurt anything to try the grounding you mentioned, first, since that won't cost anything to try.
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Jul 11, 2005 at 6:28 PM Post #4 of 4

JimmyDean

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I ran the ground, but it didnt help anything. Would the filter caps be large? Currently, I only see 2 caps in the whole amp (im sure there are more, but they are hidden under an inverted board). They are very large and silver. They have way more than 2 leads though. The bottom almost looks like how youd solder a tube socket. Ill take pictures if I get a chance later.

EDIT: Found 2 rather large caps on each channel board. BTW this is not a tube amp, I dont know why they use the word valve in its description. Im going to take some pictures later.
 

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