I fitted some old amps in my car today.
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I have a 4 channel amp and a 2 channel amp.
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The 4 channel amp was to run the speakers, and it has a low-pass preout which I ran to the inputs on the 2 channel amp.
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The 2 channel would then run a dual voice coil sub.
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When I fired it all up for the first time it worked great, but as soon as I shut/slammed the bootlid I started getting mad hum from the sub, which built and built louder and louder, so I switched off, checked my wiring to see if anything had come loose, and switched on again.
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Same problem.
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I took the 4 channel amp out of the equasion and ran the output from the stereo to the 2 channel amp, which works fine.
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I did some more testing with just the 4ch amp and the sub, and found that I get the same building hum
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(which I think is DC voltage, but I didn't have my meter so I don't know if it's 12VDC or internal rail voltage).
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I get it from any of the 4 speaker outputs and it's obviously outputting it down the preout to thw downstream amp as well, so I've narrowed the fault down to the input stage of the 4ch amp.
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I have previously replaced the transistors in the power stage in this amp
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(4 or 5 years ago I expect) but as the fault seems to lie with the input stage I'm not sure where to start.
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All the transistors test OK, there's no obvious shorts
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(it doesn't blow any fuses).
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The amp is a Soundstream USA 364 and I can't find a schematic for it anywhere.
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Does anyone have any suggestions to help me find the fault?