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Both hum and radio interference in an audio circuit
Both hum and radio interference in an audio circuit
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Old 4th July 2003, 04:45 PM   #1
mnw21 is offline mnw21  United Kingdom
diyAudio Member
Join Date: Jul 2003
Location: Cambridge
Question Both hum and radio interference in an audio circuit

I have an audio circuit that I built a while ago (ten years), and I have been recently trying to upgrade it, and seem to have got into all sorts of problems.

My audio system is split into two separate boxes - a stereo amplifier, with satellite speakers, and a bass amplifier. The bass amplifier has a transformer to provide +-70V DC to a 50W power amplifier attached to a 300W bass speaker in the same case, and another transformer to provide +-12V regulated DC to the preamplifier. The stereo box has a transformer to provide single rail 14V DC to a couple of 8W amplifiers to power the speakers. Until recently, the input to these two 8W amplifiers was completely passive, but since then I have incorporated an input stage, which uses the +-12V power from the bass amplifier, supplied over an umbilical cord between the two.

The intention of the new circuitry is to allow more than one input to the system, and to provide those inputs as balanced receivers, to eliminate an earth loop problem I had whenever I plugged the system into my computer or video machine (interestingly the CD player was fine).

Once I had put this circuitry in, the system was fine, but it hummed a little - I assumed this would be due to an earth loop between the stereo and the bass boxes. Therefore, I made another balanced receiver circuit and placed it between the two systems, disconnecting the earth connection on the umbilical cord. Both systems have their own earth connection. Now the hum is much reduced (but still bad), but I get a large amount of radio interference from all three speakers. The interference is never intelligible, but consists of whistling, squeaking, and other similar sounds.

I currently have two ideas, but I'm not entirely confident of them:
1. Connect a low resistance across the inputs of everything - at the moment, all the inputs are relatively high impedance. This might reduce the interference a little by shorting it out as such.
2. Power the new circuitry from batteries temporarily, to see if the power umbilical is to blame.

Any help or advice would be much appreciated,

Matthew Wakeling
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Old 4th July 2003, 10:15 PM   #2
paulb is offline paulb  Canada
diyAudio Member
Join Date: Jun 2001
Location: Calgary
See if you can borrow a copy of the Radio Amateur's Handbook. It includes good advice on RF suppression.
I just bought some transformers because I intend to wire my PC audio to the rest of my audio system. Your findings have verified I haven't wasted my money...
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Old 5th July 2003, 09:08 PM   #3
mnw21 is offline mnw21  United Kingdom
diyAudio Member
Join Date: Jul 2003
Location: Cambridge
Default Hmm. Possible misinterpretation.

The system produces this noise when there is nothing plugged into the stereo box except the bass box. Plugging other things in doesn't seem to make any difference - I assume this is my balanced line receivers working.

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