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Re: Cary 300 SEI with HUM in Noise Floor

Posted by Paul Joppa (P) on March 9, 2007 at 23:32:33

In Reply to: Re: Cary 300 SEI with HUM in Noise Floor posted by daleda on March 9, 2007 at 18:55:33:

OK, so it's electrical - it appears in the electrical output signal. And it's a buzz rather than a low-pitched hummmm. That helps identify the problem - it's not going to be power supply filtering or magnetic field pickup.

The first question then is, does it in fact meet the specs? You can measure the output voltage with a voltmeter set on "AC volts". Most meters are flat up to several kHz at least, so the measurement is likely to be accurate. The 300 SEI is specified to have hum and noise 90dB below full output. Full output is specified at 15 watts, which is 11 volts rms. 90dB below that is 0.35mV. This is quite a good hum level, though it may still be audible on sensitive headphones The point here is to determine if the unit is operating correctly - that affects what solutions might be possible or appropriate. Note that the meter leads can pick up hum; check the voltage reading with the meter and leads in the same position but the leads connected to each other. You may need to twist the leads together to make this measurement accurately.

I'm assuming from Cary's response that they think the amp is operating correctly, so if it does not seem to meet specs there is likely to be some environmental source of the problem outside the amp itself. This may or may not be true, but in either case you'll have to determine the answer in order to make progress. If the measured noise is louder than it should be, it's time to isolate the amp. Disconnect everything - all input cables, speaker cables, and/or headphones, and repeat the test. Other things connected to the amp can introduce buzz from ground currents (most common) or by acting as antennas.

If it still buzzes, it is always possible the source is on the power line. Do you have a power line filter? The alternative is to run around the house makeing sure all light dimmers, flourescent lights, computers, and other possible power-line pollutants are turned off.

In my experience, by the time you've done all these things the odds are at least 9 out of 10 that you will have found the problem, and can start working on solutions.

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