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Old 3rd December 2003, 11:49 PM   #1
P901 is offline P901  United States
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Question AC Hum

Got a very slight, sounds like, 60hz hum at the speakers with the volume down.
Suggestions were to add a .02 or .05 cap on the primary input to the power transformer and go to ground.
Being that this amp does not have a three wire power cord I would need to put the other side of the cap to the chassis and I am hesitant to do this since this is where all the circuitry derives its ground reference.
Unit is a Sansui QRX7001 Quad unit that has been completely recapped with Panasonic FC series eletrolytics, Panasonic P series Polypropylenes everywhere I could add them and Panasonic AA series computer grade caps on the power supply, these things have ultra low leakage numbers and this is why I can`t understand why I still have any hum at all.
Could it be that I have 8- 100w Sansui speakers in a circle less than 6 ft. from my head and even then I need to put my ear right against the 12 inch woofer to hear the hum.
Am I expecting too much?
I would like to remove all traces of the AC hum.
Any and all suggestions welcome,
Thanks, andy
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Old 4th December 2003, 10:57 AM   #2
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Putting any kind of device between a live mains terminal and the chassis on a non-earthed piece of equipment is a VERY VERY bad thing to do. It is a massive safety hazard as if the device goes faulty the case will become live and no fuses or RCD will protect against this until somebody touches the case. And dies. It's also against regulations and will fail a Portable Appliance Test (PAT).

Mains earth loops cannot be a problem with your amplifier as it has no earth! Therefore the problem lies somewhere else. I suggest that the PSU smoothing capacitors may need increasing in value (but this will probably end up being a costly exercise for little gain and can be taken to extremes), or hum is being picked up by leads or even poor wiring practice inside the amp. I personally think you are asking too much, as with that number of speakers the output will be quite high and you are loading the amp quite heavily. Also, if you have to put your ear right up close...

Whoever told you to do the cap thing has given you VERY bad advice. Even ignoring the safety issue, this is a very poor and ineffective way to try and get rid of hum (on earthed equipment). The usual way is to add a 100 ohm 'ground lift' resistor bypassed with 100nF between the chassis and 0V. You MUST NOT 'lift' the chassis mains earth connection.
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Old 4th December 2003, 11:14 AM   #3
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This is important: is it really hum (sounds like a pure tone, 60Hz) or is it more a buzzy like tone (hum + line freq harmonics). In the first case, it's not a supply problem, but may come from the un-earthed mains lead. In the second case, no amount of caps or whatever on the xformer primary (or secondary) will help. What's your assessment of the tone?

Jan Didden
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