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Old 6th October 2006, 05:30 PM   #1
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Default Elliot P101 humming

I finished my P101 a while back and was concerned about the output voltage . Now, I have another concern. The amp is dead silent until I connect the input RCA cables to a source. It doesn't matter what the source is it will hum. I'm wondering if this is a grounding issue of some sort or the fact that every input device I've connected has been on a polarized plug while the amplifier uses a grounded IEC plug and there might be a possible ground reference issue between the neutral and ground (earth) in my house wiring. Any thoughts on this?
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Old 6th October 2006, 08:21 PM   #2
jaycee is offline jaycee  United Kingdom
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Install a ground loop breaker between chassis earth and 0V on your power supply. Ensure your RCA connections are isolated from the chassis.

If it's possible you have an earth leakage issue, get an electrician to do an earth leakage and EFLI test on your house.
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Old 6th October 2006, 10:41 PM   #3
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I've been doing some more reading about ground loops and I'm tempted just to cut the ground pole off a cord to see if the hum goes away (testing purposes only). It stands to reason that this might make it worse (allowing it to pick up noises elsewhere) because then it would be floating. Anyway, I measured between the neutral and ground with the breaker off and I only see a couple tenths of an ohm of difference.
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Old 7th October 2006, 12:16 AM   #4
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Smile Humming P101

Are your RCA jacks isolated from the chassis ? I mounted mine on a piece of plastic then ran the center conductor to the amplifier boards and the shields to the star ground.
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Old 7th October 2006, 12:51 AM   #5
Joe1976 is offline Joe1976  Thailand
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I concur with the above, as I have finally got rid of, to paraphrase Rod Elliot, "the @$%ing hum."

ISOLATE EVERYTHING from the casing. In particular the RCA jacks, AND the transformer. Then earth/ground your box at one point (only) via a 10 ohm 2 watt resistor, to kill off any hum. (You will need to scrape away paint and use a tight bolt).

An alternative to isolating RCA jacks is to use balanced XLR connectors, (without necesary using balanced circuits). They have the metal body, which shouldn't be electronically connected (but they will be bolted to the chasis, and therefore earthed via the 10ohm resistor). Of the 3 pins, one can be used for the screen (usually pin 1).
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Old 7th October 2006, 09:00 PM   #6
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The RCA jacks are isolated from the chassis. The transformer came with rubber pads. I have one on each side. I don't have a resistor between the earth and chassis ground, it's just a direct connection. I'll try that next. Thanks for the input.
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Old 7th October 2006, 10:33 PM   #7
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Default hummer

Is the hum coming from both channels ? (Assuming you have more than one)
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Old 8th October 2006, 01:00 AM   #8
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The hum is only in the right channel. There's a very, very slight hiss in the left that I'm fine with as your ear must be right against the tweeter.
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Old 8th October 2006, 01:02 AM   #9
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I'd like to post some pictures of it (probably tomorrow). It's my first DIY, so I'm rather proud even if it is a bit ugly.
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Old 8th October 2006, 02:46 AM   #10
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Default hummer

Have you tried swapping the source inputs around.If thats not the problem. check your p-grnd connection to that channel or there is cold solder joint on that PC board. Heres a pix of one of my amps.
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