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Old 28th February 2009, 02:34 PM   #1
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Default Hum issue

Hi,

I am aware that hum are hard to resolve over the internet but at this point I do not know what else to try.

Here is the set-up;

Stereo amp using TDA8920BTH, each in bridge mode using differential input mode. The amp is from Connexelectric.

With 1 RCA connected, either the left of right one channel one at a time I've got no hum at all. As soon as I connect the second RCA into the input and I pause the CD player there is a nice big hum.

I have changed the RCA connector on the amp, also the cable from CD output to amp input.

The amplifier chassis is made ot of 1/2 inch MDF. The unit is obviously not grounded.

I have 1 transformer (400VA) to power both amplifier. Each amplifier module has it's own power supply so I feed them each with +- 20Vac.

Why is everything OK when only 1 channel is playing (either left or right) but when the second RCA is connected I get hum..

Kinda strange, any help would be welcome.

Eric
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Old 28th February 2009, 07:31 PM   #2
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Hi,
A thorough wiring diagram of your amp would help.

Good luck,
M
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Old 1st March 2009, 11:22 AM   #3
col is offline col  Australia
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sounds like a common variety ground loop, have a read of Rod Elliot's article on earthing:

http://sound.westhost.com/earthing.htm

col.
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Old 1st March 2009, 09:36 PM   #4
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Hi,

Thanks for the link but my amp does not have a safety ground. My source is an MP3 player (battery powered).

When I disconnect the source i.e. floating inputs (I have tried many source) the hum goes away but as soon as I connect BOTH RCA jack into any source (dc power or not) the hum comes back.

Both the amp and the source have a 2 prong power cord set (non -gronded)

I will post a schematic tomorrow

Eric
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Old 2nd March 2009, 01:39 AM   #5
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A little more info;

Each channel has a gain of 36db and each amplifier module are configure in differential mode (inversion 180 degre of the second input to get bridge)

The amp does not have any volume therefore whatever noise is sees on the input is amplified by a gain of 36db.

My speaker is 94db eff and when I move about 4 feet away I do not hear the hum anymore, could this just be amplified noise floor instead of hum.

Thanks for your opinion.

Eric
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Old 3rd March 2009, 03:31 AM   #6
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Problem solved

I added a wire between the output (Gnd) of channel 2 to the input (Gnd) of channel 1.

Daed as it can be..I'm a happy camper (after spending 3 night trying to find this)

Thanks for the help.

Eric
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Old 3rd March 2009, 03:42 AM   #7
col is offline col  Australia
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ahh right, so it was a signal gnd loop.

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