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Brownlow 31st May 2004 05:02 PM

(I'm sure that was used before - sorry)

I have a grounding issue that has been threatening to tear my hair out.

I have 2 X P3A used in Bi-amped configuration with P09(X-Over) and P88 is the preamp. When the system is switched on, there is hum that's annoying coming mainly from the left channel(It's driving me mad!).

If I disconnect XO and Preamp, the amps are quiet enough. When I connect up the Preamp or XO directly to either of the power amps, the hum can be heard again. But as I said, when both amps are not connected to anything else nothing is heard.

I have searched through the site, it has given a number of excellent tips on grounding. But, not got the success I am after.

I've seen a few threads on grounding where connect of star to chassis is via a resistor and diode. What diode should this be and what value R?

Any help would be greatly appreciated.



audiofan 31st May 2004 05:17 PM

You may have ground loops from power line, are you using 3 prong plugs ? If the amps have their chassi grouded through power cord ( usig a 3 wires cord) this connection to ground may complete a groud loop, but I am not familiar with mains wiring in UK

Brownlow 31st May 2004 05:29 PM


Thanks for the reply. I have got 3 pronged plugs, which go to the IEC connectors. Is this the problem?

audiofan 31st May 2004 05:57 PM

Yes it could, because if the chassi of each units is connected to ground via mains connection you may have a voltage present between differents units. The connection to groung with main is not perfect there may be already some voltage present from one point to another , then when you connect your amplifier system some current will flow through the shield in the coax interconnect and will cause

You may keep one unit connected to ground for safety ( via 3 prong plug ) then all the others unit should not be connected to ground from utilities, they will be grounded by the connection betwenn units , those connection should spread as a star from the initial point, do not make loops or rings.

Geoff 31st May 2004 06:09 PM

If you haven't already done so, have a look at Doug Self's page on ground loops:

and Rod Elliot's article about earthing:

One of these should give you the answer to your problem.

Regarding audiofan's last post, for use in the UK all items of equipment must have an earth wire connected to the three pin plug unless they have been specifically designed and constructed so as to only require a two wire connection to the mains, ie they are double insulated and tested/labelled as such.

Brownlow 31st May 2004 06:49 PM

Those pages look interesting. I need to read through and try and follow these examples.

Geoff and audiofan, thanks for your time and the links, I will report back to you my progress.



jaycee 31st May 2004 11:51 PM

Disconnecting safety earth from a device in a metal chassis is illegal, or at least heavily frowned upon (not to mention dangerous!) in the UK.

If you read Rod Elliot's article on earthing there will be a section describing an earth loop breaker using 1 resistor, 1 capacitor and a bridge rectifier. I would recommend you install earth loop breakers in both your pre and power amp.

Brownlow 1st June 2004 03:37 PM


Thanks for the reply. I often thought that if there was a hum issue, there would be a problem with both channels, but this is a little strange (to me) in that the hum occurs in my left channel of my power amp.

Before I implement the earth breaker, is there anything else that I might try - or should I just go full steam ahead.



Jean 1st June 2004 03:42 PM

Can you post details on how you have power wired on the DC side ? Is everthing in one case (2x p3a + preamp etc) or do you have separate power supplies ? I use a small bridge with resistor between DC ground and chassis ground and it helped me eliminate hum in my stereo p3a. Chassis ground is connected to the earth ground, shouldn't disconnect it ;)

Good luck

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