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-   -   Ground loop hum (https://www.diyaudio.com/forums/pass-labs/7901-ground-loop-hum.html)

jag 22nd November 2002 03:50 PM

Ground loop hum
 
There are several threads discussing hum problems. But, I am still confounded (pl bear with me).

The Zen4 is finally up and running:

When switched on, but no input signal connection - silent.

When connected to portable CD player (running from a wal-wart transformer) headphone out - silent, great sound!

When connected to pre-amp - hum (both channels), it is not just 60Hz, modulates at a higher freq (sorry, no scope available).

The preamp, and CD player both have 2-pin plugs - cheater plug is useless. So, the problem has to be with new Zen.

The other power amp (Adcom) also has 2-pin plug and when connected to pre-amp is silent.

I tried connecting all components on a common extension chord - still humming.

I have both monos in one chassis. I have followed the wiring scheme as shown in the Zen4 article: the power supply grounds are connected to AC pins of common bridge (used for grounding). The + & - of common bridge are connected together and then to the ground pin on power connector (14ga solid copper). The ground pin of power connector is connected to (both) heatsinks (14ga solid copper). The PCBs are connected to heatsinks via 22 ohm 3 W resistors.

Sparhawk 22nd November 2002 04:07 PM

It's definitely a ground loop, and not a problem with the ground wiring within the amp itself, since the hum only appears when connected to the preamp.

Have you tried running the amp without the ground connected to the outlet?

If the hum goes away, then it is a problem with your house wiring. The ground is bonded to the neutral bus in your breaker panel, so if the ground wiring has a problem, and a potential develops between ground and neutral at your outlet, it can cause the hum. This would explain why you still had the hum when you used a common extention cord for all components.

jag 22nd November 2002 04:30 PM

Quote:

Originally posted by Sparhawk

Have you tried running the amp without the ground connected to the outlet?

That is the next thing I was going to try today evening (got to buy a cheater plug).

Quote:

Originally posted by Sparhawk

If the hum goes away, then it is a problem with your house wiring.

That would be bad - live in an appt. Time for moving to another appt? Is there any way I can prove it to appt people that ground is bad (other than making them listen to the hum - that would not be very convincing I guess).

Sparhawk 22nd November 2002 04:51 PM

Quote:

Originally posted by jag

That would be bad - live in an appt. Time for moving to another appt? Is there any way I can prove it to appt people that ground is bad (other than making them listen to the hum - that would not be very convincing I guess).

You could try measuring AC voltage between the ground and neutral pins of the outlet. If it isn't close to 0V, the wiring is bad. :eek:

eLarson 22nd November 2002 05:09 PM

Safety First
 
All the usual disclaimers about how your first electrical shock can easily be your last apply, especially when contemplating defeating a safety ground connection.

Make sure that your outlet is wired properly (in the US black is usually used for hot, white for neutral, green for ground). They are sometimes reversed. I'm not sure which of the "skinny" or "fat" slots of a US outlet is supposed to be neutral. I use a circuit tester that lights up one way if it meets code, and another way if it is reversed.

If your preamp and amp are plugged into different circuits (two different outlets) you can also try to measure potential between the cases of the two devices.

Erik

jag 22nd November 2002 06:00 PM

Quote:

Originally posted by Sparhawk


You could try measuring AC voltage between the ground and neutral pins of the outlet. If it isn't close to 0V, the wiring is bad. :eek:


Will something like this tell me about that? Thanks for all the help

jag 22nd November 2002 06:03 PM

Re: Safety First
 
Quote:

Originally posted by eLarson
All the usual disclaimers about how your first electrical shock can easily be your last apply, especially when contemplating defeating a safety ground connection.
Erik

Definitely not going to run the amp without ground on a permanent basis with the baby running around.

Quote:

Originally posted by eLarson

Make sure that your outlet is wired properly (in the US black is usually used for hot, white for neutral, green for ground). They are sometimes reversed. I'm not sure which of the "skinny" or "fat" slots of a US outlet is supposed to be neutral. I use a circuit tester that lights up one way if it meets code, and another way if it is reversed.
Erik

the small slot is black - hot, the large is white - neutral.

Quote:

Originally posted by eLarson

If your preamp and amp are plugged into different circuits (two different outlets) you can also try to measure potential between the cases of the two devices.

Erik

already tried running all components from the same extension cord, still humming.

Sparhawk 22nd November 2002 06:08 PM

Quote:

Originally posted by jag
Will something like this tell me about that? Thanks for all the help
That's definitely a good tool to have. But it will only tell you about major faults or miswiring. If the ground connection is correct, but not very good (ie. high resistance due to poor connections, etc) this tester may not indicate a fault. A multimeter would give you more information.

wuffwaff 22nd November 2002 06:39 PM

Jag,

do you have a tuner connected to your preamp? If this is the case try disconnecting the antenna.

william

Sparhawk 22nd November 2002 06:47 PM

Quote:

Originally posted by wuffwaff
do you have a tuner connected to your preamp? If this is the case try disconnecting the antenna.

william

That's a good point... I had a strange ground loop problem once that was resolved by disconnecting the cable TV from my tuner. The cable is supposed to be connected to ground somewhere, usually at the main breaker panel. But if it isn't, or if the connection isn't great, or the resistance of the cable is high, it makes for a very nice ground loop. :(


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