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biff584 14th March 2003 01:23 AM

P3A and ground loops
I just finished my P3A. I'm very impressed with the sound- I really love this amp. Since I'm at school and was working in an electronics lab, the only source I had to test it with was a portable CD player- everything sounded great so I brought the amp back to my room and just tried playing a DVD through my laptop. The sound was terrible- loud, distorted and ringing. I thought this might be a ground loop so I tried floating the ground on the amp- it solved the problem and everything sounded perfect once again.

My connections are as follows: (I have an aluminum chassis)- the earth ground comes in through a filtered IEC socket and is immediately grounded to the chassis. The signal comes in through RCA jacks and is grounded to the chassis. The binding posts are connected directly to board which, in turn, is grounded to the chassis. The hot and neutral enter 2 transformers and the center tap of each is grounded to the chassis.

I would like to eliminate the ground loops, but I want to find a safe way of doing it. Simply floating the ground sounds very dangerous-- any advice on a better way to eliminate the ground loops safely with this design?



biff584 14th March 2003 01:37 AM

Just had a though- if I eliminated the earth ground and just fused the hot and neutral would that be safe enough?

dwjj 14th March 2003 04:41 AM

Not really. The ground is for faults. If you lost the neutral (and weren't double insulated), your chassis could become energized. Bad news.

Try doing the ground lift circuit using a bridge rectifier that's on the elliot site. It introduces some impedance to the ground path, but the bridge allows fault currents to flow to safety ground.

Cradle22 14th March 2003 07:39 AM


Do yourself a favour, build a grounding star point, isolated from the chassis. Connect earth to chassis (you already did that), all ground connections (from supply caps, amplifier, maybe input signals - if you don't wire them to amp) go to the star point. Then run a single connection from star ground to chassis with a 0.47 uF (rated at least your mains AC voltage, to be on the safe side take a little bit more) polyester capacitor, maybe bypassing a 100 ohms resistor.

And / or try using something like the current breaker circuit from ESP website.

That got rid of all my ground loop probs on my End Millenium amp...

Hope this help,


biff584 14th March 2003 12:07 PM

Thanks for the replies- I'll give that a shot.

With the two separate grounds- why do they need to be connected at all? How come I couldn't just earth ground the chassis and then ground the powersupply and all signals in some other place and just leave it alone?


dwjj 14th March 2003 02:21 PM

Faults again. If your rectifier fails with a short, the ground would again be energized.

Personally I'd definitely have a resistor between the grounds rather than just a cap if you go that route. I think Elliot uses 10 Ohms 20W. You'd want low enough impedance and high enough power rating to make sure any fault currents are shunted through earth.

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