Trace Elliot V-type preamp noisy ground

[Alex R]

Hello again. I have yet another problem with a circuit feature I don't quite understand...

Below is a schematic of the Trace Elliot V-type preamp (sorry about poor quality). As you can see it has an AC ground, coupled to the chassis via three caps.

The complaint is that the amp hums and buzzes and picks up atmospheric noise, but when you touch the metal toggle switches the noise goes away. These are connected to the circuit ground. It's not the usual 'grounding-yourself' thing, as the switches are connected to circuit ground not chassis ground, and a jumper between the two grounds solves the problem regardless of where you stand and what you touch. When you connect the circuit ground to chassis ground the noise all goes away. So touching the switches simply provides them with enough grounding to sort the problem.

Could someone explain the rationale of cap-coupled grounding to me? The caps are 0.1uF and test ok; hard to believe all three might have failed and there's continuity between all circuit grounds and a good ground on the chassis.

Would it be Very Wrong to just connect circuit ground to chassis ground? Worth trying new grounding caps?

[cbarrow7625]

According to the schematic there is an "earth lift" switch already in the preamp that connects circuit ground & chassis ground when closed.

The reason for doing it that way is to avoid ground loops when connecting multiple pieces AC powered, grounded, gear together. The caps provide an RF ground for the circuit so you don't get the "squeallies" when the amp is around other gear but it lifts the ground for DC-to-60 cycles (or higher).

If connecting the circuit ground to chassis fixes your problem, then do it. There is nothing wrong with it. It is, infact, the preferred way to do it whenever possible. But, like I said, it appears that there is supposed to be aswitch to take care of this difficulty for you. I wouldn't hard wire anything on this piece unless you can't find that switch. you may find a piece of gear one day that causes you a ground loop ant you'll wish you had the ground lift switch back.

Good luck.

[Alex R]

According to the schematic there is an "earth lift" switch already in the preamp that connects circuit ground & chassis ground when closed.

The reason for doing it that way is to avoid ground loops when connecting multiple pieces AC powered, grounded, gear together. The caps provide an RF ground for the circuit so you don't get the "squeallies" when the amp is around other gear but it lifts the ground for DC-to-60 cycles (or higher).

If connecting the circuit ground to chassis fixes your problem, then do it. There is nothing wrong with it. It is, infact, the preferred way to do it whenever possible. But, like I said, it appears that there is supposed to be aswitch to take care of this difficulty for you. I wouldn't hard wire anything on this piece unless you can't find that switch. you may find a piece of gear one day that causes you a ground loop ant you'll wish you had the ground lift switch back.

Good luck.

Thank you! The ground on pin 1 of the DI socket maybe has a bad ground - or the lift switch is bad - oh a little understanding goes a long way... I'll take a look tomorrow and report back.

[Alex R]

OK, the ground lift switch is fine as are its associated connections. It only lifts the ground on that DI pin, nowhere else. The preamp ground is AC only whatever you do with the ground lift switch. What are the advantages of that?

...So anyhow I connected preamp circuit ground to chassis ground and the amp is nice and quiet and works fine.

Shout now if this looks like a problem to anyone!