Capacitor in Series of Earth Wire

[dmeier]

Hi all,

I've got a Matchless Vibrobox - a tube powered tremolo effect stomp box. It uses mains voltage of 115/230V.

To prevent a ground loop in conjunction with an amplifier the earth wire of the mains input is not connected directly to the chassis but has a .02M 2KV capacitor in series.

I wonder how safe and legal this wiring is. Or what are alternatives to prevent ground loops?

Cheers, David

[Dave H]

I would have thought that the mains earth wire should be connected directly to chassis for safety and the circuit 0V should be isolated from the chassis by a 100 ohm resistor to prevent earth loops.

[loudthud]

I can tell you how to fix the ground loop problem, but I can't tell you if it would be acceptable safety-wise in your country. I don't think the capacitor in series with the ground wire would pass inspection or certification in any country. In my country, products are not required to pass any safety test (by government), but if someone is injured, they can take you to court and make you pay damages. If Matchless has any kind of product liability insurance, the insurance company would be the ones who require safety testing. If you modify the product, you could be held responsible in court for any damages or wrongfull death.

[R.G.]

Unless the chassis and audio grounds are inaccessible by a user, that would not pass any safety certification I know of, and I have had some experience with this topic, although I am NOT an expert at it in any way.

I don't know if Switzerland has a legal requirement for formal testing. The USA does not, as LT mentioned. However, I do know that Switzerland has requirements for electrical safety inspections for houses at 20 year intervals or when sold, so it seems like it would have some requirement for testing electrically powered appliances and such.

If you can touch the chassis, or if the audio signal ground is tied to the chassis and you can touch the chassis, I personally do not believe that is a safe setup.

There are ways to do this, having to do with how the grounding is done for safety and audio signal, as noted. You could also install an isolation transformer for the audio signal on the output to help eliminate hum.

But I would be suspicious of the setup as well.

[Jazz P Bass]

Was the capacitor added by someone other than the manufacturer.
I don't see it on the schematic.

[dmeier]

You can't rely on those schmatics, the circuits of Matchless products differ a bit through time. But the cap in series is definetely installed by the factory. I've seen a Hot Box schematic with it but the one I own doesn't have it.

I don't worry that much about how legal it is or not - as long as you don't sell anything you're responsible for your own safety and health only.

Therefore, I wonder more how safe it is. Isn't it rather safe as long as you are earth grounded through your amplifier? As far as I know, the simple solution when using two amps in parallel is to disconnect the earth on one. I guess there are safer methods - what do you recommend that is also "road-ready"?

Or what needs to be done within the stomp box, so the earth wire can be attached to the chassis without a ground loop? The solution posted by Dave H?

[Jazz P Bass]

[QUOTE=dmeier;292174]As far as I know, the simple solution when using two amps in parallel is to disconnect the earth on one.
QUOTE]
That my friend is NOT the way to do it.
First off, let's identify the purpose of the "ground" wire from the mains.
If for any reason the metal chassis becomes 'live' (mains potential) the ground wire will divert that current to the outlet breaker & open the line connection.
Safety first.
As to your loop fix, that is not safe (disconnecting one of the amps ground leg).
The disconnected amp is now putting someone or something at risk.
Now what Matchless did is truely assinine.
They are relying on the cap to never fail!
What could be done is to attach the circuit ground to the chassis via the cap & secure the mains ground to the chassis.
To 'break' a loop the safe way is to use an 1:1 isolation transformer box or lift the output jack ground.

[Bruce / Mission Amps]

Seems like I've seen a number of old vacuum tube Ampeg amps that lift the common ground point off the chassis with a small value resistor, 1A 1N4??? diode and maybe a .01uF cap across the two.