Cutting ground prong on 120V cord for pedal.

[Burstbucker]

I have a couple of stompboxes that are powered by a 120volt cord, inside of these pedals there's a small stepdown transformer.  There seems to be a fair bit of noise added because of ground loops though.

I was reading somewhere that you can cut the ground going to the pedals as long as the amp is still grounded, this gets rid of the ground loop problem.  I know for a fact that this will really lower the noise issue but is it dangerous even though there's still a ground going through the cords and jacks back to the amp?

[R.G.]

It is quite dangerous unless the pedal is built to the standards for double insulation. No pedal I've ever seen has been built that way.

Here is what to do instead.

Isolate signal ground from the metal enclosure. This will mean changing to isolated jacks, isolated power jacks, or using shoulder washers on the existing ones. It may mean reworking the board mounting and/or control mounting to be sure that it's insulated. Verify isolation with an ohmmeter.

Now connect one and only one 100K resistor paralleled with a 0.01uF 1kV capacitor between signal ground and the safety/chassis ground. If I remember right, this should give you safety (the chassis and control knobs and switches are hard-grounded to safety ground) and hum freedom ( signal ground is tied to the amp).

[rubberlips]

I wouldn't recommend cutting any earth pins on the 120V AC. Instead, take one of the ground wires off the effect output jack if that helps with ground loops. The earth lead is there for protection and shouldn't be messed with

Pete

[object88]

Now connect one and only one 100K resistor paralleled with a 0.01uF 1kV capacitor between signal ground and the safety/chassis ground. If I remember right, this should give you safety (the chassis and control knobs and switches are hard-grounded to safety ground) and hum freedom ( signal ground is tied to the amp).

Why use a resister and cap; why not tie the chassis directly to ground at a single point?

[R.G.]

Why use a resister and cap; why not tie the chassis directly to ground at a single point?

If the effect was the only AC powered unit, that is the way to go. However, if you have multiplt AC powered ones, leaving them all attached hard to AC safety ground creates ground loops. See "Build the Spyder" at GEO for some background.

[toneman]

don't "cut" the gnd prong.
use one of those 3prong-2-2prong adapters with the wire hanging off.
that way U preserve the cord. *and* can lift or tie gnd.
Gend problems like yours could B from phaseing differences in the
isolation xfmrs.
staybuzzfree
tone

[Burstbucker]

Thanks all.

Yeah, I was thinking of using one of those 3-prong to 2-prong adapters which defeats the ground.  But, I guess I'll live with the little bit of noise and avoid possible death!

8^)

[R.G.]

You know, it occurs to me that battery power is not as much of an issue with AC powered effects as it is with battery-only ones.

You could just deal with the ground isolation problem with a sledgehammer.

How about you put one more opamp and an isolation transformer inside the box? You simply include the circuit of one output of the GEO improved hum free splitter/A/B/Y box and an isolated jack. No hum if you are careful to place the isolation transformer where it doesn't pick up hum from the AC power transformer in the box.

See http://geofex.com/FX_images/humfree2.gif