Foreplay Hum Problem

[mudiwater]

Hello all,
I'm sure this has been hashed over a million times, but I have to ask because I can't find the cure. I put together a Foreplay kit many years ago, loved it, eventually giving it to my son when he moved away.

So..........finally I dig out the old book and build another one. I used a three prong a/c cord, ran all the ground to one point that ends up at the ground wire of the a/c cord. Ok, so everything checked well (voltages all a little high +10%), I have a pretty bad hum that does not increase with volume control, as soon as I "lift" the ground ( use a two prong adapter) the hum completely leaves and the Foreplay sounds great!

I have checked the wall socket, everything checks out good. There is only hum if the input cables are connected, it doesn't matter what source is connected, still hum until I use the adapter to "disconnect" the ground wire. Then its very quiet and sounds really good.

It seems that its probably something simple that I don't understand. Sorry if this is an old beat down problem, any help would be greatly appreciated.

Thanks

[Doc B.]

Are you saying that you built another kit, or just used the schematic to build a copy? If you built a copy it's pretty hard to guess where you may have create the ground loop you are experiencing.

[mudiwater]

I built a copy from the original plans. I'm a complete novice, I realize now that I have asked a nearly impossible question without you knowing what I have done exactly. But it sounds like I have a created a ground loop? The only thing I can think of that may be different from the original, is that the transformer frame may not be grounded very well.

I have gone over all the solder joints, re-flowed a few, all look good, everything measures good. The pre-amp is very quiet and sounds very good as long as I don't "ground" it .

thanks

[Paul Joppa]

There are two different grounds - safety ground (the power line ground) and the signal ground. It is important (for hum) that the signal ground is connected to the chassis at only one point. Way back in those days, the signal ground (at terminal 14) was floating - but we soon realized that was a problem, and suggested people try connecting T14 to the adjacent T13, which is bolted to the chassis plate.

It is important for safety that the safety ground also connects to the plate, but that should happen closer to the power line input. These days we mount the transformers with fiber washers and run a ground line to the safety ground/chassis attachment from one of the transformer screws.

These are things we learned over the years (Foreplay came out in 1998!)