You got me floatin' (jacks and pots)

[jfutral]

Of two builds that gave me the most trouble with hum (present one included) one thing I remembered reading about was to float the speaker jack off the chassis with something like nylon washers. Both times this either greatly reduced or (the first time) just about eliminated the hum.

I can't find the article where I read that. Are there any thoughts for the rest of the components that connect at the chassis like pots and input jacks?

Thanks,
Joe

[GregS]

A good way to do the grounds is to have one circuit ground connection to the chassis. This requires insulating washers for all types of jacks (speaker or input). A wire bus is used with one end connected to the chassis. All circuit grounds connect to this wire bus at various points along it. The noisier power supply and power amp grounds at one end followed by the phase inverter ground and then the more sensitive pre-amp grounds. It is important not to get this order mixed so that noisier ground currents don't mix with the more sensitive ones.

This grounding scheme is detailed in Kevin O'Connor's books which he refers to as a galactic grounding scheme and also in Merlin's book on designing tube pre-amps for guitar and bass.

Greg

[jfutral]

Which section is closest to the chassis? The power supply/amp?

Joe

[GregS]

Which section is closest to the chassis? The power supply/amp?

Do you mean which end of the wire bus is connected to the chassis? I think it can be connected at either end (but not both of course). I got good results connecting the ground wire bus near the input jack in the pre-amp end of the chassis with my latest build.

[jfutral]

Sorry about that. I was trying to sort this out:

"The noisier power supply and power amp grounds at one end followed by the phase inverter ground and then the more sensitive pre-amp grounds. It is important not to get this order mixed so that noisier ground currents don't mix with the more sensitive ones."

Right now, I don't have one point, but I do have each section separated—120v in has it's own point right at where the wires enter. Then the power section has its own point close to the 120v in, the PI has its own point a little further away, and furthest is the pre-amp parts all running close to the inputs. Not exactly single point ground, but each section is isolated.

Joe

[GregS]

From your description I understand you have multiple chassis connection points, with each part of the circuit having its own connection point so the chassis itself acts as the ground bus. The sequence of your circuit grounds is consistent with what I mentioned.

Also, the grounding scheme I described actually has two connections to the chassis - one for the safety ground located where the AC comes in and the circuit ground at the other end.

So back to your original question:

If you don't have an isolated input jack then that will impose another ground connection there in addition to your other pre-amp ground connection. With an insulated jack you would just connect the ground lug of your input jack to your general pre-amp ground connection. I'm not sure if this will make a lot of difference one way or the other.

The pots should not matter unless your using them as grounding points to the chassis. In that case they can't be isolated.