Signal Ground

ptah

Member
2006-08-07 8:48 pm
What do we do with signal ground? Is it best to leave the signal side grounded to the source ground ONLY, or is it better to have it tied ALSO to central ground in the amp or is it better to have signal ground tied to central ground ONLY?

I'm try to fix hum. I think I brought it on by grounding signal to the chassis via the RCA plugs and at the other end through central. Sort of makes a loop?
 
Hi ptah,

The signal input 'ground' should not really be thought of as a ground. It is only the reference voltage for the input signal.

Typically, the signal input 'ground' is NOT connected to the chassis at the point of entry (use insulated RCA jacks), and is only connected directly to the signal ground reference point in the input stage. From there, there should be a separate signal ground-return path, all the way to the main 'star ground' point (where all of the separate ground returns meet at only one point).

The input signal ground reference point might also include a feedback divider ground and a lowpass RF filter ground, and a highpass DC-blocking filter ground. The signal input reference ground's star-ground-return path could also include an optional 'ground separator' resistor of a few ohms, which should not be needed if the whole grounding scheme is ideal.

A star ground scheme is used because any ground return current, flowing through the parasitic resistance and inductance of a ground-return conductor (wire or PCB trace), induces a voltage back at the non-ground end of the ground-return conductor. Large dynamic return currents sharing the input signal's ground return, for example, would cause a changing input ground reference point voltage, which would directly sum with the input signal voltage, which could cause distortion or hum. It would also be another form of feedback, and could cause other problems, or even oscillation. Usually, at least the signal ground, the power/decoupling ground, and the output ground should have separate return paths, probably for each stage.

The only connection of any circuit ground to the chassis should be through the single connection from the star ground point to the chassis. This is often done through a 'safety disconnect' network, i.e. a 10 to 100 Ohm power resistor, a 0.1uF or so capacitor, and a hefty rectifier bridge, all in parallel from the star ground point to the chassis. Instead of a bridge, two high-current anti-parallel diodes could be used. (When the disconnect network is not connected to the chassis, an ohmmeter should show an open circuit between any circuit ground point and the chassis.)

Note that the chassis should always be connected to Earth Ground, using a mechanical connection (i.e. not solder, or at least not mainly solder).

Do some searches for 'star ground', to get more information about this important subject.
 

ptah

Member
2006-08-07 8:48 pm
Thanks Tom. That made complete sense. The first amp I built used plexiglass for the rear panel. The RCA jacks were the non-insulated type. There was no hum problem there. Here, after insulating the same jacks from the aluminum chassis and disconnecting signal from star - no hum!
 
Cool!

I guess I could have just said 'Don't ground the RCA jacks to the chassis.'. :)

By the way, just for clarification (probably mainly for future readers of this thread), when you said ..."disconnecting signal from star", were you referring to a 'safety disconnect' network between the star ground point and the chassis, or maybe referring to a low-value 'ground separator' resistor between the input ground reference point and the input ground's star ground return path? I ask because it also sounds like maybe you were saying that the input ground is now floating, i.e. not connected to any ground.