Buss grounding scheme

[Enzo]

They explained the 13.11 thing. They added the resistor to help keep ripple currents out of the audio ground.


No one is saying screen resistors are not interesting. Just the thread seemed to be about killing hum, which is why no one picked up on the other items.

[R.G.]

Please take a gander at fig. 13.11. Anybody got any ideas about why you would want to purposely add resistance to the ground side of the power supply?

Enzo's right (big surprise!). Here's another way to look at it. "Ground" is wherever you decide it is and then all measurements shift to be relative to where you say ground is. When you add resistance to the ground side of the power supply, the ground reference shifts to the outside of that resistor, where the amplifiers are.

If you think about it, you can rectify and filter into a capacitor and have that voltage be floating, not connected to anything we'd call ground. It's like a battery which isn't connected to anything. We can connect that "battery" to things any way we want. Since we're trying to get rid of ripple, we probably want to use more filtering to do it. A simple lowpass filter is a series resistor and a shunt capacitor, which is what is usually used for the second filter cap circuit.

However, since the negative and positive terminals of the first filter cap are both places where there are high pulse currents, we'd like to get away from the chances of where we connect our wires. Since we're using a series resistor into a second cap anyway, the DC voltage on the second cap does not change if we split the series resistor into two halves and put one in the + and one into the - sides.

If we then take our signal ground as the negative of the second cap, what HAS changed is that the ground point itself is no longer a place where big power rectifier pulses mix with delicate signal ground returns. The first filter cap negative is where the pulses are, and the resistor to the second filter cap negative gives you some isolation from the noisy first filter cap negative.

And the signal ground point moves to the second filter cap negative. The first filter cap negative is just a place where pulses are integrated before being sent to the real circuit ground on the second cap.

And Raybob's comment about not needing any ground connection to the chassis at all is very interesting, but I can't help but think there may a be a problem with radio interference.

There would be. I think he was referring to the buss bar not needing to be connected to chassis. Ultimately, the chassis needs to be connected to signal ground for shielding or you do get hum and noise, but I think Raybob was making a different point.

I see a plethora of things that cause hum, and although my amp is extremely useable, I can still hear it. That deep, 60 cycle MMMMM! lurking somewhere probably in the preamp circuit. It's not loud, and at real world settings the amp is totally quiet, but it's there.

The Hum Demon lurks everywhere around us. It's more like an octopus or squid, sending tendrils of hum every where, than it is like a demon with claws and horns.

[Raybob]

[HTML]... And Raybob's comment about not needing any ground connection to the chassis at all is very interesting, but I can't help but think there may a be a problem with radio interference. ...

I might not have said what I meant. I meant the bus doesn't need to terminate where all other grounds terminate. The buss will ground to chassis by input jacks with ground wires from them going to buss. Point is, each stage of preamp and PI get their current from (-) of filters, attached to buss. Buss gets ground from input jacks. Should keep any loops from ever happening, buy separating current flow between 1st stage power, and 2nd stage. Clear as mud?