"DIRTY GND"?

[Axtman]

I saw this on the Ampeg SVT 3 Pro schematic. What is a dirty ground?

[Chuck H]

Hmmm...? It's even more strange because the symbols used would indicate that the "DIRTY" ground is a signal ground and the "SIGNAL" ground is earth. Though I'm not sure what a proper symbol for a dirty ground should be.

I did some reading and a "dirty ground" would be any ground that has a higher potential for current or voltages, intermittent or modulated, than would be acceptable for a signal ground. In a nutshell.

If you have it maybe post an image for the power amp board so we can see where it's going.

[Jazz P Bass]

There are three grounds used on the amplifier.
Dirty, Chassis and Signal.
As far as I can tell from the power amp schematic, Dirty Ground and Signal Ground are separate and distinct.
Dirty Ground is used on anything 'power' (ie: power supply voltage) orientated. It also attaches to chassis ground.
Signal Ground is just that. It is used on all signal voltages.

[teemuk]

"Dirty ground" conveys all those ground return currents you don't want to interfere with "signal" ground return currents.

By quick glance is see power supply filter ground returns, speaker load ground return, and footswitch ground return in the "dirty" category, which makes sense. Power supply filter ground returns can carry high surge currents, speaker ground return carries all load current, and footswitch ground return may have long wire run to an external foot pedal and carries current surges of switching. You do not want any of these currents to develop voltage potentials to place that should be the steady reference point for the audio signal, the "signal ground".

More enlightening might be a diagram that shows how and where those connectors and "grounds" also "terminate". There are not many true "ground" reference points in a circuit and most "ground points" are actually just resistive connectors to it in practice. Proper routing of return currents is therefore important.

[R.G.]

You're right teemuk - that's the return of currents you don't want polluting your signal ground returns. I have a list of various I've typed in here at one time in the past. I called it "sewer ground" instead of "dirty ground" but the idea is the same.