Ground bounce in mixer.

[nigelwright7557]

I have designed myself a 6 channel audio mixer with digital pots.

Got the circuit boards through today and I managed to get it working ok except for a hum when the pots were turned right up but with the input shorted. The preamp does have a gain of 200 so the slightest bit of hum on the inputs gets amplified massively.

Turned out to be the ground line travelling all around my pcb being too long and becoming resistive. So put a wire on the pcb from ground on the right hand side to ground on the left hand side and the hum went away.

Not sure if the problems is just the ground track too long or cheap pcbs made with cheap copper.

Of course I should really have added a powerplane but my supplier cant do them for some reason.

[stephensank]

This is actually a well known potential problem in mainly large analog mixing consoles, particularly from Soundcraft. You may have either had inadequately wide ground traces, or perhaps your boards were clad with half ounce copper(most cheaper pc makers use this), whereas one or two ounce copper cladding may have prevented the problem.

[nigelwright7557]

It looks like there is resistance in the ground line.
I have added a couple of wires to short the ground line out and the noise is now minimal.
Will have to take care next time to request 1 or 2 ounce copper.
I guess if I leave it up to the manufacturer he will go for the cheapest option.

[marce]

Always specify copper wieght, also if you have through plated holes the base wieght will be increased due to the plating process. Most manufacturers also have a list of their capabilities, these will usually show what size feature they can etch for a given copper weight.

[nigelwright7557]

My mixer has a gain of 200 so it only takes 2mV of noise to get 200mV of noise on the output which is unacceptable by miles.