can a bias supply introduce noise into the audio?

[leadfootdriver]

Perhaps from a lousy diode or bad filtering?

[The Dude]

Anything is possible, but you're more likely to notice inconsistent bias current or an inability to bias the amp correctly if there are problems in the bias circuit. If you have a scope, look at the bias supply and see if anything is amiss.

[Gingertube]

Absolutely it can - in fact any noise on the bias supply will appear on the grid of the output tubes and you are then relying on the Common Mode Rejection of the Output Stage to get rid of it. That will depend upon tube balance, idle current balance etc.

Last HiFi Amp I used MOSFET Source followers direct coupled to teh output tube grids and with bias applied to the mosfet gate. Replacing teh source follower load resistor to the -ve rail with a curret source made a quite dramatic reduction in noise. (The current source isolated the -ve rail noise).

Cheers,
Ian

[R.G.]

Yep. Any signal on the bias supply is going right into the grids. This can be the origin of hum if the bias filter cap is going bad, or the bias rectifier diodes is getting leaky.

[soundguruman]

Perhaps from a lousy diode or bad filtering?

You don't usually notice it because the bias voltage is shutting off the output tubes.
When audio is present, the tubes turn on, and you notice it as distortion or noise in the background, of the music.
That's how the design gets away with some ripple in the bias supply.

If you hear noise or hum, it's usually from the preamp...
and the bias ripple is the least of the problem.

Another difficulty is if the output tubes are out of balance...
You hear noise or hum, but it's not from the bias supply.

[leadfootdriver]

Thanks for the replies!

[Tom Phillips]

You don't usually notice it because the bias voltage is shutting off the output tubes...

I'd describe it more like the bias is throttling the tube back. In Clase A or AB designs the tube is still definitly on at idle and any ripple in the bias supply will show up in the output as a hum component.