Variable pot on preamp boards ...

[craigen]

So far I've had good luck with buying preamp boards from overseas. The next one I am buying has integrated "variable pots," one for each channel. The description of the board implies that these have already been set. But what ARE they, really? What they adjusting? And how do I know that they have indeed been properly adjusted?

 

[ron68]

Hi,

Do you have the link for this product?

 

[craigen]

Thanks for responding. Here it is

https://www.aliexpress.us/item/3256...t_main.64.638718020HVaR4&gatewayAdapt=glo2usa

 

[rayma]

Probably for output DC offset adjustment.

 

[ron68]

Thanks for responding. Here it is

https://www.aliexpress.us/item/3256...t_main.64.638718020HVaR4&gatewayAdapt=glo2usa

Hi,

From the specs, I've got the following:

"Field tube input to promote the gold seal tube output, the quiescent current and the midpoint can be adjusted, work in class A amplification"

It looks like the DC offset and bias current normally found in any amplifier.
In this case, just a pre-amp, but it looks like a pre-amp with a robust output stage.

 

[craigen]

So why would that be used instead of specific value?

 

[rayma]

Circuit is fully direct coupled, so it needs an adjustment to compensate for all the variables causing DC offset.