Values of volume pots

[SimonConnell]

Just a quick question I'm hoping someone can answer. I've got a Alps Blue Velvet pot in my pre-amp at the moment, I'm not sure but I think it's 50K. Now, if I were to replace it with a 600 Ohm studio stepped attenuator, what would happen

1. The thing would blow up
2. Absolutely nothing
3. Volume levels would change (or something similar)
4. Something else I can't think of?

Now I know you'll argue if I don't know the answer to this one I'm not really safe to be fiddling with thing, but please bear with me
Thanks in advance,
Simon

 

[Andrew L Weekes]

That depends a little on what you are putting it into.

Volume controls are another one of those compromise areas in audio design - there's a balance between a number of electrical factors that results in the final value chosen.

A high impedance pot is noisier, electrically a lower impedance pot presents a heavier load to the driving circuit and can increase power dissipation. It would require larger value coupling capacitors in AC coupled design, which is in itself usually a bad thing, sonically.

The change you propose from 50k to 600R is huge, electrically, and I would expect that the driving circuit will, at the very least be loaded more and have it's output attenuated more. It is also likely to affect frequency response if there are DC blocking capacitors in the signal path - the lower terminating impedance will cause a change in bass response, rolling off earlier than intended in the design.

At worst it will also reduce linearity, increasing distortion, this is especially true if an op-amp driving stage, almost all opamps have poor output stages and when loaded, even comparitively lightly, they distort and sound worse.

Andy.

 

[SimonConnell]

It's going into a Williams Hart headphone amp. It doesn't sound like its a very good idea given what you've said
Regards,
Simon

 

[Andrew L Weekes]

Simon the Chiara uses a 50k pot as you suggested and this is in effect 'passive'.

It is therefore being driven by the source component you are using.

There's not many components that are truly happy driving a 600R load, since they are almost all designed to operate into a higher impedance then this, so I'd suggest it may not be such a good idea.

Andy.