[sdiy] 1 Meg input level pots?

[Seb Francis]

I would say that they are most suitable for audio input use.  If you 
have a 1M linear pot going to a 100K resistor into a summing (inverting) 
opamp configuration then you will get almost exactly an ideal log pot 
response.  I don't think thermal noise will be a problem with 1M into 
100K into a FET type (e.g. TL07x) opamp.

You can see the response for yourself here:
http://www.mindspring.com/~clist/PotGraph.html
(also see the lower schematic on this page)

Seb


Neil Johnson wrote:
> James,
>
>> Since I would be basically using them as voltage dividers, I have to 
>> ask, what bad thing happens if I go ahead and throw caution to the 
>> wind and use these things for input level control pots ?
>
> If you're using them to mix signals into an inverting summing amp 
> (most likely) then you really need about 10:1 ratio of input resistor 
> to pot resistance, otherwise the input resistor loads the pot and you 
> end up with a non-linear (log-like) response curve.
>
> So with a 1M pot track you'd need an input resistor of at least 10M.  
> Which is not very practical and will very likely introduce noise (both 
> thermal and stray pickup, like a taxi radio) into your circuits.  Ok, 
> there are ways to avoid this (e.g. in sample-hold amps), but they add 
> complexity where you don't need it.
>
> I did something like you did a while back, but went for 10k pots so 
> that I could use 100k input resistors, which is a bit of a standard 
> (although not cast in stone!).
>
> Hope this helps in some way.
>
> Neil
> -- 
> http://www.njohnson.co.uk
>
>
>
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