[sdiy] Dead Unobtainable Concentric Pot, What to do?

[ShedSynth]

A quick way to test this idea might be to try Steve's suggestion with 2
separate 500K pots on flying leads, hear whether it sounds right.
Then swap in easier-to-find values.
Al

-----Original Message-----
From: Synth-diy <synth-diy-bounces at synth-diy.org> On Behalf Of Steve Lenham
Sent: 20 January 2021 14:06
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] Dead Unobtainable Concentric Pot, What to do?

On 19/01/2021 19:26, James Coplin wrote:
> This was my plan re the curve but I have not been able to find any 
> concentric pots except those for guitars which are the wrong size (I 
> think I can work with that) and none in the 10k value. 500k seems 
> avail but its the 10k I'm stumped by at the moment.

Hi James,

It's terribly difficult to find much in the way of dual-concentric pots
nowadays, doubly so for lower and/or non-matching values. I feel your pain,
having had to do it myself for the Lexicon Prime Time delay which has one
10K linear section and one 500K log section from the old A-B ModPot range.

So I looked up the schematic for your Orban 111B.

Firstly, I believe the level controls actually have standard log tapers. 
  It's the BANDWIDTH controls that are dual 100K CCW LOG (or reverse-log).
So that makes things a bit easier from the off.

Secondly, there is an easy way out. The input level pot section has been
made 10K in order to make the input impedance of the unit 10K "for free".
The pot wiper actually drives the very-high-impedance non-inverting input of
a TL072, so could be of a much higher value.

I suggest that you simply fit an available dual-concentric 500K log pot,
then add a 10K resistor across the outer pins of the input pot to keep the
input impedance unchanged. The TL072 will be perfectly happy with its
increased source impedance. How do I know? Because the existing 500K log
output section feeds a TL072 in exactly the same way (the output pot is fed
from a passive EQ network, so has been chosen to be a high-impedance load).

Hope this helps!

Cheers,

Steve L.
Benden Sound Technology

_______________________________________________
Synth-diy mailing list
Synth-diy at synth-diy.org
http://synth-diy.org/mailman/listinfo/synth-diy
Selling or trading? Use marketplace at synth-diy.org