[sdiy] Pot Survey

[MBEDTOM at aol.com]

Before changing pots I would ask: "What problem exists for which a change in 
potentiometer is the solution?"

Alpha pots (Mouser 31VJ401) cost $0.96 if you buy 10
Spectrol 248-series (Mouser 594-248-7104) cost $5.50 each, $4.82 (25)
Spectrol 249-series (Mouser 594-249-7104) cost $5.50 each, $4.82 (25)
Spectrol 148-series (Mouser 594-148-7104) cost $7.88 each, $6.30 (25)
Spectrol 149-series (Mouser 594-149-7104) cost $7.88 each, $6.30 (25)

Alpha pots are carbon composition, 1,000PPM/degree C, 15,000 turns rotation 
life
Spectrol 248 pots are conductive plastic, 1,000PPM/degree C, 50,000 turns 
rotation life
Spectrol 249 pots are cermet, 100PPM/degree C, 25,000 turns rotation life
Spectrol 148 pots are conductive plastic, 1,000PPM/degree C, 50,000 turns 
rotation life
Spectrol 149 pots are cermet, 150PPM/degree C, 25,000 turns rotation life

Now for the kicker... Mouser does not stock 148/248 series Spectrol pots in 
anything but linear taper.  You want audio taper?  T.S. Mr. Eliot.  You will 
get linear taper and you will like linear taper!  But those "deer nuts" Alpha 
pots are linear, audio, single, dual, and about any flavor you want.  (I liked 
that ... "deer nuts".  Hit my funny bone.)

For controls that adjust "frequency" (VCO, filter cutoff, LFO, etc.) a nice 
stable cermet pot would be better due to lower temperature coefficient.  Best 
would be a 10-turn wirewound pot with a temperature coefficient 1/5 that of 
cermet (20 PPM).

Now the great debate... should level controls be linear taper or audio taper? 
 For example, should a mixer have linear pots (and possibly reversing 
attenuators) or audio taper like most any other volume control on the planet?  Which 
do you prefer?  The MOTM-830 mixer has 6 inputs.  3 of those inputs use linear 
pots and are more suited to mixing of control voltages.  The other 3 inputs 
use audio taper pots and are more suited for audio inputs.  Do you tweak by 
sound or knob position?  A linear pot sitting in the middle of its range is easy 
to determine what voltage levels you'd see if you scoped it out.  Great for 
not overloading subsequent stages.  But audio taper pots give a finer adjustment 
at the lower ranges of signal levels (more pot travel for less change in 
perceived signal level).  But audio taper pots make it harder to figure out if 
you'll overload subsequent stages.

If I had the money to stock up, I would go with Spectrol 248/249 series pots 
and special order a boatload of audio taper pots at the values I anticipate 
that I would want.  That's a risky proposition, obviously.  Maybe you buy some 
1M audio taper conductive plastic pots for use in planned ADSR modules.  After 
you order them and they arrive 8 months later, you figure out that you really 
wanted a voltage-controlled ADSR generator and those panel controls must be 
linear taper because the taper change is the result of an exponential convertor. 
 Arrggh.  That is one of the problems one faces with special orders.

Going the other way, I started using the Alpha pots from Mouser because I can 
layout PCBs to accommodate 248-series pots, but use Alpha 24mm pots to do 
things on the cheap.  I can still change my mind if I pop the lottery!  Besides, 
the "deer nuts" 24mm Alpha pots work very well with Stooge brackets for 
MOTM-format modules.  That was important to me.

YMMV

Peace.
Tom Farrand
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