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Author Topic: Simulating Log Pots with Linear pots + parallel resistor: a bad idea.  (Read 898 times)

J M Fahey

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This way it´s easier to understand:


Suppose a pickup loaded by a 1M potentiometer.

We use a true 1M Log pot and a 1M Lin one with 100k strapped from wiper to ground.
We calculate 3 positions to make it easy and in any case are more than enough to show the heavy loading effect and wildly varying load impedance shown by the fake/simulated one.
A terrible idea I might add, sadly wildly popular because it looks "easy" .
I´ll simplify some values to closest round number, the idea here is not to get numbers accurate to 6 decimal places but to understand the concept of what´s happening.

Ok, let´s start with wiper on 0

True Log shows pickup load=1M , signal at wiper 0
Fake Log shows pickup load=1M , signal at wiper 0

Hey!!! it looks good!!!! ;)

Repeat with wiper on 5


True Log shows pickup load=1M , signal at wiper 11% 
Fake Log shows pickup load=590k , signal at wiper 18%

Mmmmhhhh, load has varied significantly,  maybe it won´t hurt pickup sound that much, now if pot were after a small coupling cap (as in VOX and Marshall) I would be losing almost the full lowest octave. :o
And Log simulation is not that good either, we are almost 6dB above what a true Log would offer.

Repeat with wiper on 10


True Log shows pickup load=1M , signal at wiper 100% 
Fake Log shows pickup load=90k , signal at wiper 100% .... IF pickup or earlier stage can happily drive 100k , 10X smaller than expected.

If it were a volume control after a small coupling cap, a common trick in respected Guitar amps, equalization would be a mess.

Same if following a tone control stack.

If following a triode stage, you would lose at least 6dB gain, because plate load resistor (typically 100k to 220k) would now be in parallel with 90k.

If following a pentode stage, which has high internal impedance and typically drives a high value load (220k and 470k are common values) effect will be a mess, easily losing 15dB signal.

Not enough time now to show the effect of a fake Log Pot used as variable NFB gain control in an Op Amp circuit, but believe me it´s even worse than this.

But is it absolutely useless then?
No, there is *one*  very limited case where it works, sort of, at least as a Saturday afternoon stopgap until shops open on Monday and you can buy a proper Log one: IF you use it as a passive Volume control after a low impedance driving stage, say an Op Amp, which can easily drive the much reduced impedance shown on 10 , then it´s acceptable.
Not accurate but human ear is so imprecise that it won´t complain.

Not even those who "can hear the influence of power cable Oxygen content" or whatever :D

ASTN

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I agree.
Though it'd be better to mention that 'Simulating log pots... is a bad idea IF you're dealing with high output impedance devices'

 

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