Simulating Log pots with Linear ones + resistor in parallel: bad idea.

J M Fahey

Member
Messages
2,411
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. :eek:
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
 

J M Fahey

Member
Messages
2,411
No, the way to go from Linear to Log is with extra resistor from wiper to ground.

If you go from wiper to hot, you make it "less Logarithmic" ....if it was logarithmic of course.
 

Tone_Terrific

Silver Supporting Member
Messages
33,048
No, the way to go from Linear to Log is with extra resistor from wiper to ground.

If you go from wiper to hot, you make it "less Logarithmic" ....if it was logarithmic of course.
Yes, I see, running it to ground is clearly going to produce loading problems.
 

Kyle B

Member
Messages
5,074
Not even those who "can hear the influence of power cable Oxygen content" or whatever :D
What are you talking about??? ANYBODY can hear the difference using oxygen-free cabling. It's the guys who think they can hear the gold plating on their speaker jacks that are full of crap!

[/sarcasm] ;)
 

HotBluePlates

Member
Messages
10,670
But you don't run it to ground.
You run it from hot to wiper.
No, the way to go from Linear to Log is with extra resistor from wiper to ground. ...
Scroll near the bottom of R.G Keen's article. One way is for faking a log taper, the other way is for faking a reverse-log taper.

Keen said (in 1999), "However, neither the load seen by whatever drives Vin or the source resistance as seen by the input of whatever is connected to Vout is close to what would exist for a real log pot of value R."

So usually faking the taper is a poor choice, unless you're having to fake a reverse-log pot that's unobtanium.
 

Kyle B

Member
Messages
5,074
Necro thread. Score one for the new guy....

Juan hasn't been on TGP since Nov 2017 :( That's a shame, I liked his candor.

I just noticed though, Juan's analysis is not correct. Taken by itself, i.e. a guitar not plugged into anything, the analysis is, of course, spot on. But once plugged in, there's gonna be one or two 68k resistors (or thereabouts) from wiper to ground due to the resistor at the amplifiers input. For example, the classic Bassman circuit.....

So that 100k he says "don't use" may essentially already be there.


 

Badside

Member
Messages
1,415
Necro thread. Score one for the new guy....

Juan hasn't been on TGP since Nov 2017 :( That's a shame, I liked his candor.

I just noticed though, Juan's analysis is not correct. Taken by itself, i.e. a guitar not plugged into anything, the analysis is, of course, spot on. But once plugged in, there's gonna be one or two 68k resistors (or thereabouts) from wiper to ground due to the resistor at the amplifiers input. For example, the classic Bassman circuit.....

So that 100k he says "don't use" may essentially already be there.


68k to ground is only when using the Low input
 

HotBluePlates

Member
Messages
10,670
But that is 1M to ground not 68k.
68k to ground is only when using the Low input
:facepalm

Input 1 only: 1MΩ to ground
Input 2 only: 136kΩ to ground
Input 1 & Input 2: Input 1 sees ~120kΩ<Input-Z<207kΩ (for 500kΩ Volume pot or ~167kΩ for 250kΩ pot); Input 2 sees ~136kΩ<Input-Z<247kΩ (for 500kΩ Volume pot or ~195kΩ for 250kΩ Volume pot)

They're different input impedances depending on whether you plug into Input 1 or 2, and different again if a 2nd instrument in plugged into the other jack (changing yet again depending on the setting of that other guitar's Volume pot, with a max when the other guitar's volume pot wiper is at half-resistance).

Anyway, I still stand by my advice to skip faking a log pot and just buy a log pot...
 

HotBluePlates

Member
Messages
10,670
My advice: if you can make it work you're job is done.
An amateur will expend time to save resources. A pro will expend resources to save time.

(A pro might use a resistor & a linear pot when the customer is too amateurish to know the difference, saving time & resources. My primary customer is me, and I know the difference, so I just buy the pot. :D)
 

Kyle B

Member
Messages
5,074
68k to ground is only when using the Low input
:facepalm

Input 1 only: 1MΩ to ground
Input 2 only: 136kΩ to ground
Input 1 & Input 2: Input 1 sees ~120kΩ<Input-Z<207kΩ (for 500kΩ Volume pot or ~167kΩ for 250kΩ pot); Input 2 sees ~136kΩ<Input-Z<247kΩ (for 500kΩ Volume pot or ~195kΩ for 250kΩ Volume pot)

They're different input impedances depending on whether you plug into Input 1 or 2, and different again if a 2nd instrument in plugged into the other jack (changing yet again depending on the setting of that other guitar's Volume pot, with a max when the other guitar's volume pot wiper is at half-resistance).

Anyway, I still stand by my advice to skip faking a log pot and just buy a log pot...
I know that. To keep it short I didn't completely describe the circuit. This is TGP. I knew you guys would do that for me ;) Had to leave one as an exercise for the reader LOL
 

Kyle B

Member
Messages
5,074
Lol! I don't see too many guys working out Ohm's Law puzzles on TGP. That's a shame, as we'd probably have more informative discussions if they did. :D
I like working out the puzzles ;) It's the creating and posting of a picture that's a PITA !!!!!
 

Johndh

Member
Messages
407
I've seen the 'resistor-from-wiper-to-ground' idea on a few mixer circuits I've looked at. As noted above, its ok if driven by a low impedance and onwards into a high impedance. Good for fader slide pots which are often linear.

Another idea for a lin to log work-around could be to stack the two halves of a dual-ganged linear pot, one after the other. It would give a very smooth 20% (I think) mid taper. But like many ideas, I've never actually thought of a real application for it!
 




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