"Accelerated" Anti-Log Pot / Variable Resistor advice

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thermionic

Well-known member
Joined
Jun 3, 2004
Messages
1,671
Hi,

I'm working on a project that needs a swept HP shelving filter. It uses an op-amp with a cap and a pot wired as variable resistor - a simple design, sweeping from <20Hz to 3k at full travel. The trouble is, even with a reverse-log pot, you don't really 'feel' the HP until it's well passed 12-o'clock, and the region that is most noticeable is squashed into the last half rotation. Towards the last 50-odd degrees of the travel it gets really sensitive / twitchy. So, if I stay with this HP design, to get a more even feel, I'll need an 'accelerated log' pot, i.e. a reverse-log pot, but with an exaggerated curve.

Can anyone see a way around this, short of paying for an unusual custom taper?

NB - feel is really important as the project needs to be used live, and the user doesn't want to twiddle the knob half the way around to get any effect, with the last few degrees of travel being twitchy.

Thanks in advance.

Justin

 
I'll have to scan it. The thing is, I've tried different RC combo's within reason, i.e I can only find conventional pots. I've tried 50k with a 470n, 100k with 270n - and 470k with 47n (from memory - the cap values could be slightly out). If the cap is too small, I get roll off with pot fully anti-CW. To big, and I don't get anything happening until pot's turned right around. Even with a smaller cap, the actual curve is still similar (whether I use reverse-log or lin pot) - it gets progressively more twitchy as I rotate. As the ear is more sensitive to midrange and frequency doubles every octave, this isn't surprising. My goal is to try and find a solution that feels more useable.

Thanks.
J
 
It's a tad difficult to answer precisely without a schematic - in fact I don't understand exactly what you mean by HP shelving, do you mean something like a variable-frequency Baxendall fully cranked?
I think you may have to resort to a more complex filter, namely a first-order state variable filter, in which the frequency control would be provided by a grounded pot (instead of a rheostat) and a law-steering resistor (R43 on the attached schemo). You may have to move the law-stering res to the upper half of the pot to have more resolution at the end of travel. Then you need to combine with the original input to make it shelving. The values ther give a range of 300-13kHz, you may want to increase the cap for a lower range.
 

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abbey road d enfer said:
It's a tad difficult to answer precisely without a schematic - in fact I don't understand exactly what you mean by HP shelving, do you mean something like a variable-frequency Baxendall fully cranked?
I think you may have to resort to a more complex filter, namely a first-order state variable filter, in which the frequency control would be provided by a grounded pot (instead of a rheostat) and a law-steering resistor (R43 on the attached schemo). You may have to move the law-stering res to the upper half of the pot to have more resolution at the end of travel. Then you need to combine with the original input to make it shelving. The values ther give a range of 300-13kHz, you may want to increase the cap for a lower range.

I'm talking about an HP that, when the control is fully anti-CW, is unnoticeable, i.e. is shelving inaudible frequencies <20Hz, and moves the shelving point proportionally with CW rotation. The same thing as an HP on a synth - but without resonance.

I will breadboard your idea when I get a chance and play with it. Many thanks - that's really cool of you.                                     

Justin
 

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