Super POT

[markseel]

Put this in your pipe and smoke it ...

I was messing around with an analog distortion circuit that has multiple clipping stages.  Each stage has a gain adjustment and a bias adjustment and a tone (low-pass filter) adjustment.  So that's a lot of adjustments and I don't want too many knobs on the effect so ...

How about a super pot.  It's a small 10-pin board that has connections for:

1) Power and Ground
2) One ADC input to connect to a 10K or so potentiometer
3) Six digi-pot / output pins for
    - Two digitally controlled potentiometers with three connections each OR
    - Three digitally controlled potentiometers with two connections each
4) Serial programming pin

You download a program to FLASH using the serial pin.  The program is a short sequence of instructions that determines how the input value maps to the two or three pots.  Similar to a simple DSP program that reads an input and writes to multiple outputs, only at a relatively low rate of 100 Hz or so.

Thoughts?

[ExpAnonColin]

So you mean the single pot would kind of morph through pre-sets?  That might be cool, but I don't see it as something that tons of people are antsy to get their hands on.

-Colin

[markseel]

No, more like this;

Say  want my distortion circuit to exhibit lots of asymmetrical clipping when the gain is low and symmetrical clipping when the gain is high.  One digipot controls gain, the other digipot controls a bias.  As you move the control pot (input) from low gain to high gain the bias is slowly brought to normal/symmetrical as the gain approaches max.  Like this:

Gain (set by digipot #1) = 50K * Input
Bias (set by digipot #2) = 50K * (1 - Input)

Or perhaps you have a hi-pass filter for tone control.  A single pot could control both the amount of attenuation as well as the cut-off frequency such that the single 'real' pot has three regions of operation.  Like this:

If Knob Position ranges from 7:00 to 10:00:
Cutoff freq is set to f1 (set by digipot #1) while attenuation varies from 0db to -12db (set by digipot #2)

If Knob Position ranges from 10:00 to 2:00:
Cutoff freq is set to f2 (set by digipot #1) while attenuation varies from 0db to -12db (set by digipot #2)

If Knob Position ranges from 2:00 to 5:00:
Cutoff freq is set to f3 (set by digipot #1) while attenuation varies from 0db to -12db (set by digipot #2)

Make sense?

[JKowalski]

Sounds like a really complicated replacement for two pots. I can usually get by fine with individuals.

Why would you sample the pot at 100Hz? 

[ExpAnonColin]

Make sense?

Sounds like morphing through pre-sets to me   That's what I was thinking - you are essentially defining control curves so that only "good" pot settings exist.  Like I said, sounds interesting, but I don't think there's a big demand/market for it.

-Colin

[markseel]

Ok. Doesn't like a usefull approach for anyone
Thanks for the feedback guys.

[Taylor]

Mark, I think there's potential here (pun unintended). Don't get discouraged right away. I've seen some of your ideas and none of them have been bad - I think you just need to explain a little more how this would be useful to people.

The other thing is that when you post in the digital forum, you mostly get people reading who already work with digital stuff. If you posted in the general forum, and aimed your pitch at a more general user, and gave a more detailed explanation of real-world uses, you might get more interest.

In this case, I think such a thing could be useful, but people would be wondering about price as to whether it's worth it, and also they need to really have it spelled out why they would want this, and why it's better than other, simpler, analog solutions.

You might also get some interest if you made a whole project that used this. For example, if you designed a fuzz that used a single pot to morph through some very different fuzz types, that would be cool. You should be able to define pot curves that are bell-shaped, so you could have a pot that fades from Fuzz Face, increasing gain, then blends into a Big Muff-type fuzz, then increases gain of that.

...or something.  Basically my experience has been that
1) super-knowledgeable folks like Colin and Chris above don't think this stuff is useful because it's obvious to them
2) those without this kind of knowledge would be impressed and interested, but they need the significance really spelled out for them. I belong to this second category in some ways.

[Galego]

I was using a dual digital pot to try and control the level/repeats of a PT-80 delay. Before I learned that Microchip MCP41xxx pots can only handle audio within a Vss-Vdd range, which usually means 0-5v. What kind of digital pots are you going to use?

[ExpAnonColin]

I was thinking about this a little more, and the Line 6 "4" series pedals do this, except with only two settings.  You set the beginning and end settings of the expression pedal and can morph between the two settings.  This sounds like that on steroids.  I didn't mean to give the impression that it wouldn't be useful (it definitely would be), I just don't think it's something people think of as needing - in other words, not a huge market.  It's a really smart idea and probably a useful thing.

-Colin

[potul]

A related item I think would have some acceptance would be a set of let's say 3 digipots and some way to store presets for them. This way we could add presets capability to some analog pedals, and if you add MIDI control to it, it would be even better.

The problem though, is how to decide on the digipot values....

I have a similar project in mind since a long time ago, I even bought the digipots, but couldnt find the time to do it.

Regards,

Potul.

[Galego]

Why no answer to my question? Isn't the voltage range an issue for you guys? Wont 0-5v be an issue for most applications, wont that force you to make changes to the circuits and lose some headroom?

[markseel]

It's a good question Galego.  There's various applications in which the voltage range of the pot has to be considered:

1) When it's in the signal chain.  Dynamic range might be an issue for some.  I was using it in the feedback path of a an opamp in a diode clipper arrangement.
2) When it's not quite in the signal chain.  I was using a pot to adjust a voltage bias on an amp circuit operating in class-A.
3) When it's not in the signal chain.  Say for a LFO circuit for a BBD-based delay where you want the LFO's characteristics (frequency, shape, etc) controlled in some way.

Not all arrangements cause concern for the pot's voltage range for the analog signal.  There are some digit pots (by analog devices I think) that have separate pins for Vdd and Vaa such that the pot's analog range can be much larger than the 5.0 volts and can be positive and negative with respect to common ground.