Programmable amps and effects

[valdiorn]

I've been looking into some ways of building circuits that can be controlled digitally from a microcontroller. Basically I want to replace the potentiometers in a normal circuit with one or two variable resistance devices that can be controlled either digitally or with voltage. I've come up with a couple of solutions but none of them are really that great. What I have so far:

Digitally controlled resistors - Easy to use with a uController and they're cheap. Problem is they only work with small voltage signals (0-5V) so would require attenuation and bias before input and then gain after output, really a hassle.

JFets as Voltage controlled resistors - Thought of this first but this would be completely unusable where large resistances are needed as they would distort the signal alot.

Photoresistor + LED (Vactrol) - The best solution I've found so far. But they are hard to come by and cost alot. I could build my own but they would probably vary in resistance too much to be useable and I'd have to hand-pick the ones that fit my criteria. Also, most photoresistors / photocells I can find don't give a resistance lower than 5k at maximum brightness... (I know they used this method in the Mesa Triaxis, but the chips are very hard to get).

Anybody have the "ultimate" solution to programmable audio ciruits?? Know where I can find a good source of LDRs (light dependent resistors)?

[R.G.]

Anybody have the "ultimate" solution to programmable audio ciruits?? Know where I can find a good source of LDRs (light dependent resistors)?

Sure. I think I put it in the original guitar effects FAQ some years ago. I know I've posted it here or in the previous life of the forum.

The answer is a computer-controlled pot turner. The track stepper motor from many 5.25" diskette drives (ACK! What are those???) is/was both tiny, had lots of steps per revolution, and had a 1/4" shaft on both ends. You put a knob on one shaft end. That's the human interface. Then you clamp/glue about 1/2" of 1/4" rubber fuel tubing between the shaft on the other end and the pot shaft. Ideally, you use a dual pot, one section of which is used by the uC to read pot position. Now you hook it up so the uC can step the stepper motor, turning the pot shaft. When you get to the desired position, turn the stepper phases off. That lets humans grab the knob and turn them. The second pot section lets the uC read where it is so the uC can come back to that position.

Real pots, human or programmable. Doesn't get much better than that. 

Size? Weight? Power? Complexity?

Huh. We don't need no steenkin' simplicity.   

[The Tone God]

As this "ultimate" question is just a variation on the common "best" question I will respond with similar answer which is there is no "ultimate". Each option has pros and cons. The skill is in selecting and implementing the most appropriate option for the task.

Andrew

[Ice-9]

You could check out the TC9176 series of chips they can be used to control volume ,gain, tone controls etc. For an example of how they can be used check out the Marshall JMP1 rack preamp which uses these to control all the "traditional potentiometer settings"