How to replace Analog Pots with CV control?

thefunk · Joined: May 04, 2009 Posts: 23 Location: Orlando, Fl

I'm looking to use a MicroController to control a number of analog drum circuits. But, I am currently struggling to find a suitable / hassle free method to replace volume (voltage divider) pots with a voltage controlled circuit.

The methods I know of and their issues to me are:
Digital Pots: Only linear. Only allow voltages within the power supply rails: 0-5V for the most part.
OTA: Just seems like overkill. According to the 13700 datasheet I would need a whole a chip for 1 pot.
LED / Photo-resistor / Cell: Would need 2 pairs for each pot and would need to find a way to increase 1 resistance while accordingly reducing the other... non-linearly.
FET: same as photo-resistor.
VC Pot chips: NTE still makes one for $10 each.

Are their any other methods I am missing? How is this normally achieved? Digital pots look like the best method out of the lot, but I'm hoping there is an easier way.
thanks.

Clack · Posted: Tue May 01, 2012 12:06 am Post subject:

Yes pretty much

An experimental idea I have been playing with is to switch an analogue gate on and off at more that audio rate put a simple filter on it and then adjust PWM

Also a Note that you only need half a lm13700 if it is grounded or single resistance.

slacker · Posted: Tue May 01, 2012 10:49 am Post subject:

You can probably get away with using one LDR for each pot. You make the top of the divider a fixed resistor and use the LDR for the bottom or the other way round.
This will work like a volume control, the only thing is the impedance changes with the changes to the LDR's resistance. Depending on what is driving the "pot" this may or may not matter.
You can do the same thing with FETs.

JingleJoe · Posted: Tue May 01, 2012 1:27 pm Post subject:

What about a D to A converter?
Simply put the output of that into the input of the voltage controlled circuit.

thefunk · Joined: May 04, 2009 Posts: 23 Location: Orlando, Fl

Thanks all.

@Clack
Yes 1/2 an lm13700 will work. I missed that. That means I'll only need 14 of them to be able to control the TH Quad Bass++. That, with pwm, a filter and a s&h circuit should work nicely. This is sounding more enticing.

@Slacker
I'm sure I could get away with 1/2 the resistance and a 6dB drop in these circuits, but I would like to find a method I could use reliably without much thinking. Smile

Though, these methods are definitely worth experimenting with in single ended pots. (The decay knob in the Bass++)

@JingleJoe
I'm not sure I follow... creating an analog signal from a digital one should not be a problem. Though, a serial digital decoder might be worth looking for... a digital pot without the resistors... hmmm... they must exist.

elmegil · Posted: Tue May 01, 2012 6:41 pm Post subject:

So which microcontroller?

If you take Arduino as an example, the analog outputs produce PWM signals, which can be filtered with appropriate caps & resistors to provide a steady voltage level for a given value. There are some example circuits "out there" though I haven't looked at them recently because I didn't take that approach.

I think we're assuming that the pots are hooked up with one side on a positive voltage source, the other on ground or a negative source, and the control coming out of the wiper. If that's the case, I'd assume that you could hook the voltage through appropriate resistance to ground and then just connect to where the wiper was connected.

If the pot is being used to control gain on an op amp or transistor circuit, then you'll need something else, I'm not sure what Smile

.

thefunk · Joined: May 04, 2009 Posts: 23 Location: Orlando, Fl

Netduino. So pretty much an Arduino.
Sorry for the confusion, to clarify... the full volume audio wave at one end, ground at the other end (for the most part), and the adjusted volume at the wiper. What I would like is an added control voltage to set the position of the wiper.
I'm starting to see why OTAs are so 'beloved'. Smile

The LM13700s look like the best way to go. Though, digital pots might be worth messing around with.

This is the circuit I'm beginning with...
http://www.birthofasynth.com/Thomas_Henry/pdf/Bass_Plus/basspp_schematic.pdf I'd like to be able to control the pots programmatically.

stewpye · Posted: Tue May 01, 2012 8:40 pm Post subject:

You really need to rethink the circuit design rather than just replace pots with something else.

For things like the decay control you could just use a PWM output to switch a transistor that discharges the decay cap.

Wherever there is a simple volume control just use a VCA (LM13700).

Regards,
Stewart.

JingleJoe · Posted: Wed May 02, 2012 4:09 am Post subject:

richardc64 · Posted: Wed May 02, 2012 4:14 am Post subject:

stewpye · Posted: Wed May 02, 2012 7:44 pm Post subject:

For the impact tone you could use two LM13700 VCA's. The input to one of them comes from the high pass section of the tone circuit and the input to the other one comes from the low pass section of the tone control circuit. You may want to make the input impedance of them similar to what the tone pot is. The control input for one of the VCA's would have to be inverted (and level shifted) in regards to the other.

The shell volume could probably be done by just having an attenuator (similar to the sweep control) between the emitter of Q1 and R18.

Regards,
Stewart.

thefunk · Joined: May 04, 2009 Posts: 23 Location: Orlando, Fl

Ah. So the 909 uses transistors not tied to ground. I hadn't thought much about the Impact tone, but using a switch or mixing the outputs seem like good ideas to me.
I guess I should just start tinkering and try them all. It's more fun that way anyway Smile

Thanks for the ideas all!

richardc64 · Posted: Thu May 03, 2012 12:59 am Post subject:

oldmanfury · Joined: Oct 20, 2006 Posts: 57 Location: CA

thefunk · Joined: May 04, 2009 Posts: 23 Location: Orlando, Fl

Thanks. That's cool! I've got one of those pedals... somewhere, from my 'grunge' days Smile

.NET Micro is about as simple at SPI control.