digital solution to an analog problem?

[Barcode80]

I think this is the place to post this.

I've got a steel player friend who wants to be able to run a control box out from his amp to his steel and have controls for 2 or 3 pots on his amp so he can adjust them on the fly. I thought about just mounting a switching jack on the back of his amp for each control he wants on the box, running a snake of cables to the box, and operating them from there (like a multi version of a plug-in expression pedal, but instead of a pedal just using regular pots). This would technically work, but clearly there are HUGE disadvantages to this setup, namely:

1) bulk of cables
2) finding room for/drilling holes for all the jacks
3) noise
4) buffering the controls so that the long cable runs to the control box don't cause tone loss
5) even heavily shielded cables would likely pick up unwanted noise with amp controls
6) myriad other issues

So, my next thought was to do one of 3 things:

1) do some sort of midi setup that drives step motors, with his control box sending the midi signals to control the step motors which would be mechanically coupled to the pots
2) make some kind of arduino control that can drive either step motors or plain old dc motors that are coupled to the pots

If I did either of these mechanical solutions, I would either have to have some sort of feedback as to the position of the knob so that I could stop at the knob's max tortion, or i would need the motors to be belt driven so that they can just be allowed to "slip" if the pots go too far.

right now, i'm imagining one other solution, which would simply involve 9v DC motors, a couple of buttons (Up and down), and sending battery voltage to the motor (Switching power polarity for up and down) that i could couple to the pots. it would have the same drawbacks of the arduino/midi solutions, except no programming. I'd need the belt drive setup mentioned above though, because there would be no feedback of knob position.

SO.....

I think the safer would be midi or arduino instead of the plain old power buttons, but I have NO idea how to program any of this. Never used Arduino or programmed an IC in my life, and MIDI is almost foreign to me. So I'm coming to you guys for guidance. What do you think?

[Gurner]

There are many ways to crack this nut....the easiest is to tell him to sit within arms reach of his amp!

Seriously, the solution to deploy depends how much time/effort/dosh you wish to commit.

Personally I'd avoid simple DC motors like the plague....a complete bar steward to control precisely.

So, I reckon you're then your faced with two options...

1. Use stepper motors to turn the pots on the main amp
2. Break into his amp circuit & fit digital pots (this would be the most robust solution but would need major hacking & time spent)

If going with steppers, I'd say midi is overkill....the user interface could be as simple as 5 core cable out to an 'interface box' with three pots on it & located were he sits (ground, VCC & three pot 'wiper return's back to the control box at the amp). Each pot has ground & VCC on the outer lugs, the wiper has a voltage somhwere between the two (depending on where the pot is turned)...simply ADC the three voltage levels at the main control box & have this result drive three steppers accordingly.

this wouldn't be trivial & if you've not done any of this stuff before I'd say it's quite ambitious for a first project (not to mention *very* time consuming as you learn all the necessaries).

[Barcode80]

There are many ways to crack this nut....the easiest is to tell him to sit within arms reach of his amp!

Seriously, the solution to deploy depends how much time/effort/dosh you wish to commit.

Personally I'd avoid simple DC motors like the plague....a complete bar steward to control precisely.

So, I reckon you're then your faced with two options...

1. Use stepper motors to turn the pots on the main amp
2. Break into his amp circuit & fit digital pots (this would be the most robust solution but would need major hacking & time spent)

If going with steppers, I'd say midi is overkill....the user interface could be as simple as 5 core cable out to an 'interface box' with three pots on it & located were he sits (ground, VCC & three pot 'wiper return's back to the control box at the amp). Each pot has ground & VCC on the outer lugs, the wiper has a voltage somhwere between the two (depending on where the pot is turned)...simply ADC the three voltage levels at the main control box & have this result drive three steppers accordingly.

this wouldn't be trivial & if you've not done any of this stuff before I'd say it's quite ambitious for a first project (not to mention *very* time consuming as you learn all the necessaries).

Well, I generally tackle things outside my realm of knowledge in a robust fashion! Think big, fail fast... secret to success!

I've been researching, and I'm thinking stepper motors driven with arduino, physically coupled to the pot shafts with a coupling collar. I'm pretty sure I could find a way via programming to set the stepping range by coding some sort of range calibration test in the arduino code. More to come...

[Gurner]

Well, I generally tackle things outside my realm of knowledge in a robust fashion! Think big, fail fast... secret to success!

I wasn't suggesting that it would be insurmountable  ....just a heads up becuase I had no idea when I entered all of this, just how long it would take to get to grips with (even the most basic "hello world", lighting up an LED took me ages...not the code, but all the supporting bits - but there again, I went the PIC route because I'm unfeasibly tight & arduinos seemed a whole lot more costly)

[G. Hoffman]

Just in case you didn't know, there are companies out there that make pots with built in stepper motors.  I've never bought any, and I believe they are quite expensive, but they are out there.  That might be a better idea than trying to tie your own motors to the existing pots, though.


Gabriel

[Barcode80]

Just in case you didn't know, there are companies out there that make pots with built in stepper motors.  I've never bought any, and I believe they are quite expensive, but they are out there.  That might be a better idea than trying to tie your own motors to the existing pots, though.


Gabriel

This is true, but I'm trying to not modify the existing amp if I don't have too. Popping the knobs off and coupling a motor to the pot shafts is completely reversible and in no way requires modding the amp. Plus, the harder route brings greater satisfaction when completed! I think I may have found an old post from RG about Neil Young's Whizzer unit. R.G.'s info will be a good starting point.

[derevaun]

Robot builders need steppers with accurate position info. And there are bound to be code snippets for Arduino that do just that sort of thing. A nice thing about Arduino is the wealth of people hacking new code out of each other's code, and often it's not too abstracted to sort out quickly.

[Pablo1234]

This is always a fun project, Stepper motors are easy to drive but having them encoded for feedback would be better IMO, this gives you feedback from each motor to dial in a preset. For the interface you could do a simple grey-encoder as their much cheaper than potentiometers. I use Bourns PEC12 - 12 mm Incremental Encoder their $1.05 per encoder and you can get them in Detented or smooth, I like the smooth myself and with or without switches.

Also the Propeller uC is $7 at mouser and is 8 core 32 bit, no interrupts with several languages to write in, its awesome for this type of project.

[Galego]

Using pots like these?

http://pt.mouser.com/ProductDetail/Bourns/PRM162-K420K-103B1/?qs=sGAEpiMZZMuarXojuOTsc%2fyBotqZoOf5mUx8Bbhpd8Y%3d

I don't see how you could use digipots, I don't know of any that handle the voltages used in a tube amp (assuming it's a tube amp).

[G. Hoffman]

Using pots like these?

http://pt.mouser.com/ProductDetail/Bourns/PRM162-K420K-103B1/?qs=sGAEpiMZZMuarXojuOTsc%2fyBotqZoOf5mUx8Bbhpd8Y%3d

I don't see how you could use digipots, I don't know of any that handle the voltages used in a tube amp (assuming it's a tube amp).

Well, except most pots in a tube amp don't really see much voltage, or current for that matter.  They are almost always on the strictly AC side of the coupling caps. 

Also, he said he didn't want to mod the amp, however many months ago this was, so changing pots would seem to be out.


Gabriel

[Galego]

Using pots like these?

http://pt.mouser.com/ProductDetail/Bourns/PRM162-K420K-103B1/?qs=sGAEpiMZZMuarXojuOTsc%2fyBotqZoOf5mUx8Bbhpd8Y%3d

I don't see how you could use digipots, I don't know of any that handle the voltages used in a tube amp (assuming it's a tube amp).

Well, except most pots in a tube amp don't really see much voltage, or current for that matter.  They are almost always on the strictly AC side of the coupling caps. 

Also, he said he didn't want to mod the amp, however many months ago this was, so changing pots would seem to be out.


Gabriel

How much is not much voltage? 0-5v?

[wavley]

You mean like Neil Young's Whizzer??

[pjwhite]

You might want to consider using servos like you would use in a model airplane or R/C car.  Probably a high torque type.  These are easy to control with a modulated pulse width signal, which can be easily generated with an analog circuit (555 timer).  A pot on the timer circuit would control the pulse width signal which would adjust the position of the servo which would control the position of the pots on the amp.
There should be plenty of circuit examples for controlling servos with a 555 timer out there on the web. 

[PRR]

> How much is not much voltage? 0-5v?

The swing at the first plate can be 20V even 50V peak.

It IS far beyond simple CMOS solutions.

You can re-design the tube amp to keep signals small... but you lose most of the reason to use tubes.