Variable resistors in a tube guitar amp controlled via MIDI

[Loogie]

This is for a tube guitar/keyboard amp I'm building.

Is there device that would behave like a pot in my circuit, but would take its commands from a MIDI message? I would use an exisiting MIDI controller to handle communication and storage. Not a motorized pot, but a chip, I suppose, that I could use in place of a pot.

As an example: I use a thing called a Nord Electro. Its a keyboard controller. It has controls that look and feel like pots. You dial in volume, EQ, etc and then you save the settings. When you recall a patch it doesn't move the pots, but when you roll a pot it clears its current setting and reverts to its real-time value which you can store as a new patch or as an edit to an existing one.

One of the guitar players I'm working with is finding some real interesting sounds in the amp, but he has to do a lot of knob fudging. I know I could put in some switches that could change interstage attenuation or cathode RC, but what I'd really like to do is dial pots.

Does this make sense? I know there are amps with motorized pots, but I was thinking of something more along the lines of the way my Nord works.

[Gregg]

There's a long thread concerning this question. Search for "LDRs as pots".

[Alex/Tubewonder]

Just google "digital potentiometer", also check page 5 of this EDN Design Idea

[Loogie]

Thanks. I'm digging around. I found this:

http://highlyliquid.com/kits/mpa/


Promising. I think this is doable, though not trivial. Any basic programmable midi controller could handle all of the business upstream. Messages, patches. I might try one of these just to see how long it lasts before I fry it. 100k ain't quite enough, but there are probably ways...

[Don Symes]

Those digital pots are neat, but they tend to catch fire if you feed them a signal that swings outside their power rails (+/-5 or 5/GND).

[Enzo]

Your Nord pots are plain pots. Instead of being in the signal path, they are controlling analog inputs to a digital circuit. Basically they are changing a parameter setting. The software in the digital circuits ignore the pot settings until they change. SO the pots themselves are not what reacts that way, it is the software program. In other words, the pots are not what you are looking for.

If you got digital pots, there would still be the problem of coming up with the digital circuit to control them.

[R.G.]

Actually, you can do a good job of high voltage variable resistors using an LED driving a matched pair of LDRs. You put use a digipot or some such to receive the incoming signal and create a control voltage across it. You then analog-servo the LED and one LDR section to the digipot. To the extent that the other LDR tracks the one you're servo-ing, you have a high voltage variable resistor controlled by a digital logic signal.

It's much harder to do a digital-controlled attenuator, particularly with a specific taper.

The easiest way is to get miniature 200 step/rev stepper motors with dual 1/4" shafts. You mount the motors on the user panel and the control knobs on those. The other shaft couples to the knob of the pot with something like fuel line hose and clamps. When the motors are not being driven, you get a nice, high-inertia and slightly detented rotation. When you want drive, you flip on the stepper drivers and away we go. The knobs turn to the appropriate position. If you don't turn the stepper drivers off when you're through moving with motors, it is really hard to turn the knob though.

Actually, this might need to be coupled with an encoder or a slave position sensing pot, as a useful thing to have is a "remember this!!" button on the amp. When you tweak it in, push the "remember this!" button and the amp reads all the pot settings and stores them away.

Frankly, I don't know why some savvy amp hacker hasn't broken into ultra high end amps with this kind of setup. Maybe it just takes two different kinds of skills and those are rare in the same person.

[Enzo]

I perceived his problem as the MIDI part. SOmething has to decode the MIDI signal and use it to control whatever circuit we come up with.

[R.G.]

Trivial. A $2.00 PIC microcontroller and a few hours coding. There are places on the web with downloadable PIC MIDI code you can just splice right in.

The tricks are all in how you get one PIC (or other controller, like the AVR, etc) to do it neatly, efficiently, and without a huge build and debug bill.

I programmed a PIC to servo-adjust the bias voltage on some output tubes once. Had it look at the voltage on a 10 ohm 1% resistor in the cathode of the output tubes and diddle with the bias voltage until the tube current was right. The PIC was holding off incoming signal through an optoisolator clamp and when the bias was right, it unclamped the incoming signal and held bias steady. It worked fine, but the tubes drifted too much to make it practical on a production amp.

[Steve Conner]

Writing an engine to save and recall patches under MIDI and front panel control isn't "trivial" unless you have done many such things before.

When you add in the difficulty of modifying a tube circuit for digital control, and the age-old problem of handling differences between the front panel controls and the state in memory after a patch change, the whole project would be enough time and effort that you should seriously consider just buying a used Mesa Boogie Triaxis or Fender Cyber-Twin, or even, God forbid, a Pod. I know that's what I would do.

If you're determined to do it, the midibox project from these guys http://www.ucapps.de/ might make a useful starting point, rather than having to start from scratch.

[gdgross]

Thanks. I'm digging around. I found this:

http://highlyliquid.com/kits/mpa/


Promising. I think this is doable, though not trivial. Any basic programmable midi controller could handle all of the business upstream. Messages, patches. I might try one of these just to see how long it lasts before I fry it. 100k ain't quite enough, but there are probably ways...

It also says the max V on the pots is 5V. Not quite enough for us tube freaks...

I am watching this thread with interest. I have been wanting to build a MIDI controlled stereo power amp for about six months now. MIDI control over drive level, power scaling, presence, etc. would be really cool. The highly liquid stuff looks promising.

I can build the audio stuff no problem, but when it comes to MIDI and PICs and coding, I'm kind of in the dark.