# MIDI controlled variable resistor?



## JHarasym (Mar 27, 2007)

Here's a question based in ignorance, but born of a dream.
I am working on a G-System rig, and am wondering if the MIDI-out from the G-System (which can control almost everything else) could somehow be used to adjust amp settings (eg all those thing controlled by pots, including gain, volume MV, EQ).
I imagine replacing the pots on the front panel of an amp with these "MIDI-varistors" and programming all the amp settings into each patch on the G-System.
I tried googling this but am probably not wording the search in a useful way, and came up with nothing.
Any tech wizards up on this?
Has any amp manufacturer implemented MIDI controls?


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## jb welder (Sep 14, 2010)

There are 2 ways that I can think of. One is digital pots. This is probably what you are thinking of.
Second is motorized pots which are electronically controlled. The cyber-twin used these, also Neil Young's 'whizzer' set up.
I think Mark Hammer is probably familiar with this kind of stuff.


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## dcole (Oct 8, 2008)

Its doable, but there are a lot of restrictions with digital potentiometers that need to be studied and applied to the design of a midi-controlled variable resistor for a guitar amp. Some of these restrictions are described here: http://www.analog.com/media/en/news...ection-guide/Choosing_the_Correct_Digipot.pdf


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## Guitar101 (Jan 19, 2011)

I'm not familiar with the G-system but I remember many years ago when we were using sequences for the drums and bass, our lead player would insert patches into the sequences that would change his Boss ME10 effects processor to the desired setting as the sequences played. Pretty sure he did this through the midi ports of the ME10. It didn't change the amp but did change his Boss ME10 to the desired sound for leads etc.


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## JHarasym (Mar 27, 2007)

Thanks for the input. This at least has led to some more productive search terms.

Found the following on diyaudio.com (posted by the operator of Tubelab | Dedicated to advancing the state of the art in affordable high end audio.). 
The link is : Digital volume/tone control in Tube amps - diyAudio

Looks like a bit too much for me to bite off, but the solution may eventually be developed. Here's the text of the post:

"I have been tinkering with this on and off over the past 5 years and can offer the following:

The usual pigital pot that operates from a 5 volt supply just doesn't cut it. There is not enough headroom and they sound really nasty when they clip. Analog Devices to the rescue! The AD5290 chip will work on a 30 volt supply while its SPI port works from a 5 volt PIC chip. You can get these in a 100K ohm flavor which can be used in a tone stack, just don't put too much gain in front of it.

Build your favorite tube preamp circuit that has the usual overdose of gain, and use mosfets to switch the cathode bypass caps in and out. This technique can be used to adjust the overall gain by about 30 db. 

Reed relays can be used to switch circuits in and out of the signal path. Careful circuit layout is needed to avoid a pop when you turn the relay off.

I am slowly developing these circuits to be installed right inside the guitar using subminiature tubes. It will all be posted on my web site when it is done, but I just don't have the time to work on it right now. I want to use a PIC chip and an EEPROM memory so that your favorite settings can be saved and then called up by slapping a button on the guitar."


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## greco (Jul 15, 2007)

Extremely interesting concept and thread. Thanks. 

The Tubelab website in the previous post has some very cool pics and information.

Not intended to be a derail from the topic....just a short break from a lot of complicated electronics.....a creative idea for a chassis!


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## JHarasym (Mar 27, 2007)

Bet you that would cook!


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## JHarasym (Mar 27, 2007)

So these guys were actually working on a modern digitally controlled tube amp (including using midi) : Klonz-Labs

It appears they made a deal with JOYO to make an amp using their technology.


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## jb welder (Sep 14, 2010)

I forget the actual element that was used, but the ADA MP-1 was a digitally controlled (midi) preamp with tubes.


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## High/Deaf (Aug 19, 2009)

Progress makes me cry sometimes. 

With my mentor and a couple buddies, we designed and built some digital volume controls 30 years ago, before they were commercially available. We used 3 IC's per channel that each had 4 op amps in them. The op amps in each chip were selectively activated through address buses. We set gains of each op amp at 1, 4 and 16dB increments and were able to get a total of 63 dB of attenuation in 1 dB steps, plus a mute. You could parallel multiple controllers to the up/down inputs and there was even a front panel display. This was before stereo's or anything else really did that, except your TV. My mentor even built a wireless remote for his - I kept it simple and used a couple of hard-wired remotes. They cost us around $200 in parts per box and we sold a few for around $800 (hardly covering design and labor). 

Now every cheap AV receiver has this feature. Maybe amps pretty soon too.


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## mhammer (Nov 30, 2007)

I'll wager that Molten Voltage has a module that could be used for this purpose...or could be made interested in creating one: PedalSync .:. audio control chips and modules .:. by Molten Voltage


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