Remote MIDI volume control....

[G. Hoffman]

I'm trying to figure out a way to do this quickly, because I have WAY too many time consuming projects on my hands right now, so if it's not small I can't do it, and will have to buy something, which I really don't want to do....

I'm making a wet/dry/wet rig, and I'm trying to figure out a way to control the level of the wet channels without having to run wires back from the rack to the pedal board.  What I want to do is a  MIDI controlled stereo volume control in the rack that I can control from an expression pedal on the board, and I THINK I've figured out the way to do it, but wanted to run it by others.

I have a couple of Highly Liquid MPA boards, which I'm using for the 8 digital outputs, but they also have three digital pots on each board, but I want to limit the risk of digital noise getting through to the audio signal.  So, can I just use the pots as voltage dividers to drive a couple of opto's set up like the audio path of a Tremulus Lune (i.e., an Opto between two opamp buffers).  Like so (kinda, this is ten minutes, blatantly ripping off the Tremulus Lune schematic - I'm sure that some things would have to be updated, and I might go with an opamp to drive the opto instead of the transistor - either way, I'm not sure if I've got 10k or 100k digital pots, but I know they CAN'T drive an LED):



So, do that twice, and I've got a stereo MIDI controlled volume control, right.  Right?  And for that matter, this is all coming right after an opamp mixer, so do I even have to put in the first buffer, or can I just put it immediately after the mixer?

Thanks!


Gabriel

[slacker]

Seems like it should work to me. You could probably get rid of the first buffer if it's driven by an opamp, you'll only have one inverting stage then though so the output will be inverted compared to the input. that may or may not bother you.

[Processaurus]

Nothing wrong with the first stage being the inverting amplifier, it's generally good to keep music processors phase coherent, rather than flipping the phase willy nilly.  There are plenty of respected analog designs that use two inverting amplifiers in series, when you want a variable gain of less than one out of a stage (or where one is an inverting summing stage, like a mixer), but want the phase to be correct on the output.  In many cases, that's the only way to get what you want with opamp design.

For a second I was wonering if you would want a cap across the digipot wiper to ground, to filter out any zipper steps from the control signal, but realized the response of the LDR is slow enough it should filter out any zipper artifacts from getting into the audio.

[G. Hoffman]

For a second I was wonering if you would want a cap across the digipot wiper to ground, to filter out any zipper steps from the control signal, but realized the response of the LDR is slow enough it should filter out any zipper artifacts from getting into the audio.

Still, an idea to keep in mind, and maybe a good idea to design an optional cap into any the board.


Gabriel

[Tightpants]

Using two optocouplers in a series/shunt arrangement works better than a single one in series. Have a look at this document:http://www.cresttech.com.au/pdf/Silonex/levelcontrol.pdf, it explains things much better than I could. I have built the circuit in Figure 12 and it works great (I used a PWM output from a microcontroller to drive the leds directly and got no digital noise or interference). For stereo it is probably better to use matched optos. I'm not sure if these are still available, try Googling "Lightspeed Attenuator matched LDRS" to get you started but be prepared to lash some cash if you can find them!

[Tightpants]

Using two optocouplers in a series/shunt arrangement works better than a single one in series. Have a look at this document:http://www.cresttech.com.au/pdf/Silonex/levelcontrol.pdf, it explains things much better than I could. I have built the circuit in Figure 12 and it works great (I used a PWM output from a microcontroller to drive the leds directly and got no digital noise or interference). For stereo it is probably better to use matched optos. I'm not sure if these are still available, try Googling "Lightspeed Attenuator matched LDRS" to get you started but be prepared to lash some cash if you can find them!

Here ya go - not cheap but easier than matching your own!
https://www.shoplocket.com/products/HhFU8-matched-ldr-pairs-nsl32sr2s

[Processaurus]

what about a dual vactrol, one LED polygamously married to two LDR's, like this:

http://www.smallbearelec.com/servlet/Detail?no=349

[David]

Gabriel, did you ever build this?  How well did it work?

[G. Hoffman]

Gabriel, did you ever build this?  How well did it work?

Not yet, but I am revisiting it.  Tonight, in fact.  I had to take some time off of these sorts of projects to focus on my CNC rebuild so I could use the machine for making faceplates and the like.  But I'm planning to do some breadboarding and maybe design a board for this tonight.


Gabriel