Goals for a TW Theremin

Hi Gordon,

Ok, seems I am not up to date on pedals! ;-)

Assuming that the interface to the pedal is simply resistive (variable resistor, however this is implemented - be it a photo-resistive sensor or whatever), and there is a common standard for these (as in, what the variable resistance range is etc) (I will see what I can find on-line about this and get back - so check this post for additions)..

Then it seems another "standard interface" should be added to the TW theremin specification - this being that resistive control..

Ok - How to do it? Get the voltage output (from a Smirnov sensor or TW module), and use this voltage (via a buffer of some kind if required) to drive an opto-isolator such as a H11F1, or to drive a led (or bulb) which is looked at by a simple cadmium sulphide photo cell..

There are some really nice parts like the NSL32SR3 which encapsulate a LED and CDS photo-resistive cell.

If one uses a beefy opamp (there are some low cost amps capable of more current than is needed - you only need maximum 50mA to drive the LED) one could have adjustable gain and bias, allowing you to trim the span and 'tuning'.

Tell you what - if you can get the specification for the pedal input on your effects unit, or give me a few names / model numbers of units which have an expression input, I will see what I can work out..

At the moment I have a bench full of bits (in fact, it is hard to find my pen, calculator or even sometimes my mouse under the pile of components and notes ;-), and have everything required component wise.. I should be able to lash a board together for you to play with, or at least give you a simple circuit to add to a Smirnov board.. I am actually playing with my voltage output modules at the moment to use in my Clara-min clone for volume (they are not Smirnov) but hadnt wanted to let them out of my sight or publish them - but I could build one for you if you kept it under wraps.. You would need to box it up etc..

Fred.

Here's an article with a circuit diagram for a typical expression pedal. ("Anatomy of an expression pedal". Scroll to the bottom of the page.)

http://www.strymon.net/tag/expression-pedal/

 I naively imagined that one could do something like the article at the top of the same page ("Volume Pedal as an Expression Pedal") - substituting a Theremin Volume Module for the Volume Pedal, but reading it gives me the impression that this might not work, or may work but be unsatisfactory.

Here are the service notes for a Boss DD-7, which uses an expression pedal, including circuit diagrams.

http://www.synfo.nl/servicemanuals/Boss/DD-7_SERVICE_NOTES.pdf

 Also, I have a theremin volume module already, built into my etherwave, courtesy of Thierry. It would be good if we could make it work with that.

Hi Gordon,

Had a look at the schematics, and it should be damn simple..

Ok - the wiper of the potentiometer outputs a proportion of the voltage across the pot .. so if there is 10V across the pot, at the 'top' position one should get about 10V, and at the bottom avout 0V.. I see that the pot is 10k on the standard TRS configuration, but seems non-critical (how critical will depend on the input Z of the appliance) and 250k is shown as usable.

My advice would be:

Get a 10k pot, wire up a plug as shown, and measure the voltage across it ..

You have a voltage output from your EW.. If you can (using resistive devider) get this output to be the same range as the output from your pot, you should be able to connect the two units together with nothing more tan a couple of resistors.

Probably though, the best (most universal) solution would be to take the CV from your EW into a circuit consisting of a dual op-amp driving a pair of photo-resistive cells configured as a potentiometer.. This way the interface to the external equipment would be completely isolated from the EW.

All you (or Thierry) need to tell me is what form the volume CV output takes - as in, does it come out via a buffer, what its output Z and drive current / voltage span is, etc.

You will probably need power for the external circuit - and depending on the answers to the above, this may need to be a split supply.. If there was a way to have a socket carrying the EW supplies (+12V, GND,-12V) and the CV signal, fitted to your EW, a small box with a knob to set the range, and a opto-isolated jack TRS output should do everything you need.. There is nothing to the electronics - one dual opamp, a few R's and C's, couple of Pots, plugs, socket, and a couple of CDS Isolators (the most expensive components) and perhaps a couple of transistors.. Component cost £20 max (depends on cost of CDS isolators - these were about £6 last time I got some, and there was a £15 shipping fee - but I have a few left over.. The H11F1's might work, and they are cheaper.

Easiest route - Fit a power / signal socket to your EW if its not already there (I know Thierry has modded your EW but dont know exact details), give me tha data I need, I will then design the interface board and build it if you wish - You have the job of all mechanical assembly etc.. I will only charge you for components (after all, I lost your PSU and have never located it ;-).

Fred.

https://pbs.twimg.com/media/A_ZMeflCAAER4dk.jpg:large

I don't have CV on my etherwave. This is my etherwave as I understand it. It may not be an accurate reflection of the actual circuitry - I'm happy to leave that level of detail to people who actually understand electronics.

Once I an done with this wretched cold I'll buy a multimeter or whatever I need to take measurements for you.

i thought you gave me back my PSU at one of Lydia's workshops. I have one just like it and I don't remember buying a duplicate. :-)

Since the VCAs in the Etherwave are realized with OTAs, there is basically no volume CV available because the LM13700 are driven with currents, not with voltages. In a normal Etherwave you can use the free darlington transistor pair in the LM13700 to do a current to voltage conversion and buffering as shown in the Hot Rodding Manual.

But things are completely different in Gordon's Etherwave. The original VCA is pulled up to maximum level to allow an already waveshaped but unattenuated audio signal. The initially free transistor in the LM13700 is used as a current buffer and drives both control inputs of a second LM13700 which is mounted on top of the first one. One half of it is now the "pure" VCA (without waveshaping since it's done before) for the theremin's signal and the other half is an identical "pure" VCA for the external signal input. 

Thus it will not be easy to find a point from which a volume CV can be derived without having an effect on the existing circuit, but I will think about that. Another problem will be that the bottom of Gordon's EW is already a Swiss cheese, there are already three 1/4" jacks and three potentiometers and I actually don't see where to put another jack for an eventual volume CV output without affecting either the pitch or the volume response...

I warned Gordon already in summer 2010 after adding the whole VCA mod and once more one year later when I still added an ESPE01 module on the original extension connector that his Etherwave was more than "full"...

I'll write back here when I'll have found out how and where I can derive a volume CV. Then, one could most probably put a 5-pin DIN audio connector (since there are flat versions which are not deeper than the thickness of the wood) in the bottom, just below the extension connector and wire it to +12V, -12V, GND and a yet to define point and then build an external box with an op-amp which will make all needed conversions, level scalings and shifts.

@Gordon: btw, I really wonder why you did not ask me directly and first...

My apologies, Thierry. I only thought of applying this idea to my own instrument as I was writing the post where I mentioned the idea. I was planning to consult you when I had clarified my idea with Fred and learned what to ask.

Why are we talking about CV?

I am still certain that I don't want CV - it would be too expensive in the long run - I like cheap pedals without CV and find it easy to resist the temptation to buy lots of moogerfoogers and a slim phatty while I don't have CV.

Can we not pass the signal that the flanger normally passes through the expression pedal through the theremin's secondary op-amp instead? Would it be a bad thing if I connected the expression pedal socket of my flanger to the audio in and out sockets of the secondary volume circuit of my etherwave with a stereo-jack-to-two-mono-jacks cable?

In my naivety I guess that the voltage might need some adjusting to be in an appropriate range to go through an audio channel, and I would have to be careful to plug it in the right way round, but other than that I cannot envisage problems.

Aha. I've just re-read this thread and spotted the bit where Fred wrote

"you should be able to connect the two units together with nothing more tan a couple of resistors.

Probably though, the best (most universal) solution would be to take the CV"

That answers my question "Why are we talking about CV?"

Please, let's go with the "couple of resistors" solution. CV may be better, but I don't want to go down that path.

"@Gordon: btw, I really wonder why you did not ask me directly and first..." - Thierry

Thierry, I am "to blame" for this, not Gordon! .. Gordon merely posted a suggested "TW Theremin" variation, I jumped in with my comments, which led to this deviation from the main topic.

"That answers my question "Why are we talking about CV?"Please, let's go with the "couple of resistors" solution. CV may be better, but I don't want to go down that path." - Gordon

Sorry Gordon,

But the "couple of resistors" solution is entirely based on CV - Effectively, if one looks at the expression input, one can view it as a voltage input (the wiper has a voltage determined by its position and the voltage across the potentiometer) or one can re-create the potentiometer (using opto resistors for example) and control the wiper position from an isolated external voltage..

IF the voltage "seen" on the wiper can be directly derived from the theremin, by using a few resistors to scale it, then one could use theremins CV output directly..

But in all cases, one needs a CV from the theremin.

Fred.

ps - looking at the diagram, you have a VCA which is controlled by the volume antenna and has in and out.. If this VCA is DC coupled, you should be able to use it, probably with a bit of external circuitry, to generate a CV which could drive the opto-isolators or whatever.

If this VCA is AC coupled, things become a bit more difficult, but not impossible.. You could drive a fixed frequency into the VCA (something quite HF - probably near the top end of the available bandwidth) and then (from the output)  have a rectifier + filter to convert this to a DC CV.

But it is only Thierry who knows how the VCA is constructed, what its specifications are, and if AC is required, what the potential problems might be with regard to any signal leakage into the audio circuits.

If you or Thierry want any help from me, please contact me directly.. But I am not looking for work, have no interest in encroaching on anyones "territory" and have absolutely no wish to get into any kind of tussle with anyone.

Hi Fred, glad to see that we are now already two grumpy old men... :-)

I see no territorial conflict - Fred, you and me share in some way the same balcony as our common friends Waldorf and Statler do in the muppet show. ("Booooooo!")

Just to satisfy some eventual academic curiosity: Gordon's VCAs are AC coupled, seen the limited space for all the additional circuitry I saw no way to implement offset compensations for both VCAs and it was much easier to put two electrolytic capacitors behind the output buffers to kill eventual offset voltages, "bleed through" of the control currents and the 1.2V DC voltage drop across the darlington buffer pair, a common issue with these OTAs.

Thus I see no direct way to "modulate" the DREF voltage (3.3V ?) from the ring of the TRS jack and feed it back into its tip. Out of that we have to see that the Etherwaves internal volume control current has a pseudo exponential characteristic thanks to the diode in the feedback path of the volume CV processor (first half of the original LM13700) which makes it a little more difficult to simulate a linear voltage divider as required...