Control Voltage Whatchamacallit?

[elmegil]

As I continue on my journey through Analog Synth for the 21st Century, I realize that the only way I can set up the patch shown at the end (clearly not the only one possible, but anything will have a similar challenge) is to have a way to take a CV in and send it to multiple places out.

I've read enough to understand that passive "wire the input jack to these 4 output jacks" methods don't scale well. And I've looked at Ray Wilson's CV and Gate Distributor (http://www.musicfromouterspace.com/analogsynth_new/CVANDGATEDIST/CVANDGATEDIST.html).

But I haven't seen any other circuits for buffered distribution, and I've spent a while this evening googling "control voltage distribution" and "control voltage multiple" and "control voltage processor" and all I get are Ray's ckt and a bunch of things I don't need at this point (like And/Or or + - processing of separate CVs, for example).

I am thinking I must just not have a common term that is used for these beasts, because I know I've seen them offered from synthesizers.com (the Q124 though that's called a "multiple").

There are a few reasons I don't want to just use Ray's ckt as is...First I can't fit it on a 1U wide panel with 1/4" jacks, but if I put it on a 2U wide panel there's lots of wasted space. Second, one of the things I want to do with extra space is a quantizer bit, using Barton Musical Circuits' teeny simple quantizer.

Bear with me, I'm getting to the point

So all those mods are easy enough (more TL07* chips, mostly), and I worked out a panel layout I like (see attached)...and then I realized I'd left off the voltage offsets. And my panel layout doesn't really have a good place to put those without cramping the quantizer section (which still needs some tick marks to indicate the 8 positions in each bank).

SO that's when I started googling around trying to find any other example circuit/board layout that I could be sure was essentially the same thing, and failed miserably. The Q124 is sorta kinda the same, but I am not certain it's buffered. It is quite a bit simpler .

But I'm curious whether the offsets are something 1) that are commonly added to this sort of module and 2) people find to be useful? Because I want to leave them off at this point, but I've already found in the past that I'm regretting things I've left off (I cut the MTS-100 to just CV/Gate/Trig, and later realized that I could do many other cool things with it even if I didn't have a pitch wheel to play with )

Anyone have any advice or opinions?

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[elmegil]

Well....

I'm still interested in people's opinions about offset, but I went back and looked and there's no way I could get all the way to that patch scheme with this layout, as cool as it looks to me. I'm going to have to go to something much much simpler, though I can still use the basic circuit elements. But if I want to hook it all up that way, I'll need 5 different channels to do multi on; Gate, Trigger, and three different sorts of CV (CV from kbd, EG, and LFO).

Not just Gate and one CV. I'll have to leave the tricks for another time.

[JingleJoe]

I'm not entirely clear on what you're doing here but splitting up a CV isn't hard, infact no splitting is involved at all- just connect multiple things to your CV output, simply add more jacks where there was previously only one.

Then the only thing you need to avoid is overloading the CV output, if the inputs you connect to it are too low impedance, then you will draw too much current and voltage levels will start to get discombobulated.
To resolve that, simply add a voltage follower which outputs more current.
You could do something like this; (see attachments)

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[elmegil]

The CV Gate Distributor on MFOS is exactly what you describe there, Joe: basically buffer followers so you can get a better fan-out than you'd get wiring it direct (passively).

Except there's a bit more to it than that. Ray's circuit includes voltage offsets for the CV input, an inverting summer and then an inverting amp stage (to get us back to positive), a passive LPF to get rid of glitches, and then the followers to send the CV onward. The Gate circuitry uses a comparator to bump the gate voltage a bit, in case it's a low input (anything over 10% of Vcc gets sent on as a ~10V output signal).and also has followers.

I guess the core of my question at this point is how useful do people find the ability to add a DC offset to your control voltages in a modular? The search for example circuits was to see how others might have done it and what they had to say about why.

I can definitely see how this would be simpler if I just did straight voltage followers. I plan on doing some testing anyway, so I think I'll try both ways and see what looks cleaner/works better (of course I won't have anything to fan out TO, really...but I can test individual stages going to the VCO and VCA on my breadboard....)

[Sebo]

I did a buffered multiple using a voltage follower for every output, I need it to distribute CV to my VCO. I originally used TL072, and dicovered that the offset of that OpAmps was to high for that use. I ended using LF412, but I think that LM358 will do the job.

[elmegil]

Can you explain more about the offset being too high? Ray's circuit uses TL074's, and I was planning to add a TL072 in one place to follow his pattern.

[Uncle Krunkus]

The biggest reason I'd put the DC offsets in, is in case you do get loading problems. Then you'd just retune the VCOs across the outputs by tweaking their appropriate offsets. Does that make sense? They give you control over the CVs which are coming out, and, that's a good thing.

[elmegil]

Yah, that makes sense, thanks

Having spent some time today messing with panels (since I dont' have parts to make headway most other places, though I could with this once I have a final decision), Here's what I came up with that fits in 2U that meets the patch scheme needs...

I'm assuming a trigger channel should work the same way as the gate, my MIDI->CV generates 5V for each. I'm still wondering if I ought to try to drop the output on those back down closer to 5V; the original bumps them to 10.5 or so....

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[Sebo]

[elmegil]

Aha, thank you.

I will have to check that as I set this up then.

[mph]

I had the same problem as Sebo when patching VCOs with a passive multiple to distribute CV.
I've ended to design an active multiple with one buffer per output based on TL071 and each output has it's own onboard trimmer to setup the offset to minimum.
Using 071s + trimmers allows me to keep a very simple front panel with no knob and just jacks.

Here you'll find the pcb and layout. Note there's one inverting stage which allows to have an inverted output (somtimes useful). So there's one input and five outputs. Trimmers are Vishay type 43.
I've two pcbs in one module with a serial/parallel switch, it's a very useful module!

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[J3RK]

I usually use the LT1112 or LT1013 for this sort of thing, and they work pretty well.

[mph]

Yeah those Linear ICs are cool, but in europe somtimes it's hard to get their products.They don't even give some samples: got a phone call from a sales manager and he was not very friendly towards diy or education purposes

Anyway the 071 + trimmer combo are way much cheaper than the 1013 alone:
http://www.reichelt.de/ICs-LT-LTC-/LT-1013-CN8/3/index.html?;ACTION=3;LA=2;ARTICLE=10860;GROUPID=2913;artnr=LT+1013+CN8;SID=10T8-eVn8AAAIAAHNpNiEe02d8388c30dc4562bca3b2f2b1c245a

[Sebo]

I think any low offset OpAmp will work well without trimming.
As I said LF412 worked for me, but LM358 are really cheap, easy to find and should work OK.

[prgdeltablues]

I built something similar - just a straightforward inverting summer followed by an inverting buffer, to combine cv's. Using LT1013's worked out cheaper for me as well as simpler - Farnell (UK) stock an LT1013 for 97p (as opposed to over £4 from Rapid!). That worked out less than the combined price of a TL072 and a trimmer. I believe they sell across Europe too.

Peter

[JingleJoe]

[prgdeltablues]

Farnell's minimum order is £20, and so far, I've not been charged postage. Rapid's minimum has gone up to £35. I find Farnell is generally a bit more expensive, but some items, like this one, are cheaper, and they do seem to carry a much bigger range (eg 4000 series logic ICs)

Peter

[JingleJoe]

[prgdeltablues]

Sorry, me being imprecise. Rapid has no minimum order, but I believe you only get free postage if your order is more than £35 ex VAT.

[JingleJoe]

Ah I see I tried to find some info on farnell's minimum order on thier website but couldn't my advice is: go to your nearest privately owned old electronics shop