Buy this board and an SSM2164!
see the board layout
Single VCA (with) wiring suggestion
A stereo panner (with) layout and wiring
Son of Dual! Stereo panner w/less parts... (with) wiring diagram
Using the Irwin VCAs to mod a Steiner VCF
When reading the tons of materials about musical synthesis, you start hearing linear this, exponential that, exponential converter this and that, etc etc...
The exponential devil doesn't just bite when it comes to 1V/OCT VCO’s.
Another area where this comes up big time is when building your VCA’s. Lookee here:
I built an AR that takes 5 seconds to decay after the gate signal disappears. When the gate signal first disappears, indicating it's time for the VCA to start "fading out", the AR's control voltage output is producing 5V relative to ground. The AR is based on an RC circuit fragment, so if you plot time vs. voltage on a simple graph, what you'd see during its release phase isn't linear.
To match sounds in nature, like plucked strings, you want an exponential AR, but a linear VCA hooked up to it. The linear VCA produces gain or attenuation in direct proportion to the CV coming in, instead of based on its own exponential response to incoming CV. The result, to my ears, is a more "natural" sounding envelope.
I was buzzed when I found an article in EDN magazine that showed a linear VCA based on Analog device's SSM2164. This brilliant little circuit was created by a certain Mike Irwin, who must be a very, very smart person. The first stage bends the CV signal in such a way that if you run this output through another identical stage, the next stage's exponential amplification qualities are canceled out, and the amplification becomes linear. Cool!
I wire wrapped a prototype--basically, an exact steal of the EDN circuit, with a few minor modifications to get a reference voltage from the V-- rail and was very pleased with the sound: fast, smooth, and quiet.
Here are some things about Mike's design that are "cool". In its basic form, it's low parts count, for me, that means less to go wrong, and you can have a working dual VCA in your rig in a few hours using this design as a basis. Also, by hooking up different REF voltages (-5VDC, -10VDC, etc) you can quickly condition this for different CV standards out there...ie: to hook it to a 0-7 CV based synthesizer, put in -7V DC rel to GND into REF--get it? And if you use a pot for REF that varies between, say, -15V DC and -5DC, you can very easily set your overall gain for the VCA from the front panel.
But most of all, I liked the way it sounded!!!
Well, I may never be clever enough to come up with the kind of engineering marvels that Mr. Irwin can, but, I know a good thing when I see it....so, I created stereo version of his design using EAGLE on a single-sided PCB. You can review a layout of the board. Also, take a look at the parts list. I have included an already upsidedown, ready to press and peel pdf of the bottom side of the PCB.
The dual VCA is not just good for scratch-building; I also think of it as a building block to speed up modifying other tech's DIY projects. For instance: adding a VC mixer to the input of Ken Stone's near-legendary, you gotta build this if you don't build anything else, CGS35 Steiner filter. I have done this, and it sounds cool IMO...details are here.
I am really happy with this board--rereading this page, it sounds like a Hairclub for Men type ad from a few years back, where the guy with the tupe says " I am not just the president, I am a member." Actually I could use the extra hair, and unlike those guys, I really AM enthusiastic about this, and am making a lot less money to boot! Anyway, if you don't want to wirewrap, etch, or scratch build this design, I have some PCBs of it for sale, they are good to go, so please buy a few and get building! I got a basement full of the damn things! Go to a link here for more info and to buy the PCB.
What is going on with the 500pF caps and 500ohm resistors at the input?
That's a lowpass filter, I believe it's in there so as not to have fido-only hi frequencies drive the input stages of the VCA crazy and to get rid of any clicks and pops that may occur at the input. In it's present form it provides about -3db at 11K and -6db at 22K. Looking at one of the 2 channels, you can change the values of R6 and C5 to vary this if you want, and the values of the cap andthe resistor between the cap and ground isn't totally critical(ie, 470pF will sound almost the same as 500pF). I have built CV attenuators out of these VCA's and left these components out altogether, and built audio VCA's with a much higher cutoff frequency and both worked. When computing your -3db point, remember that the value of R1 + R6 form the "R" of your RC filter: the classic formula freq = 1/x where x = 2piRC.
Does it have MOTM type power connections (1.56, vertically arranged, etc)
NO! It has wirepads for V--, V++ and Ground. Maybe someday I'll clean up all the wiring, where it comes into the board, etc., but for now, I have a day job, and do this in my spare (??) time. However, the board runs good, did for me, anyway.
What the heck are the trimmers for?
Anything you want. Since I am going to use this as a daughterboard for other projects, and had some extra space, I put 'em in there, along with some other wirepads for returns to ground and the like. I used one on my prototype to derive the –5V DC "REF" signal that feeds pin2 of the 2164; see the EDN schematic. Another clever (??) thing you can do is to wire together the trimmer pads with a single jumper and then use the trimmer contacts to the trimmer as a "mult" to distribute things like V++ to other parts of your circuit.
Is the board cap coupled? Does it "do" DC?
No, and yes. My latest experiment to date is to use it to control attenuation of a slowly (or not so slowly) rising 0-5V EG signal from another EG, and it works well for this. In this arrangement, using one ADSR to modulate the output of another, I can get some interesting EG output and control the whole thing from velocity at the same time. If and when I ever get a minute to write some details of this build.... I will post what I come up with here.
IMPORTANT.... When working with DC type applications, remember that the VCA's invert the signal present at input as well as attenuate or boost it, and this becomes a big factor when dealing with control voltages, but can be easily "fixed" by using an additional inverting op-amp at output.
What are all the V--'s for?
More ways to derive the DC "ref" voltages which will set the overall gain structure of the VCA. If you look at the layout, these are nothing more than feeds from one of the V-- rails, and the REFRET are returns, through a resistor (I suggest 47K, but you can use anything reasonable), to ground. If you have other ideas of how to get -5V DC or whatever into the REF input, don't use these.
Do I have to use a TL084 in there?
Nope. However, this chip worked for me, and I had a bunch of them around, so I designed that into the board (as opposed to, say, 2 TL072's or some Burr Brown craziness) However, make sure to use a high-impedence, low-offset opamp. And if you're setting up an inverter to flip flop the CV going into a MOD IN or whatever, you'll have to get rid of offset or else current will leak into the CV ins and make for a VCA that never goes "off".
Can I add more CV ins?
Yes. Just add more 100K resistors to pin 2 and/or 13 on the TL084, and add your CV on the other side. You may have to do some drilling, sweat soldering or use a daughterboard.
Can I use it as basis to build a (LFO? VCF? Bread Toaster? Reverse Dual Whammy Widget?)
Maybe. Probably. The more I use this circuit fragment, the more things I figure out it's useful for (something like that). Anyway, the SSM2164 and this Mike Irwin design are, if I say so myself, hours of fun. A hundred and one uses. I think so, anyway.
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