Today I finished the Octo VCO. Fortunately there where only some minor design flaws I could correct without having a new PCB produced. Currently I have nothing to drive all eight channels separately so I hooked two channels together and connected it to the Midimuso. What should I say, it’s a beast. Hear for yourself:
Very impressive! How do you plan to to drive all the channels? Can you send one input to all of them, or does it require 8 independent v/octs? Do you have a panel in mind?
…and does it work without those transistors soldered in?
I think I will build a midi controller with an Arduino and some DAC. Or buy a second Midimuso.
Currently I’m tying two VCOs together with one voltage input. I see no reason why it shouldn’t work with all eight. The CV input voltage is buffered by an op-amp.
Not yet, but I think I will build one. On the long term this should be all in some custom casing.
No, it’s optional. You either put two matched transistors in or one BC847BS. It’s a dual transistor in a SMD package. It is below the NTCs on the picture. I have very good results with the BC847BS and not that good with matched transistors. But maybe it’s my poor matching…
Exactly. Specs are hard to say, they will change a lot on the way I guess. Now I know how much time it takes to create a board like this, like hundreds of hours, so it’s really a long term plan. Basic guidance is to keep component count low and they should be broadly available.
Next board will be a VCA/ADSR. I think I’m going for a simple transistor based VCA and a 555 timer ADSR. After that a VCF, either a simple Steiner or some 13700 based. Also an Arduino with Midi input and some DACs to drive the thing.
That’s sweet! Sounds fat and the github is a nice touch.
Some question on your use of the NTC - firstly, they’re the 4k7 resistors in your BOM? What kind of temp coefficient do they have? I used PTCs in my VCO, but they’re in a negative feedback path - so I guess it has the same effect as an NTC? Any particular reason you did it that way?
If you’re working on VCAs, here’s an lm13700 linear VCA I recently built - two ICs and two transistors for two channels. It seems to more or less work:
I mainly choose the NTCs because they are cheap and broadly available compared to tempco resistors. Kassutronics [1] and René Schmitz [2] have some background on the usage of the NTCs. PTCs should have the same effect when used in the op amps feedback path to change gain on temperature change (higher temperature = higher gain).
I don’t know about the LM13700 regarding the VCAs, they are even to expensive for my taste. I think I’m going for a simple op amp/transistor solution. I’m very interested in the JFET variant, I would definitely like to try that out!
An LM13700 (2 VCAs) is $2.08 at Mouser. Then you need a TL074 and some transistors for the input current source, probably also ends up about $5 or $6 for a quad.
Here’s dual linear JFET VCA. It’s not perfect - getting it to fully turn off is almost impossible due to non-zero on resistance - but it seems to do the trick. However, the transistors are something like 25c each.
I decided to go with the simple Yusynth VCA [1] because it only uses standard components. The schematics is from Kassutronics [2]. It strips out even more not really needed components like input mixer and level led. I changed some resistor values to match input and output level. Even without matched transistors it works quite nicely.
