Polykit Octo VCO

DIY STUFF
#1

vco-8
vco-82032×2112 1.62 MB

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:

The VCOs are based on a design I posted earlier in Non 3340 oscillators - #72 by jkb

For the brave I uploaded everything to Github: GitHub - polykit/vco-8: Analog Octo Voltage Controlled Oscillator VCO PCB and schematics
I hopefully corrected all problems in that version.

Some documentation will follow soon.

Edit: added another example

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My build progress
My build progress
#2

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?

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#3

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…

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#4

So you are designing / building your own poly 8? What are the specs you are aiming at?

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#5

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.

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#6

This is incredible. Bookmarked for sure!

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#7

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:

lm13700 vca schematic

I’ve got a JFET variant that’s a single IC and two transistors I can share if you’re interested.

Cheers,
Jared

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#8

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!

[1] Kassutronics: VCO part 1: core
[2] Synth schematics--::--Exponential Convertors

#9

On the other hand one VCA is only using half of the LM13700 and the rest looks quite simple. Maybe I try it out.

#10

Or you can use a quad VCA chip :
AS3364 (4 VCAs for $5.40, and only a few resistors and caps)
https://syntaxis.com.pl/en/voltage-controlled-amplifiers/73-as3364.html
or AS2164 (same but Log CV)

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#11

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.

#12

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.

dual_jfet_vca
dual_jfet_vca1123×794 61.3 KB

I picked up my LM13700s from RS - €1.52 each when you buy 10.

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#13

YuVCASimplified_FixInversion
YuVCASimplified_FixInversion1247×578 19 KB

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.

[1] index.thml
[2] Kassutronics: Discrete VCA

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