Simple VCA with bonus S&H/Glide on one piece of stripboard

[nicolas3141]

To make it sample as quickly as possible, reduce R103 to 10k or even 4.7K (don't go any lower than 4.7k or you might blow up the 13700) and use a TL074 or similar for IC1d (R106/107/108 will not be necessary) which will then allow you to significantly reduce C101. A smaller C101 will reduce the hold time before you notice sag, but you could experiment with different values between 1n and 50n to find a good compromise. Those 3 changes will improve the high freq performance dramatically.
R103 --> 10k
IC1 --> TL074
C101 --> 10n (experiment a bit with this value and let us know what you find)
And of course make sure P101 is right round at the fastest setting possible.

[Bogus Noise]

That's great advice, thanks

I will test it out this week and report back. I already have a dual glide module and will be using this mostly as a sample and hold so shouldn't need to compromise too much, but I'll post up how things go regarding the glide response as well.

[Bogus Noise]

Well, have given it a bit of a test. 10k and 10nf capacitor works well at high sampling rates, but drifts quite a bit if there's say a second or so for each cycle. I changed to a 100nf, still drifts a bit. It's more noticable on higher voltages (both + and -), the closer it is to 0v the longer it holds. It's good enough for the moment but when I get a chance I'll experiment with some more values to try and get the performance at higher voltages to be a bit more stable.

I'm also thinking about about adding a gate to trigger circuit on the input, could be a handy extra!

[mrand]

Hi there,

I've been using this VCA circuit (thanks Nicolas!) for a few little projects, my first synth circuits, really, and am quite happy with it. But it must have shortcomings, since it's so wee... can anyone let me know what they are?

[nicolas3141]

The main improvement a more sophisticated circuit would offer is the option of a log/expo response. This one is linear only, which is what you want sometimes, but not all the time. A good VCA offers both options. In terms of a linear VCA this one should sound as good as any other.

Nicolas

[mrand]

Thanks, Nicolas.
A thought: Given that VCAs are frequently used to dynamically attenuate audio signals, using an EG with expo curves, wouldn't it make sense to pepper a great number of these simple, low cost, VCAs throughout one's (modular) synthesizer? (Most of the synths I've used dont seem to have enough VCAs.) Is there a reason this isn't common practice? Or maybe it is. Sorry if this is off topic.

[mrand]

I've done some more reading, and pretty much answered myself with "yes, good idea". I guess I just I wonder why people don't piggyback these on other circuits, all over the place, just for the convenience of having one nearby.

[bubzy]

likely parts cost, to get a "good" VCA(linear/log&other fancy bits) you are looking at getting a nice OTA chip or perhaps even a purpose built VCA chip, while they are not bank-breakers, they can soon add up if you put them "all over the place"

but it is true, theres never enough VCA's
or VCO's
or LFO's
or ADSR's
or.......

*walks off mumbling*

[mrand]

Okay, "nice OTA chip" might be the answer to my question. I'm sourcing from tayda, so all the parts for the simple Nicolas VCA are coming to three or four dollars. Granted, at least two jacks and possible inclusion of a pot will up the price, but is there something I should know about the ~$2 Lm13700s I'm using?
Thanks!

[bubzy]

oh heh, I didn't look at the design, the lm13700 are nice ones

[mrand]

So maybe this sdiy stuff can quite inexpensive, after all

Thanks for your help.

[nicolas3141]

My experience is that generally the component cost for most circuits is pretty affordable. Where things get expensive tends to be more on the front panel - sockets, pots, switches, knobs, etc

Nicolas

[bubzy]

[did_poutrator]

hello, I just finished the VCA part, it works well and I was wondering if it's possible to add an LED before or after Q1?
I have not tested yet and I prefer to ask first.

[nicolas3141]

Yes, or between IC1a and IC1b. The voltage in both places will usually be negative so the led will have point away from ground.

[killingzoe]

First of all I´d like to thank you for this amazing forum! This is my first post, I hope it not to be redundant (:


I´d like to open again this thread with an information request:

I´m a newbie in this field, I was building this project but I have a doubt about the cv part. I am planning to control it from an arduino, using an mcp4922.

The datasheet of the lm13700 present a vca schematics, which ibias should be between +- 15v (if I understood it correctly).

Do I have to first transform my 0-5v cv signal from the mcp4922 to a +-15v signal?

I tried to simulate this solution here, reaching +-12v (that is the power coming from my small modular case) :

http://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+10.200277308269968+50+5+43%0Ar+896+464+976+464+0+10000%0Aa+976+480+1104+480+0+12+-12+1000000%0Aw+976+464+976+416+0%0Aw+1104+480+1104+416+0%0Ar+1104+416+976+416+0+20000%0A172+784+480+720+480+0+6+0+5+0+0+0.5+Voltage%0Ar+976+416+976+320+0+20000%0AR+976+320+1120+320+0+0+40+-12+0+0+0.5%0Ag+976+496+976+544+0%0AO+1328+496+1456+496+0%0Aa+1168+496+1328+496+0+15+-15+1000000%0Ar+1104+480+1168+480+0+10000%0Aw+1168+480+1168+416+0%0Aw+1328+496+1328+416+0%0Ar+1328+416+1168+416+0+10000%0Ag+1168+512+1168+560+0%0Aa+784+464+896+464+0+15+-15+1000000%0Ar+704+384+784+384+0+10000%0Aw+896+464+896+384+0%0Aw+784+448+784+384+0%0Ar+896+384+784+384+0+22000%0Ag+704+384+656+384+0%0Ao+9+64+0+550+20+0.00009765625+0+-1%0A


Is this the right approach or I am lost in a useless thought?

Thanks again to all the community and to Nicolas for these schematics that are perfectly suited to learn this amazing subject!

[eeling]

Can anyone doodle me up a diagram of the wiring??

[eeling]

Glide side working totally fine, VCA side making not a hiss nor peep with audio, cv, etc. Where should I begin looking for issues?

[tinfoilcat]

eeling: I guess you are the one I responded to on Reddit, hope it helped!

I noticed an error in the stripboard layout, pin 5 should not be bridged to the V+ pin! The 10K resistor should be between pin 2 and V+. The schematic is correct. I made a video showing mine and talking a bit about the design: https://www.youtube.com/watch?v=NdxPAccjdoU

I want to thank Nicholas for the awesome designs that he shared!

And one more caveat: Don't be like me, remember to connect the output pin (5 or 12) to the buffer input (pin 7 or 9) if you are building the simpler VCA from this thread.