hi, its me again, since i now have midi to multiple cv, i now need multible vca’s. i’ve found this schematic on the forum, but there was a discussion under it, so i wanted to know if would work as a normal vca, without amplifing, but opening via the gate? and if it will work, which hFE count should i take for the transistors?
thanks 
Sam himself says in his description that “it’s not a clean VCA”, I think it works for circuit bending or whatever, but for a modular synth I’m not sure it’s the best choice for a VCA module.
i’m curious, what means hFE please ?
If ever there were a question that called for “breadboard it!” as an answer, this is it! I’m not just “not sure it’s the best choice”, I’m sure it’s not the best choice, at least if by “best” you mean performance quality. But it’s definitely cheap and definitely easy, and only you know exactly what you want to do with it and what kind of results you’ll be happy with.
As transistors go the BC107 is not the cheapest, $2 each at Mouser. Still, wouldn’t break the bank to get a couple and try them.
If you support Sam on Patreon he just (today) posted KiCad design files and a PDF schematic for a dual VCA PCB. Each VCA uses half of two ICs, plus a transistor (a $0.02 one!) and maybe a dozen other components, so it’s definitely more cost and effort than this, but it’s a real VCA that may be much more suitable for what you have planned.
i saw that, it looks very useful!
okay, i will try it. thanks!
wouldn’t an optokopler times 4 (for 4 vca’s) do the job as well? and would it be better?
no answers ?
so i searched myself and find it, thx to me 
sorry i haven’t seen your reply. i don’t know either so i’ve asked.
No problem ! 
(more machin characters …)
hFE is current gain through a bjt in a specific configuration - usually a common emitter amplifier. You might see it referred to as β or beta. Normally, if you put in 1mA to the base, you’d expect 100mA of collector current if the hFE was specified as 100 i.e. Ic = hFE * Ib.
All that said, hFE can change depending on a lot of things. From transistor to transistor it can be different. It also depends on which “region” you are operating the transistor in i.e. linear or saturation or whatever - audio circuits usually have them operating in the linear region, but if you’re using a transistor for switching it’s usually in the saturation region and hFE no longer applies. It can also vary depending on the voltage across the transistor, the collector current or the device temperature.
In general, you should avoid designing circuits that rely on a specific hFE - it’s not an easy parameter to control. If you’re looking for substitute transistors, getting something within the same ball park as what you want to replace is usually good enough.
Cheers
It’s possible to use a vactrol (optocoupler). I found this schematic:
and I would say: Don’t do that. For one thing, it’s using a CV to drive the LED, and that’s not good for a couple of reasons: CV sources are not meant to supply current, and LEDs don’t respond linearly to voltage; in fact they won’t even turn on until the voltage is high enough. As for using the LDR in series between input and output, what that will do will depend heavily on what your input and output modules are. It might sort of work in certain situations but it’s very overly simplistic.
This is better:
This uses op amps to buffer the audio and CV inputs and to drive the LED for better linearity, and to actively alter the output voltage (instead of just throwing a resistor in series and hoping for the best). It’s designed for a guitar pedal, so there are things you’d change for a synth. Leave out the whole part on the lower left and replace “VREF” with a connection to ground, assuming you’re using a bipolar power supply. I’d replace the 1M resistors with 100k and the 0.22 uF output capacitor with 2.2 uF.
Vactrols are still not greatly linear, and they vary from one to another, and they’re slow, so they’re not ideal for all purposes, but they can be used.
ok, thanks 
!
if i have the time, i will test all of them, then i will say which works best.
