Envelope Generator - VCA issue. please help!

injektilo001 · Joined: Aug 21, 2020 Posts: 8 Location: Chile

Hello, so I have this VCA from Modular in a week series

The VCA CV input reads -10V when nothing is connected to it.

When i connect my LMNC envelope generator output to the VCA CV it stays in -10V and when I trigger it goes to +11V, so the VCA open up, like it supposed to be.
Now, i recently build a Roland System 100 ADSR VCA, and i when I connect the output to my VCA input, the voltage goes to 0V, without triggering, so the VCA opens quite a lot. Seems like something in the ADSR output is activating the LM13700 and brings the voltage from -10 to 0 just connecting it.

Here the schematics to the VCA, LMNC EG and Roland ADSR.

https://www.dropbox.com/s/85l1fvs0xroi79h/Schematic_13700-Dual-VCA.pdf?dl=0

https://easyeda.com/itsnotworkingatall/lmnc-adar

https://electro-music.com/forum/phpbb-files/rolandsys100m_adsr_168.bmp

Hope someone can enlight.

JP

Blue Hell

injektilo001 · Joined: Aug 21, 2020 Posts: 8 Location: Chile

Hello!
Thanks for your reply, my friend.
Well, indeed i disconnected the led to have more voltage range on the output, how did you suspect this?
So, what do you recomend me to do with this VCA. How do i keep it closed if the behavior of the OTA is to raise to 0V when i connect an "well behaved" EG.
Or i'm saying nonsense stuff?
Please enlight me

Blue Hell · Posted: Thu Nov 12, 2020 9:02 pm Post subject:

Well .. when the LED is in it will clamp the signal on the low side to zero volts ... which is what is wanted behaviour ... the diode after the opamp will prevent the output from going low then, but when the LED is out the current to the VCA can go to zero, cutting off the sound.

What would be needed is some level conversion on the the CV inputs of the VCA .. or actually a voltage to current conversion that is a little more complex than just a resistor (the 22k one in the schematics you posted).

https://electricdruid.net/design-a-eurorack-vintage-vca-with-the-lm13700/ shows some ideas for how to do that, with an extra opamp a transistor and some other stuff. The one in figure 6-15 doesnt look too bad to me.

The thing is that an OTA is current controlled and not voltage controlled, a zero current will set it's gain to zero, but because the refrence is at the negative power supply there will still flow a current when the input voltage is zero.

Hence a voltage to current converter is what is needed. The 22k resistor in the simple one actualle does that, but it will not set the current o zero for a zero volt input as the other end of the restor is at the negative supply voltage.

Fig 6-15 in the link above does some trickery to get that right.

Oh .. and a 3080 is roughly the same in it's behaviour as 1/2 of a 13700.

Anyway, hope this will help a bit :-)