Strange VCA and CV mixing

Discussion in 'Homework Help' started by omissis, Apr 2, 2012.

  1. omissis

    Thread Starter New Member

    Apr 2, 2012
    Hi all, first post here..
    I think that the questions I'm about to ask could have been set even in the Electronics Chat forum , but I understand they're more related to the basic scheme of how does something work.
    I found a quite weird VCA in a synthesizer: it's called iG00151 , made by Yamaha; unfortunately they never have released the datasheet of it and I'd like to try and understand what's the deal about it. I have a block diagram of it here below, named "current-voltages" : you can see that it looks like an OTA with an Expo-to-Linear converter, so it gets a voltage (green line) to output a current.

    How can I do the converting calculation? Also, what's the role of the negative feedback?

    Main problem for me, though, concerns the voltage calculation at the output of this VCA: see the "VCO control" picture for reference. Looking at the picture I can see the currents mix with voltages (initial pitchbend, subosc); but I need to know how to render the current's magnitude into voltage! Can anybody help me to understand what's the correct calculation at the "N" node?

    Last question: what's the role of the 0.01KΩ capacitor at the VCA's output?

    Thanks for any help.
  2. omissis

    Thread Starter New Member

    Apr 2, 2012
    Edit . Capacitor goes at 0.01uF : sorry for the hurry.
  3. crutschow


    Mar 14, 2008
    It would appear that the voltage on the "N" node is determined by the combined current outputs of the two VCAs and the voltage output of the Buffer amp summed into the "N" node which has an equivalent resistance of about 90KΩ. (I presume the VCA outputs are current and thus have a very high output impedance).

    The problem is that unless you know the value of the VCA "K" gain constant you can't calculate the current outputs of the VCAs.

    The capacitors roll off any high frequency noise on the VCA's outputs.