I am working with an interesting limiter project...
Until now I have interfaced VCA control ports like Symetrix does in the following schematic:
http://www.symetrixaudio.com/kb/565E_sch.pdf
The small resistor at the control port causes some unbalance and therefore distortion, but part of it can be eliminated by adding another resistor at the opposite port. If the resistances are small enough this circuit works fine and provides low distortion. A huge amount of limiters and compressors are made this way.
VCA manufacturer THAT Corporation however recommends in all their application notes another solution. It connects output of an op-amp directly to the control port and grounds another port. You can find an example on page 1.
http://www.thatcorp.com/datashts/dn122.pdf
The THAT circuit has two bad sides:
1. The inductive impedance of the op-amp output at high frequencies is poison to the VCA (can cause oscillation). It can be cancelled by a capacitor, but still.
2. The benefit of attenuating the op-amp output noise by resistor divider is lost.
Plus connecting an op-amp output directly to the control port feels like quite a harsh solution...
Which to believe?
Until now I have interfaced VCA control ports like Symetrix does in the following schematic:
http://www.symetrixaudio.com/kb/565E_sch.pdf
The small resistor at the control port causes some unbalance and therefore distortion, but part of it can be eliminated by adding another resistor at the opposite port. If the resistances are small enough this circuit works fine and provides low distortion. A huge amount of limiters and compressors are made this way.
VCA manufacturer THAT Corporation however recommends in all their application notes another solution. It connects output of an op-amp directly to the control port and grounds another port. You can find an example on page 1.
http://www.thatcorp.com/datashts/dn122.pdf
The THAT circuit has two bad sides:
1. The inductive impedance of the op-amp output at high frequencies is poison to the VCA (can cause oscillation). It can be cancelled by a capacitor, but still.
2. The benefit of attenuating the op-amp output noise by resistor divider is lost.
Plus connecting an op-amp output directly to the control port feels like quite a harsh solution...
Which to believe?