Doepfer

Does anyone know if there is any way to mod the VCA on the A-107 to
reduce the noise level?  Right now, the output contributes quite a bit
of hiss to the signal, especially noticeable when working with
low-passed signals that don't mask the added hiss.  And there doesn't
appear to be a way to bypass the VCA signal path.  Ideally, I'd want
it to be at least as clean as the A-132-3 or close to it.

Kindest regards,
John

Hi John

> Does anyone know if there is any way to mod the VCA on the A-107 to
> reduce the noise level?  
To my knowledge the A107 is using a Curtis Chip CEM3372 which integrates 
VCF and VCA.

If you are knowledged in measuring signals, you might check at pin 17 
whether the  VCF signal itself is noisy already. pin 12 is the VCA input 
  pin 14 is the VCA output. There should be a buffer opamp after the 
output, which might be noisy too. In theory you might skip the VCA, by 
connecting pin17 directly to this buffer, and use an external VCA.

To avoid noise at the VCA, you should assure to feed a full signal level 
to the VCF input already. And keep in mind, that some filter settings, 
which reduce the energy of the signal (Bandpass), will provide lower 
signal levels to the VCA.

Florian

Hi John, Florian,

> > Does anyone know if there is any way to mod the VCA on the A-107 to
> > reduce the noise level?  
> To my knowledge the A107 is using a Curtis Chip CEM3372 which integrates 
> VCF and VCA.

It is actually the CEM3379 which is used in the A-107, so the pin-outs for Florian's good ideas aren't applicable: the VCA 'in' is pin 13, and 'out' is pin 11; the 'out' however is a current, feeding an op amp virtual-ground summing node, so unfortunately you won't be able to measure any signal there.

However, it should be possible to take the signal _before_ the VCA, from jumper 'JP16' - this is hidden by the ribbon connecting the two boards, and the pin nearest the edge of the board is the signal output from the op amp summing the outputs from all four filter sections.

The signal paths are going through quite a lot of chips, so it might not be too much of a surprise that it picks up noise on the way: after the '3379, each stage is amplified (approx x10 last stage; x75 the others); then through the SSM2164 VCAs for the morphing; summed; then the '3379 VCA, and the final op amp converts to a voltage for output!

Tim