CA3046 VCA

[jorgen.bergfors at idg.se]

>>P.S. Yesterday I worked on a CA3046 VCA that looks very promising.
Linearity 
>>looks extremely good on the 'scope. It uses one transistor in the 
array as

>>linearizing diode, I suppose.

>Which VCA circuit is that ? Are there schemos anywhere ?

>JH.

It is based on the circuit in Modulus. But I have modified it in a couple of 
ways. The Modulus circuit has the following problems:

1. Terrible CV bleedthru (ca 0,5 V) and no way to trim it. I used av CA3086. 
Maybe it is better with a CA3046 but it still ought to be trimmable.
2. Negative control voltages cause a DC voltage on the output
3. Input is AC-coupled, which makes the VCA unsuitable for control voltages
4. Some ringing with faster op-amps
5. No headroom above 10V p-p
6. Too much gain
7. Linear control response is a problem if you want log.

1. Is fixed by adding a 10k trimmer. The ends are connected to the 47k 
resistors that used to be the control voltage input. The input signal is 
connected to the wiper. CV bleedthru can now be adjusted to as low as 4 mV.

2. A diode is added to the CV input. This does not mean that nothing happens 
below 0,6 V CV, as you might suspect. The VCA starts to output signal at 0,15 V 
CV. That is quite acceptable in my opinion.

3. Corrected by simply removing the cap on the input. This does not disturb the 
function of the VCA.

4. Added a 22 pF cap in the feedback loop of the output op-amp. An even smaller 
cap might be sufficent, but I had no smaller. This of course is dependent on 
what op-amp you use. I use a  TL072.

5. Increased the input resistor to 68k

6. Lowered the feedback resistor in the output op-amp to 56k

7. Mine is still linear but I plan on making a log version too.

/Jorgen