[sdiy] VCA troubles, NE5517

[René Schmitz]

Hi Fredrik,


> I have truble with what I thought would be a simple VCA circuit. I use half 
> a NE5517. The trouble is that the control current feeds thru to the output 
> in a very distinct manner. Sometimes the feedthrough is even stronger than 
> the actual signal. 
> 
> Vdd is 5 volts. Vdd/2 is 2.5V. I didn't use the buffer since I 
> am using a single +5V supply, and I didn't want the 1.2V drop over the 
> darlington.
> 
> Can someone please help me remove the control current feedthrough?

Make sure that both inputs see the same source impedance. I.e. insert a 
1k resistor between the - input of the 5517 and Gnd. Additionally you 
might include a 100k resistor from the - input leading to a trimmer 
connected between the supplies. With that you can minimize the 
feedthrough. THat would be normal procedure for a +-15V powered circuit.

I have no experience with these running from +-2.5V, and I suspect that 
some of the problem _might_ be due to that there isn't sufficient 
voltage available to all the internal current mirrors.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159