[sdiy] VCA troubles, NE5517

[Fredrik Carlqvist]

Hi again!

I have tried different tricks, but none seem to help. My question now is:

Is the darlington buffer necessary to remove the control current feed thru?

When I look on the schematics in the NE5517 data sheet, the base of the
second transistor in the pair is connected to something that seems to depend
on the control current. Some kind of current mirror perhaps? Perhaps this is
some intelligent device to remove feed thru?

Referring to my schematic, the inverting input goes to a very low impedance
(OP output at 2.5 Volts). The non-inverting input sees 1k. René, are you
saying a resistor between the inverting input and the 2.5 V reference would
improve the feed thru? 

Is the output of the NE5517 current or voltage? It says in the data sheet
"Voutput" at pin 5. Is it only the CA3080 that has a current output?

Anyway, using the CA3080 at 5 volts is no problem, but the 5517 seem more
complex. It works well in the filter and some other stuff, but here in the
VCA circuit it fails. Perhaps I will have to go back to the 3080. There will
be no more 5517's anyway (he said bitterly).


Fredrik C 


PS
The web site is up, so now the schematic is available there;

http://www.minod.com/vca.gif



-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Fredrik Carlqvist
Sent: Wednesday, April 14, 2004 16:56
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] VCA troubles, NE5517



Hello all you brilliant people!

I am very sorry to attach the image, but I have trouble with my ISP today,
so I cannot put it on the internet. At least it is very small.

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?


Thanks,
Fredrik C