[quote author="Viitalahde"]Ok, the datasheet of the THAT21xx vca says it's designed to output in to a negative input of an op-amp. I understand the statement that it just has to be this way - outputting in to a virtual ground.
Why? It's just an I/V -conversion, wouldn't any kind of an amp/buffer do the same function? As an example, would the Hamptone block work here - with a normal level pot after the VCA, outputting to the Hamptone block.
???[/quote]
I haven't looked up that specific VCA, but if it's some variant of the Blackmer VCA core, then while putting out a current, the voltage swing for that current source is very restricted. So you need a virtual GND
summing node. (You could most probably also feed that output current
into a very small resistor to get the I/V conversion done in a passive way, but you'd have to amplify the resulting tiny voltage a lot, which involves all kinds of problems, mostly creating an unecessarily high noise gain without any andvantage over the virtual-GND-input inverting amp.
JH.