Help me understand one thing about THAT vca's..

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As I understand your alternative, it's probably not an I/V-conversion as the VCA would like to see (guessing here). If I understand it correctly you present the resistance of the pot to the VCA (realizing the I/V) and then sense the resulting voltage with the HiZ JFP.

I thought to have seen things like this when people want to follow a DAC with a tube stage. They add a small value resistor and go to the tubegrid. The resistor may need to be small in value to avoid eventual influence back to the DAC (or VCA) since the signal develops across the resistor. This might be the reason for the VCA as well, so that it prefers to deliver its outputcurrent into a fixed voltage.

The virtual ground topology ensures that the VCA output current (in which format the 'signal-information' is captured at that point) is flowing into the feedback-resistor of the opamp and realizes the I/V-conversion
-while the virtual ground stays at the same potential. But you know all this.

So I guess it might be OK to use a small resistor but then you need more amplification; likely that resistor has to be a lot smaller than an OA-feedback-R.

Why not use the suggested OA+R_FB-stage and put the JFP after that ?

If you still want to go the small-R + JFP route you need to determine how large that resistor may be. And if you want to make it a pot at the same time you might need to // a small R if you need a lower value than pots can give you.

Bye,

Peter
 
[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.
 
What JH said.

The voltage swing at the output is only a few tenths of a volt, and it may distort some even with millivolt signals. So you could try a super-low resistor, like 100 ohms, but you get a super-low voltage and poor signal to noise ratio.

If you detest op-amp I/V converters, use a transformer. 50 ohm to 50K ohm ought to jack-up your FET amp, though I have not thought about what the transformer's low DC resistance will do to the VCA.

Use an op-amp, chip or any high-gain inverting amp. Yes, even the Hamptone plus a 10K-20K feedback resistor, though that's an odd choice for a medium-Z I/V converter.
 

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