[quote author="Curtis"]
No I was talking about the opamp right at the VCA. While I may be reading it wrong, it looks like the - input is actually connected to the VCA control port and the output not connected to anything but the - input trough a parallel R & C? As drawn a unity gain buffer with a strange source impedance characteristic, but I probably need to shut up until I can read a schematic I can see.
No, you're reading that right, and that's how it is in the original too. It gives a handy point to break into the CV for things like stereo de-essing and other special sidechain functions (or at least that's how the manual describes it). I left it as-is as I intended it as an option for combining two units as a stereo pair.
New high-res schematic located
here.[/quote]
Thanks, I can now read the schematic better, and that opamp still looks unconventional to me, to put it kindly.
From the 2181 data sheet they advise keeping the drive impedance at control ports to 50 ohms or lower. Further they caution if using an opamp to buffer that point the source impedance of an opamp which rises at HF due to falling loop gain margin may cause instability. The recommend adding a R (100 ohm) in series with a C (1.5nF) from the control port to ground for stability.
Driving the control port (as drawn) from the minus input of the (tl074) opamp will be moderately low impedance as long as the opamp can keep up with current drawn at the node though the feedback resistor. That point will be higher source impedance than a typical opamp "output" like THAT warned against, and who knows what else is hanging off that - input to bother the opamp stability. The better node to use IMO for stereo de-essing or sidechain tricks is the + input, before it is buffered.
Another tidbit from the 2181 data sheet is that the noise of a unity gain connected 5532 will increase the the noise modulation floor from -94 dB to -92dB. OK only 2 dB but the 5532 is far quieter than a TL074 assuming it was connected right, not to mention TL074s unless selected or graded parts can have DC offsets of several mV perhaps significant at the control port of a VCA.
While I would have used a resistor divider with the final termination resistor < 50 ohms, I appreciate the opamp mentality and modest voltage available from the RMS chips. This is one application that screams for modern low noise, low DC offset super opamps but it seems a waste of $ to throw them in a control side chain. The old general purpose bipolar opamps were a somewhat better match for the task than older bifet technology.
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Looking at the rms/log/control voltage circuits, It looks like the 2252 is designed to interface with the VCA 1:1 so is only hundreds of mV full scale.
That said, all the opamps in that area would benefit from low noise and low DC offsets, while there is a dedicated trimpot just for DC offset this circuit wasn't designed for bifet sized offsets. The 4.7k resistors to ground from the + inputs on several of the opamp sections are a holdover from an earlier design version that obviously used old school bipolar opamps, and the Rs were there to correct for input bias currents.
Since all of the control voltage crunching is done at 1:1 or close to it the layout is important. The ground end of R42 (12k) should be close to the VCA ground.
Without scaling up the control voltage I suspect the TL074 is not a great opamp for this application, something with better DC performance, lower noise, etc might be better. The opamp that really makes me uncomfortable is the one driving the VCA control voltage. If the VCA is unstable because of impedance there consider adding the RC they mention in the spec sheet.
Considering that PCB is already finished perhaps a better high performance quad opamp might drop in.. while I'd take my exacto to that one opamp, if it works and you're happy enjoy..
JR