People don't seem to care about mass-production costs here John.I am not sure what exactly a balanced VCA configuration is. Using two (one for positive zig, and another for negative zag) will sum to +6dB more signal, while noise should combine incoherently to raise noise floor +3dB. So net 3dB S/N improvement for 2x cost.
I don't know if terkio was going to actually make his own VCAs - he didn't say that.
My answer was related to turbo-charging THAT VCAs.
I did a cost no object high headroom VCA for Greg Hanks that used a pair of 2180s operating differentially.
He didn't care about the cost so I figured it was wise that I didn't either.
At high levels there is some really effective even-order THD cancellation provided by a differential topology in addition to the modest 3 dB noise improvement.
I used differential in/out on the audio ports which were I-V-converted by two OA stages and then followed by a cross-coupled dual diff amp.
The VCA was balanced in/ balanced out with common mode rejection in-between.
The Ec control voltage was also differential drive giving it high "off-ness" and low control feed-through.
THAT2180's work best.
Though the 2181 can be trimmed for lower THD, the THD tends to drift more with temperature than the 2180.
One factor is the tempco of the external trim versus the internal closely-matched tempcos of the 2180's internal trim.
Nulling multiple VCAs also seemed silly.
The VCAs being current in and current out aren't voltage-limited but the I-V op amp outputs limit swing.
Having cross-coupled output doubles voltage headroom if you want or need a balanced output.
I agree with Duke not liking rail-to-rail op amps in audio.
The two exceptions might be the OPA1612 (bipolar) and OPA1642 (FET) which have nearly-identical "rail-to-rail" common emitter outputs.
The one odd thing about the 1612/1642 outputs is that the output impedance actually rises below 100 kHz.
Zo open loop at 100 kHz is about 1Ω but at 10 Hz its 2KΩ. Why?
That's weird - is it some thermal phenomena?
Never seen that before.