I've watched a few videos comparing different effect designs. Phasers, Envelope filters, tremolos etc. In all the videos, the sound of the OTA effects sounds strange against the background of circuits with conventional op amps. It seems that the OTA passes a smaller frequency range and slightly reduces the signal dynamics. I have already figured out that OTA requires a large voltage divider at the input, has a very small input impedance and a very large output (we usually want it the other way around). perhaps this is the reason for the strange sound. no i don't want to say that the effects on the OTA have a bad sound, but when compared directly it seems plastic and unnatural. Who thinks about it?
It depends on the specific OTA. For many years, the only OTA one could buy was a CA3080/LM3080. It would distort at fairly low input signal levels. Jim Patchell has documented this extensively ( http://www.oldcrows.net/~patchell/archives/ca3280_2.html ). Craig Anderton even exploited this characteristic for a voltage-controlled distortion module ( https://hammer.ampage.org/files/Device1-8.PDF ). The CA3280 improved on this but was still imperfect. The LM13600 and 13700 went one step further, and the Rohm BA6110 and BA662 (that Boss/Roland had made for them) went even further. The THAT corporation developed OTAs that were even less susceptible to distortion. The SSM2164 also has very good distortion and noise specs. So, basically, not all OTAs are created equal, and the trend has been for manufacturers to produce cleaner and cleaner OTAs.
But you ARE correct in finding that older OTA-based designs (that would use either 3080s or 13600s) were susceptible to distortion, and would attenuate the signal being fed to them, resulting in noise issues.
Hi Mark I meant classic pedal chips like 3080/3280 or 13600/13700 In addition to noise and distortion, in comparisons I hear that there is less sound, as if there is not enough high and low frequencies. This may be due to compression due to input signal limiting. I wanted more to make sure my ears weren't deceiving me.
It is a tried-and-true strategy for keeping noise low to feed a circuit the highest possible input signal. This maximizes the "signal-to-noise" ratio, since the circuit itself will produce at least a little bit of hiss and noise. When the headroom of a chip or other device is low, however, you can't do that without producing distortion. So if you can't boost the input to keep circuit-noise proportionally low, the "fallback position" is to try and filter out any circuit-noise. This is not only true of older OTAs, but is also true of BBDs, that also have low headroom, generate noise, and use LOTS of lowpass filtering.
The noise with those is really high, too. I always hated the way OTA phasers whoosh even when there's no input signal.
Hammer brought up THAT Corp., and these days there are MUCH better options like their VCAs. They have really good sample designs and data sheets you can check out on their website.