"Second, I think that we have to understand that in the 80ies and 90ies, these OTAs were Bob Moog's "daily bread" which he used in almost all his designs. So, he was for sure very familiar with these and preferred their use, even if other variants or choices would have been possible." - Thierry
Yes, go with what you know. And bulk prices can be considerably lower. And less manufacturing inventory (that can get stranded) too.
First, the raw audio in the Etherwave is about 550mVpp only with a -300mV offset. Second, you'll definitively have to use a wave shaping circuit because no external instrument tuner will give precise readings with the u-formed wave and the offset. Third, you should not load the raw audio with anything lower than 50kOhm. Fourth, the circuit has to operate from symmetrical +/-12V (which is not the same as unipolar 24V!) and might not draw more than 20mA on each line.
So, this 1A to 3A driver module is only suitable for one thing: To ruin your Etherwave.
There is still another reason why an OTA is the best solution here: It does the soft clipping without feedback onto the raw audio signal. Using a standard op-amp in whatever schmitt-trigger or limiter configuration to obtain a similar output signal would forcibly bleed the square wave back into the Theremin's audio and it would then have the same ugly sound like the Theremax.
I highly value Christopher's scientific mind and all the experiments he does, but here, he is completely off the rails.
Thierry said" "I highly value Christopher's scientific mind and all the experiments he does, but here, he is completely off the rails."
That is so funny, completely? ruin the theremin, still value in experimenting with the 358 while collecting input from others. Remember Thierry drives on the wrong side of the road. His thinking comes from too many years of sniffing solder smoke or the other kind. Commercial tuners are designed for noisy signals, a theremin is as pure as it gets.
I prefer our Russian friends input on the LM358 as they have less self interest, less secrets and are great innovators. I wish their president would kick our presidents ass. 
Christopher
BTW: Although I'm also developing sometimes around the Teensy ecosystem, I found their audio library not suitable for my purposes. But as an instrument tuner, it's very good and you'd even not need the audio shield but you could feed the signal directly into the Teensy's analog input (with the help of just 2 resistors and one capacitor) and run the audio library's tuner routine from there. Speed is not a matter when it does come to that use. A digital tuning display does not need to be updated more than 15/sec, the eye and brain can't follow.
...as an instrument tuner, it's very good and you'd even not need the audio shield but you could feed the signal directly into the Teensy's analog input (with the help of just 2 resistors and one capacitor) and run the audio library's tuner routine from there.
That's very good to know. Unfortunately in my haste I already soldered the audio shield to my Teensy, but that can easily be undone. I'll give this a try. I sure wish I hadn't forgotten everything I learned getting an EE degree 20 years ago!
Thierry, I had bookmarked these articles for an input circuit, but are you saying that I only don't need to amplify/buffer the input because the signal is already hot enough?
https://learn.sparkfun.com/tutorials/proto-pedal-example-programmable-digital-pedal
Fantastic - thank you again! Does the Etherwave Plus use the same tuner output circuit? Rather than continuing to modify my ancient signature series Big Briar Etherwave Standard, I'm also considering just buying a used Plus instead.
The tuner output of the Plus is similar in terms of level and impedance, although the inside circuit is different. Thus, you might go with that, too without modification. But for heaven's sake, make sure that the internal jumper on the Plus board near the outputs is really in pitch preview position. If it's jumpered to headphone operation, the signal might become too hot for the poor Teensy when you turn the volume up.
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