op-amp based, high-gain, high rail voltage, preamp

[Monte McGuire]

The circuit looks really interesting. I tried something similar for a phono input with a zero drift amp to correct the offset from the differential Jfets on the input. Remarkably DC coupled with the offset removed by the chopper it worked really well with no feedback problems from the really extended bass. And the zero drift amp kept the output offset around 100 uV at 80 dB of gain.

However the hiss from the zero drift was audible. I was using an older zero drift with a switching frequency in the 5 KHz region. These newer ones may work better in that application. I need to figure out how to get the cancelling working in a simple amp stage.

The ADA4522-2 switches at 4.8MHz and also 800kHz, but also shows a little bump at 200kHz that is only a little bit above the noise floor when you look at an FFT. So, it's easy enough to filter its noise away over the audio passband. It also has very low distortion, especially at LF, so if you lowpass the sense signal, you won't tickle its HF distortion, which rises above the noise around 1kHz. It's also available in a 2 channel format, which makes it easier to use without beat frequency problems from two unsynchronized choppers.

Yes, the overall circuit is a bit complex, but in my world, I prefer balanced signaling, as it has other benefits for my circuits. You may be able to just unwind the circuit and use each servo on two series stages, and get the same response cancellations - I haven't tried that, but since that's most of what's going on here, it may work. Or, you could just go balanced! ;-)

Have fun with it!