Am I correct in assuming that you differentially add two input phases from the XLR input jack at or near the input and then use the Circlotron for the output?
@kevemaher I’m not exactly sure what you are asking about here. I wanted to see what would happen if the entire circuit was balanced/differential from input to output. So no single-ended circuits; the signal pins (2 and 3) of the XLR connection drive a differential amplifier in both the phono section and line stage. The Circlotron circuit is the output of the line stage. We use a servo to detect any DC Offsets and correct them. The servo is not responsive anywhere near audio frequencies. In this manner driving low impedance inputs has no effect at all on the preamp’s ability to play low frequencies.
The advantage of using differential amplifiers is you get noise rejection; if there is noise (perhaps a buzz) to which the interconnect cables are subjected, that noise is common to both the inverting and non-inverting inputs. So the differential amplifier has a property called Common Mode Rejection Ratio (CMRR), which is measured in deciBels. Noise introduced to the interconnect cable tends to not be amplified, unlike a single-ended connection.
In addition, differential amplifiers also have immunity to power supply noise.
FWIW a balanced circuit that isn’t differential has no CMRR at all unless the input is transformer coupled. Circuits like this are simply two single-ended circuit duplicated, one for each phase. They really do require at least double the parts.
