The 270A is a quad preamp designed for high or low impedance microphones. This would also work with tape heads although there is no NAB equalization, or with an electric guitar.

It consists of three gain stages. In lo impedance, the Microtran M1A transformer would provide a voltage gain of 14.14:1. It is followed by an op-amp stage with a gain of 11X, followed by the attenuating slider, followed by another op-amp stage with a gain also of 11X. In lo, the total gain is 1711X and in hi the gain is 121X.

This MEMS module is built with the Microtran S105S audio transformer which has a voltage gain of 15:1, so very close to original.

Working backwards from a standard 4V pk-pk sine wave signal, the output would be 1.414 Vrms. Dividing this by the gain, we have an input level of 826 µV for the low input and 11.6 mV for the hi input.

Low impedance microphones typically are in the range of 1 to 15 mV so this preamp is certainly designed for microphones in this range. A typical high impedance output might be a guitar pickup and they are more in the range of 15 mV to 250 mV depending on how many pickups and if they are in series. Certainly the low end of 15 mV is in the range for this preamp. This preamplifier was designed for microphones and magnetic pickups in ranges of  1 to 15 mV.

270A schematic 1973

The module consists of two PCBs with PCB1 for the front panel controls and PCB2 for the circuitry. A separate bracket mounts the transformers.

I ran the shields over the top of the PCB to solder to the ground plane on the rear.

The transformer bracket is folded down to view the lower portion of PCB1.

There are 5 wires from each of the transformers of which the shield (brown) and secondary (blue) are grounded. You can also see the jacks normalled to ground.

Operation

This mixer clearly wasn't designed for mixing standard Buchla 4V pk-pk signal levels nor even a consumer line level output of 0.316 Vrms. A 3.16Vrms sine input would require the slider to only be up 3.6% of value.

This design uses 5% resistors. The first op-amp is non-inverting so can have a gain of 10X to 12X. The second op-amp is inverting so has a gain of -9X to -11X. The All Output stage can have a gain of -0.9X to -1.1X. The Outputs then  can range from -90X to -132X while the All Output can range from 81X to 145X. Don's notes on the schematic says the hi input has a gain of 40 dB which is 100X, but the first op-amp is 11X, not 10X, so he might not have added the "1+" for the non-inverting stage gain using nominal resistor values. 

I did not build this module but modified it with shielded cables and normalled 1/4" input jacks to ground. With these the module is much quieter. I also would have used 1% resistors to keep the gain of each channel closer together.

I used my signal generator with a ~20 mV pk-pk signal and measured the hi gain on the Channel A Output of -96.8X. The Outputs are out of phase with the input while the All Output is in phase. That gain measures -93.6X so the All Out stage gain is -0.96X.

Another note says the maximum input is 2V pk-pk, which, with a gain of 12X, is 24V pk-pk. The op-amp can likely swing to +/-13.5V so there is some headroom with 5% resistors. Of course the slider has to be at 1.5% of value for a 4V pk-pk output. 

The low input measures a gain of  -1380X. That corresponds to a gain of 14.2X for the transformer, a bit lower than the specified 15X.

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