Channel 7 and 8 are now balanced input

I have installed and tested the INA217 boards on channel 7 and 8 and it seems to work just fine. Channel 8 is just the preamp board with unity gain installed were the transformer is on the stock boards. On channel 7, after reading the schematic, I have done some further modifications.

Removed R4 and shorted R3, taking S1-1 out of the picture.
Removed the voltage divider formed by R7 and R8 by removing R8 and short R7.
Added a 10K gain set resistor on the INA board, getting 6dB gain in this stage.
Replaced gain set resistors on U1-1 that are switched by S1-2
R10: 10K 19.4dB
R9: 2K7 10.24 dB
R11(stock): 540R 3.23 dB

Replaced first coupling cap with 47uF elyt + 1uF poly caps paralleled.Added ceramic decoupling on the rails at U1.

As this is just testing right now I just lift the component lead that are in the signal path for easy reassemble if the mod is not beneficial. Here R4 lifted out of circuit.


R8, eerie close to R12 and the jumper

R3 and R7 shorted on solder side. If you do permanent mods, replace the resistors with proper jumpers. That's what I am going to do when I am satisfied.

Since I am doing this on spare time when geek junior is asleep I have only done some very basic testing. Comparing the noise and estimating the gain by tapping a very crappy dynamic mic. The mod works, no doubt about that, channel 8 is a bit less sensitive than stock and channel 7 is some dB less sensitive than channel 8. Since I am mainly intending to record slamming drums on it I suspect that less sensitive inputs might be beneficial. A real situation test will clear that out.

My intention with spreading the gain over two chips was to reduce noise floor and Transient intermodulation distortion. However I estimate the noise to be about the same, taking into account the different gain of the channels.