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Old 11th February 2010
1600's and 2400's used a standard instrumentation amp design for the mic preamps. It's a pair of noisy 2N4403 switching transistors in front of a 072 opamp.
Modern Japanese transistors will lower the noise, but THD is high with this design at higher gains. It's the same design as used in the TS-24, early 600's and others of that era.

They can be rewired into trans-amps by cutting the opamp feedback loop and tying that feedback resistor to the inverting transistor emmiter. That will lower preamp noise below that of a mic and will lower THD levels considerably. You can get -129.6 EIN with that preamp if reworked.

Jim Williams
Audio Upgrades