Among the earliest 2001-era transitor experiments was building and characterizing the voltage adder in the Moog 901 VCO. I liked the circuit initially because it offered differential outputs, and Moog’s use of +12V and -6V power supply rails was interesting approaching very practical.
After some work with this circuit, and a reprise examination in 2003, even with higher-performance modern silicon transistors, I really could not obtain accurate DC performance — something crucial for pitch accuracy and stability. This outcome lead to a stronger DC amplifier developments, with some detours on the way. The Type X DC Amplifier followed in 2004. Then finally, after much more insight into how to use discrete transistors, the Type X1 DC Amplifier was designed in 2017, with much better performance in all aspects.
Here’s the Moog Voltage Adder notes from 2001:
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