I replaced the thermistor in my x0x, following Open Music Labs’ Thermal Compensation of Analog Exponential Converters. The aim was to reduce the oscillator’s sensitivity to temperature.
The x0x board on the workbench for the thermistor change.
Why the thermistor matters
An analogue oscillator’s exponential converter turns pitch control voltage into an exponential current. Its behaviour depends on transistor thermal voltage, Vt = kT/q, where T is absolute temperature, k is Boltzmann’s constant, and q is the elementary charge. A thermistor adjusts the control-voltage scaling to compensate for that dependence. See Section II, pages 1–2.
we need a resistance that also varies linearly with absolute temperature.
— Open Music Labs (2015), Section IV, page 5.
For the simple op-amp scaling circuit discussed there, the target temperature coefficient near 25 °C is about +3350 ppm/°C. A resistive divider changes the effective coefficient, so the required component depends on the circuit. Thermal contact with the transistor pair also matters: the compensating resistor needs to follow its temperature.
Board detail, with the component area marked in the photograph.