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Fairchild μA726HC

The Fairchild Semiconductor μA726HC or UA726HC, has been used in a prolific amount of synthesizers spanning from the 70's up until the early 80's. Intersil (Renesas) then created the CA3046 to succeed it, which is now also an obsolete part and no longer actively manufactured. For some reason it's sometimes said not many synthesizers used this component (a staggering amount do), but considering it was one of the only solutions at the time for temperature stability, it found its way into many things.

What the hell is it?

The μA726HC is a matched transistor pair with both a heater (oven compensation) and temperature sensor. The IC resides in a potted epoxy compound to ensure thermal stability. In the context of an analog synthesizer, this is to provide pitch stability of the voltage controlled oscillator(s) as the machine undergoes changes in temperature; the oven compensation will keep the transistor pair operating at a higher reference temperature than the ambient temperature to reduce flucations that may affect tuning of the oscillators.

Obviously, the μA726HC was used in other devices at the time (generally military equipment), and in fact, I don't suspect its original intention was pitch stabilization for voltage controlled oscillators, it's likely this component predated the concept.

Synthesizers That Use μA726HC

Here's a list of synthesizers I know that use the μA726HC along with the (suspected) component count needed. I couldn't find a list like this anywhere so started building my own.

Replacement Solutions

Curiously enough, the way Moogs, Buchlas and Rolands all use this component varies a bit which makes building a 'universal' replacement more complex.

There doesn't seem to have ever been a reliable/consistent replacement created for the μA726HC, however, some alternatives exist:

Modifying VCOs to Omit μA726HC

Rob Keeble took the System 100m oscillator design and rebuilt it to exclude μA726HC, I'm not certain how this was accomplished but in theory it may be possible to build something to retrofit existing synthesizers to omit it.

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