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An OTA (operational transconductance amplifier) circuit is one that converts an input voltage to an output current. This is a popular amplifier design as it can be less prone to going into saturation (clipping), has good bandwidth, and is also known for a “warm” sound. Therefore, you may find it in VCAs (voltage controlled amplifiers). The venerable CA 3080 chip – long since replaced with the 3280 and then the 13700 chips – is an OTA design, and appeared in many early synth circuits. Indeed, I have a custom Gentle Electric cabinet designed in the 70s that is full of 3080-based VCAs. An OTA circuit can also be designed with discrete transistors.

As alluded to above, the “transconductance” in “OTA” means the process of converting voltage at the input to current at the output. Current can be thought of as the inverse of resistance, so what you have in essence is a very handy voltage to resistance device that makes it possible to add voltage control to circuits such as filters. Many classic filters – from the SSM chips used in the early versions of the Prophet 5, to the filters in early Roland synths, to the infamous Korg MS-20 filters – are based on an OTA design. In general, when someone touts they have an OTA based filter, it usually means it has a “warm” sound…unless it’s an MS-20 filter clone, in which case it’s thinner and more edgy. In reality, an OTA makes circuit design easier and more efficient, so their real impact is on convenience rather than sound.

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