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How to design a VCA with the LM13700?
A Eurorack VCA with the LM13700 Let’s assume we want +/-5V (10Vpp) input and output signals, and let’s design for a 0→5V control voltage (gain CV) level. The audio signal levels are pretty standard, but Eurorack envelope modules vary in the output level they provide.
How is the output level of the LM13700 determined?
Working out the output level of the LM13700 The LM13700 isn’t a simple op-amp. It’s an “Operational Transconductance Amplifier” or OTA. I’m going to avoid trying to explain what the hell that means. The short answer is it has an extra input that allows us to set the gain of the chip using a current.
Is the LM13700 a good synth chip?
The LM13700 is a bonafide classic synth chip, plus it’s cheap and *still* available decades after it was originally introduced. Ok, it’s no longer the state of the art, and its noise and distortion figures are pretty poor, but that’s actually a part of its charm.
What is IABC in the LM13700 datasheet?
This control current is known as Iabcin the LM13700 datasheet, and the relationship between the input, the gain control current, and the output looks like this: Iout = (q / 2kT) x Iabc x Vin Ok, hold on! What on earth is all that lot?! Brace yourself…kis Boltzmann’s constant, Tis temperature, and qis the charge of an electron. Yeah, seriously.
How does the LM13700 convert current to voltage?
Converting the LM13700’s output current to voltage In cheap’n’cheerful 13700 circuits, you’ll often see a resistor hanging off the output down to ground. This resistor acts to take this current to ground, and if a current is following through a resistor, there must be a voltage across it, so the resistor converts the current to a voltage.
How are the output buffers of the LM13700 different?
2 Applicationsespecially designed to complement the dynamic • Current-Controlled Amplifiers range of the amplifiers. The output buffers of the LM13700 differ from those of the LM13600 in that • Stereo Audio Amplifiers their input bias currents (and thus their output DC • Current-Controlled Impedances levels) are independent of IABC.