OTAs and pots

1A pot

Write a pot as P, then one end, its wiper, its other end, and its value, the whole of its track. After that its position, and maybe its taper:

setting volume: 70%
P1 a out gnd 100k volume audio

At 100% the wiper is at the first end, at 0% at the last. So this is a volume control: turned up, out is a; turned down, it's ground. Write the end you turn towards first.

A pot's position is a setting in percent, from 0% to 100% unless you give it a range (from 20% to 80%). Its knob turns in a straight line, and a wire's 1 turns it half way along. One setting can turn two pots: a dual pot.

2A tone control

A famous fuzz's tone control is one pot between two filters. From the clipped sound, a low-pass (Rlp, Clp) and a high-pass (Chp, Rhp) run side by side; the pot's wiper picks how much of each. All bass at 0%, all treble at 100%, and half way, a dip in the middle frequencies, the sound that made it famous. After it, a volume pot on the audio taper.

Rlp a lp 39k
Clp lp gnd 10n
Chp a hp 4n
Rhp hp gnd 22k
P1 hp t lp 100k tone
P2 t out gnd 100k volume audio

In Scooped Tone an LFO is wired into tone, so the pot turns slowly back and forth on its own.

3An OTA

An op-amp's out is a voltage. An OTA's is a current: the difference between its inputs, times its transconductance, and that is set by another current, the one into its bias pin. Write it as A, then its + input, its − input, its out, its bias pin and its model:

A1 p m o abc CA3080
Rb env abc 47k
RL o gnd 47k

The bias pin is a junction down to ground, so feed it as a real OTA is fed, through a resistor from a control voltage: Rb turns 10 V into about 0.2 mA. Its out is a current, so give it a load (RL) to make a voltage from it.

Its current is Iabc tanh((v+ − v−) / 2 Vt), two transistors' law: a straight line for a few millivolts, then it bends and stops at Iabc. So the sound goes in through a divider (100k and 220 Ω make 6 V into 13 mV), and driven harder, it rounds off softly.

4An OTA as a VCA

Wire an Envelope into a named input, in env: from 0 V to 10 V, and through Rb it sets Iabc: how loud the note is, the way synths of the seventies did it with a CA3080. U1 buffers the out. Turn input up and the OTA's tanh starts to round the peaks. Ned builds one, then pushes it until it growls, in the lesson An amp from one OTA, pushed until it growls (opens in a new tab).

5Two OTAs make a filter

Into a capacitor, an OTA's current makes an integrator: its voltage follows the input, as fast as Iabc lets it. Two in a row, each with a buffer after it (U3, the band-pass; U4, the low-pass and out), and the low-pass fed back into the first one's − input, make a 12 dB low-pass whose cutoff is a current. Rq feeds the band-pass back too: the bigger it is, the more it rings (Q is Rq over 100k).

The cutoff is the current into both bias pins, from the 15 V supply through Rk1 and Rk2. One cutoff setting turns them both, a dual pot: from 1M (about 100 Hz) down to 33k (2.9 kHz). With its range made wider and an Envelope on its cutoff, it plays an acid line (opens in a new tab).

6A random filter

Wire Circuits 3's sample and hold, a circuit of its own, into cutoff: sh -> rf.cutoff. A wire turns a setting from where it's set, half its travel for a 1, so each held voltage of the noise turns the dual pot to a new place, a new random cutoff on every clock: the burble, from a filter whose every part you can see. Ned builds it, chip by chip, in the lesson A filter from two OTA chips (opens in a new tab).