Precision attenuverter / mixer

The word attenuverter, a combination of attenuator and inverter, has become pretty standard in the modular synth world.  The attenuverter does exactly what it's name suggests: it is a single control that can be used to attenuate and optionally invert a signal. Turn the control clockwise to get an attenuated version of the input signal, and counter-clockwise to get the inverted version. With the control at center position, the output is simply off.

Quad Attenuverter module, described in more detail at the end of this post

In it's basic form, the standard attenuverter circuit looks like this:

This is a clever little circuit. The opamp is configured with both inverting and non-inverting inputs, having the gain equation

Vout = 2a Vin - Vin,

where a is a number between 0 and 1 representing the potentiometer position. For a = 1 (fully CW), Vout = Vin and the circuit is just a voltage follower with some unused resitors (no current flows through either of the 100k resistors in this case). With a = 0 (fully CCW), the opamp's + input is grounded and we have a simple inverting amplifier with a gain of -1. With a = 0.5 (center position), the positive and negative gain cancel and Vout = 0.

Attenuverters are great for flexible modulation routing. However, it is this center position where they can be very annoying: it can be difficult to dial in a small amount of modulation. Turn it down too far, and you end up increasing it again on the inverting side.

The precision attenuverter is designed to help with this problem. By adding load resistors to the potentiometer, the response curve is made non-linear, increasing the sensitivity near the center. This idea is not new, but rather an extension of a common technique to create quasi-logarithmic behavior with linear potentiometers, and I wouldn't be surprised if others have also applied it to attenuverters.

The precision attenuverter circuit is as follows:

Note that the potentiometer itself still is a linear type. Depending on the choice of Rs, the response curve of the total circuit can be chosen more or less nonlinear, as shown in the graph below. I typicaly choose Rs = 47k, which gives about double the sensitivity in the center region compared to a linear attenuverter.

Quad precision attenuverter and mixer

I designed a PCB in Eurorack format with 4 of these precision attenuverters and an integrated unity gain mixer. The attenuverters have individual outputs, and the mixer takes its inputs from the attenuverter output jacks normal switches. Hence, by default the Sum output contains the sum of all attenuverter outputs. However, if a cable is plugged to an individual attenuverter output, that attenuverter is removed from the mix. Finally, there is a unity-gain Aux input to combine for example mutliple mixing modules together.

This PCB is my first endevour in surface mount technology, with 0805 passives and SOIC opamps. Soldering the SMD parts is quick and easy, faster than filling a through-hole board once you get used to it. I included space for trimmer potentiometers, which are useful only if you use potentiometers with center detent to set the gain to exactly zero on center, but I did not mount these in my own modules.

Update August 2019: PCB version 1.1 is now available. It now features a 5V reference voltage, which is normalled to the first input jack. Hence, the first knob can be used as an offset voltage if nothing is connected to it's input. If desired the other input jacks can be also be connected to the 5V reference with solder jumpers on the PCB. The front panel layout has not changed with this revision.

Build documentation

I offer the PCB for this module for sale. The build documentation below contains everything you need to know to complete a successful module. Please read it through carefully before starting, and especially check the notes in the Bill of Materials when ordering parts.

• Build documentation (github)