Update : may 13th, 2007

Mini-Maxi Voltage Processor

En français
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The purpose of this module is to process multiple sources in order to retain two signals which are respectively the minimum and the maximum of the input signals. Basically, each submodule has two inputs that can be fed with either CVs or audio signals, and two outputs, one is the maximum value and the second is the minimum value. The two submodules can be chained to achieved more complex functions or combine more than two inputs. 
The design of such a min-max processor can be addressed by two different approaches : current processing (see reference at the end of the page) or voltage processing. As far as I am concerned, I preferred using the voltage one. This way the min-max processor can be built with elementary blocks such as adders, subtractors and full-wave rectifiers which only involve ordinary components (OPAs, resistors and diodes).
This circuit is mainly dedicated to processing control voltages, as an example it can be used to creat a new sequences out of the outputs of two sequencers. The outputs of the MIN-MAX modules are for one the highest pitches of both original sequences and for the second the lowest pitches... In the second case, it extracts the bass line out of the original sequences.
Also it can be used as a nice wave modifier if you feed input one with a sinewave or the triangle signal of a VCO and connect input two to a LFO, a ADSR or another VCO...

The max function or MAX(a,b) can be simulated with addition, subtraction and absolute value operators, it reads :

The min function or MIN(a,b) reads :

The absolute value operator is obtained using a full wave rectifier !

Besides, as a default the inputs are grounded, therefore if one plugs a signal in an input and nothing in the other, the signals collected at the MIN and MAX outputs are the negative and positive halves of the signal, respectively.

Following is the result of a spice simulation of the circuit based on my

Spice3 simulation

Input signals

Output signals

Schematic diagram

*R8, R12, R17, R29, R33 and R38 are 50k, in fact the PCB has been designed such that you can either use a single 50k 1% resistor or two 100k 1% resistors connected in parallel.

Printed circuit board and component layout


Component layout

Download the schematic as a PDF file

Download the PCB as a PDF file

Building instructions

LM324 or TL074
470 5% (off board)
50K 1% (or two 100k 1% in parallel)
R1 to R7,R9 to R12,R14,R15,R18, R19 to R28,R30 to R34,
100K 1%
JK1 to JK8
jack socket
SPDT switch

The 470 ohm resistor are connected offboard

Front panel
Panel design

Download the silkscreen mask as a PDF file

Download the silkscreen mask as a JPEG file

Settings and trimming

This circuit requires no trimming, it must work as is.


J. Castillo, J.L. Pérez, Instrumentation and Development, Vol 3 Nr 9, 1998:43-55

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