mm33 mods…

The mm33 matrix mixer is designed to perform the mixing of audio or control signals without too many ‘bells and whistles’. Sometimes, however, it might be required that something different is needed, and that’s where this page comes in. We’re going to list a few things that you can change if you like — the intention is that this page might grow over time…

Gain Mod

One thing that the mm33 provides (unlike a passive matrix mixer) is the ability to amplify the signal as it leaves the mixer. By design, the mm33 has a gain of just over 1 so that an input signal can appear at its maximum amplitude at the selected output, but sometimes — particularly when using feedback loops — it might be desired that the signal gets boosted a little more…

The gain of each output (X, Y, or Z) is defined by three pairs of resistors: R2 & R1 for the X output; R5 & R6 for the Y output, and R3 & R4 for the Z output. The ratio of these determines the gain for each output (since there are three inputs, the calculated gain is also divided by three — take a look at the section headed ‘Non-inverting Summing Amplifier’ on this page if you want to see the maths!).

The upshot of this is that if you change the 220k resistor (R1, R6 or R4) for a larger value then the gain can be increased. For example:

GainResistor Value
1 (default)220k
2510k
3820k
41M2
51M5
Increase the resistor value by around 300k for each integer increase in gain!

Input Decoupling

The ‘a’ inputs of the mm33 are for audio signals, and therefore have DC blocking capacitors inline (this is particularly important when dealing with patches containing feedback as the DC values can quickly reach the maximum of 5v which will stop any further audio until the mix is reduced). The capacitors form high-pass filters with a default cutoff frequency of something less than 1Hz, but it might be that you would like to change this (since audio frequencies don’t start until 20Hz).

Changing the values of C1, C2 and C3 to smaller values will increase the cutoff frequency of the high-pass filter:

CapacitorCutoff Frequency
2.2µF (default)0.7Hz
220nF7Hz
82nF19.4Hz

Knob Pointer

As supplied, the position indicator on the mm33 controls is a simple indent — it is of course possible to make this more visible by using a white ‘paint marker’ like we use to write serial numbers on the PCBs themselves. These pens from Pilot seem to work well…