DIY Mutable Instruments

Mutable Instruments Rings: Should the SJ1 pads be bridged?

So you want to DIY a Mutable Instruments Module

This is going to be the first in what I anticipate will be a series of posts I’m tentatively titling ‘So you want to DIY a Mutable Instruments Module’. When I started designing Eurorack modules I decided to put together a few DIY modules to learn about common components and design patterns. The modules I picked to DIY were designed by Mutable Instruments. Mutable Instruments has designed a line of top quality Eurorack modules, and because they’re better people than I am, all of their designs are open source. That’s right schematics, bills of materials, PCB layouts, cover panel designs, and software source files are all easily downloadable from GitHub. I generally consider the documentation to be quite good. However there always seems to be something fiddly that doesn’t work quite right for the first one you assemble. And I’m going to discuss the solutions to the questions and problems I had while I was assembling the modules.

Back to the actual topic, the solder jumper on Rings

On the Rings module there is a solder jumper, SJ1, with no clear indication if it is supposed to be shorted or left open. Since the pad is wedged between two mono jacks it is best to pick its state during assembly. What should we pick?

Short version: short the jumper

The longer version:

To answer this question we need to know two things: What does SJ1 do? How is this module expected to function when it passes QC at Mutable? The first question should be pretty easy to answer; we’ll just take a quick look at the circuit schematic.

This section of the schematic covers SJ1, IC8 (output opamp) J9 (Odd output), and J10 (Even output). SJ1 connects the switched pin on Odd output to the amplifier mix for the Even output. Interestingly there is no solder jumper in the connection between the switched pin on the Even output and the Odd amplifier mix. So SJ1 causes the Odd output to be mixed with the even output if no cable is connected to the Odd jack.

Is this desired behavior? Fortunately we’re in luck and this is covered in the official manual for Rings no digging required.

Mutable Instruments rings manual

5. Odd and even audio outputs. In monophonic mode, these two outputs carries two complementary components of the signal (odd and even numbered partials with the modal resonator, dephased components due to picking position and pickup placement with the string resonators). In polyphonic mode, splits the signal into odd and even numbered strings/plates. Note that you need to insert a jack into each output to split the signals: when only one jack is inserted, both signals are mixed together.

The final sentence tells us what we need to know. “Note that you need to insert a jack into each output to split the signals: when only one jack is inserted both signals are mixed together.” So the factory default is both signals are mixed when only one output jack is used therefore we need to bridge the solder jumper SJ1.

Equipment Fix

Heathkit IP-2718 Fix

A few weeks back I acquired a Heathkit IP-2718 triple output power supply. I fired it up and discovered that only the fixed 5 V output worked, and the two variable outputs were stuck at 0 V. So I did the traditional dance of my people.

This particular power supply is from that magic time between vacuum tubes and ICs when transistors reigned supreme. So I didn’t have to try and find a replacement for an obsolete IC or a binary for a microcontroller.

Inside the Heathkit IP-2718
The inside

After opening the case it didn’t take long to spot the problem. Six power resistors had gone bad.

Since resistors don’t tend to fail in isolation. That is frequently some other component fails and causes the resistor to dissipate power in excess of it’s rating and thus release the magic smoke. I checked the capacitors and diodes for shorts and they appeared ok. The transistors appeared ok as well. Since all of these parts were probed while attached to the board it’s hard to say for sure. So I got some replacement resistors. Pulled out the bad resistors and replaced them. Fired up the power supply and everything seems to be working fine.

In this close up of the bad resistors it’s easier to see the cracking and discoloration. Also note that the new green resistor on the right is smaller and has a higher power rating. I’m not exactly sure how that trick was managed. I’m guessing it’s 40 odd years of materials and process tweaks.

Bad resistors (left) next to a replacement (right)

One of the most interesting parts of this repair for me was seeing the traces on the PCB.

PCB Close up

The copper tracks on the board are wild, almost artistic. compared to modern PCBs where CAD software has made all the tracks uniform width and fine pitch packaging has caused them to get packed as close together as possible. Like this one

Modern PCB from Wikimedia,
Modern PCB, Courtesy of Wikimedia


Power Supply Noise Non-Analysis

When I started designing modules I decided it’d be handy to have a case to demo modules in. So naturally I decided to build one.

Not too long ago the power supply for this case arrived, and I thought, ‘wouldn’t it be great to compare the ripple of the new supply to the ripple of the ATX power supply I’ve been using to test modules.’

The ATX supply

Let’s start by looking at the ATX supply.

Where I out myself as a Rhode & Schwartz user

Wow look at all the garbage on there. Spikes all over the place. There’s so much high frequency noise that the oscillations blend together to form what looks like a solid 22 mV wide line. So what, if any of this, is coming from the power supply? Let’s start by taking a closer look at the high frequency noise. Enhance!

It’s funny because Oscopes over sample unlike cameras.

Well the high frequency noise looks suspiciously periodic. But with a frequency of 95.2 MHz, I’m pretty sure that’s a local radio station, not my power supply. How about the bigger spikey bits?

The frequency (~380 kHz) on these artefacts is a lot warmer, but they’re not the right shape to be power supply ripple. Power supply ripple should be triangular; this looks more like switch ringing. And sure enough just about all of the spikes go away when I turn off my bench lights, which are dimmable LEDs so they’re being switched on and off at some ridiculous rate.

Bench lights on
Bench lights off

But what’s this with those big spikes from the lights gone it’s easy to see an 85 kHz triangle wave. This could be power supply ripple; it’s impossible to know for sure without knowing more about the supply circuit. And the magnitude is *drum roll* 31 mV; barely larger than the background radio noise. nothing to be concerned about.

New Case supply

Now that we’ve established how little ripple there is with the ATX supply it’s time to fire up the new case supply and see what happens. I did hook it up to a few bus boards before taking measurements because who cares about consistency in experimentation.

Great, turns out a few bus boards more than tripled the background noise level to 70 mV. ~30 mV of ripple will easily be lost in this. At this point I stopped the experiment having determined that in order to meaningfully distinguish between the ripple of the two power supplies I would need to dig my power resistors up from wherever I hid them and try to limit background noise.

Parting thoughts

For me the most enlightening part of this to note that the power supply ripple is dwarfed in magnitude by ambient noise sources. Does this mean everyone with a mod synth should go buy a faraday cage. No, a few 10s mV ripple can be easily filtered by board level supply caps. And since the lowest frequency we looked at today was 80 kHz it’ll get filtered by our ears if nothing else. And we haven’t even started talking about the ambient noise suppression of all those op-amps in modules.

Is this a vote for or against using an ATX power supply to power synth modules? Neither, my understanding of why Eurorack systems shy away from using ATX power supplies has more to do with the current rating of the -12 V rail and the extraneous 3.3 V rail which isn’t in the Eurorack specification. Here’s the rating sticker from the supply I’m using.

300 mA of current at -12V is pretty reasonable for a small rack, but an ATX power supply would take up a substantial amount of space in a small case. So it’s usually a better idea to get a dedicated Eurorack case power supply. For a bigger system an ATX can’t supply enough current at -12 V to be useful.

LRR Eurorack Modules

Mid Side Module

This Eurorack module enables mid side processing. The top portion of the module converts left and right stereo signals into mid and side components for filtering or other effects. The bottom portion of the module converts the mid and side signals back into left and right stereo components. Mid and Side out are internally connected to Mid and Side in respectively (normalized). These connections are severed if cables are connected to the Mid and Side inputs respectively. The module uses professional audio quality amplifiers which are both fast and have low noise insertion so the module imparts minimal color to the processed signals.


Width4 HP 
Depth54.5 mm
Power0.38 W
+12 V16 mA
-12 V16 mA

Channel Definitions

L<Left in (Lin)
R<Right in (Rin)
M>Mid out (Lin + Rin)/2
S>Side out (Lin– Rin)/2
M<Mid in (Min)
S<Side in (Sin)
L>Left out (Min + Sin)
R>Right out (Min – Sin)


The module uses a standard 10 pin eurorack connector with polarity marked. 

Filter ideas

Uniform Filtering

Separate Mid and Side apply similar filters to both channels before remixing. This minimizes unwanted distortions in left and right channels from using filters that aren’t perfectly matched.

Center the base

Separate mid and side, then apply a high pass filter to the side channel before remixing to left and right. 

Add space

Separate Mid and Side, then apply reverb to the side channel, before remixing to left and right

Control Voltage Mixing

Half Differential+