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SUMIX is a modular audio summing/mixing system for modular synthesizers, initially targetted at the Eurorack format. It provides a range of facilities, features, and specifications comparable to mid-range professional audio mixing consoles, with specific additional features to match the modular synthesizer environment and usage.

The original SUMIX idea was for a monolithic 5-input stereo mixer. But this seemed to be an anethema within the context of a modular synthesizer! So while the original idea has been shelved/cancelled, the idea of an audio mixing system for the modular synthesizer lives on.

Goals and Objectives

First and foremost SUMIX is designed to be an open system, in that the full specs of the summing bus are published for all - DIY and commercial - to use freely.

Secondly, SUMIX should provide a low-noise and low distortion signal path from channel inputs to master outputs. Within the context of the modular synthesizer this requires careful attention to circuit topology, component choice, and architectural decisions.

Thirdly, a SUMIX system must be able to grow as the synthesizer grows. Growth both in terms of the original housing, and on to additional cabinets, even to modular synthesizers of differing formats.

Fourthly, SUMIX is designed to be cost-effective. Standalone mixing desks achieve fantastic economies of scale due to the tightly-integrated nature of the design. A modular mixer is not so blessed, yet customers remain cost-conscious. There is a careful trade-off between the convenience and cost of an integrated audio mixing system compared to an external mixing desk.

My hope is that SUMIX achieves all this.


System Architecture - The various components and how they can fit together

Specifications - Detailed electrical and mechanical information

02.04.2016. 23:20

System Architecture

SUMIX is designed to be a flexible audio mixing system. To progress the design the following system architectures are presented. They serve two purposes: they provide concrete examples to further discussion and analysis, and they provide a starting point for exploration of alternative structures and applications.

The SUMIX system comprises: four aux buses, four mix buses, mono PFL, stereo master buses (Left/Right). All buses are balanced to minimise interference both from coupling into the bus lines and from common impedances in the 0V rail (a well-known problem in the Eurorack modular synthesizer format).


A mono bus system treats the four buses as separate mono channels. Each channel can have different audio content, and can be placed in a different position in the stereo field.

(Click for a larger version, opens in another window)
SUMIX System Diagram

CHANIN is the traditional channel strip, tailored for SUMIX. It follows the usual signal path: input preamp, optional EQ, pre- and post-auxes, PFL, main fader, mute, and bus assignment.

AUXIO handles a single AUX bus, providing an insert point, PFL, fader, mute, and bus assignment.

BUSIO performs a similar function to AUXIO but focussed on the specific duties of directing the four mono buses to the Left/Right master buses. Instead of a bus assignment following mute BUSIO provides a pan control.

There are two output blocks. STOUT is the master output module, providing a stereo fader and balanced output drivers for feeding balanced lines to external equipment (mixing desk, amplifier, etc). PHONS performs two functions: it provides the PFL function - switching headphone monitoring to whatever source(s) are feeding the PFL bus - and it provides a stereo headphone amplifier.

Finally, the EXTENDER allows for expanding the SUMIX buses across multiple systems/cases/cabinets/rooms/etc.


* Should AUXIO and/or BUSIO have their own EQ sections? Perhaps simpler hi/lo shelving?
* Where to add automation? Channel faders? Pans? Mutes?
* Level monitoring: could put small (say, four segment) VU meter on CHANIN, and larger ten-segment VU meters on STOUT.


A stereo bus system pairs the four mix buses as two stereo buses. Each stereo bus can have different audio content, with content positioned in the stereo field at the source (either channel input or aux I/O), with the BUSIO modules providing insert, level and mute only.

(Click for a larger version, opens in another window)
SUMIX System Diagram

BUSIO takes one of a pair of buses, representing left and right stereo channels. It provides a stereo insert, but loses the pan control since that function is now controlled by the audio sources.

There is the potential for a stereo version of CHANIN for handling stereo material. Again, no pan required since this would be determined by the source. There is also the matter of supporting stereo EQ, which is difficult if the stereo position is not to be compromised.


Perhaps the ultimate application of SUMIX, we treat the four buses as one quadraphonic mixing bus.

19.05.2016. 23:07



Note: Specifications subject to change, especially those marked "TBD"

Audio/control signals only. No power.
Audio signals onto virtual earth busses.
All audio busses balanced.

Audio signals:
10k bus resistors (0.1% tolerance or better)
-4dBu nominal feed level into bus resistors

PFL Sense
Connect sense lines together with 1k0 resistor for PFL active

Main buses: L, R, 1, 2, 3, 4
Aux buses: 1, 2, 3, 4
PFL: audio, sense
Other: REF0 (common reference across all modules)

26-way IDC header, gold-plated contacts, latching, right-angle
Support for 25-way D-sub for inter-cabinet bussing (wire #26 not connected)

26-way 0.05in-pitch PVC-insulated IDC ribbon, flat-n-twist (e.g., Amphenol SpectraStrip part 132-2801-026 or 3M part 1700/26)


Output impedance: < 60 Ohms
Frequency response (line input to main output): 10Hz to 40kHz (+0, -0.5dB) @ 0dB gain
THD+N @ +4dBu Output (line input to main output @ 0dB gain): < 0.005% (@ 1kHz)
Noise : Residual noise (main output fader at 0): < -90 dBu
Bus noise (8 input channels, muted, main output @ 0dB): < -80 dBu
Crosstalk/bleedthrough (all @ 20kHz): Adjacent channel: < -90 dB
Channel muted: < -90 dB
Channel to unassigned bus: < -90 dB
Phase : line in to main out: +30 to -30 degrees (20Hz to 20kHz)


Line input : -27dBu to +13dBu nominal
Aux Send : +4 dBu nominal
Bus Insert : +4 dBu nominal send/return
Maximum input : Channel input: +20dBu (10Vpk-pk sine)
Maximum output : Main output: > +18 dBu
Aux send: > +18 dBu

Backbone Signal Assignment

1PFLSENSE-ABridge with 1k0 to enable PFL
3PFLSIG+PFL audio send (mono)
13B1+Bus #1
15B2+Bus #2
17B3+Bus #3
19B4+Bus #4
21R+Main R
23L+Main L
26REF0 [1]

[1] - Not carried across when using 25-way D-SUB connectors for inter-cabinet links

02.04.2016. 23:41