AeDiy / TheMASTERModule

The MASTER module

The master module is an important element in an AEM system, since it interfaces to the outside world, so it is worthy of further discussion.


The master module supplies the regulated 5V from one of 2 sources:

  • DC Jack with 9V DC, positive centre, minimum 1 amp.
  • Eurorack via the Eurorack connector (+12V, or 5V?)

The Master module (and the smaller 1U Power module) can supply up to 800mA without any problems. This is more than sufficient for 4 full width rows.

(check: max current consider, only PSU, or also regulator ?)


  • AUD.1|2 - audio input or output
  • CTRL 1|2 - cv input or output
  • BUS CV (2) - MIDI note on/off pitch (v/oct)
  • BUS GATE - MIDI on/off
  • BUS CTRL - MIDI cc 20

Audio Interface

There are two audio I/O 3.5mm sockets.

These are both bi-directional, so can be used either as input or output.

The input and output can be up to 5Vpp, outside this it will be clipped.

(check: DC coupled?) (check: additional filtering?) (check: Vpp , I thought it's not 5Vpp, but much lower 3Vpp?)

CV Interface

There are two CV IO 3.5mm sockets used for Eurorack connectivity. These are both bi-directional, so can be used either as input or output. The input and output can be up to 5Vpp, outside this it will be clipped.

The primary difference between this and the audio IO is these are DC coupled. You can use the CV I/O for audio input too.

(check: does a bipolar/audio get offset to 2.5V)

Midi Interface

Serial MIDI is provided via a 3.5mm jack, with a simple MIDI DIN converter. It is input only (unidirectional) and monophonic

The following midi messages are understood:

  • Note on/off -> Bus CV & Bus GATE
  • MIDI CC 20 -> Bus CTRL
  • MIDI Clock -> Bus CLK
  • MIDI Transport start-> Bus START
  • MIDI Transport stop-> Bus STOP

All these signals are present on the bus ribbon, even though CV, GATE and CTRL are the only ones present on the master IO header. (CTRL is present on v2+)

Racklink module

If you already have a Master module in one AEM rack, and wish to extend to another rack, then you can use the Racklink module as an alternative to a master module in the second rack.

The Racklink connection is actually 2 Racklink modules, and a ribbon cable (20 pins). One racklink modules goes in the ‘master’ rack (the one with the master module) and the other in the slave rack. So this uses 1 U in each rack. Each Racklink is then connected to the bus cable in the rack (so is the source of the bus for the slave rack)

The ribbon cable links the normal ‘bus’ connections, so power + signals, and also 8 additional signals which you can use as you wish (the connection is bi-directional, and can be used for both CV and Audio)

Additionally, the Racklink module has a dip switch bank to turn off the connection for some or all of the normal bus signals.

This has 2 use cases:

  • If you have a master module in both cases, and want to use Racklink to just send the 8 additional signals and ground(!)
  • If you don't have a master module in the slave case, and would like the Racklink to just supply power, ground and the 8 additional signals, this would mean that in the slave rack you could also use the CV bus as you wish (since there is no master sourcing the signals)

Notes on connectivity


  • Pro audio line level ~ 3.4Vpp
  • Consumer line level ~ 0.9Vpp


Whilst Eurorack modules/racks interact very well, the Eurorack standards are fairly ‘loose’ and open to interpretation - so variations exist. Doepfer introduced the Eurorack standard, it's the closest there is to a specification, See here for their ‘specification’ -

Audio voltages - up to +/- 10V, but are more commonly +/-5V, this means you will want to attenuate before bring into AEM to avoid clipping/distortion.

CV general - theoretically +/-10V, which you need to attenuate, however, often its limited to either unipolar 5V (0..5V) or bipolar (-5V to +5V), outside 0..5V, AEM CV inputs will clip. LFOs vary, some are bipolar, some are unipolar, some switch, often 5V. Envelopes usually unipolar 5V, but some may have an inverse env, which could be 0..5V or 0..-5V. (check: lfo/env ranges)

CV pitch - 1V/octave, but range varies. Fairly common is 0V..5V, which allows for 5 octaves, others extend to give a greater range e.g. Mutable use -2V to +5V.

CV gate / trig - voltage above +3V are considered high

(check: Eurorack connector specifics - pinout?)


When connecting different systems, be it Eurorack or 5V systems, you need to share a common ground to ensure voltages share a common reference. Since the ‘Sleeve’ of a 3.5mm Jack is ground, this can be done by connecting systems with at least one 3.5mm Jack (doesn’t matter if its audio/cv or from master or 4IO all are the same). If you do not want to do this for some reason, you can take the ground from the bus ribbon. As most uses 3.5mm patch cables to connect AEM to other systems this is normally not an issue, it only becomes relevant if you start patching patch wires directly which only carry signals not ground.

Do I need a master module?

Theoretically no, but see the above information for why you might want to use one.

If you can supply a regulated 5V + GND supply, you could connect these directly to modules. If you don’t have a master module connected, you can then freely use the other bus signals for your own use.

For audio output you would need to convert from unipolar 0..5V to bipolar -2.5V to 2.5V. You could use a 4IO module for this.

NOTE: if you already have one master module and this is for a new case, an alternative is the Racklink module.

Page last modified on June 30, 2020, at 06:55 PM
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