Eurorack modules are powered from +12V and -12V bus rails

All Doepfer A-100 (Eurorack) modules require +12 V and -12 V from the system bus; a few older modules additionally need +5 V (generated by an A-100AD5 adapter or supplied directly by the A-100PSU3). Three generations of Doepfer power supplies cover the format’s history, differing in max current (e.g. the A-100PSU3 gives +12 V/2 A, -12 V/1.2 A, +5 V/4 A). Bus cables are keyed by convention, not by a physical key: the colored wire marks -12 V and must always face the bottom of the bus board (‘red strip down’). Misconnection or reversed polarity destroys the module instantly. This matters when budgeting current for a system and when mixing module generations.

Examples

Building a system with 20 modules averaging 50 mA each on +12 V totals 1 A — within spec. Adding a power-hungry digital module that draws 500 mA could push the total over an older supply’s limit.

Assessment

Which two rails power every A-100 module, and which modules need a third? What is the ‘red strip down’ rule and why does it matter?