Eurorack · 5HP · sigterm.co
Megaphone connects a vintage GPO rotary telephone to a Eurorack case. Lift the receiver and the module wakes. Tap the hook to choose what you are setting. Then dial it - a tempo, a voltage, or your own voice - and hang up to commit.
The idea
A rotary dial is a precision pulse generator. It was engineered to send a countable number of clean electrical breaks down a wire, and it has been doing that reliably since before modular synthesisers existed. The hook switch is a mode selector. The carbon microphone is an instrument input. All of it is passive, mechanical, and still works.
Megaphone reads those three contacts and turns them into modular control. An ARM Cortex-M7 counts the pulses, and the outputs are proper Eurorack clocks, control voltages and line-level audio. The phone is not on a network. It is an instrument.
Playing it
The interaction is the one the phone was built for. Nothing to learn, nothing on screen to navigate.
The module wakes from idle. The display comes up showing the current mode.
Each tap steps through the five modes and wraps back round to the first.
One rotary digit at a time, with a brief pause between them. A tempo, or a voltage.
That commits it. The output starts immediately and holds until you change it.
Five modes
No menus and no shift functions. Each mode owns a jack, and dialling 999 always clears it.
The number of digits you dial decides the resolution.
Signal path
The phone presents a single carbon-mic line carrying three things at once - the audio, the hook state and the dial pulses. Megaphone separates them and sends each where it belongs.
The instruments
These are working GPO handsets, sourced individually and checked over before they go out. They are not decoration - the phone is the control surface, and each one dials with its own particular feel.
A striking blue variant of the 746, made in 1962 and originally installed in Sherborne. Bluebell is a rare wall-mounted unit, so she will sit happily on a bracket beside the rack rather than taking up bench space.
Fire-engine red, rugged and dependable, with a tight finger dial and a sharp ring. Ringo is built for the stage as much as the studio - keen to dial in awkward polyrhythms and put your voice somewhere it has not been before.
Already own a rotary phone? A GPO 746 or 8746 works natively with a standard BT-to-RJ11 adapter. A 706 needs rewiring to BT standard first. Other vintage phones will often work with a little calibration.
Specifications
Clock outputs are 3.3 V, not 5 V. Modern modules with Schmitt-trigger inputs - Doepfer, Mutable, Make Noise - trigger reliably on that. Older modules with a higher comparator threshold, or very long patch runs, may occasionally miss a trigger.
CV is unipolar, 0–10 V. Ideal for pitch and for unipolar modulation. If you specifically want a bipolar ±5 V source for panning, this is not that - yet.
The bell does not ring. The phone is wired for its passive contacts only. Ringing the bell needs around 90 V AC, which has no business inside a Eurorack case.
Enquire
Each module is assembled, tested and paired with a phone before it goes anywhere. If you would like one - with a handset or without - send a note and tell me a little about your rack. I will reply personally with what is available and what it would cost.