mxm-folded-spring

Sound

A spring tank sends sound down metal springs and picks it up at the other end. Different lengths return at different times, and the high frequencies move more slowly through the spring than the lows. The spread and dispersion make the familiar boing.

mxm-folded-spring uses the Roland System-100 spring reverb code already inside mxm-mono-00. The standalone effect added tank choice without changing the instrument’s original render. A digest captured before that change still matches.

Medium is the reference tank. It uses three spring transits at 41.6, 50.4, and 60.0 milliseconds, with a 3.5-second decay and a narrow midrange drive. Short scales that model to two brighter springs and a quicker decay. Long stretches it to three darker springs that ring for almost six seconds. Short and Long are chosen variations, not measurements of other boxes.

Feedback changes the circuit

Level is the wet return. Tank chooses Short, Medium, or Long. Feedback sends the tank output back into its input.

That feedback path was not part of the System-100 mixer. It is a deliberate addition drawn from dub routing and spring modules that put the return under a performer’s hand. Most of the control lengthens and darkens the tail. The top tenth can hold a tone with no new input.

The loop passes through the tank’s band limit on every lap. Its gain falls as the return gets louder, which keeps the self-oscillation bounded without turning the spring into a clipped square wave. Each tank is calibrated to sing at the same place on the knob even though their loop gains differ.

Interface

The editor uses one card. Level and Feedback sit beside a three-position Tank selector. The display draws every spring on one fixed time scale, so a longer line always means a longer transit. It brightens from the wet signal the audio thread has just produced.

Changing tanks cannot be a normal crossfade. The old delay lines contain returns timed for a different set of springs. The plugin fades the wet signal out, swaps the model at silence, then fades it back in. Dry audio stays untouched.

Level at zero is Off. The tank empties instead of freezing, then the DSP stops running. Raising Level cannot release audio stored ten seconds earlier.

Presets

Four factory patches ship with the effect: one for each tank and Regenerating, where the long tank feeds itself. Init uses Medium with Feedback at zero, which is the spring circuit already used inside mxm-mono-00.

User presets use the shared browser. The Tank names describe their length rather than borrowing product names from hardware that was never measured.

Status

In stereo, the two channels sum into one tank and the mono return is added to both sides. Each dry channel stays where it was.