An electrical loudspeaker at one end of the unit makes the springs wobble; at the other end of the unit a circuit converts the wobbles back into sound, but because the signal has been wobbled with springs the sound is wobbly. Wobbly in a complex way. That's how a spring reverb works.
The Doepfer A-199 comes in two parts. There's the faceplate, which has the controls and power supply, plus a separate reverb tank. They connect up with some cables.
Here are some isolated examples. In the first sound clip I play a loop with my Behringer RD-8 drum machine, gradually turning up the effect volume. Mid-way through the track I introduce some more effects in order to show how the spring reverb can be used as part of a mix:
In the second example I'm using the A-199's feedback input to send the signal into a filter, then back into the unit again:
The result is a distinctive metallic sproing sound. Ordinarily the feedback circuit feeds the reverb signal back into itself. You'd expect this to create an enormously long reverb signal, but disappointingly it just makes a feedback howl. In moderation the howl is soothing, but it gets old quickly. The other control is emphasis, which boosts the mid-range a little bit.
In this video I mess around with the unit for five minutes while playing a bassline through it.
In this video I use it in a piece of music. It's part of the effects chain for the swoopy noise that flies around the other instruments. The sound is coming from a mixture of Plaits and a Behringer TD3, fed through a spring reverb, then a filter and some digital reverb:
Surprisingly for a modular unit the A-199 doesn't have any voltage controls. If you want to modulate the reverb you'll have to feed the signal through a mixer or filter, and modulate that instead.
Of note my A-199 is the third model. The original A-199 was 10hp wide, with the "feedback" and "emphasis" labels written out in full. The second model slimmed the panel down to 8hp, and the third model added jack plugs instead of soldered connectors for the spring tank. If you're listening, Dieter Doepfer, I would be very grateful if you could add a little cut-out in the front panel - perhaps with a plastic tab, or a rubber grommet, or a flap or something - so that the cables can route out of the case.
Does the A-199 make any sense? At low levels it works perfectly well purely as a means of thickening the sound without changing its character. At higher levels it sounds lonely, distant, metallic, like the bits of a Godspeed! You Black Emperor song in between the violins. At 8hp it's not excessively big, although the need to find somewhere to put the tank is awkward. It's not particularly expensive.
On the other hand almost every digital reverb unit made in the last forty years has a spring reverb simulation, and the howling sproinging crashing noises that are characteristic of spring reverbs are a novelty that quickly wears off. Nonetheless there's something psychologically appealing about having actual springs. Actual physical springs. Just like the BBC Radiophonics Workshop.
Actual. Physical. Springs. And that's the Doepfer A-199.