The A-199 is a spring reverb module. The reverb is produced by sending the signal down 3 spiral springs. Spring reverb systems have a very characteristic sound that is based on the (insufficient) mechanical properties of the springs: like signal delays, audio resonances, limited frequency range, acoustic feedback behaviour, sensitivity to mechanical shocks and others. The 3-spring system used in the A-199 ensures a "dense" reverb because of the different properties of the three springs.
The A-199 implies some special features that are not self-evident for spring reverb units: The reverb signal can be fed back to the input using the Feedback control. Even self-oscillation of the springs similar to the self-oscillation of filters is available. The feedback loop can lead even via external modules like VCA, VCF, phaser, frequency shifter, vocoder, distortion/waveshaper, ring modulator and others. In this case the reverb output of the A-199 is connected to the input of the external module(s) and the output of the external module(s) is fed back to the Ext.Feedback In socket of the A-199. This socket contains a switch that interrupts the internal feedback loop as soon as a plug is inserted. Another feature is the Emphasis control. This enables the adjustment of the accentuation of middle frequencies (around ~ 2kHz).
Since 2007, the new version of the A-199 has a new smaller front panel and a detachable spring line tray, attached to the modules with a detachable lead.