Module A-199 is a spring reverb module. The reverb effect is electronically simulated by means of 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 similiar 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).
With the Mix control the relation between original and reverb signal appearing at the mix output is adjusted.
Using all these features very extreme and unusual effects can be generated with the A-199.HP : 8