The original Moog 905 Reverberation Unit design dates back to the late 1960’s and is a simple four transistor circuit that drives a traditional Gibbs (then Accutronics) Spring Reverb Tank actually mounted inside the module! It is a medium delay time (1.75 to 3s).
With just one control potentiometer for wet/dry MIX, I am sure this basic (and noisy, hum prone) reverb did not impress Klaus Schulze as he quickly replaced it and used a high quality AKG Reverb and then Dynacord digital reverbs. However other electronic musicians, such as Keith Emerson, did embrace the Moog 905 spring reverb as part of their sound and with some modern upgrades it is a great sounding module!
AMSynths AM905 There is no room for a spring reverb in a EuroRack module or even the case, so this is a modern digital reverb that uses the Belton/Accutronics BTDR-3H. The reverb “brick” just fits in the 8HP wide module and is surrounding by Op Amps buffers to bring the signal down and then back up to the right levels.
The design keeps to the original Moog theme of simplicity, so tone controls or feedback control have not been added, but there is an input level control and the decay time can be adjusted from 0.5s to 3.2s. The module has dual audio outputs which are out of phase with each other and create a pseudo stereo effect.
The AM905 module has been carefully designed to be compatible with the lower signal levels of a Moog Modular and its replicas.
Outcomes and Availability A prototype was built in November 2020 with the BTRD-2H but it never made it into production because I did not like the panel which had white silk screen lettering.
I updated the module with the BTDR-3H in January 2025, and used SMD assembly with a new etched black anodised aluminum panel, and Moog style control knobs. The result is a very nice spring reverb for the Modular III system.
