905 Reverb

Live Rig 1976

Introduction In August 2020 I bought a set of Behringer System 55 modules, as my first journey into Moog modular. My plan is to recreate the Klaus Schulze Moog IIIP synthesizer and Dual Sequencer setup that Klaus bought on 22nd December 1975, and used successfully for many years.

The Behringer System 55 provides a great basis for developing a replica of the Klaus Schulze “Big Moog”, but there are some modules missing such as the 905 Reverb. I decided to make one.

Klaus quickly removed his Moog 905 and replaced it with a very useful frequency tuner in 1976. He also used a more sophisticated (studio quality) AKG B15 dual channel spring reverb, and then moved to a Dynacord DRS78 digital echo and reverb in 1978.

I am using digital reverb and delays from 1984 with my Klaus Moog, but I still wanted the classic sound of a spring reverb in the modular system.

Moog 905

Original 905 The Moog 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 use one control potentiometer for wet/dry MIX, I am sure this basic (and noisy, hum prone) reverb did not impress Klaus! However other electronic musicians, such as Keith Emerson, did embrace the 905 spring reverb as part of their sound.

AMSynths 905 There is no room for a spring reverb in a EuroRack module or even the case, so I have used a modern digital version – the Belton BTDR-2H. 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.

AM905 Reverb – Rear

I have restrained myself from designing lots of new features such as tone controls or feedback and just added a second output to my module. The circuit fits on one PCB and is held to the front panel with spacers. The prototype PCB was ordered on 01 November 2020 and the module quickly built and tested perfectly.

Outcomes The BTDR-2H is more impressive than I was expecting and this is definitely a useful module, with the clanking and noise of a true spring reverb. I am using the Long (L) version of the reverb and it provides 2.85 seconds of reverb time which is similar to the original.

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