EMS 8 Octave Filter


Overview In August 2020 I started building a replica of the “big Moog” owned by Klaus Schulze in the later 1970’s and recreating the same synth setup as he used in 1976. One of the more unusual pieces of kit Klaus had at this stage was an EMS 8 Octave Filter Bank. Whether this came with the Moog IIIP or when he swapped his EMS VCS3 with the AKS, we do not know.

It was usually placed on top of his Dual Moog Sequencer and it is not obvious which synth is being processed by the filter, possibly the ARP 2600 or the Big Moog. It was displaced by the set of PPG modules that Klaus bought in 1976, but was used on the Mirage album of 1977. I was curious to see if this filter could be recreated as they are too expensive and rare to buy secondhand.

EMS 8 Octave Filter

Original EMS introduced the Octave Bank around 1973 along with a random noise source as 1U racks in nice wooden cases. There are two versions; the original version uses a VCS3 like PCB card and connector, with a rear mounted power switch and VCS3 style Cliff knobs. The later version has a single large PCB, a toridial transformer, the same circuitry but a front mounted power switch, bypass switch and different (Sifam?) control knobs. The later version makes more use of polystyrene capacitors in the higher frequency bands, but both have tropical fish capacitors as well.

The circuit has 8 active Op Amp based (1458) band pass filters, set at octave frequencies from 63 Hz to 8kHz. The band pass filters are constructed from a 12 dB high pass filter followed by a 12dB low pass filter. Whilst there are no schematics, but there are photographs of the PCB inside the later version, and it has components values marked on the PCB. By tracing out the the PCB it is easy to recreate the design.

AMSynths Replica With the circuit decoded it was an easy job to recreate the schematics in Eagle CAD and lay out a PCB with the pots on board, to avoid hard wiring. I kept the power supply separate, and replicated the original front panel with Cliff KS knobs. I omitted the Facilities port which gave access to the individual and main outputs. I already had a nice Oak 1U 1U rack case, so this was reused. The main PCB is 285 mm x 100 mm, and its connects via DIL cables to the 10 1/4″ jack sockets on the right of the panel.

The metal 1U case is a standard Don Audio N-mod case with a front panel manufactured by Beta Layout. The panel design replicates the original EMS version. I ordered the PCB on 11 September.

 

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