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 first built the 8 Octave Filter as part of the Synthi 100, where it was located in the second panel with large blue control knobs. In April 1971 EMS introduced a series of 1U Rack units derived from the Synthi 100 at £150 each;

  • Pitch to Voltage Converter
  • Triple Slew Limiter – voltage controlled
  • Eight Octave Filter Bank
  • Random Voltage Generator

They were housed in rather nice 1U racks with wooden afrormosia (African Teak) cases.  There are two versions of the Filter Bank; the original version uses a VCS3 like PCB card and connector (derived from the Synthi 100 cards), with a rear mounted power switch, traditional chassis transformer and VCS3 style Cliff K1 control 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 polyester capacitors as well.

EMS Inside

Original Circuit The EMS circuit has 8 active Op Amp based (1458) band pass filters, set at octave frequencies from 63 Hz to 8kHz, accurate to within 10%. The band pass filters are constructed from a 12 dB high pass filter followed by a 12dB low pass filter. The filter takes 6V p-t-p audio input signals and has a maximum gain of 10dB, so the filter works well with modern synths and even Euro Rack, without any buffering or amplification.

Output signal noise is reasonably low at -80dB, but would probably be lower if modern Op Amps were used. Whilst there are no schematics, 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 I made a set of schematics in Eagle CAD and laid out a PCB with the pots on board, to avoid hard wiring. I kept the 15V power supply separate, and replicated the original front panel with Cliff KS knobs. I have used a slim toridial transformer and a well regulated linear power supply. The circuit only draws 30mA.

I omitted the original Facilities port which gave access to the individual and main outputs from the rear. I already had a nice Oak 1U rack cabinet, so this was reused. The main PCB is 285 mm x 100 mm, and its connects via a 10-pin DIL cable to the ten 6.35mm mono jack sockets on the right of the front panel. I initially used Cliff sockets but smaller Neutrik versions are needed which I have mounted on small custom PCB’s.

The metal 1U case is a standard Don Audio N-mod case with a front panel I designed and had manufactured by Beta Layout. The panel design replicates the original EMS version and its a tight fit for those 6.35mm jack sockets!

I ordered the PCB on 11 September and built it on 09 October – everything works fine but a couple of minor mistakes. A second and production version was ordered on 18 November.

EMS Random Voltages

Random Voltage Generator This 1U rack unit is rather more interesting than a traditional noise source or even S&H. Whilst there are no schematics available, it is possible to work out what the circuit does. An interior screen shot of the PCB is available but there are no values marked on the VCS3 style “card” PCB.

At the core of the module is a traditional S&H working from a noise source for random voltage levels. The twist is that the noise output goes through a low pass filter to create low speed random voltage levels (100:1 ratio) that change the S&H clock rate, which means the length of any output voltage step is random.

The S&H clock rate is varied by a front panel potentiometer (MEAN RATE) from 0.2Hz to 20Hz, and the step length randomness varied by the VARIANCE potentiometer. There are two signal outputs (V1 and V2) each which its own LEVEL control. A trigger output is driven from the signal output, rising to +4V when a step starts and lighting a front panel lamp (NEW SELECTION).

The module has two modes selected by a toggle switch:

  • INHIBIT – disconnects the internal clock with manual stepping from the RESELECT push button or +5V on the SELECT input.
  • FREE RUN – connects the internal clock with voltage controlled from the V.CONT MEAN input (0.5V/octave).

Recreating the RVG I may build a replica of the EMS circuit using an ARP 2600 S&H & Noise circuit as I have a populated PCB in stock, which is a spare from my 2500 clone. A new PCB has been designed to hold the front panel pots and switches, and the low pass filter for the VARIATION control.

Pitch to Voltage Converter This 1U rack unit seems to change hands secondhand quite frequently, I suspect because of the limitations of an 1970 analog design on tracking pitch accurately. There is a set of schematics available, but its hard to read the component values in some places. This makes a recreation time consuming and for not a lot of benefit.

Copyright AMSynths 2019