In 1976 the EMS Eight Octave Filter Bank was a key part of both Klaus Schulze’s and Tangerine Dream’s sound. The 1U rack unit with beautiful wooden case can be seen one top of Peter Baumann’s customised Moog 960 Sequencer as well as Klaus Schulze’s IIIp, where it was usually placed on top of his “Complement B”. It is not obvious which synth is being processed by the filter, possibly the ARP 2600 or the Big Moog. It was displaced by a set of PPG modules that Klaus bought in 1976, but was used in the studio 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 second hand.
The 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.
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.
I ordered the PCB on 11 September and built it on 09 October – everything works fine but a couple of minor mistakes. An accurate production version was ordered in June 2021.
My original plan was to use a Don Audio N-mod case but this has metal lips at the front edges that made fitting the jack sockets difficult. Instead I re-purposed an Alesis Data Disk (cost £25) which works a lot better, no lips and less depth. There is room for the main PCB as well as the linear power supply and transformer. The top of the case becomes the bottom, and vica-versa. This ensures the studs on the bottom don’t interfere with the PCB’s.
A hole was cut in the rear panel for a connector to an external 16VAC power supply (Yamaha PA-30). I designed an accurate replica of the EMS front panel and had it manufactured by Beta Layout. I built two of these filter banks, and one was sold to a customer, whilst the other retained for my studio.
2024 Update AMSynths ported the filter design into Eurorack as the AM1013 Octave Filter Bank, using SMD components except for the capacitors which are all high quality WIMA polypropylene. The module has ARP 2500 style knobs and proved very popular. In 2025 I updated the design to use 2% PPS SMD capacitors and found the original EMS schematics here.
EMS Synthi 100 Processors Alongside the launch of the Synthi 100 in 1971 EMS launched three sound processors, sound cards of the 100 in 1U rack form. As well as the 8 Octave Filter Bank, there was the Pitch to Frequency Converter, Triple Slew Limiter and the Random Noise Generator.
The 1u Random Noise Generator has been carefully replicated by AMSynths, more details are here. EMS went on to launch the Quad Ring Modulator and Phase Filter as 1U rack mounts. Unfortunately none of these proved popular and few examples exist.
