Analogue Systems


EMS Synthi Filter

Genuine Ems synthi filter. This legendary filter can be switched between pre 1974 and post 1974 filter type and response. For the first time response can be voltage controlled which was designed on the circuit board but not available on the matrix. This module is made under license from Ems.

The various models of EMS VCS3 and Synthi rank among the most desirable of all vintage synthesizers. Designed in England between the late 1960s and mid 1980s, these eschewed common designs and control mechanisms, and performed very differently from synthesizers manufactured in America and Japan. The earliest commercial Synthi (although not called by that name) and by far the most famous, was the VCS3. This was later joined by the Synthi A, Synthi AKS, the monstrous Synthi100, and the Synthi E. (For further information about these,please refer to the chapter entitled "The EMS Story" found before the appendices in this manual.) The RS-500e low-pass filter for the RS Integrator was recreated by former EMS engineer Steve Gay and is, as far as possible, an exact reproduction of the filter within these synthesizers. Although there are minor differences between the circuitry in the RS-500e and the originals, most have been made specifically so that the EMS circuit will fit the RS Integrator format. Other modifications allow it to handle modern signal levels and control voltages.In addition, the RS-500e offers three facilities not found on unmodified EMS synthesizers. These allow you to use the filter in ways not possible on an original Synthi. The additional facilities are:

Voltage Control of Response

"Response" was the EMS term for filter resonance. EMS synthesizers allowed you to alter this using a knob, but they did not provide for voltage control. Fortunately, it has been simple to add a CV input to control the Response. This has not altered the filter's sound.

Rapid Cut-Off Frequency response to incoming CVs

Due to a capacitor that slewed incoming control voltages, the cut-off frequencies of early EMS filters responded slowly to changes in incoming CVs. The consequence of this was obvious when you tried to create snappy, percussive sounds on a Synthi... you couldn't. The RS-500e allows you to switch this capacitor out of the circuit, allowing you to program significantly different types of sound.

18dB/octave and 24dB/octave filter slopes

Until early 1974, all VCS3s and Synthis were fitted with filters that rolled-off at 18dB/octave. But in response to market forces, later models offered the more common slope of 24dB/octave. Although this was in some respects an improvement, it came at a price... the sound of the filter changed, and many players stated a preference for the earlier design.

Happily, you don't have to choose between one or the other when configuring your RS Integrator. The RS-500e offers both filter characteristics, selectable using a simple front-panel switch.

Note: The CV slew switch and the 18dB/24dB filter options were - and remain - standard modifications for vintage EMS instruments.

In use :

The RS-500e is a low-pass filter with variable resonance. It is modelled on the filters produced for EMS synthesisers, and shares their principle sonic characteristics.

Cutoff Frequency

You control the filter cut-off frequency (Fc) using the FREQUENCY control. In its fully anticlockwise position, Fc is approximately 20Hz. As you rotate the knob clockwise Fc will increase until, it its fully clockwise position, it reaches approximately 20kHz. These extreme positions are called 'Low' and 'High' respectively. You may also control Fc using one or both of the CV inputs:


    Unlike other RS Integrator filters (which will track an incoming 1V/oct CV accurately over a wide range frequencies) the fixed input on the RS500E is not strictly a 1V/oct CV input. It is calibrated to respond to 1V/oct as well as possible, but it is one of the quirks of the original EMS design that it is not linear enough to track over a range greater than about two octaves


    You may wish Fc to track incoming CVs at >100% or <100% relative to 1V/oct, so a CV-IN VARY input is provided. This socket and its associated LEVEL control allow you to specify the filter's sensitivity to CVs within the range ∞V/oct to approximately 0.2V/oct. The former of these makes the filter invariant to incoming CVs, while the latter makes it over-sensitive compared to the FIXED input.

Cutoff Frequency Slew

The RS-500e offers two CV response modes: CV SLEW FAST and CV SLEW SLOW (STD). You should select the correct mode for the type of sound required.

  • CV SLEW SLOW (Standard):

    When high frequency CVs are applied to the unmodified Synthi filter they are low-pass filtered by a slewing capacitor. This means that the filter will not respond to high frequency CVs, and will respond slowly to rapid changes in incoming CVs. You can use this mode to recreate the bubbling and warbling effects for which early VCS3s and Synthis are famous.


    With the switch in this position, the slewing capacitor is removed from the CV input circuitry, so the filter will respond to high frequency CVs. Use this mode for sounds such as percussion and effects that require a rapidly varying filter cut-off frequency

Filter Slope

Changing the filter slope alters the way in which upper frequencies are removed from an audio signal passing through the device. This change can be subtle or obvious, and can be the single most defining characteristic of an analogue synthesizer.

  • PRE 1974:

    The filter attenuates frequencies above Fc at a rate of 18dB/oct. This recreates sounds of EMS synthesisers built before 1974.

  • POST 1974:

    The filter attenuates frequencies above Fc at a rate of 24dB/oct. This recreates sounds of EMS synthesisers built from 1974 onwards.


The RS-500e has variable resonance, which on early EMS synthesizers was called "Response".

As this is increased from its minimum, the filter will accentuate Fc, and will attenuate frequencies below Fc as well as those above it. As the resonance is increased further, the RS-500e will exhibit 'ringing', and will strongly color any signal passing through it. At high resonance, the filter will begin to oscillate, and it will then act as an audio signal generator, the frequency of which is Fc itself.

You can control the resonance using the RESPONSE control. In its fully anticlockwise position, the resonance is approximately zero, and there is no emphasis of the signal at Fc. As you rotate the knob clockwise, the resonance will increase until, it its fully clockwise position, the filter oscillates. You may also control the filter resonance using the RESPONSE CV-IN, which accepts and responds to all standard RS Integrator control voltages.

Audio Input and Output

The RS-500e has a single audio input: SIG IN, with an associated LEVEL control. This input acceptssignals in the range ±10v.

The LEVEL control offers unity gain in approximately the 2 o'clock position, marked '4' on the panel. At its fully anticlockwise position it attenuates the signal fully (MIN = -∞dB gain) while at its fully clockwise position it offers a small gain.

There is a single output with an associated LEVEL control. This carries a signal in the range ±10v.



The filter circuit incorporated in the RS-500e is used under a licence granted to Analogue Systems Ltd by EMS Ltd. The name EMS is used under a licence granted to Analogue Systems Ltd by EMS Ltd. All rights reserved.

Audio Files :

AB Jazz Pad

Bass is a one oscillator patch using the RS-500e filter, played by hand. Lead is a 2 oscillator patch using the RS-500e EMS filter and the RS-510e Trapezoid, some echo added externally.


Bass patch uses the RS-500e EMS filter and the RS-310 reverb/chorus, sequenced by external MIDI. The lead patch also uses the RS-500e with the RS-510e trapezoid, played by hand.


Two sounds using the RS-500 EMS filter, played by hand plus some reverb.

From Putney with love

Using the RS-500e and RS-510e and other RS modules

Analogue Systems

Custom vintage analogue synthesiser system equipment.