Analogue Systems RS-500E EMS Synthi Filter

Controls

  • Frequency
    controls the cut-off frequency of the filter. 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.
  • CV Slew (Fast/Slow)
    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 RS500E allows you to switch this capacitor out of the circuit, allowing you to program significantly different types of sound.
    • 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.
    • Fast: 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.
  • CV-In Vary Level
    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.
  • Sig In Level
    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.
  • Sig Out Level
    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.
  • Response
    "Response" was the EMS term for filter resonance.
  • 18Db/24Db
    (Pre74/Post74)
    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 RS500E 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.

Sockets

  • CV-In Fixed
    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.
  • CV-In Vary
    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.
  • Response CV-In
    controls the response. EMS synthesisers 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.
  • Sig In
    audio input, accepts signals in the range ±10v.
  • Sig Out
    audio output, carries a signal in the range ±10v.

Various

the manual

The various models of EMS VCS3 and Synthi rank among the most desirable of all vintage synthesisers. Designed in England between the late 1960s and mid 1980s, these eschewed common designs and control mechanisms, and performed very differently from synthesisers 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 Synthi 100, 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 RS500E 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 synthesisers.

Although there are minor differences between the circuitry in the RS500E 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 RS500E offers three facilities not found on unmodified EMS synthesisers. 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 synthesisers 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 RS500E 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.

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