JonDent - Exploring Electronic Music: EMS Synthi

Wednesday 4 March 2015

EMS Synthi - Tweaking the filter

I'm just playing around with the EMS Synthi filter today.
When I saw it for the first time, it looked unfamiliar until I realized that RESPONSE is the EMS version of RESONANCE. Sadly there is no voltage control for resonance.

As you can see, it resonates in the higher ranges so can be used as an extra oscillator. The Synthi & VSC3 are not as famous for their filters as say the Korg MS-20 LP or the Moog ladder, though I still like the sound.
The unedited video is here:
http://youtu.be/Xhvb8RD2KW8

Before 1974, the EMS Synthi filter was officially described as a 3-pole diode ladder with a frequency response of 18dB/octave. In '74, EMS changed their design to a 4 pole 24dB/Oct response. (There is still much debate as to whether the pre-74 design is actually a 4-pole and the post 74 a 5 pole design). Anyway, I'm not sure why, though many say the pre 74 version sounded better. My filter is I believe a post 1974 design.

This schemo is of the pre-'74 four-pole diode filter

Schematically, the Synthi ladder filter is very similar to the Moog ladder, except that Moog used transistors, and EMS used diodes. Another famous diode latter filter was made by Roland for their TB-303.

The audio enters via the matched pair of 2N5172 transistors at the base and exits via the two lines
above the group of 8 diodes.

I wonder whether this abundance of diode ladder filters is a direct response to Moog's dispute with ARP over their use of a transistor based ladder filter in their early 2600s.???

Beware ...the response knob is very sensitive -- before you know it you are into self oscillation mode.
The response/resonance also effects the audio level a lot. ... it (loudness) can fall quite dramatically
as you approach the Resonance/Response's upper limits