REV -1: The worlds first synthesizer downgrade

Introduction

This modification for the Sequential Circuits Prophet-5 reverts the Rev 3 sonically to the earlier, much bigger-sounding Rev 2! The two Prophet 5 versions sound different due to the nearly complete redesign of the voice architecture to use Curtis’ CEM chips instead of Dave Rossum’s SSM chips for the transition of the Rev 2 to the Rev 3 model. This difference in sonics has made the Rev 2’s more desirable - and significantly more expensive.

Rev 2

The main factor in the Rev 2’s superior sound is its filter chip, the now revered SSM2040 which features unique asymmetric distortion. Due to the inverting nature of its four filter stages, this asymmetric distortion switches direction after each stage, while the input signal propagates through the SSM2040 and is progressively filtered. An excellent benefit of this is that it makes synthesizers built around it much more moldable from a sound-designer’s perspective as the filter reacts dynamically to outside parameter changes. For example, increasing the level of a waveform changes resonance, the sum of waveforms sound different than their individual parts and at higher levels, the pulse width also controls the resonance symmetry. The oscilloscope plots in the 'waveforms' section illustrate all this.

Rev 3

The CEM3320 filter chip of the Rev 3 features four linearized filter stages which are pretty much distortion free - a remarkable engineering feat, but resulting in an almost digital, less animated sound. Another issue is that internally it works on only -2V negative instead of the full -15V of its predecessor. To accommodate this, level shifting circuitry was added that basically sums an unfiltered voltage straight from the power supply with the audio signal of each filter stage. This pollutes the audio with all the noise and hum generated by the power supply, as well as all the junk from all of the ICs that share this supply.

History

The idea to ‘downgrade’ a Rev 3 to use SSM chips started more than a decade ago, but was put on hold due to the scarcity of the discontinued SSM2040 chips. These rare old parts should be kept for repairs instead of being used for new designs. While, there are schematics circulating on the net of discrete-component clones, none of them come close to the real thing as they all lack key aspects* of it design and ignore the thermal effects that prevent non-integrated versions from working properly.

Now that Sound Semiconductor is releasing their new SSI2140 chip, which is a redesign of the SSM2040 with some extras by its original designer Dave Rossum, this project finally became feasible. A/B comparison, done by mounting an engineering sample of this new SSI2140 in a RSF Kobol Expander (See pictures of the adapted circuit board and test setup) synth and comparing it with another unmodified Kobol Expander showed that its sound is absolutely dead on.

*See footnote #28 on page 22 of the AN701 application note on filter design with the Sound Semiconductor SSI2164 IC.

Features

Fully working prototypes have been built and tested, and have the following features:

No mods to the Prophet 5 PCBs are required.
A REV -1 simply plugs into the IC sockets, replacing each CEM3320.
Only 1 extra wire for the -15V supply needs to be soldered.
Electronics have been added that cancel out the pollution from the CEM3320’s level shifting circuitry. The little that remains (30 to 100dB attenuation depending on frequency content and Prophet component tolerances) is also moved entirely to the filter's input. So, it get's further attenuation of 24dB per octave beyond cutoff.

Extras:

The input signal has been increased a bit, since the 2040 likes to be driven a bit harder. This is no issue for the purists as it can be attenuated in the ‘mix’ section of the prophet-5.
By design the SSM2040 is also self-limiting, so the output level has been increased a lot as ample headroom was available, and this gives a much better signal-to-noise ratio and dynamic range.
REV -1 also adds two new features not present in the original Rev 2:
- variable Q-compensation that restores the loss of bass when resonance is increased (14dB is lost at the onset of self-oscillation in the original filter design).
- the option to use the REV -1 as 2-pole filter instead of the original 4-pole configuration. Most non-linear effects happen in the first two filter stages, and the 2-pole mode lets you hear these effects unfiltered by the last two stages.
The combination of these two extra features will make a Prophet 5 sound absolutely monstrous!
Switches to select 2-pole or 4-pole mode and the amount of Q-compensation will be included with mouting hardware.

Waveforms

Resonant behaviour of the filter is affected by the waveform's amplitude:

Resonance at low level sawtooth and high level sawtooth.

The waveshape also affects it:

Pulse width controlling resonance symmetry at higher levels.

Summing waveforms results in unrelated new shapes depending on the amplitude:

Sum of Sawtooth, Square and Triangle wave.

Demo

If the world wasn't turned upside down a while ago, you should have been able experience it first-hand at:

Synthplex 2020 (Syntaur booth #45)
Superbooth 2020 (Analogue Renaissance booth H254)

But, since these were both postponed, you'll have to do with this short demo by Syntaur

(Skip back to the beginning of the video to see how it is installed)

and this short one for now:

More extensive ones should appear once the pre-production models are out.