Michigan Synth Works SY-1 Pearl Syncussion remake

First of all, wow! Second of all, just the vibe my old Pearl had… BUT with this one I’m not as afraid to muck stuff up, so I eventually did a whole bunch of modifications 🙂

Two modded syncussion lines with a bit of string action from a Yamaha MODx on the side:

and here’s a review: https://www.amazona.de/test-michigan-synth-works-sy-1-dyi-syncussion-klon/#comments

Sources

Scans of the original Service Manual. Ninni Bergfors’ redrawing of the OG schematic and her Super Syncussion (expanded SY-1) with a corresponding moddwiggler thread. The old but good Remörk mods. MSW BOM (look under MODS in google doc). My own stuff: https://maffez.com/?page_id=218; https://maffez.com/?page_id=250; https://maffez.com/?page_id=3048. There is a wealth of information on the modwiggler forum, where some sound modifications are discussed, especially by user @paperCUT. 

Basic Information and Circuit Discussion

We all know, that while the SY-1 was meant to be played as a piu-dong-type drum synthesizer, it’s a secred kick drum weapon and main ingredient of vibey plonks in techno music. The underlying synthesizer structure is a common subtractive type: waveforms are filtered and amplified/attenuated. And while the most basic sound element is a triangle wave, so not the most exciting stuff, some east coast synthesis ideas where oscillators modulate each other in order to generate more sonically complex material. Hence, apart from large controls (easy to operate when busy playing paradiddles), you get cool presets (called OSC MODES) that control pretty wild cross-wirings.

Since the technical building blocks for oscillators and the filter are not super common, I discuss them now in more depth – if that’s too nerdy, simply skip to the modding section… 🙂

VCO – BASIC VCO OPERATION

What you see is a “basic” TRI core VCO. A variable current source charges a capacitor (at the VCO core, the integrator) – when the capacitor is fully charged, a complex sensing system (comparator and bias circuit) tells the source to witch from charging to discharging until the capacitor is empty and the next cycle begins. In contrast to a saw core VCO, where the capacitor is zapped empty instantaneously, resulting in the typical sawtooth like waveform, the rise-fall-pattern is pretty symmetrical, resulting in a triangle shaped wave.

What’s pretty nifty in the Syncussion is that the basic building blocks of the VCO are neither entirely transistors nor op-amps but so-called hexinverters (CMOS chip 4069) that are creatively used as amps and comparators. These circuits (you know them from the Roland TR909 Toms or the TR808 cymbal/hats, for instance) work like: input voltage is low –> output voltage is high and vice versa. There is a threshold where lo flips to high and the other way round, yet, unlike in op-amp comparator circuits, which do similar things, you cannot change this threshold externally. That’s why you get a bias circuit here that senses the point where an inverter cell in the 4069 flips its hi/lo state and utilises this information for the charge-discharge cycle. TLTR: elaborate stuff for a cheap CMOS chip!

VCO – COMPLEX WAVEFORM GENERATION (ON VCO2)

As just discussed, the basic VCO waveform on the SY-1 is a TRI wave. There is a SQUARE WAVE as well (available at TP1 and TP2), but these are not used in the audio path. For bringing in some garden variety, the different OSC MODES not only switch between different volume ratios or envelope and pitch settings for VCO1 and VCO2. They also introduce some cross modulation (exponential FM from VCO1 to VCO2) and some trick to switch VCO2 to a SAW wave.

As you see on the illustration above, in mode B VCO2 is in the audio path while being crossmodded by VCO1. On mode C, the general pitch of VCO2 is raised quite a bit and there is cross modulation from VCO1. Now, mode E is fantastic and deserves a bit more description.

Mode E allegedly turns TRI into SAW. Now, there is no wave shaper circuit in this synth we usually expect from a TRI core VCO… On Ninni’s schematic I found the only real comment on this, yet she suspected some feedback from VCO2 onto itself. What happens, in fact, is as simple as mischievous…

When the SAW circuit is switched on, some +8V are fed into TR18. This voltage is passed though the transistor every time the comparator output is LOW (and the comparator output is low, when the capacitor on the TRI waveform core is empty and ready to be charged up again). The voltage through the transistor is forwarded to the exponential converter that sets the waveform PITCH, and that’s the cool/odd thing. The rise portion of the TRI is sped up so much that this part of the waveform cycle falls out of hearing range, and as soon as the integrator cap is fully charged again, transistor TR18 closes and we’re back to the normal fall portion of the waveform. A bit like fast forwarding a part of a song you don’t care for that much. I don’t know of any other VCO using this trick.

VCF SECTION

Again, we find 4069 inverter cells used as amplifiers in the filter, and a bias circuit. The filter itself is a voltage controlled state variable filter setup used as a second order low pass. Cool so far but now stuff gets weird: there is no filter resonance, and both low pass units are slightly unequal as the integrator capacitor of the second stage is half of that of the first (1nf vs 2nf), and the resistor limiting the control current into the second filter is also roughly half of that of the first (47k vs 100k). This means the cutoff frequencies of both low pass units move around differently when modulated. The closest thing I know of in the synth world is the pole spreading thing in the OSCAR synthesizer, but this here is definitely its own thing!

Although this filter is not set up for resonance control, the latter can easily be added:

And yes, since this is a state variable filter, you can easily get a bandpass response too – here’S what it sounds like with resonance (first LP then i switch to BP):

VCA

As the VCA goes, you have a pretty standard transistor pair-diff opamp thing going.

MODULATORS (ENVS, LFO)

The envelopes are a bit more interesting. While EG1 modulates pitch and filter cutoff together (drum sound principle – bright and high vs dull and low), EG2 has decay CV that is affected by a decay slider but also the tune slider (i.e. different decay per base pitch of sound).

EG1 is kind of an upside down circuit. Normally, decay envelopes are fed by a trigger impulse that charges a capacitor, and the discharge of that capacitor to ground (usually through a decay slider that sets the fadeout time) is buffered and ready to serve decay env purposes. Here, a trigger impulse opens a transistor that makes the timing capacitor discharge in a short burst, and the charge time (as set per decay envelope), makes the actual decay sound. Hence you get the UP and DOWN direction (DOWN being created by inverting op-amp).

EG2, on the other hand, is the conventional “trigger charges capacitor” type, yet a set of transistors and some extra inputs allow you to instantaneously MUTE the envelope (here’s your drum synth again) or variate the decay time. If I got this right: Base of T42 that connects to CONNECTOR PIN 4 of the VOICEBOARD (via a 100k resistor) shortens the decay with rising CV input, and base of T42 that connects to CONNECTOR PIN 4 of the VOICEBOARD (again via 100k resistor) lengthens decay with increasing CV input.

What’s with the LFO? Basic relaxation oscillator – R21 sets minimum cycle time and R20 sets maximum speed.

The Sample and Hold is pretty classic style with a FET switch engaged by a trigger impulse. First I thought the LFO TRI input is a bit lame, since the classic computer room do-dee-do-dee-sound is done with noise as an SH input and LGO as trigger, yet, to be honest. I find that thing good as it is.

MICHIGAN SYNTH WERKS VERSION

MSW always do this cool thing where you can get a through hole DIY version and a SMD-based assembled version. Usually, I’m totally game for DIY projects but with this circuitry I made really good experiences with Pharmasonic’s SY-CORE and my own homebrew SY-1 VCOs for Eurorack, both based around commonly available parts. Encouraged by direct comparisons between the MSW-810 in through hole original Roland ICs and my SMD version, I happily opted for my pre-built extended I/O unit. As for op-amp choices and transistors etc. all is very carefully done, and sounds right, period. Never been a friend of nos part magic cap fetishism, and this unit again shows me that with careful choices you get the right sound.

one voice board
the artwork’s based on scans AFAIK and very beautiful

Modifications

some modifications showcased

These modification are for non-commerical use only (CC-BY-SA-NC; this excludes mods that were previously developed & documented by others, of course). Collected schematics are at the very bottom of this page.

Note, part numbers in my schematics refer to “MOD” part numbers (not SY-1 part numbers).

1. TUNE MODIFICATIONS 

VARIABLE VCO 2 TUNE: If the fixed VCO2 tune setting bores you, replace VR1 with a B100k potentiometer and R59 (470k) with a 200k resistor.

REWIRE MUTE INPUT FOR TUNING VCO2: One classic mod is to replace the sensing knobs with manual tune knobs for VCO2. I wanted to go beyond this and have CV modulation possibilities too, so I rewired the MUTE socket accordingly. Simply disconnect the TRS  socket for MUTE form the MUTE connection (black wire to main PCB. Then rewire rewire T and R of the MUTE SOCKET each via a 200k resistor to VOICEBOARD CONNECTOR PIN12 of your respective voice board. Now you can send in static CV for simple tuning or envelopes, LFOs, VCO signals… If you want to play safe, clip your incoming signal *pre-200k-resistor* from/to -8V and +8V. (Do an online search for power clamping, if need be).

EXTENDED LFO – XMOD: The LFO wave selector switch has a centre-off position that can be fed a third modulation source besides SQUARE and TRI wave. This means you need to pull the DEPTH slider down rather than flicking the switch into the off position, for deactivating any modulation, but with this easiest modification ever, you can already cover so much more sonic ground!

Here’s one voice on Mode A (the only TRIANGLE mode):

On my unit I wired VCO2 output (from the ANODE of C11) of the OTHER VOICE via a 1K resistor to the LFO “OFF” pin on the backside of the control board. Now you get nice cross modulation between voices going! (PS try also noise generator output 😉 If especially daring, you can also wire each voice output (post VCA, at VOICEBOARD PIN 19,w hick is pre-output volume slider) as a modulator. A similar (although simpler) trick is used in the Division Department’s O1/IV drum synth.

2. VCO WAVEFORM MODIFICATIONS

SWITCH VCO 1/2 TO SAW WAVE: For VCO 2 this is really easy. Simply switch +8V (tapped from VOICEBOARD CONNECTOR PIN 14) von a a diode and a 1k resistor to the cathode of D3 on the voice board. You then activate the “SAW” circuit. For VCO1 you need to reproduce the circuitry of R51, T23A, R60, and C11. Use and on/off switch between +8V and cathode of C11 for activating saw wave, then. 

ADDED SQUARE WAVE: Tap either TP1 (VCO1) or TP2 (VCO2) and wire them via a n on/off switch and a 560k resistor to VOICEBOARD CONNECTOR PIN5 (VCF IN). Awesome on OSC MODE A.

Here is a heavily processed (compression, delay) demo of square wave osc with resonant filter…

Here’s a demo with hardsync on one channel and squared up OSC MODE A on the other:

CRUNCHY SQUARE WAVEFORM: This little trick is nicked from the Roland SH101 subs circuit. We use the output of TP1 and TP2 for mixing two square waves through diodes that make a simple AND gate. When detuning both VCOs against each other, then, we get some different flavours of square wave. Connect TP1 and TP2 each to a 200k resistor connect these two resistors to the anode of a diode each. The cathodes of both diodes are tied together and connected via another 200k resistor and an on/off switch to VOICEBOARD PIN5 of the voice board. If you find this too loud, use a final 300k resistor instead of 200k.

First double square, then double crunchsquare –

PWM WAVEFORM MODULATED BY LFO: The TRI wave of the VCO can be turned into a width-modulated pulse wave by using a simple op-amp comparator setup This is a very common way of deriving square/pulse waves from a triangle or saw wave core. The intensity of the effect is fixed – I scaled this to my personal taste.

NB: If you want the waveform width to go through zero, reduce the value of the resistor between LFO TRI out and op-amp (R24 in my schem).

VCO HARDSYNC

DIFFERENT SYNC SOURCES: Instead of using an A/B switch, use A/OFF/B, so you can switch between VCO1 of the same and of the OTHER VOICE as syncing VCO – this further extends your sonic palette.

MODE D AS HARDSYNC PRESET: A cool sync “preset” that doesn’t require external VCO2 pitch CV is MODE D with a slight alteration. In this mode, the VCA envelope is routed to the pitch of VCO 1 and VCO2. By removing R70, we remove the pitch sweep effect on VCO1, which remains stable while VCO2 plays higher and sweeps nicely. Turn on sync now and smile…

VCO RESET ON START OF SOUND: First off, my apologies and sincere compassion to Raf, whose patience I must have tested quiet a bit by sending shite schematics of this mod. Winter 23 was a crazy time, and I seem to have had zero proofreading skills.

Now, this mod makes individual sound onsets uniform in that it resets the VCO and thus also reduces clicky sounds. usually, the Syncussion VCOs are free running, which is basically neat. Yet, imagine recording a four to the floor beat and the attack portion of your sound starts drifting around… That’s not the “lively” you’re after; at least I’m not. With this simple modification here, the trigger impulse that starts the VCA and pitch envelopes also resets the core of VCO1 briefly (Q2) while pushing the sound frequency very high (Q4) – that’s how it’s also done on the TR-909 kick drum reset, which inspired me to do this. As a result, whether you play a short high blip or a thundering bass, your attack phase is always neat and clean.

Why would this consistency be good? Techno kick drum! (this mod is in fact inspired by the TR909 bass drum VCO reset). boring demo, but listen to sound onsets…

Here’s Raf from MSW demoing this very modification on his wonderful cream coloured version…

AUDIO RATE OSCILLATOR MODE SWITCHING: The OSC MODE selector is a nifty arrangement with the rotary switch simply sending some +8V to different transistor switches. There is fun to be had by using the LFO square or the VCO square (which is not in the audio path but available for use at TP1 and TP2 on each voice board) in order to do some fast switching while your sound continues playing. Try this: connect TP1 (for VCO1) or TP2 (for VCO2) via a silicone diode (anode to TP1/2) to one of the following voice board CONNECTOR PINS – 7, 8, 9, or 10. This one sounds cool with MODE B (10) and MODE E (7) as targets. Personally, I prefer using VCO as a modulator here too. 

PS: if you want shorter ON cycles, use the TRI out (ANODE of C9 or C11 – cathode won’t work here) as a switching modulator. If you want to go really fancy, mess around with a CD4017 counter circuit. Feed your trigger input to the clock input and have each step of the counter activate another OSC MODE.

3. VCF MODIFICATIONS

There is already a Filter Cutoff CV modification (documented Raph’s BOM as well), so I leave this out. Apart form that not much to add… Here’s an example that also features resonance…

FILTER FEEDBACK: This is inspired by Ninni’s awesome Super Syncussion and easy – VCF output (C24 ANODE) is connected via a 100k resistor + ON/OFF switch to the VCF input (C18 ANODE). Gives you some thick, brutal sounds. Personally, I don’t use this though.

FILTER RESONANCE: Inspired by Ninni’s feedback modification, I though negative feedback should work well and introduce some resonance. Going filter out to filter in did not have the desired effect, as an inverted feedback signal simply thinned the sound out, so I tested a resonance setup as you find it on state variable filters and yup…!

NB I tested without the 1uf cap (but that one might help).

FILTER FM: What’s cool about the modulation setup in this machine is that sweep and tune and envelope affect pitch AND filter cutoff alike, so simply doing some Devilfish 303 style VCA out to cutoff modulation… I think there are more suitable mods.

4. VCA MODIFICATIONS

VCA CV INPUT: This one is easy and useful if you want to play sounds that fade in slowly or use some VCO/LFO for amplitude modulation.

5. MODULATOR (ENV, LFO) MODIFICATIONS

CV OVER (AMP) ENVELOPE DECAY: this one is well documented (see links at top of page)

LFO RETRIGGERS ENVELOPE 1: While the amp envelope is decaying as per normal, you can have Envelope 1 retrigger in the frequency of the LFO:

LFO RETRIGGERS SAMPLE AND HOLD: I forgot the details but think same schematic as retrigger of envelope 1 except target point is cathode of D0.

6. REST

FIXED SENSE SETTING: this is useful if you want to repurpose the sense knobs for something else. If the sense setting is too high, I get double triggers when feeding in gate signals instead of short trigger pulses. With teh fixed resistor settinsg sketched below, all fine without bounce blip…

Consolidated schematics