After almost two years the SE1 mod is finished. Now intense testing can start.

Here’s a quick shot of the new inside. The aluminium back extension contains the six jack boards with all the audio signal amplifiers and CMOS switches. Those boards are connected to the analog PCB of the SE1X directly via shielded cables.
The control board for all the new circuitry resides on top of the SE1X digital and analog boards, shielded with a large copper-cladded FR4 sheet.

The new SE1X has 44 1/4″ jacks – somewhat more than the original two connectors. The meaning of the abbreviated information shown on the labels is explained on the very bottom of this page, so scroll down for a reference until I’ve finished the block diagram.

Hopefully the last time I had to remove the back and all the modules for some minor fixes, the SE1X shows all its new wiring here.
The 4o wire ribbon cable has been tapped by the new controller board on top of the original boards. All switching signals and control voltages are handled here.
Shielded cable has been used to interface with the audio signals right on the analog board (which means removing some resistors and cutting some traces – ouch!)

All boards have been tested successfully now with this setup, two potentiometers turned out to be of wrong value for the possible parameter deviation and will be replaced soon.
Goooooooooooo!

But hey, what’s that?

(first of all, for the younger readers: this is a digital photo of an analogue scope screen, with a real cathode ray tube. Yes, I like those things when working with analog signals.)

The lower trace is the output of the RMOD gates (4070 CMOS device) with only VCO2 running, the upper is the triangle wave of VCO2 routed to the output.
First you might think this is a shot-through of the VCA controlling the RMOD amount to the master mix – nope, it is not.
It’s the hard switching of the 4070 causing a spike on the +10 volts rail, making its way into the analogue circuitry.

I’ll have to think about that later.

Recently I checked the mod boards in the SE1X to find out that there still was some level mismatching. The intended 0.775 volts RMS on any stage output and external input were not yet achived.
So I removed the boards and did some additional modifications. Here’s a photo of the updated boards. No, these are not the boards for 6 SE1X’s but the complete mod for one single unit!
And it’s not even the full mod, the controller board shown above as well as some meters of additonal wiring belong to the upgrade too.

During re-assembly I will check the levels starting at the VCA back to the VCOs and have the last missing parts installed, once the required values have been determined. Stay tuned!

As circuit design is finished and most PCBs are populated now, it’s time for assembly.
First of all the controller boards need a place:

Here you can see the Controller Board on top of the original SE1X CPU and analog boards.
An additional copper-cladded PCB should prevent digital noise from the new microcontroller to disturb the analog board, although the small ATmega @1MHz with no fast clocked outputs would not make much EMC trouble either. The Controller Board gets its +/-15 volts supply directly from the PSU, all other voltages (+/-12, +/-7, +5) are generated on board.

The board lying on the table is one of three VCO jack boards. The functions of its 8 jacks is alrady outlined in the previous post.

I thikn it’s time to wire & fire things up…

The MIDI jacks from the SE1 CPU board have been removed and extended by shielded cable to allow the DIN jacks to be fitted in the new rear panel, same will apply to the IEC power connector.
Yes, it’s supplied from an isolation transformer, and yes, the open frame PSU is a nasty little devil and will be replaced by something more safe and silent.

The VCO1 Jack Board is now connected to the Controller Board by several (yellow) cables, carrying control voltages of different kind. The two shielded cables are hooked up to the analog board – one in between the VCO output and VCO Amount VCA input, the other one delivers the triangle output to the sine converter on the Jack Board. The sine wave is mixed to the VCO output under control of the Controller Board, eventually overriding the command from the SE1 CPU.

On the photo above I was adjusting the Middlebrook & Richer triangle to sine converter that any of the 3 VCO Jack Boards features to allow all three VCOs producing sine waves. I was quite surprised that I got the 2nd harmonic down to -50dB and the 3rd to -40dB, measured at 440Hz. Overall THD will be in the 1.5% range, maybe even better when final adjustment are made.

Studio Electronics’ SE1X is a nice 19″ synth module featuring 3 VCOs based on a Moog concept, one Moog-style 24dB ladder filter and a 12dB Oberheim-like switchable lowpass / bandpass filter. The VCOs are capable of sawtooth, triangle and variable-width pulse waveforms; with the addition of a triangle-to-sine converter according to Middlebrook & Richer, the SE1X version generates a sine output instead of triangle on VCO2. Everything  is mixed together by classic LM13700 OTAs and finally controlled by two cascaded discrete Moog-style VCAs.

So far everything was strictly analog. The control of all the analog circuits including LFO and envelope generation resides in the digital domain of the central microcontroller. Although the D/A converter used features rather high resolution for internally generated signals, MIDI-only control restricts the actual precision to 7 bits for external control.

Needless to say that the SE1 needs an option for external CV control, stricly analog. Once we started drilling some holes, why not making the whole thing modular? To cut the story short, after all the component count of the modified SE1 has doubled, a new rear panel was necessary to allow 43(!) jacks to be fitted and 7(!) additional circuit boards to be installed.

Ready for a peek inside?

Two out of 43 1/4″ jack holes in the new laser-cut aluminium rear panel grant a peek on the old circuitry

How modular it finally got is probably best described with some photos – stay tuned!

1. The controller board

The controller board for the additional circuitry sitting on the SE1 analog board, waiting to be fitted.
By chaining into the 40 conductor flat cable between the SE1 digital and analog boards, the new board has full control over all analog control voltages and digital switch controls signals.
27 OP Amps and 18 CMOS SPDT switch functions take care of proper signal routing. Almost all analog CVs are buffered and routed to output jacks. Each CV has an input jack assigned, followed by protection circuitry and an input buffer. The mechanical contacts in the jacks do not get in touch with the CVs, but control the CMOS switches. Each VCO has an additional CV input which allows to override the internal waveform setting. The same applies for the VCF; a dedicated CV determines whether to use the internal setting or override to one of the three filter types. For ease of A/D conversion of those latter four CVs, the controller board features a small Atmel AVR microcontroller.

2. The VCO jack board (1 out of 3 identical boards)

A VCO jack board in a not fully populated state. Most parts reside on the bottom side.

Each VCO gets one of these jack boards with the following features:

VCO CV OUT – a buffered version of the internally generated VCO CV (pitch, frequency, whatever you like)
VCO CV IN – override the internal VCO CV by an external 0..5V control voltage
WAVE SELECT – override the internal VCO waveform setting by an external CV. With this jack board, all three VCOs now offer saw, tri, pulse AND sine and any combination thereof!
WIDTH CV OUT – the internal version of the pulse width control for rectangular wave form. Dim your studio light with it under MIDI control…
WIDTH CV IN – externally control the pulse width of rectangular wave
VCO RAW OUT – the raw output of the VCO, buffered to 0,775Vrms into 10k ohms in triangle mode
MIX IN – disconnects the VCO from the internal mixer and allows an external signal to be inserted (or an effects enriched version of the raw out)
AMOUNT IN – an external 0..5V CV overrides the internal VCO amount setting

There’s no amount out as this control is not modulated or influenced by the envelope generators, but only a representation of the 0-to-63 value set in the SE1’s menu. Too low resolution to be of real use, sorry.

Important: all intermediate outputs (read: all except the main output) are trimmed to 0,775 volts RMS with a triangular signal at 440Hz.
The inputs are adjusted the same, to a patch wire from any intermediate output to its adjacent input will not change the overall signal amplitude.
This allows for transparent insertion of effects, assuming they provide a 0 dB gain.

3. The RMOD/Noise Jack Board

This is the smallest out of the six jack boards of the mod. It provides the following features:

RMOD RAW OUT – the raw output from the ring modulator, trimmed to 0,775 volts RMS with an 440 Hz square wave (50% duty cycle)
RMOD MIX IN – allows to insert an external signal to the RMOD amount VCA, instead of the ring modulator
RMOD AMT IN – override the internal RMOD AMT control by a DC CV of 0..5 volts
NOISE MIX IN  – disconnects the noise generator from its amount VCA and replaces it with an external signal (like the external input on a stock SE1X)
NOISE AMT IN – override the internal noise amount control by a DC CV dod 0..5 volts
MIX OUT – a buffered copy of the “master mix”, the sum of the amount VCAs of OSC1..3, RMOD and noise, right before it enters the VCF

4. The VCF jack board

Here’s the board with all the filter controls and signals, look here for its capabilities:

VCF IN  – remove the VCF input from the “master mix” and insert an external signal – 0,775V RMS, input impedance 10kOhms, nothing new here…
VCF TYPESEL – a control voltage on this jack selects the VCF operating mode, one out of 12dB LP, 12dB BP or 24dB, overriding the internal setting
CUTOFF OUT – a buffered version of the internal cutoff control voltage (in the range -5..+5 volts!)
CUTOFF IN – overrides the internal cutoff control, same voltage range as for the output of course
RESO OUT – a buffered version of the internal resonance/Q control voltage (0..5 volts)
RESO IN – overrides the internal resonance control
VCF OUT – the buffered and normalized output of the VCF before it enters the VCA

5. Final! The VCA board

Although two parts are missing, here’s the VCA jack board! Featuring those features:

VCA IN – disconnects the VCA input from the VCF output and allows to insert an external signal. 0,775V RMS, you may have guessed it.
FUZZ AMT – something new: the SE1X’s Fuzzzz has gone variable. Apply a 0..5 volts CV here to control the amount of distortion
VCA1 CV OUT – the internal control voltage for the 1st VCA (controlled by ADSR)
VCA1 CV IN – overrides the internal VCA CV for envelope control
VCA2 CV OUT – the internal control voltage for the 2nd VCA (which is mainly the volume knob’s position)
VCA2 CV IN – overrides the internal volume control
OUTPUT – finally, here’s what you are listening to. The level remained unaltered, which means it is normally somewhat lower than 0.775V RMS.