Oberheim SEM clone - Oberheim Four Voice clone

[analogmonster]

Hello everyone,

as work is making progress, I decided to open a thread concerning my Oberheim SEM cloning activities. And – perhaps this will be my Oberheim 4-Voice thread someday, who knows…

By the way – I will clone this thread in other forums as well.

All started with an SMT finger exercise, and I decided to do this by cloning sub modules / sub functions of the Oberheim SEM as mini projects, just to try SMT in small functional units.
Yes, I know, there are a lot of SEM cloning projects out there, and affordable cloned instruments, but I wanted to dive into and gain experience in SMT and I want a clone which is quite close to the original, to learn and understand how the original SEM works, the pros and cons of the SEM concepts, so to say to “look under the skirt” of the SEM technology etc.

By the way - I did this by cloning the Moog System 55 the same way.

And although I try to keep the original analogue circuitry of the SEM, I changed / added some concepts surrounding the SEM like Autotune, additional wave shapers and probably some more I don’t know yet.

I published some information in the 2024 Build Pictures thread already, so I start my thread with chapter 4.

Chapter 1: SEM LFO clone

Chapter 2: SEM ENV clone

Chapter 3: SEM VCA clone

So this is Chapter 4, the SEM VCO:

Things I changed:

I simplified the Init Frequency – circuitry.

I adapted component values and types to SMT availability.

Things I added:

I added a triangle wave shaper together with offset and amplitude adjustment and the possibility of morphing between saw and triangle (see video below).

I added a “Tune Fine” option, as the dual pot of the original is not available anymore, so I split the “Frequency” adjustment into two pots “Tune Coarse” and “Tune Fine”.

I added an offset adjustment for the saw wave.

I added an amplitude adjustment for the saw / triangle – mix ( = “Ramp Output”).

I added a 30 kHz low pass filter to the ramp output to reduce the distortion of the reset slope of the saw core.

Things I kept:

The saw core uses a tempco based exponentiator with a 3046 transistor array. It needs a warm up time of 15 minutes like the original does. Before that the scale and the tuning do not fit; the difference is up to a semi-octave. I will add Autotune functionality to compensate that.

The rest of the circuit I kept as well, beside the changes mentioned above.

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VCO_Board_Populated.jpg
Description:	SMT VCO board populated
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VCO_KiCad.jpg
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TSEM_VCO_Waveforms.mp4
Description:	Video: Waveforms and Morphing	Download (listen)
Filename:	TSEM_VCO_Waveforms.mp4
Filesize:	52.86 MB
Downloaded:	47 Time(s)

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[analogmonster]

Further improvements of the VCO:

I protected the expo-tempco arrangement on the board with a small block of polyurethan to increase temperature stability. As the SEM VCO does not provide a chip heater by concept, the PCB, especially the tempco <=> transistor-array arrangement, needs at least a bit of thermo isolation. If I would not plan to add autotune to all SEM VCOs anyway I would place the scale trim pot on the front panel to allow climatic adjustments more easily.
So one disadvantage of the original SEM VCO clearly is the lack of a chip heater.

Another correction I had to do was to add another 30kHz low pass filter behind the triangle wave shaper to reduce the distortion of the reset slope of the saw core as well as I had to do behind the ramp output / saw output.

These adds will be integrated in the TSEM Clone PCB.

[analogmonster]

In contrast to my original plans I designed a front panel for my TSEM VCO clone and added the VCO as stand-alone module to my Analogmonster Modular System. Now it can be used as voice in my modular environment.

All internal ins and outs are provided as external connectors.

Here a short demo video:

YouTube

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TSEM_VCO_Front_small.jpg
Description:	TSEM VCO
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[analogmonster]

Hello everyone,

now I finished the fifth chapter of my Oberheim SEM cloning activities, cloning the SEM VCF.

This VCF is basically a state variable filter. It creates highpass, bandpass and lowpass outputs in parallel.

Following the original SEM my clone processes three input signal groups of two input signals (see schematic below). Each input group can morph between its input signals with a balance pot (in this stand alone version of my clone I use only one of them ("PULSE IN" <=> "RAMP IN")).

Each filter stage is implemented with a standard OTA cell.

An interesting aspect is the way the resonance function works: it works as damper, not as amplifier. That means that omitting the resonance circuitry means maximum of resonance!

Additionally the module processes three selectable modulation sources for voltage control.

Remark: The "NOTCH" filter function of the original SEM module is created by its VCA by addition of the high- and lowpass VCF outputs. My VCA clone of the SEM sub functions offers the same possibility with morphing between lowpass, notch and highpass.

The resonance adjustment is done manually and can't be voltage controlled.

Component changes:

- OTA CA3086 is LM13700
- All OpAmps are TL072
- N-Channel FET are MMBF4391
- All NPNs are BC847C
- All PNPs are BC857C

The basic functions of the module are the same as in the original. I added some enhancements though:

- Adjustable zero symmetry of the filter outputs (LP, BP and HP) for internal post processing (like being used in a complete SEM clone)
- Adjustable output amplitude of the filter outputs (LP, BP and HP) for internal post processing (like being used in a complete SEM clone)

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VCF_Schem_KiCad.jpg
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TSEM_VCF.mp4
Description:	DEMO VIDEO	Download (listen)
Filename:	TSEM_VCF.mp4
Filesize:	249.64 MB
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[analogmonster]

Chapter 2 and 3 of my Oberheim-SEM-Clone-2-SMD - activities handle the VCA- and ENV- sub function of the Oberheim SEM (see above). At the beginning just a feasibility study, I integrated them into my third modular system now as combination module, or better to say as module with two functions, because I built it, so I use it.

If you compare the front panel of this clone with the original (see below), you see the additional control elements and connectors at the front panel immediately, which you can't find at the original. These are the outer equivalents of the internal connections and functions of the original SEM which are not needed as external elements there. The TSEM VCA ENV as stand-alone module must expose them obviously to be usable.

Please find any technical description and details, circuit explainations etc. at
https://www.analog-monster.de/tsem_vcaenv_en.html

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tsem_vcaenv_module.jpg
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[analogmonster]

So this is the next combination module of my Oberheim SEM Clone goes SMT activities. This time I built a combination of the TSEM LFO and TSEM ENV in one module.

As an original SEM contains two envelopes I built another TSEM ENV to add it to my modular synthesizer representation of an Oberheim SEM. Together with some helper modules this will be my test platform for SEM evaluation.

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SEM_clone_20241111.JPG
Description:	TSEM clone and helper modules
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LFOENVFront.JPG
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[analogmonster]

Here you see the TSEM Clone ++ of an Oberheim SEM as Modular Synthesizer. It is patched like the original.

Beside the sub unit clones of the original SEM functions (VCO, VCA, VCF, LFO, ENV) and a "nice to have" AUTOTUNE I added helper modules to be able to patch the unit clone modules like an Oberheim SEM: A CV Mixer and a Distributor for passive multiplying of control voltages like ENV or LFO outputs which are all multiplied and connected internally in an original SEM.

I added Attenuators to the Distributors to control the impact of control voltages like LFO or ENV outputs though. This is an ability the original SEM obviously does not offer.

And obviously as well is the ability of changing the patch to enable other sub functional interrelations which are not possible in an original SEM as they are hard wired there.

Beside these advantages and enhancements the disadvantage of a patchable modular realization of an Oberheim SEM is obvious as well: Patch Cable Spaghetti.

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TSEM_patched.JPG
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[analogmonster]

4 short sound demos:

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TSEM_DEMO_1.mp4
Description:		Download (listen)
Filename:	TSEM_DEMO_1.mp4
Filesize:	30.5 MB
Downloaded:	24 Time(s)

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[analogmonster]

I updated the project documentation / technical module description of the new modules, functions and features:

Analogmonster Modular

TSEM LFO ENV Combination Module

Attenuator Distributor

[analogmonster]

4 new demo patches arranged as mini track:

Youtube

[analogmonster]

Visit my PCB stock for building your own TSEM sub function module.

[analogmonster]

Now my clone of the famous Oberheim SEM is complete - at least as modular solution.

With adding the second VCO the amount of SEM sub functions cloned as modules is identical to the original.

This is the second version of my VCO clone. This time I added a chip heater solution for expo stabilization, in contrast to the original SEM which utilizes a tempco approach.

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chip_heater.jpg
Description:	Heater for 3046 transistor array
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tsem_clone_complete.jpg
Description:	TSEM clone complete
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tsem_vco2_pcb.JPG
Description:	PCB for TSEM VCO version 2 chip heater variant
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[blue hell]

[analogmonster]

[analogmonster]

I made further experiments.

My first try was the tempco approach of the original SEM VCO. Next was adding a chip heater and keeping the tempco. Last was chip heater only.

So what are my findings?

The chip heater approach is definitely the best. Neither the tempco nor the combination appraoch could nearly stabilize as good as the pure chip heater. You switch on the power, and everything fits, like I've noticed when switching on my Moog Prodigy which operates with chip heating as well.

The optimum temperature for a SMT 3046 based expo seems to be +40 degrees Celsius / +104 degrees Fahrenheit. The amount of heating current in standard temperature environments is moderate. The chip is not becoming too hot (which decreases life time).

I abused a non-contact thermometer I once bought for my little children to perform the chip heater adjustment for VCO calibration (see video) - may all mothers in the world forgive me

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ChipHeaterAdjustment.mp4
Description:	Chip Heater adjustment - fever temperature check...	Download (listen)
Filename:	ChipHeaterAdjustment.mp4
Filesize:	55.14 MB
Downloaded:	18 Time(s)

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[blue hell]

Ok, the synth needs a good fever - good to know that

[analogmonster]

Here a new video showing the infinite sound capabilities of the TSEM clone, just with the standard patch, just playing a repetitive 4 note sequence and some delay added:

https://www.youtube.com/watch?v=ta_8Lw0N4z8

[blue hell]

nice