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Oberheim SEM
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Oberheim SEM

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Oberheim SEM
Oberheim SEM
image - Wikipedia - lic. under CC 2.0


Oberheim SEM - Introduction

The 1970's were an interesting time for synths. To start with, this was the first time the music world got to access synthesizers in smaller forms than the large and bulky modular synths, that, previously to the 1970's defined the word synthesizer. Lot's of the terms, forms, and ideas pertaining to synths that we still use today were developed and implemented in the 70s. This is also the era dominated by two big names: Moog and ARP.

Moog, first gaining fame with their gigantic Moog Modular in the late 60s, started to apply their know how on designing a portable version in the early 70s. Thus, in the year 1970, the music world got their fingers on the Minimoog Model D. Mini, because it was not the size of a wall, as the previous Moog Modular. This, expectedly, revolutionized the music world, especially in the arena of live performance. Keyboard players, in droves, bought the Mini and finally had a weapon of sufficient fire power to compete with the guitarists who turned up to 11.

But what would the world be without a bit of competition. Enter the ARP 2600, the Minimoog's closest and first major competitor. ARP took a slightly different approach with the 2600, making it semi-modular and consisting of a separate keyboard module from the main synth body. Also, unlike the Mini, which was essentially a close your eyes and turn the knobs operation, some know-how was needed to really get the most from the ARP, despite it being semi-hardwired (no patch cords were needed to get a basic synth patch going). Understandably so, as ARP's approach was to provide education institutions a suitable learning synthesizer.

And while these two companies were attempting to best each other, a man by the name of Tom Oberheim decided to get into the synthesizer business and invent a device that was a peace-maker of sorts; a device that was designed to work with both Moog's and ARP's.--the Oberheim Synthesizer Expander Module (SEM).

Oberheim SEM - Basic Specs

On the surface, the SEM is a desktop type device, with a slightly angled, white panel with black legending. It's a sturdy box, housing an internal power supply and a circuit board full of discrete parts. On offer, you essentially have a basic, self contained, monophonic synthesizer voice. There are two voltage controlled oscillators capable of producing saw and square waves. Additionally, pulse width modulation, oscillator sync, and pitch modulation effects are all possible. Besides the usual pitch setting knobs, which are actually two pots on one shaft, there is a modulation knob that regulates the amount of either pulse width modulation or frequency modulation by the Low Frequency Oscillator, Envelope 1 (for VCO1) and Envelope 2 (for VCO2), or an External input. There is also a knob per oscillator for setting the initial pulse width of the square wave.

The two vco's feed a multimode 12dB voltage controlled filter that is capable of producing a continues sweep from low pass to high pass modes, with a notch mode in the middle section. Additionally, there is a switch to place the filter into a band pass mode. The filter has ample and large controls for frequency cut off and resonance (though it never quiet reaches full on self oscillation) as well as a knob for filter cut off modulation. This knob has a fixed point in the middle (12 o'clock) so as to guarantee zero modulation and also allowing both positive and negative voltage offsets (turn clockwise for positive and counterclockwise for negative). There is a three position switch below this knob that acts as a mini source matrix for filter cut off modulation and it can pull voltage from Envelope 2, the external input, or the low frequency oscillator.


There are two envelopes that are of a very simplified three stage approach: attack, decay, sustain, whereby the decay also functions as a release time. These envelopes are hardwired to the voltage controlled amp, and the filter, with modulation doubling carried out by the aforementioned switches in the corresponding synth sections (i.e. filter cutoff modulation, oscillator pitch modulation…).

A single low frequency oscillator rounds out the architecture with a simple triangle wave and a knob to regulate the speed.

Oberheim SEM - Feature Details / Architecture

Because the SEM was designed to be an expander module for another synth (such as the Minimoog or ARP2600), it had plenty of interfacing jacks beyond the simple output. There are provisions to trigger the synth via CV and Gate, as well as a Moog Gate input, so the owner of a Minimoog could fire off the envelopes with the Mini's S-trig style grounding envelope trigger. There are also provisions for feeding audio into the SEM for filtering via the wonderful 12dB filter, or for sending external control voltages. Additionally, there is an LFO output so that you could use the simple LFO to modulate other synthesizers via CV. This was particularly handy if you had a Minimoog and didn't want to use the third oscillator as an LFO.

Going deeper, and opening the SEM itself up, the user will find many connectors that are clearly labeled on both the PCB and the service manual. These connectors open the scope of modding possibilities up ten fold, as they allow for virtually any component to have an individual output or control input. Want to have a straight feed from the VCO 1 square wave on a switch? It's as simple as wiring a jack to the correct pin on one of the connectors, grounding the jack to a common ground point, and drilling a hole on the case. Ok, so maybe not super simple. But any experienced tech will be able to modularize the humble SEM into something of a patchable monster.

Oberheim SEM - Sound

And what of the sound. After all, with the name implying expansion, rather than center stage, how does the SEM fare? Well, I'm here to tell you, insane well. It could have easily been called the Main Synthesizer Module (MSM … hmmm … maybe not). The SEM has an incredible, big, powerful, warm, and aggressive sound. It is the sound of American synthesis of the 1970s through and through. Basslines pound away at low end in the same way the Minimoog does while the leads sweeten the tonal spectrum with finesse and class. Everything about the SEM is super 70s! Sequences also sound very good triggering the SEM because the envelopes are rapid fire fast!

Oberheim SEM - Applications in todays world / How it fits in hybrid modern set up

In today's world, Tom Oberheim has already re-made the classic SEM. In fact it comes in several models, one of which already has all the mod points talked about earlier broken out and put on the main panel. However, having owned a vintage Two Voice (two SEMS in one keyboard oriented package) I have to say that the new SEM, while good in its own right, is not 1 to 1 with the vintage. The original just feels more authentic and alive and part of this, I'm sure is the through-hole design, different power supply, and the overall quality of audio passing parts used. The original relied heavily on the CA3080 op amp which has a particularly pleasing harmonic distortion when driven, but is a part no longer made. But, barring a few hard to obtain parts, the original is all discrete, which means most of its innards are simple transistors, capacitors, and resistors, all things easily available today.

Oberheim SEM - Conclusion

With a little bit of maintenance, and a little bit of MIDI to CV conversion, you could easily sit the vintage SEM into your modern DAW and let the sweet Oberheim tone rip through!

Review by Matia of INHALT




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