OB-Xpander Replica

OB-Xpander

Overview The Oberheim OB-Xpander was a prototype analog synth created around 1982, and not to be confused with the later Oberheim Xpander which although similar is significantly different. There is very little information about the first Xpander, just a picture in Mark Vails Vintage Synth book (volume 1) and some text in the A-Z of Analog Synthesizers. From this limited information I have concluded that the OB-Xpander was based on the OB-8 and developed in the Summer/Autumn of 1982 (because the panel graphics reflect an early OB-8 with Page 2).

Features Here is a probable feature list:

• Part of the Oberheim System alongside OB-XA, DMX and DSX
• Four independent “multi-timbral” analog voices, using one OB-8 voice card
• Four independent LFO’s in the  modulation section
• Keyboard controls, Split/Layer, Unison have been removed
• Probably the same OB-8 CPU board with some minor modifications
• Four sets of CV/Gate inputs as well as the Oberheim Computer Interface
• Stereo and Mono Outputs with preset voice panning
• Cassette storage for patches is retained
• No MIDI, as this did not arrive at Oberheim until Spring of 1983
• Page 2 included, as the Modulation has the additional white silk screening

Modulation Section The Modulation section needs to be per voice with 4 separate LFO’s, to ensure each voice has different sounding patches. The OB-XA has two analog LFO’s for the Lower and Upper voice banks, whilst the OB-8 has two digital LFO’s with the extra sawtooth waveforms that are screen printed on the OB-Xpander panel. So could the OB-8 OS have been extended to generate 4 LFO’s in the OB-Xpander? The limitation is in the Z80 processor speed, but the keyboard scanning workload has been removed, CV/gates added and the envelopes are analog.

The OB-Xpander has a switch in the lower corner of the modulation section, which the OB-XA does not have and was used by the OB-8 as VOLUME MOD. However the labeling is shorter, so maybe this was just VOL.

Master Section This has been reduced down to one column of controls;

• MASTER VOLUME potentiometer
• AUTO switch, labelled as per OB-XA
• MASTER TUNE potentiometer

Control Section This remains as one column but has one change;

• PROGRAM VOLUME potentiometer sets Voice Volume
• PORTAMENTO potentiometer
• OSC2 DETUNE potentiometer and red LED

Oscillator Section The VCO controls are the same as the OB-8 and OB-XA and the panel lettering verifies the OB-Xpander is based on the OB-8 voice card, as it has the lettering underneath the Pulse and Saw switches for Triangle. This is how early OB-8’s were lettered before the Page 2 functions were added on the silk screen.

4 Voices on CPU board

Keyboard Section This has been renamed to three words which are too blurred to read, maybe PROGRAM VOICE SELECT. There are switches to select each voice for programming and patch save/recall. with a fifth switch (assumed) to call up a four layer program (one of 12 available, as in Double mode). The cassette Play and Check functions are retained with the corresponding red LED. The OB-8 VOICE board actually has 4 voice select signals from the CPU board (at A13, A15, A17, A19) but only two are used for Lower and Upper voicing across two PCB’s. This confirms that the OB-8 is the basis of the Xpander and that it would have used 4 voice boards, each with only one voice populated.

Programmer Section The patch selection remains the same as the OB-8 with the familiar row of A-D and 1-8 switches, with 120 patches and a WRITE/RECORD switch at the far right hand side, labelled as per the OB-XA.

Prototype Only The OB-Xpander never made it past prototype stage, probably due to being only 4 voices, and the limitations of the Z80 processor running 4 independent digital LFO’s. Oberheim reworked the idea in early 1984 as the Xpander, with 6 independent voices, lots more digital control, MIDI and CV/gate. The processing power was increased to cope with more features and the Xpander has stood the test of time and is one of the greatest analog synths ever made.

Case Dimensions Peter Forrest states this as; 90cm x 30cm x 12.7cm, the same depth and height as the Oberheim DSX and with a weight of 9Kg, which is half the weight of an OB-8. The Xpander case does not look deep enough for a set of OB-XA voice cards to fit in.

Licio Comisso OB-8 VOICE card

Replica It is possible to recreate the OB-Xpander? It would make a very nice sounding 4-voice multi-timbral synth. All the CEM chips are readily available (CEM and AS), and both the VOICE and CPU boards could be recreated from the schematics. Licio Comisso has already successfully cloned the OB-8 VOICE and CPU boards to recreate the OB-8, but when I contacted him he only had OB-XA boards available.

A rather wide 30″ case would need to be fabricated, with a silk screened front panel. Using smaller width switches might mean the case width could be useful reduced. The power supply would be mounted off the CPU board, and improved.

OB-8 OS Modification I initially considering changing the OB-8 Z80 source code to support the Xpander. The largest change is to increase the existing two digital LFO’s to four, which is no easy task as there is no source code and its quite a large operating system to reverse engineer (16 KB), however this has been done before. The rest of the changes required are smaller. I disassembled the source code but it is a lot of work and time to get the changes made.

In addition I need to find a way to retain the calibration capabilities which use the Bender Board and 1st octave of the keyboard, probably by using a set of miniature slide switches mounted internally. This means retaining the keyboard scanning.

Custom Gligli OS A possibly easier approach is to modify the P600 Gligli code to add the extra digital LFO’s, and at the same time removing the digital ADSR’s, cassette interface, and keyboard scanning. This would also give a strong MIDI capability, but the user interface and MIDI Sysex/cc needs changing to match the Xpander, and then there is all the Page 2 functionality to consider, and the loss of the Oberheim calibration software. The advantage is that the Gligli code is open source and written in C.

Conclusion Whilst an exact replica of the Xpander is possible, the limitation of 4 voices is a bit too limiting, given the amount of time and cost needed to build a replica. My preference is to build the OB-Xpander but as a desktop OB-8 with 8 voices, Page 2 and MIDI. This means no OS changes are needed and the CPU board can have MIDI built in. I will also remove the Noise circuit and add in two new filter modes (12dB HP and 6dB LP).