Oberheim OB-Xa

Above is a picture of an Oberheim OB-Xa 8 voice synthesizer.

The far left side of controls of the Oberheim OB-Xa.

The AUTO button on the left Auto Tunes the OB-Xa. Let the Unit warm up 5-10 minutes then press AUTO button so it's tuned to running temp.

The extra modulation controls next to the keyboard of the Oberheim OB-Xa.

Above: The keyboard control section and the Main Modulation section

Basically you get an oscillator LFO (low frequency oscillator) that can be Sine wave, a square wave, or a sample and hold version of a sine wave.

These can be routed to the Frequency Column with OSC1 OSC2 and Filter switches. The depth sets the level of the modulation

The LFO can also be routed to the Pulse width of the oscillators, again depth setting the level of LFO

The middle panel section of the Oberheim OB-Xa. Oscillators 1 and 2 types are selected here

The right side section of the Oberheim OB-Xa front panel

The presets are selected by choosing 1 to 4 of the ABCD buttons, and then pressing a number button on the right. A1 is the first patch.

Above 120 program upgrade plaque and Oberheim logo

The back side of the OB-Xa

Close up of the OB-Xa back left, outputs on the left, MIDI jacks on the right

Close up backside of the OB-Xa, showing the cassette interface, foot switch and pedal inputs

The Enable switch turns on the cassette interface and turns the synth OFF. So, if Enable switch is set where it says ON you get NO Sound

To ship an OB-Xa, use a Road case to ensure safe delivery

Arrange foam inside Road case and hot glue to ensure maximum protection

 

Sounds of this Oberheim OB-Xa

Five Sound examples from the Oberheim OB-Xa recorded direct to computer 1/25/2013 (mp3 and wav formats)

---The first sound below is preset A6 and shows a 5 voice chord pad. Note the slight stereo pan. This unit has been adjusted to do this

OBXaA6Pad.mp3

OBXaA6Pad.wav

--This sound is Preset C8 and is a unison patch with the modulation LFO set to Sample and Hold going to the filter cut off.

OBXaC8blurps.mp3

OBXaC8blurps.wav

--The sound below is a basic bass patch preset D3 and shows off the bass capabilities of the OB-Xa

OBXaC6Bass.mp3

OBXaC6Bass.wav

--The fourth sound is a dual preset 1. Press dual then 1. This shows how the OB-Xa can play two different sounds layered.

OBXa-Dual1.mp3

OBXa-Dual1.wav

--The fifth and last sound below is preset D3 which is some blurpiness with a long decay

OBXaD3.mp3

OBXaD3.wav

Videos of this Oberheim OB-Xa

VIDEO 1 --- This is a short video in 3 different formats showing how the Auto Tune feature works.

A few notes are played, then I switch presets. Then I press Auto and each light under switches 1-8 lights showing it is tuning each voice. When this is done it returns to the current preset. If a voice fails tuning, the light under the number will flash several times after the Auto Tune process. This means you need to get it calibrated as described in the service manual or something is wrong internally. After Auto Tuning I then switch to another preset and change the frequency and resonance of the filter.

OBXaAutotune.mp4 This is best quality format. Try this one first. Other 2 are same video, different file types

OBXaAutotune.avi

OBxXaAutotune.mov

Tips for the Oberheim OB-Xa

1. ALWAYS turn on the unit and let warm up for 5 -10 minutes, THEN press auto tune (AUTO button on far left) The OB-Xa will drift until it reaches operating temperature. It is best to just wait, let the Unit reach operating temperature, then press AUTO. After pressing AUTO each LED1-8 lights in order showing it is tuning that voice, and then it returns to the previous patch
indicating successful tuning. If a voice fails that light will flash indicating it is not passing auto tune. This is a great improvement over the OB-X, which gave no visual feedback of pass or fail.

2. The presets are selected by choosing 1 to 4 of the ABCD buttons, and then pressing a number button on the right. A1 is the first patch.

3.To SAVE a preset, press the WRITE button on the far right and HOLD it for 2-3 seconds, the light on the WRITE switch will come on and the next Preset selected will be written over with the Preset you currently are playing. There are no "Are you sure you want to erase" warnings it just does it. Hold the WRITE button until a number is pressed.

4. The Enable switch on the back turns on the cassette interface and turns the synth OFF. So, if Enable switch is set where it says ON you get NO Sound. Easy to bump when moving. The front panel will just have a light next to the word Cassette waiting to send or receive data.

 

INSIDE the Beauty

OB-Xa inside

Above the right side of the OB-Xa Power supply on the left. The JL Cooper MIDI interface is the lighter colored board on top.

The CPU board is the large greener board underneath. The battery is yellow and in the front under the MIDI board.

Close up of the CPU board

Above the two pot boards. All switches and pots for the front panel are attached to these 2 boards.

The grey ribbon cable in the middle takes all these signals to CPU board.

 

Left pot board. The red DIP switch sets which voices are active. All 8 are turned on here.

The right pot board

The Oberheim OB-Xa voice card chips

Above The 8 voice cards. Four on the bottom , four on the top

Above the top 4 voice cards

Above a HUGE picture of one board. View image, or Save as and open in viewer to see full image

The Oberheim OB-Xa is based around the CEM chips (Curtis Electromusic) CEM3310, CEM3320, CEM3340

The OB-X also had the CEM3310 and a lot of the same chips as the OB-Xa , but did not have the CEM3320 or 3340

Now lets look at each section on the OB-Xa

Above far left of one analog board, the 2 CEM3310 Envelope generators at top. Trim pots in the middle

A Few LM3080's and a TL081 general purpose Op Amp

Next, on to the the CEM 3320 voltage controlled filter chips

Above another close up of the filter chips

Oscillator CEM 3340

And the second 3340 oscillator to the far right

OB-Xa board close up

 

 

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