The Oberheim OB-Xa was released in December 1980 at a list price of $5,595. It weighs in at 45 lbs and its dimensions are 40" (wide) X 20" (deep) X 6" (high). The OB-Xa is an ‘update’ of the OB-X which was originally released in June 1978. The OB-X and OB-Xa definitely sound different and the OB-Xa offers you more features. The OB-Xa was available as both a six voice and eight voice unit. Each voice consists of two voltage controlled oscillators (VCOs) , two Voltage Controlled Filters (VCF’s - a 2 pole filter and a 4 pole filter), a Voltage Control Amplifier (VCA) and two Envelope Generators (ENV). The voices are located on two on two separate voice cards with each card containing four voices and a low frequency oscillator (LFO). The cards are independently controlled which allows the synth to offer a double (layer) and split modes.
Keyboard
The keyboard is a standard 61 note non-velocity sensitive keyboard. It has a very strange feel to it as the keys feel heavy and almost sticky and makes lots of noise. I’m not sure if this with all of them or just my board. As mention above it offers double, split and normal modes of operation. Double triggers two sounds on to one note (one from the upper and one from lower card). Split divides the keyboard into two separate parts - the upper and the lower and each part of the keyboard triggers its respective voice card. The split point is adjustable. Normal as you would expects uses the same sound across the keyboard. To the right of the keyboard are the master transpose switches will allow you to transpose the entire keyboard up or down one octave.
The front panel controls are broken into modules or sections as follows:
Programmer
Early units came with memory to store 32 programs. Later units have the ability to store 120 programs which are divided into 15 banks of 8 as well as 8 split programs and 8 double programs. The parameter controls are always active and changing the controls add to or subtract from the current setting. This allows for smooth modification of existing programs. To have the applicable parameter control indicate its current setting just turn the pot to its maximum position and then its minimum position. The unit also features a manual mode which when activated the voice parameters are set to the current physical settings of the controls.
Manual
This section features basic controls including the master volume pot. The master tune knob which tunes all the VCOs and includes a dead zone at A-440. There is a switch to engage the auto tune program which tunes the VCO’s. It takes about 2-5 seconds for the program to tune all 16 VCO’s. On later versions the program lights will scroll from 1-8 (left to right) to indicate which voice is currently being tuned. If a voice can not be tuned, the program light representing the voice will flash for two seconds and the voice will be disabled (on units with serial number 814202 and up). This disabling feature may be turned off by accessing a switch underneath the cover of the unit. Upon start up the computer does not automatically run the auto tune program so you will probably want to hit the button upon power up. There is also a balance parameter pot which is stored as part of a split or double program. This control sets the balance between the upper and the lower voices of the synthesizer when using the mono output. Since it only effects the mono output it has no relationship to the panning of the of the upper and lower voices. The OB-Xa also features a hold button to hold notes indefinitely and a chord button to play one finger chords.
Control
A portamento pot determines the rate of the portamento of each voice. On newer OB-Xa’s (after serial number 820818) both linear and quantized portamento is offered. Quantized portamento moves from one note to another in half steps (glissando) and not the traditional smooth glide. A unison switch causes all voices to be triggered by one key and produces a nice fat huge tremendous 16 VCO sound. Even better is when you are in split mode you can create two separate monophonic synths, by using unison mode, with each key triggering 8 VCOs. When in unison mode the keyboard operates using lowest note priority so the lowest note played will be sound if you try to play more than one note. Also included in this section is the VCO2 detune pot which allows you to fine tune VCO2 flat or sharp relative to VCO1. I’m not sure why its included here and not in the oscillator section but I kind of like here since I like to mess with the VCO2’s fine tuning a lot.
The synthesizer voice is made up of the following modules:
Oscillation
Two VCOs which offer saw or pulse waveforms. VCO 1 frequency is selectable between 16’ - 2’ in one octave steps. VCO 2 frequency is selectable between 32’ - 2’ in half step increments. Pulse width is adjustable for both oscillators at once. It would be nice to be able to set pulse width separately for each oscillator. As noted above, VCO2 may be fine tuned relative to VCO 1. VCO 2 may be synched to VCO 1. VCO2’s frequency may be modulated by the filter envelope and may be turned on and off with a switch, however the amount of modulation is controlled by the same pot as the envelope’s modulation on the filter. This is a shame and reduces the flexibility of sound creation. Both VCOs may modulated by the LFO - see modulation section below.
Modulation
The OB-Xa has two LFOs and one of them is located in the modulation section. The LFO offers a choice of sine, square or sample and hold (random). There is a depth pot to control the amount of modulation. There are three buttons which determine the destination(s) of the LFO modulation; VCO 1, VCO2 and VCF. Unfortunately there is no way to route different amounts of modulation to each of these three destination. Each destination is either on or off with the amount of modulation determined by the single pot. Also there is not delay parameter to delay the onset of the modulation. There is another pot to determine the amount of pulse width modulation as determined by the LFO. This modulation may be routed to either or both VCO1 and VCO2.
Filter
The OB-Xa features both a 2 pole filter and a 4 pole filter. The filter is incredibly smooth and full sounding and I think its great. Its one of the best features on the board. The filter is a low pass filter with resonance. Resonance is provided however the filter will not self oscillate. In the 2 pole mode as resonance increases so will the overall volume of the sound and decrease in the 4 pole mode. The 2 pole filter offers a more gradual (-12db/octave) filtering which produces a ‘brighter’ sound. The 4 pole filter offers a more sharper (-24db/octave) filtering which produces a ‘fuller’ sound. The filter may track the keyboard however only off, half and full settings are provided. The filter envelope (#1) may modulate the filter however this pot shares it duties in determining the amount of modulation the filter envelope will have on the frequency of VCO2 (if it is selected). The oscillator mixer is included as part of the filter module but only offers on or off for VCO1 and on, half or full for VCO2. This is a shame and really limits your mixing ability of the oscillators. There is also a noise generator that may be mixed into the signal but its either all or none - no really useful mixing available.
Envelopes
The synth offers two envelope generators one strictly for modulating the filter and the frequency of VCO2, and one that affects the voltage controlled amplifier (VCA). It would have been nice if each envelope routings were selectable. The envelope is a standard ADSR. Envelopes willnot track the keyboard and they are not invertible. There is no volume setting for the VCA so the patch volume must be set by using the master volume control which is not part of a patch’s stored parameter. This is frustrating.
Modulation Panel
This sits to the left of the keyboard is different from the modulation module. The modulation panel setting do not get stored as part of the program’s parameters. I can’t decide if this is a positive or negative feature. The modulation panel features a second LFO which offers a sine, sawtooth or square wave. There is a rate parameter knob to determine the speed of the LFO and a depth parameter knob to determine the amount of modulation. By pulling the knob up the set modulation amount is put into effect. This is really neat and I can control a lot better than by using the actual lever. The amount of the depth set by this pot is added to the depth control of the modulation lever. This is an Oberheim lever so it works backwards from the usual lever, meaning you pull the level towards you and modulation is increased. This LFO may affect VCO1, VCO2, or both. You can get some crazy effects by using two separate LFOs. However its a shame that this LFO can’t affect the filter.
There is also a pitch bend lever that may modulate both VCOs or VCO2 only. The pitch bend range is selectable between narrow (one whole step) or wide (one octave). When selecting VCO2 only and VCO2 is synched to VCO1, the pitch bend lever will affect the timbre of the sound not the pitch.
When the synth is in split or double mode the modulation panel may be routed to either or both the lower and upper keyboard or program. This is excellent and allows for pitch bends to played by one part of the keyboard (or by one program) without affecting the other part (or program).
Rear Panel
The OB-Xa has a pair of stereo outputs as well as a mono output. Each individual voice may be panned to any position in the stereo spread with the pan pots located inside the unit. On the OB-8 these pots were moved to the side of the unit for easier access but on the Xa they are underneath the hood making them a little more difficult to get to but still a really cool feature. The mono output is not affected by the pan pots.
The unit has a cassette interface for storing programs on cassette. A high and a low level output is provided for the cassette interface as well as a input jack. The cassette interface uses mini-plug cables. According to the manual the interface is designed to work with portable cassette recorders having an earphone, speaker or monitor output and not designed to work with tape recorders having only a line level output. I do my patch saving to the computer by record the data as a .wav file which seems to work quite well. The cassette interface offers a verification procedure to test the data without actually transferring the data into the OB-Xa.
There are three foot switch inputs which perform the following. Sustain causes the release on all envelope generators to be set to about 3:00 clock position. Definitely different that the usual sustain feature but the unit has a separate input for hold which is the normal sustain. Program Advance advances the program to the next program.
Two foot pedal inputs which are designed for the model P-OBX foot pedal offer the ability to modulate LFO2 located in the modulation panel and the other input is to control the frequency cutoff of all voices.
Also located on the rear is a 37 d - sub port which is used to interface the OB-Xa to the DSX sequencer. As far as I know this is the only use of the interface. A 37 d-sub cable (which is available from Belkin and Comp USA) is required to connect the two.
Finally a power supply selector (115/230V) and AC receptacle. The power cord is a standard three pin grounding connector, Switchcraft Series EAC.
Other
On the side wood panels are two screws which may be removed to open the units cover. Underneath the cover is a memory protect switch which can be set to prevent the user from writing programs into memory. The two voice cards each include LEDs to identify which voice and card is active. The pan pots described above sit on the right hand side of the boards. Unfortunately to get the lower board you must disconnect and move the upper board. Individual voices may be turned off.
My unit has a very basic midi kit installed. It has a midi in and out port (no thru) recognizes note number, on and off, patch changes and that is it. Sending a large amount of system exclusive data to the OB-Xa will make it crash and I will lose the patches stored in memory. The unit will work fine again once its restarted but my patches are gone. Kenton makes a more advanced kit for the unit if more detailed control is required.
What is inside making all that sound
The synthesizer voice is made up of the following Curtis Electronic Music Chips:
Neither the filter or VCA envelopes are software generated so they are really quick and give the OB-Xa a nice amount of ‘punchiness’
Conclusion
My OB-Xa is one of my favorite synths I own and sounds beautiful. Its definitely the fullest and largest sounding synth I own and as a result it can really eat up space in a mix. The lack of individual routings to some of the modulation destinations is disappointing and limits your ability to be as creative as you may like with the synth. But the OB-Xa just always seems so full and rich and is so easy to make great patches on. You can hear the OB-Xa all over Signals by Rush, 1984 by Van Halen and Prince records from the early/mid 1980s. The pots and switches are large compared to most and very easy to work with. The offer a slight amount of resistance and are so much easier to use then sliders. They just don’t make em like they used to!
Photographs
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