The Oberheim OB-Xa was Oberheim's overhaul of their first compact synthesizer, the OB-X.
The OB-Xa was released in December 1980, a year after the OB-X was released. Instead of discrete circuits for oscillators and filters, the OB-Xa (and the Oberheim synths to follow) switched to Curtis integrated circuits. This made the inside of the synth less cluttered, reducing the labor required to replace bad parts, and reducing the cost of manufacture. It was getting more difficult to service the OB-Xa due to the scarcity of Curtis chips; however, Curtis started re-manufacturing these chips in June 2016, which has breathed new life into the longevity of the OB-Xa and many other synthesizers that use these chips.
Aside from hardware changes, the OB-Xa had better interface features than the OB-X. These included being able to split the keyboard into two halves with different voices and the ability to layer voices to create thicker sound (essentially making two notes sound for every key pressed). Polyphony stayed the same - again 4, 6 and 8-voice models were offered.
One function that did disappear from the OB-X voice architecture was cross modulation, or frequency modulation of the first VCO with the second VCO. When done with analogue circuits, it's a unique sound made famous by the Sequential Circuits Prophet 5 and its poly-mod section which is considerably more flexible and dynamic than the basic implementation offered in the OBX. In place of this feature on the OB-Xa front panel is Filter Envelope modulation for oscillator 2, which allows the pitch to be modulated by the envelope. Another new feature to the OB-Xa is a second filter mode labeled "4 Pole".