BA662 Clone
About the BA662 Clone
The BA662 clone is a replica of the very rare BA662 OTA which was used by Roland in a number of their synthesizers and effects pedals from the 80's. The internal wiring of the BA662 was reverse engineered from an original part by applying external voltages and currents, and seeing how it responded. For example, you can find out how many transistors are at the top and bottom of the output stage by measuring the saturation voltages. From this it could be found that they used Wilson current mirrors. Also, by measuring the current consumption under varying signal levels, the value of the emmitter resistor on the buffer stage could be found. A fair bit of the anaylsis was based upon recollections from seeing a datasheet a long time ago, and the fact that OTA's have very similar topologies.
The BA662 Clone uses PMP4201 matched transistor pairs, which have a maximum offset voltage of 2mV and current matching of 98% minimum. This is probably better than the orginals, and why the originals had A and B variants. I am not sure what level they used to select between A and B, but it was probably either 1mV or 2mV, as it would have been difficult to get higher precision back then. Some synths required "selected" BA662A's, which were hand picked for even lower offset voltages.
For a full description of how the BA662 works, check out our x0xb0x page.
