MATRIXSYNTH: Behringer's Cool Audio Clones the Roland BA662A

Friday, August 14, 2020

Behringer's Cool Audio Clones the Roland BA662A

This one is in via Jose of, spotted on Future Music Spain.

Per Behringer:

"Great news. Our sister company Coolaudio who designs mixed architecture semiconductors has just released the holy grail of synthesizer chips - a clone of the BA662A.

It is the THE chip responsible for the unique sound of so many classic synths.

Not only has Coolaudio cloned the BA662 “A” version, which is the highest specified version, they also managed to greatly improve noise performance and included even two chips in one packaging.

We’re beyond excited:-)"

The BA662 was used in the Juno-6/60/106, JX-3P and TB-303, Jupiter-4, and Jupiter-8.

via AMSynths:

"The BA662 is a custom made DC controlled variable transconductance amplifier (or OTA) that Roland had manufactured by Rohm from the late 1970’s and is now obsolete. There are two types of chip with different suffices; the A version has been selected for lower offset than the B version. They are further classified by gain/gm. The BA662B can only replace another BA662B, whilst an A can be used in either application.

To quote Roland from the 100M Service Manual: 'Color has great importance in circuits of some models'. However there is nothing mystical about the sound of the BA662, it is just a reasonable good OTA chip which is often carefully matched and selected."

Note Open Music Labs made a clone as well (last edited in 2014):

"They just don’t make IC’s like they used to, and is this case, they just don’t make them at all! So, we decided to bring the BA662 back to life, in all it’s Frankenstein glory. This is a direct replica (to the best of our knowledge) of the now obsolete OTA, made from well matched transistor pairs. It can be used as a drop-in replacement for most applications. So if you have an old synth or effects pedal that is no longer synth’ing or effecting, this just might be the cure...

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 was found that they used Wilson current mirrors. Also, by measuring the current consumption under varying signal levels, the value of the emitter resistor on the buffer stage was found. A fair bit of the anaylsis was based on recollections from seeing a datasheet a long time ago, and the fact that OTA's have very similar topologies."

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