THAT 218X Current summing. Panning question.

Would be possible and/or a benefit to Pan the output/current source of a THAT 218x series.

I'm working on using the VCA as a summing system in a mixer and I'm trying to find the best way to pan.

In theory (cough) you span a pot between two VE buses, and connect the VCA current source output to the wiper to effect a pan pot, but there are several reasons why this is not a great idea. Let me count the ways (extra credit for guesses).

I believe there is an app note on the THAT website describing a way to generate pan law control voltages to pan with two VCAs per channel. 

http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=6&t=211&start=40#p2015

There is a schemo that I posted over on Wayne's forum at the link above...

Actually there's some 17 pages of related posts in that one thread.

JR

Check this
http://www.thatcorp.com/datashts/dn120.pdf
Just omit U5 and drive the bus directly from pin 8 of the VCA's.
Note that the output capacitance at pin 8 is specified at ca. 15pF; when parallelling VCA's, the bus capacitance may become quite high. Thus you will need to adjust the fb capacitor of the summing amp accordingly.

abbey road d enfer said:

Check this
http://www.thatcorp.com/datashts/dn120.pdf
Just omit U5 and drive the bus directly from pin 8 of the VCA's.
Note that the output capacitance at pin 8 is specified at ca. 15pF; when parallelling VCA's, the bus capacitance may become quite high. Thus you will need to adjust the fb capacitor of the summing amp accordingly.

Click to expand...

That seems to all ready have an input for L and R. I need to "generate" a Left and Right from the mono input?

or at least send the current source to the left and right bus... maybe it should be converted to voltage, then panned the old fashion way?

As it is, it is configured as a balance control for a stereo signal. Just feed the mono signal to both Lin and Rin.

Yup, that's the app note I was describing.

Of course you could convert the VCA output back to a voltage with an opamp and use a conventional pan circuit.

JR