I did a rigorous engineering analysis if this back in the late '70s and if you think about this making bilateral (AC) current sources using transistors is not trivial to make them low noise, low distortion etc. At best you end up with a unity gain current ratioing VCA. But even using today's, several decade later best technology VCA is higher noise and distortion, than my simple 5 precision resistors and an opamp (or should I say Wildar's topology). My secret sauce contribution for taking that well know circuit and applying it to an audio sum bus effectively was scaling down the impedance of the resistors involved to get the noise floor down where it needed to be.joaquins said:JR,
A simple collector current source would be too crappy for this, no? or any other '3 leg' device. a mirror for example.
Yes a standard VE sum amp, but now since that bus amp is running at very low noise gain there is no benefit from heroic measures to use a ultra low noise opamp there. In fact one Loft console I designed back in the 70s/80s used a DBX vca in place of the summing amp. Hows that for headroom? 8) 8) but yes any common opamp will work for the bus amp, and another benefit of feeding the bus with current sources is that the local ground differences no longer matter. The current send is referenced relative to where it is sent from, and converted back to a voltage where it is received.The sum amp for the current sources is just like any VE or have any difference?
Another tidbit, I tried to patent this back then, and ran out of personal funds to pursue, when the lame patent examiner didn't understand the difference between combining "current sources", and combining "currents". All the textbook explanations for how a virtual earth summing amp works says that the input resistors convert their input voltages to currents that are then combined. but that is far different than summing current sources. I was paying for this out of my pocket, and didn't have the resources and patience to pay to educate the wet behind the ears patent examiner about the difference. This is not that unusual with patent applications, if the invention was that obvious, it would have most likely been done already.
JR