- Nov 30, 2006
- Hickory, MS
I would be interested in seeing that too...madswitcher said:Hi Pucho,
If you can, get hold of a circuit diagram of an EMI TG console, or even the Users Manual. The designer) used a transistor as a voltage to current converter for the channel mix output and a simple virtual earth amp which simply summed the currents.
Simple, no mix resistors and did the job effectively.
In theory a single transistor (common base topology) with audio signal feeding through a resistor into the emitter could output a high compliance AC current from the collector, albeit +/- some class A current. This class A current (just like inside class A VCAs) must be large enough to absorb max signal swings and limits the noise floor performance. In a patent search decades ago I found a very old circuit the summed current outputs from vacuum tubes.
The patent office denied my patent application back in the late 70s because the wet behind the ears patent examiner didn't understand the difference between summing "currents", and summing "current sources". He cited a text book description of a VE sum amp that literally says it is summing currents (argh). At the time I didn't have enough cash sitting around to educate the examiner while paying a lawyer's hourly rates. I briefly considered driving down to VA to educate the examiner in person, but I had better things to do with my cash and time back then.
For the record I never figured out how to make a discrete low noise, low distortion, bilateral current source. I did figure out I could make an active op amp synthesized bilateral current source with 10x the source impedance of simple resistive summer, delivering 20 dB less noise gain. 20 dB less noise, less distortion, and less phase shift was a win in my book.
Apparently I am a bad communicator, lack technical authority, or both... :-[ Back in 1980 when I shared my current source summing invention with the world through my article, many at the time did not believe me. I admit it is kind of fantastic to suggest a console sum amp structure with no N+1 multiplication of sum amp ein.
As I recall exactly one fellow consoles designer (Steve Dove... sorry to name drop) shared with me when I ran into him at a trade show later that year, that not only did he get it, he copied me and used it in a small broadcast console design (high noise gain in bipolar VE bus amps can suffer from rectification when exposed to high RF environments.)