Little bogen preamp

Hello all, I am tracing out the circuit of this little bogen mic preamp and I have a question. The High z inputs are capacitor to base of the transistor, but the Lo-z inputs are capacitor to emitter of the same transistor. I haven't seen that before. How does inputting to the emitter create a low z input?

Thanks from a noob!

I suspect there is also a resistor in series with feeding the emitter. The emitter voltage will stay relatively constant and the current coming in and out of the emitter, shows up in the collector, so there will be voltage gain based on the ratio between collector resistor and emitter resistance.  Driving the emitter is a low impedance common-base gain stage.

Alternately driving the base, is a high input impedance (common-emitter) gain stage. Voltage gain will be ratio of collector resistor  and resistor from emitter to ground (actually 1+ that ratio).

Note, there will be a polarity inversion between driving the base and driving the emitter but that may not be a big concern in systems for back ground music and announcements.

JR

Thanks, John.  looking at the circuit there is no resistor in series going to the emitter. It is directly connected. Have to hook it up and try it out.

If the transistor doesn't have neither the base nor the emitter tied directly to any potential, changing the voltage on the emitter will change the voltage on the base, which is tied to a  bias voltage with some resistance, so the potential at the base will change then the current on the base changes and you have your signal. Still common base configuration but with the base actually flying in relation with the common reference voltage.

JS

dspruill said:

Thanks, John.  looking at the circuit there is no resistor in series going to the emitter. It is directly connected. Have to hook it up and try it out.

Click to expand...

The mic will have a resistive source impedance so it might rely upon that,

JR

> How does inputting to the emitter create a low z input?

"Common-Base Amplifier"

There's reasons why this is not the best idea. But you can connect mikes as indicated in the wiring diagram, and when the preacher speaks, the loudspeakers speak. What more could you ask?

Note that while it IS a fairly low impedance, it is not balanced. (If the preacher were a long way from the mixer, Bogen would sell you transformers to kill the hum on the cable.)