"Thom Kouwenhoven" wrote to "MIKE ROSS" (04 Jan 03  18:10:51)
 --- on the topic of "I/F"

 TK> A very good afternoon to you MIKE

 TK> On 04 Jan 03 00:02 you wrote something about *I/F*...

 TK>> with 5 NPN transistors on board....i was amazed it worked
 TK>> at all....but it did :-)

 MR> For simple on/off control okay but not for continuously variable...?

 TK> Exactly....only "mute"...but that is so strange...there
is no form of
 TK> powersupply connected to the collector at all...and yet it works :-)

I think it has to do with the C-E impedance of the saturated transistor.
It will conduct a low level current in both directions. I haven't
experimented the exact mechanism but I suspect it's because the B-C
junction becomes forward biased and let's a little emitter current
through, sort of behaving as the 2-diode simplified model. I'm guessing
it's probably not a linear function because the bias voltage has a
logarithm relation to the current.


 MR> Sure, I've seen bipolar transistors used as muting switches too.

 TK> And that is all that is needed....if it is switched 500000 times from
 TK> bypass to mute and back with a on/off ratio of 25% then, after
 TK> filtering,  the resulting volume must be approximately at 25%....PWM
 TK> volume control.  :-)

I don't think the Marshall amp is switching 500000 times to anywhere...
Unless its output has broken out in parasitic oscillations...

 Mike
 ****

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