Back EMF in an audio speaker is not analgous to SWR. Back EMF is
a very real voltage being generated from the physical motion of the speaker
cone causing the speaker coil to actually become a generator.
Stuart's comments about SWR are totally real and are not limited to tube
circuits.
Carl Moreschi N4PY
Franklinton, NC
----- Original Message -----
From: <AC5E@aol.com>
To: <rohre@arlut.utexas.edu>
Cc: <tentec@contesting.com>
Sent: Friday, October 25, 2002 8:24 PM
Subject: Re: [TenTec] Power Power Power - revisited..
> Stuart: Walt is perfectly correct as long as we are talking about TUBE
> circuits. Tubes represent a severe mismatch to reflected RF, with only the
> essentially trivial losses in the tank circuit keeping that impedance from
> being infinite in all respects. So for TUBE circuits most reflected power
is
> actually re-reflected.
>
> Class A, AB, and B transistors are another situation entirely. Somewhere I
> have an elementary book on transistors that represents them as a variable
> resistor with a motor on a slider for the base. The point being that while
a
> "vacuum valve" is a one way device whose current is controlled by it's
grid
> to cathode potential, a transistor is essentially a variable resistor
with
> its resistance controlled by base current. At RF of course, that is a fast
> motor!
>
> Within the limits of the text, this explanation is correct. Even when the
> transistor is cut off, some current flows. If the circuit is full
conduction
> the reverse impedance, for lack of a better term, is very low; there is a
> severe mismatch at the device output, and most reflected power is
> re-reflected. If the device is as completely cut off as transistors ever
get,
> the same conditions and re-reflection apply.
>
> BUT for most of a cycle a transistor output stage is in some state of
> conduction. So the amount of reflected power that is re-reflected varies
> according to the state of the conductive state of the output transistors.
>
> The amount of re-reflected power depends on the relative phase of the
> reflected wave and the conditions in the output stage, and range from
almost
> total to none.
>
> This situation sneaked up on many hi-fi audio designers when germanium
power
> outputs first came out. Some speakers generated enough back EMF, the audio
> analog of SWR, to drive the outputs into conduction, leading to transistor
> failures. One combination that comes to mind was several of the early H.H
> Scott amps driving the AR3. Transistors didn't last long even when volume
> levels were kept to a restrained level.
>
> 73 Pete Allen AC5E
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
|