Never, EVER connect two transistor amplifiers in parallel......

The output of a transistor amp looks like this:
+V
|
transistor 1
|
|--------------------------------output (0 VDC)
|
transistor 2
|
-V

The negative terminal of +V and the positive terminal of -V, two
separate power supplies, are hooked to "ground" the other side of the
speaker connected to "output". The 0 VDC comes because transistor 1
and transistor 2 are both conducting the same current, dropping their
respective power supplies full voltage, leaving the junction "output"
is connected to the balance of the two, 0 VDC to the common "ground"
of the two power supplies.

Audio is fed out of phase to the two power transistors, turning
transistor 2 to less current and transistor 1 to more current. The
difference current flows through the speaker to ground causing it to
move in direction A. On the other half cycle of the audio, transistor
2 turns on harder and transistor 1 current is reduced. Current flows
the OTHER way through the speaker hooked between "output" and ground.

Now, suppose you come along and connect ANOTHER of these
directly-coupled amplifiers to "output" by paralleling two radios.
Call the second set transistor 3 (on top) and 4 (on the bottom of my
drawing). If transistor 1 and 3 come on harder when transistors 2 and
4 conduct less, nothing bad happens and the speaker output current is
fine. But, WXXX, HOT FM 93, doesn't have the same audio as Channel
68. So, many times per second, Transistor 1 conducts hard at the same
time as transistor 4 comes on hard. You have just hooked +V through
two hard conducting transistors to -V, who is MORE than happy to
provide all the current you can imagine will flow. It doesn't flow
through the protective impedance of the speaker. It flows from -V
through transistor 4 to the common junction called "output" up through
transistor 1 to +V. MANY amps flow, melting one or both transistor's
junctions from the intense heat. You just blew BOTH transistor power
amps.....and the two power supplies. On the next half cycle, now that
there are two shorted transistors that have melted, transistors 2 and
3 conduct straight to + and - V of the other's power supplies. The
fuse blows right after all four transistors have fused together and
blown the power supply rectifiers....(c;

NEVER, EVER HOOK TWO TRANSISTOR AMPS IN PARALLEL!

See why?



On Wed, 4 Feb 2004 09:59:04 -0800, "Steve" wrote:

I have a pair of speakers in the cockpit coaming for the AM/FM radio..

I would like to figure out a way to connect the VHF external speaker to one
of these, so I can hear any calls on it in scan or monitoring of ch16.

Both radios are below deck so I can only hear them in the cockpit on a
remote speaker..

I know I could just use a toggle switch, but would prefer to find a way to
combine both outputs so the VHF would be heard over the AM/FM output.

Is there a simple way to do this and prevent one unit from harming the
other??

Steve
s/v Good Intentions





Larry W4CSC

No, no, Scotty! I said, "Beam me a wrench.", not a WENCH!
Kirk Out.....