Just wire the VHF external speaker output to the remote speaker. It won't
hurt anything if you have both the am/fm radio and vhf on at the same time,
just difficult to understand so you will have to turn the volume down on one
or the other.


"Steve" wrote in message
...
: 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
:
:
:

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.....

"Larry W4CSC" wrote in message
...
Never, EVER connect two transistor amplifiers in parallel......


snip large story

Actually, in high power car systems, they do exactly what you just said that
cannot be done. In such systems, each channel has two amplifiers, carrying
the signal in opposite phase. The speaker is connected to both outputs. This
way, the maximum voltage that can be created across the speaker is doubled.

So based on this same principle, it would be possible to connect the speaker
between the hot wires of the outputs of the VHF and the radio..... The only
thing that spoils the fun is the fact that the DC blocking capacitor in
series with the output expects the speaker to be connected to ground. So an
extra electrolytic capacitor of 1000uF with the minus to one output and the
plus to one side of the speaker is needed. Another possible problem is the
power: both amplifiers must be able to withstand the power output of the
other. In the above example of a high power car system, this is the case.
However, the amplifier of a VHF will probably not tolerate the much higher
power of an average car stereo.

To make a long story short: take Larry's advice and don't do it. But
technically it is possible.

Meindert

"Meindert Sprang" wrote in message ...
"Larry W4CSC" wrote in message
...
Never, EVER connect two transistor amplifiers in parallel......

Actually, in high power car systems, they do exactly what you just said that
cannot be done. In such systems, each channel has two amplifiers, carrying
the signal in opposite phase. The speaker is connected to both outputs. This
way, the maximum voltage that can be created across the speaker is doubled.


First of all, to do what you describe (double the max. voltage) the amplifiers
would be connected in series not in parallel. Secondly, as you say, both
amplifiers are fed the same input signal in opposite phase, not different
signals as has been suggested. This technique is called "bridging" and has
been used for years in concert hall PA systems and other applications where
LOTS of decibels are desired (such as ridiculously over powered car stereos).
Connecting two transistor amps in parallel with different input signals is
only a good idea if both the VHF and the AM/FM radios are old and you'd like
to replace them but you wife won't let you because "they work just fine". ;-)
Don't do it.

Bob

"Bob Gough" wrote in message
om...

First of all, to do what you describe (double the max. voltage) the
amplifiers
would be connected in series not in parallel.

No. It's a bridge configuration.

Secondly, as you say, both
amplifiers are fed the same input signal in opposite phase, not different
signals as has been suggested.

That makes no difference. In fact, when both aplifiers get the same,
opposite signal, it stresses the amps most, because if one outputs maximum
positive voltage, the other outputs maximum negative. With different
signals, the difference in voltages is always lower on average.

Which reminds me of a configuration that was quite popular many years ago:
to create some sense of depth in a stereo signal, some people connected a
third speaker between the 'hot' ends of the stereo outputs.

So again, it is quite possible, the only requirement is that both devices
have roughly the same power output capability.

Meindert

"Bob Gough" wrote in message
om...

First of all, to do what you describe (double the max. voltage) the
amplifiers
would be connected in series not in parallel.

No. It's a bridge configuration.

Secondly, as you say, both
amplifiers are fed the same input signal in opposite phase, not different
signals as has been suggested.

That makes no difference. In fact, when both aplifiers get the same,
opposite signal, it stresses the amps most, because if one outputs maximum
positive voltage, the other outputs maximum negative. With different
signals, the difference in voltages is always lower on average.

Which reminds me of a configuration that was quite popular many years ago:
to create some sense of depth in a stereo signal, some people connected a
third speaker between the 'hot' ends of the stereo outputs.

So again, it is quite possible, the only requirement is that both devices
have roughly the same power output capability.

Meindert

"Meindert Sprang" wrote in message ...
"Larry W4CSC" wrote in message
...
Never, EVER connect two transistor amplifiers in parallel......

Actually, in high power car systems, they do exactly what you just said that
cannot be done. In such systems, each channel has two amplifiers, carrying
the signal in opposite phase. The speaker is connected to both outputs. This
way, the maximum voltage that can be created across the speaker is doubled.


First of all, to do what you describe (double the max. voltage) the amplifiers
would be connected in series not in parallel. Secondly, as you say, both
amplifiers are fed the same input signal in opposite phase, not different
signals as has been suggested. This technique is called "bridging" and has
been used for years in concert hall PA systems and other applications where
LOTS of decibels are desired (such as ridiculously over powered car stereos).
Connecting two transistor amps in parallel with different input signals is
only a good idea if both the VHF and the AM/FM radios are old and you'd like
to replace them but you wife won't let you because "they work just fine". ;-)
Don't do it.

Bob

On Thu, 5 Feb 2004 16:20:26 +0100, "Meindert Sprang"
wrote:

"Larry W4CSC" wrote in message
...
Never, EVER connect two transistor amplifiers in parallel......


snip large story

Actually, in high power car systems, they do exactly what you just said that
cannot be done. In such systems, each channel has two amplifiers, carrying
the signal in opposite phase. The speaker is connected to both outputs. This
way, the maximum voltage that can be created across the speaker is doubled.

Better look closer. In "Bridged" mode, the amplifiers are in SERIES,
not in parallel....

There are four 200W amps bridged into 2-channels at 400W each in my
stepvan. These power two 12", 3-way speakers mounted to swing out on
either side of the back of the van on the full-width swinging rear
loading doors. They'll even turn around to the sides pointing the
speakers in opposite directions, if the job necessitates. Two more
speakers, massive JBL pro-sound speakers where even the 15"
bass-reflex woofers are mounted in horns, re-juvenated last year with
600W Eminence Kappa 15LFA beasts, the biggest that would fit into
these cabinets, are wheeled to go on either side of the truck, making
a wall-of-sound DJ machine I rent out to parties, car lots, anyone
needing independently-powered music/PA. A nice DJ mixer board
completes the system, powered from a 500W Tripplite inverter.

It all runs off 700AH golf cart "house" batteries I can separate from
the truck starting batteries for 12-14 hours of battery-operated
continuous music without running an engine or genset. The Honda
EU3000is 3KW inverter-technology super-quiet genset comes on to
recharge via the now-surplus 40A charger off the Amel Sharki ketch my
captain gave me.

It's all legally licensed by ASCAP and BMI to play their entire
portfolio, most of the music you hear.




Larry W4CSC

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

"Larry W4CSC" wrote in message
...
On Thu, 5 Feb 2004 16:20:26 +0100, "Meindert Sprang"
Actually, in high power car systems, they do exactly what you just said
that
cannot be done. In such systems, each channel has two amplifiers,
carrying
the signal in opposite phase. The speaker is connected to both outputs.
This
way, the maximum voltage that can be created across the speaker is
doubled.

Better look closer. In "Bridged" mode, the amplifiers are in SERIES,
not in parallel....

That is what I meant. I said: the speaker is connected to both outputs. What
I meant here is that one wire of the speaker is connected to the output of
one amp., the other wire of the speaker goes to the other amp.

Meindert

"Larry W4CSC" wrote in message
...
On Thu, 5 Feb 2004 16:20:26 +0100, "Meindert Sprang"
Actually, in high power car systems, they do exactly what you just said
that
cannot be done. In such systems, each channel has two amplifiers,
carrying
the signal in opposite phase. The speaker is connected to both outputs.
This
way, the maximum voltage that can be created across the speaker is
doubled.

Better look closer. In "Bridged" mode, the amplifiers are in SERIES,
not in parallel....

That is what I meant. I said: the speaker is connected to both outputs. What
I meant here is that one wire of the speaker is connected to the output of
one amp., the other wire of the speaker goes to the other amp.

Meindert