On Fri, 21 Oct 2005 07:06:49 +0200, "Meindert Sprang"
wrote:

"Tamaroak" wrote in message
...
I have two VHF radios, both of which have a PA and foghorn mode and will
send a 20 watt signal into a horn mounted on my flybridge. Standard
Horizon says I can't just hook the two of them to the same horn becxause
the one radio will backfeed into the other, causing it to be harmed.
The solution is to get two diodes and wire them on the + wire before it
gets to the horn.
Question: What kind, size, specifications do I use for said diode? The
guy at Radio Shack looked at me like I was from Mars, when I asked him
this.
If the horn is 20 Watt and your battery is 12V, you'll need at least a 2 Amp
diode. Ask for a 50V/5A diode and you'll be fine. Higher specs on voltage
and/or current are fine too if that is what is available. The price
difference should be negligent. We're talking about a sub-$ component here.
Meindert

The solution can't work. Who proposed it? Standard? I hope not!

You are making some assumptions here based on the output of the audio amp being
limited to 12 volts. No such limitation exists_ The radio spec will tell you
what the output stage looks like. If the 20watt output rating is into 8 ohms
then it seems unlikely the output is limited to 12 volts. If it is rated into
4ohms the it is 12.

But even still it is likely coupled by a cap or transformer as most speakers
don't handle dc very well_ This makes the diode isolation idea sort of
impossible. Well.... you might put something together to create a bias with a
couple of diodes and some resistors then sum the biased AC the couple the summed
with a cap to the horn/speaker but you are going to loose 50% of your output
power (actually a little more) sort of a foolish way of putting two signals
together and if you thought the simple diode 'or' would work this solution is
probably a bit too difficult.

I suggest picking up one of the auto stereo booster amps_ Get one designed to
take the direct speaker outputs from the stereo and has an output that can be
bridged into a mono amp. Then put one radio to the right channel, one radio to
the left and bridge the horn across the outputs. Hey_ not only will you sum the
outputs of the two radio you'll have a great setup for blaring rap music out of
your fog horn...