"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

Meindert Sprang wrote:

(Original post deleted)

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

Err... How is a diode going to isolate one radio from the other, since
the output of each radio is essentially AC? A diode will isolate the
second radio from the first during one that radio's output half-cycle,
but not from the other half-cycle. If two diodes are used, one in each
radio's output, there would still be the "backfeeding" problem unless
both radios were tuned to the same channel and were feeding the speaker
(horn) in phase. What would be the point of doing that? Additionally,
whacking off a half-cycle from the radios would not only reduce the
audio power due to the impedance mismatch (itself a destructive issue
with some solid state circuits) but would probably sound like poo-poo. No?

Ed

"Edward Greeley" wrote in message
...
Meindert Sprang wrote:

(Original post deleted)

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

Err... How is a diode going to isolate one radio from the other, since
the output of each radio is essentially AC? A diode will isolate the
second radio from the first during one that radio's output half-cycle,
but not from the other half-cycle. If two diodes are used, one in each
radio's output, there would still be the "backfeeding" problem unless
both radios were tuned to the same channel and were feeding the speaker
(horn) in phase. What would be the point of doing that? Additionally,
whacking off a half-cycle from the radios would not only reduce the
audio power due to the impedance mismatch (itself a destructive issue
with some solid state circuits) but would probably sound like poo-poo. No?

You are right, of course, when it comes to a normal speaker and AC audio
signals. But the OP was talking about a fog horn, so I assumed this radio
has an extra DC output to drive a fog horn. Right?

Meindert

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.

Capt. Jeff

On Thu, 20 Oct 2005 23:35:01 -0700, Tamaroak
wrote:

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.

Capt. Jeff

I don't think you will be able to do this Capt' Jeff since the output
is likely to be AC.

Is there no way to mount a second PA speaker? Also, why is it you
need the PA speaker for both radios? I am guessing that one is
mounted topside and the other below, but of course may be wrong.

Rob


Replace 'spam' with 'org' to reply

There are a couple of ways to do this.

1. Use resistive pads at the outputs of the two radios and a resistive
summation circuit to feed a separate amplifier which then feeds the
single speaker.

2. Use a stereo-to-monaural isolation transformer backwards. Each radio
feeds one stereo input and the monaural winding feeds your single speaker.

I am certain #1 will work, but it obviously requires purchase of an
amplifier and some soldering. Amps are not particularly expensive, but
this is probably not a job the average boater would undertake unassisted.

The second solution is more elegant, but I have no personal experience
with those transformers. Power rating would be an important
consideration, of course. It is my understanding that they provide on
the order of 20 dB of isolation, which should be more than adequate to
protect the two radios.

Good luck.

Chuck

Tamaroak wrote:
Two horns would be ugly. There has top be a way to do this.

Capt. Jeff

Right. The radio has a separate output for the foghorn/PA feature.

Capt. Jeff

Two horns would be ugly. There has top be a way to do this.

Capt. Jeff

In article ,
Tamaroak wrote:

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.

Capt. Jeff

Bzzzzt, Wrong answer, you can't Isolate AC (audio) by using Diodes, that
only works with DC Circuits. Even a RadioShack Sales Droid knows better
than trying what you advocate......


Me

In article ,
"Meindert Sprang" wrote:

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

Meindert, What are you thnking, man? Audio is AC.....You can't isolate
AC with diodes....


Me