In article ,
"Meindert Sprang" wrote:

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

Nope, most, if not all, Radio/Hailer/Foghorn combo's use a Waveform
Generator feeding the audio circuit. The "Giveaway" was he stated that
it was a 20 Watt Audio Speaker, that he wanted to drive from both
systems.


Me

A simple single pole double throw switch will work. Get a small radio
shack junction box, run the output of each radio into it, mount the switch
in the box, run the output to the speaker.

Eric

"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.

Capt. Jeff

As a follow-up, the easiest solution, by far, and perhaps the least
expensive one, is to purchase a dual voice coil speaker. Each voice coil
is completely (as they say) independent of the other so there is no
damage to equipment. I have no idea whether you can find a dual voice
coil horn, but you can search for one.

Good luck.

Chuck

chuck wrote:
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

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...

In article k.net,
chuck wrote:

As a follow-up, the easiest solution, by far, and perhaps the least
expensive one, is to purchase a dual voice coil speaker. Each voice coil
is completely (as they say) independent of the other so there is no
damage to equipment. I have no idea whether you can find a dual voice
coil horn, but you can search for one.

Good luck.

Chuck



Motorola used some really nice dual voicecoil Speakers in their
Bridge to Bridge version Modar Vhf Radios. These were designed in just
for this specific purpose, to combine audio from both the main
Receiver, and the Ch 13 Monitor receiver, that was grafted on the top
of the main radio. I always liked the simplicity of this design.
Maybe Motorola still has some of these in the supply system......


Bruce in alaska
--
add a 2 before @

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

Not everything is clear to me, however, why do you need to use both
anyway?????

We got the extra amplifier solution, as well as the dual voice coil
PA, and toggle switch method, and summing transformer method. I think
thats
it. What are you going to do?????

greg

??

On Fri, 21 Oct 2005 14:37:54 GMT, chuck wrote:

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.

Not a great idea in that you'll want to be putting something like a 3-4 ohm pad
in there. AND if you have a 20 watt output (not likely into 8 ohms) you'll want
to use a 20 watt resistors. (atually 20watt resistors will get pretty damn hot!)
Not to mention you'll be loosing most your output power in heat. Pretty poor
solution. - If you don't care about the loss then use some like 3 ohms for the
resistors. Assuming the amps are rated to drive no less than 4 ohms this will
keep you safe if the horn is 4 or 8. Seems like a rather silly solution with
this much power loss. Best you'll ever get is less than 3 watts to the speaker
at full amp output.


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

Except for the DC component this is no different than just tying the leads
together. The transformer provides DC isolation not AC isolation. The outputs
from the two radios are likely ALREADY isolated in terms of the DC. This is not
a option.

wrote:
On Fri, 21 Oct 2005 14:37:54 GMT, chuck wrote:


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.


Not a great idea in that you'll want to be putting something like a 3-4 ohm pad
in there. AND if you have a 20 watt output (not likely into 8 ohms) you'll want
to use a 20 watt resistors. (atually 20watt resistors will get pretty damn hot!)
Not to mention you'll be loosing most your output power in heat. Pretty poor
solution. - If you don't care about the loss then use some like 3 ohms for the
resistors. Assuming the amps are rated to drive no less than 4 ohms this will
keep you safe if the horn is 4 or 8. Seems like a rather silly solution with
this much power loss. Best you'll ever get is less than 3 watts to the speaker
at full amp output.


Which of us is confused here? I offered as one solution the use of
resistive pads "... to feed a separate amplifier which then feeds the
single speaker". There is really nothing to limit the power that the
separate amplifier could produce. Use a 20 watt amplifier and get 20
watts out. Pretty simple concept.

I certainly agree it's not a "great" idea. But it is not nearly as bad
as what you imagined I said. If there are volume controls on the units
being combined, then power into the pad could be as small as you'd like
and there would be no power loss to speak of. But if the full outputs
were being fed into the pad, then some power dissipation would have to
occur. Still, it is unlikely that anything like 20 watts rms would be
continuously fed into a speaker on a cruising yacht so the resistors
would not have to be rated for such high power.

This is basically an alternative version of the auto stereo booster amp
you suggested. Probably cheaper, but not certainly not simpler.


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


Except for the DC component this is no different than just tying the leads
together. The transformer provides DC isolation not AC isolation. The outputs
from the two radios are likely ALREADY isolated in terms of the DC. This is not
a option.

I think you misunderstood what I suggested and how such an isolation
transformer would work. RF power combiners are routinely used for this
exact purpose (at RF, of course) and easily provide on the order of 30
dB of isolation. Although I have never worked with a hybrid or combiner
at audio frequencies, one can easily be designed. You can find passive,
transformer-type audio combiner circuits on the Internet. although these
are generally for low-level signals. They can provide up to 55 dB of
isolation. It is my understanding that power-level hybrids are
manufactured but I have been unable to locate any on the web. A very
simple solution if a power hybrid can be found.

The dual voice-coil speaker, as I mentioned in my subsequent post, is
undoubtedly the best solution in my opinion.

Regards,

Chuck

On Sun, 13 Nov 2005 15:53:09 GMT, chuck wrote:

wrote:
On Fri, 21 Oct 2005 14:37:54 GMT, chuck wrote:

Which of us is confused here? I offered as one solution the use of
resistive pads "... to feed a separate amplifier which then feeds the
single speaker". There is really nothing to limit the power that the
separate amplifier could produce. Use a 20 watt amplifier and get 20
watts out. Pretty simple concept.


You're right.. Missed the additional AMP included. Same as the 'off the shelf'
car amp. Doesn't seem like the fella is likely to be capable of rolling is his
own here.


I think you misunderstood what I suggested and how such an isolation
transformer would work. RF power combiners are routinely used for this
exact purpose (at RF, of course) and easily provide on the order of 30
dB of isolation. Although I have never worked with a hybrid or combiner
at audio frequencies, one can easily be designed. You can find passive,
transformer-type audio combiner circuits on the Internet. although these
are generally for low-level signals. They can provide up to 55 dB of
isolation. It is my understanding that power-level hybrids are
manufactured but I have been unable to locate any on the web. A very
simple solution if a power hybrid can be found.

The dual voice-coil speaker, as I mentioned in my subsequent post, is
undoubtedly the best solution in my opinion.


Never heard of a audio combin at high power levels. Plenty of stuff around for
low or line level combining. Never heard of anything handling 20watts. Doesn't
seem like a particularly useful thing.