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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