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Hidro · 31-03-2010, 17:06

Gentlemen I am hooking up two VHF radios with PA systems to one PA horn. I would like to bring the wires from the horn in to a terminal block and hook both radios up to the same block. My concern is that the 30watt output of the radios might feedback into the other radio and damage it. Has anyone had any experience doing this? Thanks for the help. Glen

daddle · 31-03-2010, 17:24

Electronically...um...yes, probably okay....in reality....no, don't do it.

fairbank56 · 31-03-2010, 18:00

I would advise against this unless connecting through a double pole double throw switch. Modern marine VHF's use a dual output audio power amp chip set up for BTL mode where both outputs are connected to the hailer speaker output in balance. You should not generally connect two of these directly together although it really depends on knowing the exact output circuit configuration being used. You would need the schematics for each radio and know what your looking at to determine this.

Eric

unbusted67 · 31-03-2010, 18:08

Yeah two amps one speaker is a no no in the audio world.

Hidro · 31-03-2010, 18:47

Thanks for the response. Maybe I should have added that they wouldnt be in service at the same time. But my thoughts were as you have voiced and felt that the wattage from one radio might feed thru the block into the other radio and damage it. I thought of installing diodes to eliminate the backfeed, but the PA speaker also inputs sound back into the cabin and the diodes might block that feature. I am so ignorant in electronics, but seemed it would be nice to have the best of both worlds and not clutter up the boat with extra horns on the front.

VE3TIC · 31-03-2010, 19:08

If the audio has one ground lead and one hot lead, (use volt meter and measure each wire to chassis ground), you could use a blocking capacitor, probably 2 to 5 uf and it will block the dc but allow the ac audio signal to the speaker. If both leads are hot you would need 4 capacitors. Some reduction if frequency response and output level will be experienced but not much. Another option may be audio isolation transformers.

daddle · 31-03-2010, 19:16

Diode...that will be cool...all your PA announcements will sound like ZZ Top...fuzzzz...

Capacitors nor typical transformers will not help. The energy that will damage is audio frequency.

Well, if your sure they'll never both be outputting a full power you might be okay ... try it and see ... just buy new radios if they burn out :-)

VE3TIC · 31-03-2010, 19:22

Diode is a no no, it would rectify the audio and sound awful you are right.

When car stereos came out with ungrounded output's I used this trick with capacitors all the time and it works, may need to adjust the value but I seem to remember using 2uf.

daddle · 31-03-2010, 19:43

Hmmm....makes no sense...no one can say without schematics for both radios....why bother.

Who uses a PA anyway? Sounds obnoxious.

Hidro · 05-04-2010, 18:47

Guys thanks for the response and help here, as for the need of both radios and pa systems, I have fog horns that would be able to be used from either station. Plus I enjoy using it on the wife with BIG WAVE COMING!! She hates it when I do that, but it sure is fun. Im not sure how I will make the connections, but I dont want two PA horns on the boat. Will advise how it worked out. G

**GordMay** · 06-04-2010, 07:28

Quote:

Originally Posted by fairbank56

I would advise against this unless connecting through a double pole double throw (break before make) switch...

Which seems like a simple and effective solution, to me.

Fishman_Tx · 06-04-2010, 09:04

Hey Hidro,
What you want to do can be done but it's a pain in the butt and can be expensive as well. Where are the radios located? If one in the cockpit and one below, just add another speaker. Below feeds above and above feeds below. If both below (ex. fore and aft), then adding a 2nd external speaker would be the cheapest solution with the least amount of hassle.

A physical switch would be inconvenient and possibly dangerous in a crisis if left in the wrong position at the wrong time. For it to be useful, it would need to be located with the most "out-of-the-way" radio and would need to be "momentary" requiring it to be held while in use. When released, it would be active to the most commonly used radio.

But just for giggles, here's some info on bridged and paralleled amplifiers. What you need, specifically, is to "parallel" the radios' PA outputs to a single load output. It can be done, but not easily or cheaply. You would need to "pad" the power of both PA outputs and use a combiner circuit. There are no commercial parallel "combiners" available for this type of application that I'm aware of. It's all DIY.

http://en.wikipedia.org/wiki/Bridged_and_paralleled_amplifiers

http://www.bing.com/reference/semhtml/Bridged_and_paralleled_amplifiers?src=abop&fwd=1&q pvt=amplifier+bridge&q=amplifier+bridge

Fishman_Tx · 06-04-2010, 10:42

BTW, the switch needs to be a "momentary ON-ON DPDT" withOUT center-off. Radio Shack only has the center-off version, so you'll need to check around.

These part #'s fit the job....
Switches, Toggle Switches, Rocker Switches, Push Button Switches, Foot Switches

See #'s:
35-014
35-154