VHF Extension Speaker

Goldie

Goldie

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My current speaker is quietly corroding away and replacement is imminent. Having looked around, there is a choice of 4 Ohm Impedance or 8, and outputs of 5W, 20W and 30W to choose from. The current speaker is of indeterminate age and make (the labelling has long since succombed to the elements) so how do I know what I need? VHF (for which I have no manual) is a Seavoice RT550 circa 1990. Will a higher output wattage result in a better (louder for a given volume setting) output? The speaker is in a cockpit cave locker so a bit of extra 'oomph' would be nice if it can be done without having the volume setting down below ridiculously loud. All help appreciated on the basis that regarding electronics, I know nothing.... /forums/images/graemlins/blush.gif
 
I think the wattage is the maximum that the speaker can handle, the loudness is determined by the VHF. If you pick a speaker with a wattage that is lower than the VHF you may damage the speaker when you turn the volume right up.

Wattages are usually given in either Peak or RMS (if you can find a manual it may say 30Wpeak, or 30Wrms for exmple) so just make sure you compare like for like. It may be the 5W is an RMS value and he 20W and 30W are peak values, different manufactures may give the same information different ways, which can make it difficult to compare.

As for the impedance, to get the maximum power output you should match the speaker impedance to the VHF radio. Again, if you can, find out what the output impedance of your VHF is and buy a speaker with the same impedance.

I've no idea what will happen if you mismatch the impedance. The audio output may be less, but I don't know if it will damge the output stages of the VHF radio's audio amplifier. Probably not. I'm sure someone will be along soon who knows more.
 
Ian, the output of your radio for an external speaker is 3W into 4ohms so no point in going for anything above the 5W 4ohm speaker you mention.

I've got the handbook for my old RT550 here. I'll copy it and see if I can send it to you by PM. Colin. /forums/images/graemlins/smile.gif
 
If the speaker is still working why not put a multi-meter across the terminals (disconnected) set it to the ohms nearest what you think the speaker is and measure the existing one. (nearest on my meter would be 200 ohms).
 
That would not help. The figure given is the impedance, not the resistance, of the speaker. If you put a multimeter across the terminals you would expect to see a very low resistance.

It seems that the correct impedance is 4 ohms, there would be no problem putting an 8 ohm in place but it would be quieter.

The measure of the amount of sound a speaker will put out is known as the "sensitivity" that is measured in db/W, the higher the figure the louder the speaker will be for any given input. Unfortunately this figure is not often given for extension speakers and the like.
 
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My current speaker is quietly corroding away and replacement is imminent. Having looked around, there is a choice of 4 Ohm Impedance or 8, and outputs of 5W, 20W and 30W to choose from...

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A VHF will almost certainly have a 4 Ohm speaker output. Note that a meter <u>will</u> give a reasonable approximation in this case. Around 20W should be ample for most power outputs.

Have a look here for a good selection.
 
A big 'thank you' to Heigaro who has emailed me a copy of the RT550manual, and to the rest of you for your input. I think I have the information I need before parting with my cash. Many thanks one and all.
 
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