4 radios 1 extension speaker ?

[Flump]

I have 4 different ham radios and would like to use one extension speaker
which has sound from each radio all the time, so no switching.
I dont need a volume control on the speaker as the radios have that
so could I use a 4 diodes, 1 in series in each of the + speaker cables
then connect them all to the same speaker ?

The diodes would be to stop audio signal going back in to the radios
seeing as they are all connected together.

[Richard Crowley]

A diode in the audio path will rectify the audio signal into something unintelligible. 
But the concept is correct, you want to allow audio signals from the sources to be combined into the destination.

The easiest way of doing that properly is probably to use a powered speaker and a simple resistive mixer.
The relatively low output impedance of the sources (which are designed to drive speakers directly) will allow using a very simple mixer circuit without worrying about signals feeding back into the other sources.

A powered speaker could be as simple as a recycled $2 "computer speaker" from a charity shop, etc.  (or free from the trash bin).

[Flump]

Thanks very much Richard.

Would this Mixer work ?
http://www.ebay.co.uk/itm/Soundlab-4-Channel-Portable-Lightweight-Mono-Microphone-DJ-PA-Mixer-Patch-Lead/162606916680?epid=1578034833&hash=item25dc20a448:g:654AAOSwCcZaGZCz

Radios to the Mixer inputs, Mixer output to Audio amp then to a Speaker ?

[Richard Crowley]

That would work only if you pad down the signals. That mixer wants to see microphone level, and your radios (presumably?) are outputting speaker-level.  And if you are going to make pads, it would be easier to just make a passive mixer with 5 resistors.  As you say, you don't need volume controls on the inputs because you can control the audio levels on each of the receivers.

There are similar mixer products like that which are designed to take line-level inputs. Line-level is roughly equivalent to speaker-level (at least for voltage which is all that counts here).  I would look for a line-level mixer rather than fool around padding everything down to mic-level.