how to design a speaker selector

[verbion]

I want to build a public address system with more than 12 speakers. I have a 300-watts power amplifer but I can't connect those speakers straightly in parallel or serial, can I? My friend told me that I can use a speaker selector to distribute the power without degrading the efficiency of it. Can you help me how to design a speaker selector like that? I've searched the internet but I still can't find the appropriate references for this project.

Thanks in advance,
Verbi

 

[mechie]

Speaker Selector ?

What do you want from the selector ?
Are you wanting to route all 300W to one speaker?

If you want all the speakers to sound together (typical PA system) then they all need to be connected at the same time, there is no way round this.

What power rating are the speakers? (a 100v transformer is common on PA gear - do these speakers have them?)
What is the amp output? (this is normally 4 to 16 ohms for a hi-fi; 100volts for PA line-driving)
What kind of length cable runs are there to the speakers? (a few metres or a hundred metres?)

 

[verbion]

Actually, I have 20 speakers and I want to divide them into four groups (for four floors in my building with five speakers in each floor). I need to activate the speakers according to their group, so I can select only one group, two groups or all of the groups (in this case, all of the speakers turned on together).
I use speakers with max. power rating 20W (and these speakers don't use 100v transformer in their configuration) with impedance 8 ohms. There are three terminals on the amplifer output: 4 ohm, 70V and 100V. I will use NYMHY cable for my configuration and the distance between two speaker is not less than 25 m.

 

[mechie]

PA System connections?

My guess...
If you want to avoid buying 100v line transformers for every speaker then the five speakers for each area want to be in series (40R total, 100W max handling), this will stop the 300W amp giving its full output and vapourising the voice coils.
Each area can have its own switch to select the speakers, if all are selected together then the amp will see 10R, which will not overload it.
see attached sketch...

If you get universal line-matching transformers then you also have some flexibility in setting individual speaker volumes by changing tappings.
RadioSpares webpage
For this option use the amp's 100v output, all speakers (each has its own transformer) in an area in parallel. The switching remains the same.

Hope that helps :wink:

 

[verbion]

Thanks, that's really help.

 

[Gene]

you might want to consider the step-down transformers with all the wire you will be running. That is normal with PA systems. It's like what the local power company does - there is probably a transformer in your neighborhood.