Newsgroups: rec.audio.tech
Subject: Re: making a speaker selector box
References: <[email protected]>
"andrew_h"  writes:

> Hi,
> 
> I want to make a speaker selector box so I can alternate between having
> the music in the lounge (2 speakers) or dining room (2 speakers) or
> both.
> 
> The lounge speakers are both 6ohm, whereas the dining room speakers are
> 4 ohm each.
> 
> How hard would something like this be to achieve? I have often read
> about impedance matching - is this hard to achieve?

With the speakers you have and the normal hifi amplifiers this 
is a little bit hard to do... 

I your speakers were 8 ohms both, and your amplifier cna handle 
speakers down to 4 ohms, then things woudl be easy. 
Then you could just put switches that switch the different 
speaker sets connected or not connected to amplifier output. 
Separate switch for both speaker sets allows you to control 
each speaker separately (if both are put on they are 
connected in parallel to amplifier output). 
Some hifi amplifiers even have two sets of speaker outputs wired 
in this way, with front-panel swithes to control A and B 
speaker sets on/off. I have at least one amplifier that has this. 

There is another trick that some amplifiers use: when both 
A and B speakers are turned on, they are wired in series. 
So the impedance doubles (two 4 ohm speakers = 8 ohms, 
two 8 ohm speakers = 16 ohms). This will be safe for amplifier, 
but limit the maximum output power from ampifier 
(you get less total power for two speaker sets than what you 
get for one speaker set). The speaker series connection 
work quite acceptably if the both speaker sets are exactly 
same type speakers. If the A and B sets are different type 
speakers (different manufacturer, different model etc..) the 
sound quality can drop considerably (there will be interaction 
bwrween speakers when their load impadances at different 
frequencies can be considerably different.. can cause 
very sonciderable frequrncy response problems!). 

> basically the lounge speakers impedance is fine .. the amp's
> recommended and minimum is 6 ohm. So, the 2 dining room speakers
> present too little a load. Ultimately, I could add two 2 ohm resistors
> to each line for the dining room speakers, as dummy loads - yes?

Adding two ohms resistance in series with 4 ohm speakers will 
turn the whole load look like 6 ohms to amplifier. This works. 
The downside of this is that this series resistance can 
affect your speaker sound quality more or less. The ideal 
speaker wiring and amplifier output should have zero ohms. 
Other problem is the lost power in that resistor, the problem 
that you need to turn the ampifier a little bit louder to 
compensate the power loss is not typically a problem, 
but the problem is that the lost power ends up heating 
the resistors: you need power resistors capable of handling 
the lost power (around one third of what gets out of amplifier) 
and have it installed so that when the resistor heat up they 
do not cause danger. 

Even if you have all speakers "tweaked" to 6 ohms, this 
will not solve the problem. If you wire two 6 ohm speakers to 
one output, the amplifier will see a 3 ohms load. It is 
definately too low impedance for your 6 ohms capable amplifier. 

> Problem is, could there be any way I could have all four on ? 

Buy an another amplifier to drive the other speaker set is 
one option. This gives you the bonus of having spearate 
volume controls for those different locations. 

> If each
> lounge speaker and dining room speaker were connected in parallel back
> at the amp (i.e. left lounge speaker and left dining room speaker
> connected to left amp output), the impedance would be too low.

Definately too low. 

> How could I match the impedance of the load to 6ohm (or something
> higher than that)?

The methods to try to do the matching are not very practical. 
Adding series resistors will loose lots of power and definately 
cause sound quality problems. Not good. 

There are also impedance matching transformers for speaker 
applications. Those can convert impedance quite well, 
but this kind of transformers are generally quite expensive 
speacial products.. Usually not economically feasible, and 
still can have effect to sound quality. 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/