Re: whole house audio

["Keith Doxey" <ukha@xxxxxxx>]

Hi Scott.

For mismatched speakers use the following formula where a & b are the
speakers.

(a*b)/(a+b)

eg speaker a = 16 ohm, speaker b = 8 ohm

= (16*8)/(16+8) = 128/24 = 5.333 ohms.

You HAVE to work them out as pairs. The formula wont work with more
than that. for example, 3 x 16 ohm in parallel (because thay are all
the same just divide by number of speakers) = 5.333.

So for a,b and c all in parallel first work out ab and put ab in
parallel with c.

Using the formula will give (16*16)/(16+16) = 256/32 = 8
Then put the third in parallel give the first example above = 5.333
but trying to do it all in one hit give (16*16*16)/(16+16+16) =
4096/48=85 which is wrong.

If the speakers are different impedance the power will distribute
unevenly between them. For example with an 8 and a 16 the 8 will get
more power than the 16. (twice as much in fact). I could work it out
but cant figure out how to type the formulae into Yahoo :-(

Heat - will be reduced as the amp is only generating enough power to
drive the volume required. If there is nothing coming out thee is
only quiescent power going in. (I assume you are not an audiophile
using 1kW True class A monoblocks !)

Advice on avoiding AC Hum - AVOID IT ! IT sounds horrible :-)
Seriously, there shouldnt be any hum pickup on the speaker cables.
The only place would be at line level. If all the amps are in one
place then leads will be short and it wont be a problem. If the amps
are distributed around the home then Ground loops could be more of a
problem than pickup. In that case then balanced transmission using
either passive baluns (transformers) or active units (KAT5) and
twisted pair (CAT5) will overcome the problem.

Keith


--- In ukha_d@y..., Scott Crowther <scrowther@i...> wrote:
> cheers Keith, a great help.
>
> mono? no way!
>
> do you know what the score is if I use speaker pairs of different
impedance?
> it'll be a bit of a mismatched speaker collection...
>
> also, the rooms do vary greatly in size, so it seems your
suggestion of
> multiple amps connected to the 'tape out' may be more suitable so I
can
> set volume in each room. I assume this method would reduce a huge
amount of
> heat generation?


> Any advice on avoiding AC hum?
>