RE: Re: 4ohm or 8ohm?

["Keith Doxey" <ukha@xxxxxxx>]

Hi Nik,

> -----Original Message-----
> From: Nikola Kasic [mailto:nikola@xxxxxxx]
>
> Thanks Keith,
> I thought about it, but what to do if I have only 3 of them, like now,
> or 7 like I will end up with (I don't have 8th wire, maybe to speak in
> node0 :-).

You can put them all in series if you wish giving a total impedance of 24
ohms. Any modern amplifier wont complain ( you can always go higher except
for valve amplifiers where the impedance must always be correctly matched,
going too low is the dangerous thing from the poor little amps point of
view), it just wont be as loud.

> Can I just put 8ohm resistor in place of missing one?
> I think that it's not that simple, because this impedance
> changes across
> the frequency range or something like that.

Yes, an 8 ohm resistor will work fine provided it can handle the power it
is
subjected to. In the diagram I did, each speaker ( or dummy resistor ) will
receive equal power - 25% of whatever the amplifier is giving out at the
time.


> Also, I'll have two of them different for bathroom
> (waterproof). What do
> I need to put attention to when choosing those two, apart
> from being of
> same impedance.
> Do they have to be the same power rating?

No, but as per the comment above, they must be capable of handling the
power. You could put a 1W and a 400W speaker in series provided you didnt
drive them with more than 2W.

> Also, in this case of 8 speakers, how do I calculate power rating for
> amplifier?
> 8x15W = 120W
> Do I go then for 2x60W amp?
>

That is the power handling capacity of the speakers. It depends how hard
you
are going to drive it, for background use you will probably never drive
each
speaker with more than a couple of watts so even a 25W amplifier would
handle that.. BUT.. there would be nothing left in reserve.

So many things affect the power you need...

1. If you are slightly deafened from years of high level sound systems like
me you will probably want it louder so more power!

2. the efficiency (aka sensitivity) of the speakers. I have some that are
99dB others that are 87dB. That 12dB difference is quite staggering in
terms
of power. A 3dB increase/decrease in level requires a doubling/halving of
amplifier power therefore 12db = 2x2x2x2 = 16 times the power, or to put it
another way, driving the 99dB speaker at 5W would be the same volume as
driving the 87dB speaker with an 80W amplifier !

The important things to remember are...

1. Dont overdrive the speakers with an amp that is too big
2. Dont overdrive the amp so it goes into clipping.

An overdriven amp will destroy speakers quicker than an amplifier that is
too large and will also sound far worse. You could safely use a 1000W
amplifier on a 20W speaker provided it is NEVER driven above 25W max on the
music peaks but bear in mind it would have the capability to destroy the
speaker almost instantly if driven flat out. In practice, it is best to
keep
amplifier and speaker ratings reasonably close to each other but always
choose something bigger than you will ever need so that you never overdrive
anything.

> I'm a bit concerned about heating. Are those amplifiers
> generating heat even in standby mode.
> If I keep them on e.g. 50% volume all the time (I asume I have to if I
> want them to be ready to play when signal comes to input), does that
> mean that they'll consume the same power and generate the same heat,
> wheather something is playing or not?

No. All amplifiers consume some power when idle, this is known as the
quiesent power. My first disco amp was rated at 100W and had a quiesent
dissipation of about 10W. It doesnt matter where the volume control is set
if there is no signal going through it because it is still idle. When you
play some music the amplifier will obviously generate more heat, but it
would be exactly the same as if you had just physically turned the amp on.

Again, using a larger amplifier will mean it will be driven less so will
run
cooler. I always try to install a sound system at least twice as big as
needed so it goes on forever. I have never had a blown amp or speaker on a
system that I have installed or upgraded.


Keith

www.diyha.co.uk
www.kat5.tv