Advice on speakers for Home Cinema rcvr...
 

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I can't understand the meaning of ohms at all, tried looking at various
webpages with diagrams but it still doesn't make any sense.

To get a given output sound level your amplifier puts a given voltage
across the speaker terminals. The speaker then demands a current.
Impedances in Ohms are simply the ratio between these.

So when you have a load with an impedance of X 'Ohm's, when your amp puts a
voltage of 'V' volts across the speaker terminals it has to supply the
speaker with a current of I = V/R amps.

Does that make it clear? Or have I missed the point?

From what I can gather, most speakers and systems use 8ohms and if you
go lower than 4 it starts to get dangerous, risking damaging the amp??

The problem is that with most speakers the effective impedance varies with
frequency. Hence to put a given a.c. voltage on its terminals at some
frequencies requires more current than at other frequencies. This means the
impedance value quoted for a loudspeaker is just a sort of 'average' of
'typical' value for what you get over a range of audible frequencies.

With amplifiers the powers quoted are when applying signals to a resistance
whose value is the same at all audio frequencies. Thus you can use the '8
Ohms' values as a general guide, but this hides details that are usually
unimportant, but sometimes matter. :-)


Also, I need help understanding speaker power in W's.

The RX-V730RDS Home Cinema Receiver is rated 125 W per speaker.

My amp is rated 125w per channel, if i buy speakers that are rated 150w,
200w or 250w per speaker, will I lose out on the quaility? Should I
*only* be buying 125w rated speakers for the 125w amp?

Nope. As John as already indicated, these values are also only really
'guides'. The power value given for the amp is (or should be!) the maximum
amount it can driver steadily into a stated impedance. The speaker values
are 'finger in the wind' estimates of the power levels you can expect to
get away with for brief periods.

In practice, most music consists of occasional brief high-power bursts with
a much lower mean level. Hence if you are using a 100W set of items it is
quite likely that the average power is well below this.

I suspect that on both the 'Ohms' and 'Watts' points you confusion may be
coming from the *different* ways that amplifier and speaker makers use
these values. Alas, we are stuck with this due to the way these things have
evolved in practice. :-)

Thus do not take the power or impedance ratings of items too seriously.
They are useful as a vague guide, but no more than that unless you get much
more detailed info on the units.

Hope the above helps.

Slainte,

Jim

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