"John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in message
> On 31 May 2006 01:20:04 -0700, epiang@xxxxxxxxx wrote:
>>Pooh Bear wrote:
>>> What youd need to watch is the impedance. Car speaker are normally 4
>>> ohms to get
>>> more watts from a limited voltage source. Hi-fi loudspeakers are
>>> typically 8
>>> ohms. Just be sure your receiver's happy with driving 4 ohms.
>>Oh yeah... limited voltage source. So, from V=IR, is it typically
>>about 24V or so driving hi-fi loudspeakers?
> Depends on how much power your receiver can pump out.
> Assuming it's a voltage source and is rated to put 100 watts into 8
> ohms means that the voltage it needs to supply into across that
> impedance will be:
> E = sqrt(PR) = sqrt (100W * 8R) ~ 28VRMS
> And the current:
> E 28V
> I = --- = ----- ~ 3.5ARMS
> R 8R
> Since the current the amp can supply is what will limit how hard the
> 4 ohm load can be driven, we can say:
> P = I²R = 3.5A² * 4R = 50W
> It's 50 watts instead of 49 because of the rounding we did earlier,
> but the point is that with a load of 4 ohms instead of 8 ohms you
> can only get half the rated power from the amp. There are a few
> more things to consider, like how much voltage headroom the amp has
> and how hard you can overdrive it before you hurt it, but with a 4
> ohm load you should start hearing distortion with the volume control
> cranked to a little over "5".
> John Fields
> Professional Circuit Designer
That is not true if the amp is capable of driving a 4 ohm load. The specs on
my Onkio are:
70 W/Ch into 8 ohms
90 W/Ch into 4 Ohms
He could get around the whole problem by buying 2 pairs of speakers, and
having 2 in series for each channel. Still, there is the question of what's
the point? To get hi-fi sound, he would have to add a crossover network and
high frequency speakers. Everything would depend on the desighn of the
cabinet. Probably better off buying packaged speakers.