>Given a cheap 75W RMS amp, 4 ohm speakers, 8 ohm speakers >(all bass..) etc, how are things worked out??Answer: (IMHO, by Stuart Woolford, firstname.lastname@example.org)
OK. here we go...
you can *always* use a speaker of higher impedance with saftey - you will not harm anything, but (there is always a but..)
try and follow this:
1 - your amp products 'voltage' (not power), for 75W RMS into 4 ohms
using P=VI and V=IR (P=power V=voltage I=current, R=resistance)
P=75 RMS; R=4,
P=(V^2)/R (^ = 'to the power of')
V=17.3 volts RMS.
so at 75 W RMS into 4 ohms, it produces 17.3 volts RMS
I=V/R=17.3/4= 4.325 amps RMS
it will never produce more voltage, it *may* product more current.
if we go to an 8 ohm 'load' (speaker) we get:
I=V/R=17.3/8= 2.1625 amps RMS (half as much)
getting P=VI=2.1625*17.3= 37.411 W RMS, ie: less power.
this is also easier on the amp, as the 'heat' in an amp is proportional to the power, the 'strain' on the amp is proportional to the current (usually...)
NOTE: your pyramid amp may or may not be 75W RMS...
so, you will get less power delivered to the 8 ohm load,
when you bridge an amp, you get *twice* the voltage (assuming the amp can be bridged..), so *if* you could bridge the pyramid, you would get 17.3*2= 34.6 volts RMS, giving:
34.6/8= 4.325 amps * 34.6 = 149.645 W. (twice the power! into 8 ohms!)
34.6/4= 8.65 amps * 34.6 = 299.29 W (4 * the power into 4 ohms!)
of course, you cannot do this, few amps deliver 4 * the power when bridged, this is because the amp is working much harder, and the power supply (in the amp) cannot supply the extra current required, therefore the voltage drops, and you get distortion, overloading, overheating, etc.
I often find a good thing is to get a 'bridgable' 2 channel amp, bridge it, and run it to an 8 ohm load, this is easy on the amp, and allows the use of an 8 ohm speaker.
you can also add speakers in parallel as follows:
so if you parallel 2 8 ohm speakers, you get 4 ohm load.
if you parallel 2 4 ohm speakers, you get 2 ohm load.
if you parallel 1 8 ohm and 1 4 ohm load, you get 2.6666 ohm load.
if you put 1 8 ohm and 1 4 ohm, in parallel, over your current amp (1 channel) you would:
a - heat/blow the amp, 2.666 is too little load for it.
b - get the following:
17.3 volts across each speaker, giving:
17.3/8 = 2.1625 amps through the 8 ohm, and
17.3/4 = 4.325 amps through the 4 ohm.
2.1625*17.3= 37.411 W for the 8 ohm
4.325*17.3= 74.823 W for the 4 ohm
the amp must supply 2.1625+4.325=6.4875 amps= 112.23 W. RMS.
remember, the more power you use from an amp, the higher the distortion, NO MATTER WHAT!, if you go too high, you may blow something.
Feel free to ask me any other questions..Stuart Woolford