Would I be right in thinking....

[nigelwright7557]

Would I be right in thinking if I had a 3 way system working off 3 frequency bands and all speakers with crossovers were 8 ohms that I just need a 8 ohm amplifier.
Or would all these speakers be in parallel so need to have 2.666 ohm amplifier ?

 

[fpitas]

With three 8 ohm drivers in a 3 way system, a properly designed crossover will present somewhere near 8 ohms to the amplifier. The drivers are not simply paralleled, because of the reactive elements of the crossover.

 

[cT equals piD]

The only way that the amp could see a load of 2.666 Ohm would be if the voltage drop across each of the three 8 Ohm drivers was equal to the amp's voltage. That isn't going to happen as the crossover network delivers a fraction of the total voltage to each driver corresponding to the range of frequencies to be handled by the driver.

 

[wintermute]

Nigel, I usually find a graphical representation to be useful. I've attached a modified version of my two way crossovers impedance plots. Black is the low pass impedance (both are actual impedance seen by the amplifier driver + crossover) and Blue is the high pass impedance. with frequency.

In this case the crossover frequency is 2.8Khz. I took out the notch filters as they made the low pass impedance curve look very lumpy and it is not as useful as a demonstration.

The high pass levels out at around 24 ohms because there is an lpad before the crossover which has a 22ohm resistor in shunt, otherwise it too would have continued on up to infinity as frequency dropped due to the capacitors increasing impedance as frequency drops.

Tony.