Originally Posted by
Simon Barden
What's the ohm rating of the 24x5" cab? I can't see it on the PJB website. The minimum ohm rating for the Ampeg B2 amp is 4 ohms. The 8x5" is 8 ohms, so the 24x5" needs to be 8 ohms or greater to keep the combined impedance above 4 ohms. However , the 24 x5" arrangement may be less than 8 ohms, in which case it's not a good set up for the amp. So find out and we can talk about it again.
The next bit ignores the current amp and looks at maximum power usage you could get from the cabs:
Assuming each cab has an equal ohm rating, then a single channel amp's power output will be split equally between the two cabs, so the maximum power handling capacity is dictated by the power handling capacity of the smallest cab, in this case the 8x5" at 800W. So the amp output capacity used should be 2 x 800W = 1600W. Obviously this is less than the power handling capacity of the 24x5" cab on its own at 2400W.
If the cabs have different ohm ratings e.g. if the 24x5" cab is 16 ohms, then it will take about half the power of the 8x5" cab, so the total single channel amp power would be in the order of 800X x 1.5 = 1200W.
So, your cab configuration is more suited for either two mono power amps - one for each cab, or for a bi-amping set-up with a crossover, so that the smaller cab handles the higher frequencies and the bigger cab the lower ones. You'd normally feed the power amps from a pre-amp of your choice. Because the power handling of each cab is very different, I wouldn't suggest using a two-channel power amp and just turning one output down, but rather select two amps, which can each operate in bridged mono mode, and selected to match the power handling of each cab e.g. 1 at 800W into 8 ohms and 1 at 2400W into 8 ohms (or whatever the cab ohmage is). For bi-amping, you can add a crossover unit to split the feed from the pre-amp into bass and mid/treble feeds, with the smaller cab handling mid+ treble and the bigger cab handling the bass frequencies. Because the speakers will be more efficient at mid/treble frequencies, the smaller cab will be a good match for the bigger cab if used this way.
One thing that the PJB site doesn't say is if the speaker rating is RMS watts, program watts, or peak watts handling.
Typically a speaker's ratings would be: for a given RMS value, the program watts rating = 2 x the RMS value and the peak rating is normally around 4 x the RMS figure.
E.g. a 50W RMS speaker would typically be rated at 100W program and 200W (or more) peak.
The RMS power handling value is measured using a continuous signal. Unless you are putting a synth bass drone through the amp, then normal music has sufficient gaps and playing dynamics that reduce the average amount of power being handled by the speakers, such that you can use a power amp with up to twice the rated power for the speakers without causing them to overheat and fail. This is the 'program' rating. The 'peak' rating is the maximum momentary power peak (just a few milliseconds) that the speaker can stand without the voice coil insulation burning off and the speaker failing. Because this is very momentary, it's a lot higher than the continuous RMS rating.
Because 100W RMS is a lot of power for a single 5" speaker to handle, and PJB don't put RMS on the power figures, then I'd suggest that it's more likely to be a program than RMS rating, and that the RMS handling capacity of the cabs is probably half the quoted figures.
Using a solid state power-amp, you should be fine going up to the quoted 800W and 2400W of the cabs. They won't put out much more power before reaching their maximum output (though with a lot of distortion), so as long as you don't run them so the maps do clip, all should be fine. Valve amps can put out a lot more power than their rated power (which is taken on the edge of clean, typically with about 5% THD) by almost 100%, as they don't distort and max. out anything like as quickly as a solid state amp, so valve power amps shouldn't exceed the RMS rating of the speakers (obviously valve power amps of this power are very, very rare, but just keep it in mind if you ever get an Ampeg SVT head).