Question about amps, ohms, and cab pairing.

[u84six]

I'm a little confused about how companies pair their heads with cabs. I was looking at the Ampeg Portaflex, and the PF-500 amp is rated at 500W RMS @ 4 ohms, yet the cabs (both PF-115HE and PF-210HE) are rated at 450 RMS @ 8 ohms. What does this mean when using them separately or together using the same amp?

[Pollinator95]

It means they have an 8Ohm impendance, and they can take 450 W before blowing up (or so they say).
If you were using both, you´d get 250 W to each when cranking the amp.
I think that answers your question.

[cchorney]

Originally Posted by u84six

I'm a little confused about how companies pair their heads with cabs. I was looking at the Ampeg Portaflex, and the PF-500 amp is rated at 500W RMS @ 4 ohms, yet the cabs (both PF-115HE and PF-210HE) are rated at 450 RMS @ 8 ohms. What does this mean when using them separately or together using the same amp?

Amps need to send signal to the cabinets at or above the cabinets capacity.

So, a 4 ohm amp can safely send signal to a 4 ohm cabinet or a set of cabs who's total ohms is NO LOWER than 4 ohms. So you can run a 4 ohm amp into a 4 ohm or an 8 ohm cabinet, but if you run a 4 ohm amp into a 2 ohm cabinet, your amp will produce "magic smoke" (you'll fry your amp as it encounters less resistance than it was designed for and it overheats).

An example if this in my Markbass LM-III, which is 350 watts at 8 ohms and 500 watts at 4 ohms. So I can plus in a single 4 ohm cab and get 500 watts, or I can run a single 8 ohm cab and get 350 watts.

Here is where it gets science-y.

you can run multiple cabs into an amp in 2 ways - in series (daisy chained) or in parallel.

There are calculations for both, in a sticky topic under "amps", written by folks with a much better grasp on this stuff than me.

For multiple cabs of the same impedance (or ohms) the formula is:
impedance of single cab / number of cabs = total impedance
So two 8 ohm cabs gives a total of 4 ohms.

For multiple cabs of different impedance, use this formula:
(impedance of cab 1 X impedance of cab 2) / (impedance of cab 1 + impedance of cab 2)

But really, go ready the sticky to make sure (a) I wasn't wrong and (b) you get the detailed explanation of WHY.

Really, go look it up, you need to know this stuff.

[RickenBoogie]

Bass cab wattage ratings mean next to nothing. It is the point in which the voice coils will burn up and melt. Most cabs handle approx HALF their rated wattage before straining. As is the case with any amp/spkr pairing, the key is to *listen* to the speakers, and back off the volume and/or low eq when the speaker starts complaining. And, ANY amp can blow ANY spkr cab. Using good judgment is essential.

[cchorney]

Sigh, just realized I didn't answer your specific question. Sorry.

The PF-500 amp is rated at 500W RMS @ 4 ohms, so it will produce (wait for it) 500 watts when plugged into a 4 ohm cab OR 4 Ohm TOTAL worth of cabs. It will produce about 350 watts at 8 ohms. So if you got both the PF-115HE and PF-210HE, you could play either one safely with a clean signal thru that amp as you would be pushing 350 watts into a cab built to take 450 watts. If you plugged them both in, it would increase the output of the amp to 500 watts, but that would be spread across both cabs, so again a clean signal would be safe for those two cabs as they would each be getting 250 watts. This is useful when you play in both large and small venues, you don't need to bring both if you don't need the full power and might of your rig, but when you bring both you get the full power available.

[IvanMike]

click the link in my sig....

[JTE]

Basically ignore speaker power ratings. They're not telling you anything about how the speaker will work in the real world. All that 450 WRMS tells you is that at some point they tested (or calculated) that at 450 WRMS the speaker's voice coil might melt. But you'll trash the speaker well before you get to that point so it's pointless.

What you need to pay attention to is this... the amp is rated to deliver up to 500 WRMS @ 4Ω. The two cabinets are rated at 8Ω each, so hooking up two of them to that amp means the amp is going to be capable of delivering 500 WRMS into those two speaker cabinets together. Now the way it breaks down (I'm pretty sure, but not 100% positive) is that the single 15" driver is going to get at most 100 WRMS, and each of the 10" speakers in the 4x10 will each get about 100 WRMS.

There's a lot of other reasons not to get the 1x15/4x10 rig, but speaker power ratings aren't anything to worry about.

John

[christw]

Originally Posted by JTE

Basically ignore speaker power ratings. They're not telling you anything about how the speaker will work in the real world. All that 450 WRMS tells you is that at some point they tested (or calculated) that at 450 WRMS the speaker's voice coil might melt. But you'll trash the speaker well before you get to that point so it's pointless.

What you need to pay attention to is this... the amp is rated to deliver up to 500 WRMS @ 4Ω. The two cabinets are rated at 8Ω each, so hooking up two of them to that amp means the amp is going to be capable of delivering 500 WRMS into those two speaker cabinets together. Now the way it breaks down (I'm pretty sure, but not 100% positive) is that the single 15" driver is going to get at most 100 WRMS, and each of the 10" speakers in the 4x10 will each get about 100 WRMS.

There's a lot of other reasons not to get the 1x15/4x10 rig, but speaker power ratings aren't anything to worry about.

John

No! The 15" speaker will see up to 250w RMS at the same time the 210 will be seeing up to 250w RMS with each 10" speaker getting 125w.

Basically, any two 8 ohm cabs you plug into it will each get the same amount of power (up to 250w RMS) no matter how many or few speakers are in each cab so long as the cab is wired up at 8 ohms total.