350 watt Bass Amp Head into a 3200 watt Phil Jones Bass Cabs 8 x 5 and 24 x 5

Hi all,

I am not sure if this is the right area to be asking this question, if not, i do apologize.

Just wondering, i have an Ampeg B2 350 watt Bass Amp Head, and i just purchased 2 Phil Jones Bass cabs...the 8 x 5 top angled cab and a 24 x 5 fridge. The head sounds fine thru the cabs, but would i need a more powerful head or ad a power amp ?

I have been told many theories all contradicting each other and the internet is not much help either.

Any info would be much appreciated.

I have attached a pic of the head and cabs.

Attachment 22318

Regards,
Naim.

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).

I can imagine that many 5" speakers giving a very bright sound. There wouldn't be a lot of space on the back of the cone to generate a lot of low end.

Quote:

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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).

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Thank you Simon for all your info and help, very much appreciated. The 24 x 5 cab is 5.3 ohms. I have included the specs in a pic attached. According the PJB Specs the 8x5 is 16 ohms and 24x5 is 5.3 ohms and both cabs equaling 4 ohms combined. Does this sound correct Simon ?

Attachment 22329

Quote:

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Originally Posted by Fretworn

I can imagine that many 5" speakers giving a very bright sound. There wouldn't be a lot of space on the back of the cone to generate a lot of low end.

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Thats correct Fretworn, it is a very bright sound especially thru active basses, however, thru passive basses, it sounds great and brightens up a lot of muddyness. Basically making cheap basses sound quite descent ! With my active basses, i have to either reset the eq, or use a Compressor / Sustainer pedal or a Limiter pedal to boost the low frequencies.

Yes, the 8T is 16 ohms. Not sure how I misread that on their site. And yes, 16 ohms and 5.3 ohms gives a combined parallel impedance of 4 ohms (very, very slightly under but not enough to matter).

So you'll get the full 350W of power from the Ampeg, but the 8T top will sound a lot quieter compared to the 24T, and in terms of getting the best overall volume from your rig, I think you'd probably be better just using the 24T - something you can easily test yourself. Stand well back from the rig as otherwise the angle of the 8T means that it will be firing directly at your head, so making it seem louder and brighter than the 24T. An audience -or even the rest of the band - won't be standing right in front of the rig, so do the comparison at a distance.

A 5" speaker on its own is going to be pretty bright sounding as it can't reproduce a lot of bass, but the more of them you put in one cabinet, plus add tuned porting, and their ability to create bass is improved as they all act together to behave like a much larger virtual speaker. E.g. a 4x12" guitar cab sounds so much bassier than a 1x12" cab. Also, the speaker's individual smaller mass helps them to recreate transients better than a big speaker would.

But that impedance mismatch does really mean that you'd be best with two power amps, one for the 8T and one for the 24T. I see you have both the B2R and a B1-RE in your rack. So with what you've got, you can power the 8T from one and the 24T from the other. Take the pre-amp out signal from the B2R and feed it to either the power amp in jack of the B1-RE 9which means that it's volume follows that of the B2R) or the effects return of the B1-RE jack (when the master volume on the B1-RE should control its own volume). But in the long term, if you feel you need more volume, then a couple of lightweight class D power amps and a bass pre-amp (such as the Ampeg SVP-PRO) would provide a much more powerful (and hopefully lighter) package.

If you use the graphic EQ that's shown in the picture, then you could set the overall tone shaping up on that for use with passive bases, then switch in the graphic on the B2R for taming the higher frequencies without having to use an external pedal for that purpose when using an active bass.

Quote:

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Originally Posted by Simon Barden

Yes, the 8T is 16 ohms. Not sure how I misread that on their site. And yes, 16 ohms and 5.3 ohms gives a combined parallel impedance of 4 ohms (very, very slightly under but not enough to matter).

So you'll get the full 350W of power from the Ampeg, but the 8T top will sound a lot quieter compared to the 24T, and in terms of getting the best overall volume from your rig, I think you'd probably be better just using the 24T - something you can easily test yourself. Stand well back from the rig as otherwise the angle of the 8T means that it will be firing directly at your head, so making it seem louder and brighter than the 24T. An audience -or even the rest of the band - won't be standing right in front of the rig, so do the comparison at a distance.

A 5" speaker on its own is going to be pretty bright sounding as it can't reproduce a lot of bass, but the more of them you put in one cabinet, plus add tuned porting, and their ability to create bass is improved as they all act together to behave like a much larger virtual speaker. E.g. a 4x12" guitar cab sounds so much bassier than a 1x12" cab. Also, the speaker's individual smaller mass helps them to recreate transients better than a big speaker would.

But that impedance mismatch does really mean that you'd be best with two power amps, one for the 8T and one for the 24T. I see you have both the B2R and a B1-RE in your rack. So with what you've got, you can power the 8T from one and the 24T from the other. Take the pre-amp out signal from the B2R and feed it to either the power amp in jack of the B1-RE 9which means that it's volume follows that of the B2R) or the effects return of the B1-RE jack (when the master volume on the B1-RE should control its own volume). But in the long term, if you feel you need more volume, then a couple of lightweight class D power amps and a bass pre-amp (such as the Ampeg SVP-PRO) would provide a much more powerful (and hopefully lighter) package.

If you use the graphic EQ that's shown in the picture, then you could set the overall tone shaping up on that for use with passive bases, then switch in the graphic on the B2R for taming the higher frequencies without having to use an external pedal for that purpose when using an active bass.

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Thank you for your help Simon, very much appreciated. So essentially, could i run the top 8x5 PJB with my Ampeg 8x10 fridge off the Ampeg B2 head ? So i get the bright sound thru the PJB cab and the bottom end out of the Ampeg cab ? Would the ohmage still be at 4 ohms ? Because if i run just the PJB 24x5 on its own with the Ampeg B2 head, i think the head mono is 8 ohms for one cab and 4 ohms for two cabs and the PJB fridge is 5.3 ohms, therefore 8ohms going into 5.3 ohms cab. That wont work or is this incorrect ? I have attached another pic of my Ampeg 8x10 fridge cab.

Attachment 22330

The two outputs on the B2 are simply wired in parallel and it's just the speaker cabs connected to it that determine the overall ohmage and output power you'll get from the amp. Not the number of power outputs used.

The Ampeg fridge would appear to be 4 ohms (from web browsing), so the B2 can power that on its own, but if you add another cab, the overall resistance will drop further and you would be in danger of burning out the map or causing any overload protection in it to operate.

Parallel resistances are worked out using the formula (1/R1 +1/R2 +1/R3 etc.. = 1/Rtotal )

So with a 4 ohm (Ampeg Fridge) and 16 ohm (8T cab) load, you'd get 1/4 +1/16 = 1/Rtotal .

Expanding 1/4 to 4/16 to be the same units as 1/16, we get 5/16 = 1/Rtotal .

So our resistance is the inverse of that e.g. 16/5, which gives 3.2 ohms, so less than the minimum 4 ohms.

If the fridge is 8 ohms, then the overall impedance would be 16/3 = 5.3 ohms, which would be fine. If it is the stereo 4 ohm fridge cab then by putting it into stereo mode, you could have half of it powered at 8 ohms, and also power the 8T as well from the B2. It means half the fridge won't be used, but that's the compromise you'll have to make if you want to use the 8T and fridge together without using another amp. I suggest you try it and see what it sounds like. - but make sure the fridge is in stereo mode, not mono, and that only one side is being powered.

On the B2, you can attach one cabinet of 4, 8 or 16 ohms on its own, or two 8 ohm cabs or a 16 ohm plus a 5.3 ohm cab. If the cabs had their own parallel outputs, then you could attach four x 16 ohm cabs (for a 4 ohm resultant load).

To properly drive the fridge and the the 8T together, you'd need an amp that was happy driving down to 2 ohms, or another power amp to drive either the fridge or the 8T. But you can use the power amp section in one of the other amps - like the B1, to do this - especially as its already in the same rack case.

Based on Simon's numbers I smell some imperial smoke escaping from the power stage just after a loud thump through the speakers

I'm just happy I don't live next door ;D