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Question regarding slight overpowering of cabinet

L

LTDVIPER333

Apr 18, 2009
120
2
Nelson, New Zealand
#1
Hi there.
Considering purchasing either an Ampeg PF350 or a PF 500. Currently run an 80s GK 400 RB into a newish 4X8” Celestion loaded Warwick cabinet 450watts at 8ohms and a 11 year old celestion loaded Laney RB115 250 watts @ 8ohms. The PF500 supposedly runs 300 watts at 8ohms. Would this be too much for the 250 watt laney?
 
#2
Cabinet power ratings are not set in stone - you're more likely to damage it mechanically from over-excursion than from an overheated voice coil (which is how most ratings are derived), so back off the lower mid and bass EQ if you hear signs of stress.

I'm not sure you'll notice much volume difference from the GK, but then I guess I'm a little biased. ;) BTW, I have an old Viper304 that I take to jazz jams sometimes - the looks I get when they see what I'm playing is priceless.
 
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#5
Jaco who? said:
Cabinet power ratings are not set in stone - you're more likely to damage it mechanically from over-excursion than from an overheated voice coil (which is how most ratings are derived), so back off the lower mid and bass EQ if you hear signs of stress.

I'm not sure you'll notice much volume difference from the GK, but then I guess I'm a little biased. ;) BTW, I have an old Viper304 that I take to jazz jams sometimes - the looks I get when they see what I'm playing is priceless.
Click to expand...
Yea I have a LTD/ESP viper 254. Owned it since i was 15 like 16 years ago. Good to know the GK is a beast but only like an 80s 150 watts at 8ohms..
 
#6
It's likely the amp has enough power to damage the cab. Whether it will or not depends on how hard you push the amp and what sort of tone you use.

The reason I mention tone is because a cabs power rating is often based on the RMS power rating of the driver, rather than the designs actual mechanical power handling.

The RMS power is essentially how much power the voice coil can dissipate without burning out. The problem is the mechanical power handling below 100hz of many designs is actually less than the drivers RMS power rating. So, the 250W rating of your cab could possibly be a bit optimistic.

Mechanical power handling depends on both the driver's T/S characteristics and the design of the cabinet. So, if you put the same driver in two different cab designs, each design will have its own unique mechanical power handling characteristics.

Most bass cabs are ported designs. At higher frequencies ported designs behave essentially like sealed cabs. The port is tuned so that it starts to become resonant around the same frequency where the driver's response starts to roll off. The idea is the port will fill in and smoothly extend the output of the cab to a lower frequency.

Driver excursion tends to start increasing as the frequency drops below about 500hz. The excursion peaks just before the port starts to become active. As the frequency drops further, the port become increasingly resonant until Fb. Note. Fb is the port tuning frequency. As the port becomes more active it progressively suppresses cone excursion. So, at Fb the port is making most of the sound and driver excursion is largely suppressed.

Below Fb the output of the port drops of quickly and driver excursion increases drastically. Because of this, it's not a good idea operate a ported cab very far below Fb. The cab does not produce sound efficiently in the range, despite the fact that the driver is pumping back and forth like crazy.

Because of these factors, the excursion plot for the driver has sort of an S shape.

The following charts are the response plot and excursion plot for a medium ported design loaded with an Eminence 2512. The 2512 has a 250W RMS power rating, but this design is excursion limited to 175W, and it is recommended to run a steep HPF set to 40hz to protect the driver from over excursion below Fb.

upload_2022-3-16_3-41-47.png

Notice the S shaped excursion trace.

Port resonance (FB) is at 55hz. Cone excursion is largely suppressed at FB, but notice how quickly excursion increase to Xmax below Fb. The trace turns from black to gray at Xmax, which equals 4.9mm. Also notice how fast the response is falling off in the top chart.

FYI the -3dB point (F3) for this design is about 73hz. The -10dB point (F10) is around 40hz. However the HPF is set to 40hz. FYI the HPF frequency is the point where the filters response is already down by -3dB. This means power sent to the cab is cut in half to 87.5W. So, with the HPF engaged, the system response will be down a total of -13dB at 40hz.

Hope this give you a little better understanding.
 
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#7
Jaco who? said:
Cabinet power ratings are not set in stone - you're more likely to damage it mechanically from over-excursion than from an overheated voice coil (which is how most ratings are derived), so back off the lower mid and bass EQ if you hear signs of stress.

I'm not sure you'll notice much volume difference from the GK, but then I guess I'm a little biased. ;) BTW, I have an old Viper304 that I take to jazz jams sometimes - the looks I get when they see what I'm playing is priceless.
Click to expand...
Just ordered a Sterling Sub ray 4 so looking forward to having a different ax that probably sounds slightly better..
 
#9
Wasnex said:
It's likely the amp has enough power to damage the cab. Whether it will or not depends on how hard you push the amp and what sort of tone you use.

The reason I mention tone is because a cabs power rating is often based on the RMS power rating of the driver, rather than the designs actual mechanical power handling.

The RMS power is essentially how much power the voice coil can dissipate without burning out. The problem is the mechanical power handling below 100hz of many designs is actually less than the drivers RMS power rating. So, the 250W rating of your cab could possibly be a bit optimistic.

Mechanical power handling depends on both the driver's T/S characteristics and the design of the cabinet. So, if you put the same driver in two different cab designs, each design will have its own unique mechanical power handling characteristics.

Most bass cabs are ported designs. At higher frequencies ported designs behave essentially like sealed cabs. The port is tuned so that it starts to become resonant around the same frequency where the driver's response starts to roll off. The idea is the port will fill in and smoothly extend the output of the cab to a lower frequency.

Driver excursion tends to start increasing as the frequency drops below about 500hz. The excursion peaks just before the port starts to become active. As the frequency drops further, the port become increasingly resonant until Fb. Note. Fb is the port tuning frequency. As the port becomes more active it progressively suppresses cone excursion. So, at Fb the port is making most of the sound and driver excursion is largely suppressed.

Below Fb the output of the port drops of quickly and driver excursion increases drastically. Because of this, it's not a good idea operate a ported cab very far below Fb. The cab does not produce sound efficiently in the range, despite the fact that the driver is pumping back and forth like crazy.

Because of these factors, the excursion plot for the driver has sort of an S shape.

The following charts are the response plot and excursion plot for a medium ported design loaded with an Eminence 2512. The 2512 has a 250W RMS power rating, but this design is excursion limited to 175W, and it is recommended to run a steep HPF set to 40hz to protect the driver from over excursion below Fb.

View attachment 4622813
Notice the S shaped excursion trace.

Port resonance (FB) is at 55hz. Cone excursion is largely suppressed at FB, but notice how quickly excursion increase to Xmax below Fb. The trace turns from black to gray at Xmax, which equals 4.9mm. Also notice how fast the response is falling off in the top chart.

FYI the -3dB point (F3) for this design is about 73hz. The -10dB point (F10) is around 40hz. However the HPF is set to 40hz. FYI the HPF frequency is the point where the filters response is already down by -3dB. This means power sent to the cab is cut in half to 87.5W. So, with the HPF engaged, the system response will be down a total of -13dB at 40hz.

Hope this give you a little better understanding.
Click to expand...
Cheers for the info. Can always get the PF350 which runs 250watts at 8hms which would match the rating. The cabinet does say 300watts RMS on the back so I don’t know if that would help.
 
#10
Again, if you're running both 8-ohm cabs in parallel, the load is 4 ohms. The PF500 output of 500 watts is divided between the two loads: 500/2=250 watts per cab. So nothing to worry about. Honestly, the difference between 250 watts and 300 watts is negligible, and as noted previously, the power rating of a cab is nominal, not written in stone.
 
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#12
LTDVIPER333 said:
Hi there.
Considering purchasing either an Ampeg PF350 or a PF 500. Currently run an 80s GK 400 RB into a newish 4X8” Celestion loaded Warwick cabinet 450watts at 8ohms and a 11 year old celestion loaded Laney RB115 250 watts @ 8ohms. The PF500 supposedly runs 300 watts at 8ohms. Would this be too much for the 250 watt laney?
Click to expand...
No. Have fun
 
#14
Most amps can damage most speakers.
Most amps can work compatibility with most speakers.
If your plan is to dime everything you need to be cautious. But an amp that can produce more power than the load the speakers are rated for, that is headroom.
No matter how large your gear, turn up until it sounds like it’s going to fail, that’s too high, turn down a little and don’t go above that.
And also use a HPF and a compressor.
 
#15
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Exactly.
There are some ACME B2's here that get run with double their power rating. I asked Andy at ACME about it and he said "Be very careful. It will sound great if you keep control but it may be like having a NASCAR engine in a street car, one mistake and you're wrapped around a tree" or something like that. The amp(QSC 2402 with filters ON) has a dbx 166xL in front of it to protect the speakers. Spent about one hour "playing with settings" at high volume while watching and listening to the speakers. Works fine, speakers are still alive after many, many hours of use and some almost abuse.
 
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#17
LTDVIPER333 said:
Yes t

Yes this is correct. I was wondering if the 300 watts would still be too much for the 250watt speaker. The PF500 runs 500w at 4ohms or 300@8ohms.
Click to expand...
If you run two 8 ohm cabinets, they equally split the power -> 250 watts to each cabinet.

If you run just the one 250 watt rated cabinet then it is slightly overpowered, getting 300 watts nominally. But as long as you are not absolutely cranking the volume or the bass control it should be ok.
 
#20
I can't comment on the B2R, I just learned of it's existence.

V4B vs. SVT:
Do you want to play with a clean sound in a band where both guitarists play Marshall halfstacks with 4x12 cabinets and the drummer takes about a week to go through a pair of sticks?
You will need an SVT. The V4B can barely make it with an 8x10 cab, but you'll have to let it rip and you won't get a crystal clear sound. You'll get the sound of an amp that is giving its all.
 

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