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Under-powering cabs

#106
Ampeg has been famously running a 300 watt amp into an eight hundred watt Speaker for decades. I find, if you run a, say, 15 watt amp into big watt speakers, you just get a lot less bass, and probably some distortion if running the amp flat out for Volume. Also in my experience, you need at least 200 watts to get a hefty --true-- bass tone.
 
#108
Analogeezer said:
I used to burn up about one voice coil every two years with that rig, trying to keep up with 100 watt Marshall stacks, the rig was severely underpowered.
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Yes, this I saw a lot back at the time. I used to run several K130s for PA and K140 and K145 for bass cabs.
I used to power the K145 with a Fender Bassman head (100W) and never had any issues but we didn't play loud, the amp had lots of headroom and the signal was, for all intents and purposes 'clean'.

We used to provide repair services to local musicians at the time and had a lot of valve amps and speakers in for repair. We had a lot of recones done because voice coils were toasted. We were curious so we turned up at some of the gigs where these guys played, it was extremely loud.

The popular amp for guitar was the 100W Fender Twin Reverb, I still have my 1969, those amps are very loud and if they were cranked (very often) a 100W bass amp and single 15" cab could not compete. I suppose this is the reason we saw so much damage to speakers.

Today most amps will kick in the limiter when they reach the maximum clean output, you can dime the volume control but it won't get any louder, just more squashed.
 
#110
gstaats said:
Should be 10db louder. 3db is considered as the smallest difference in level that is easily heard by most listeners to speech or music. Double the power, get about 3db in gain. A 10gb gain is achieved (double the perceived volume) by increasing the power tenfold.
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Which is why I don't sweat the fact that my amps are 'only' 300W as I wouldn't gain that much by going to 600W other than clean headroom, and I have better cabs and PAs are such that I don't need to flog all 300W.
 
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#111
MultiScaleMale said:
Quilter lists "watts" on their Master knob of the Bass Block. Maybe it's not really watts and just marketing? But what I was trying to get at is that some think that if you set the master on a 1200W amp at 25%, then you'll only be putting out 300W maximum so it's safe to use on a 400W cab.
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On some amps with names starting with B I think they should call them Whos not Watts as they don't see to be very related to any Watts I know.
 
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#113
MultiScaleMale said:
Quilter lists "watts" on their Master knob of the Bass Block. Maybe it's not really watts and just marketing?...
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I think it's probably a bit more marketing than a unique design concept. The 'Master' control is still an amplifier input attenuator, but it's integration with the input limiting makes it possible to put some marks on the knob that correspond (more-or-less) to actual maximum output power. Still, I expect if you connected it to a 4-ohm load and drove the Line Input with a signal of (say) several volts, the amp would output it's rated 800 watts regardless of where the Master knob is set. At that point, I imagine the input limiting would be distorting the signal so badly that it wouldn't be usable, but the power would still be delivered to the load.
 
#115
I have no problems whatsoever running my 215 800 watt 4 ohm cab with my 600watt 4ohm amp.
Can it destroy the speakers?
Sure... but that's not a case of underpowering the cab, that's a case of overloading frequency parameters. This is the crux of the biscuit...
 
#117
TTA said:
For sure. I like to remind myself that a 500w amp is 10db louder than a 50w amp. (Double the volume).
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It takes twice the power to raise the sound by 3db.
Just so I don't have to use a bunch of X's to represent original Sound Pressure Level (SPL), let's presume that your 50-watt amp produces 100db with 50-watts of power (all hypothetical of course). Then let's step it up by doubling the power of the amplifier to find out how much it would take to be twice as loud to the human ear:
50-watts = 100db
100-watts = 103db
200-watts = 106db
400-watts = 109db
600-watts = 112db
That's all presuming everything is working perfectly with speakers that can handle the power.
​
Although 3db increments are measured as "twice as loud", the human ear hears a 10db increase as twice as loud. There's a 9db increase using 400-watts and a 12db increase at 600 watts. So it's reasonable to say a 500-watt amp is twice as loud because it probably does produce 10db more than the 50-watt version and you would indeed hear that as twice as loud.

This is why amplifier power is only half of the formula. The other half of the formula is the speakers you use or plan to use with it. The better the sensitivity of the cab, the less power it takes to bring the SPL of your gear up to gig levels. It's the combination of the amp and the speakers that give you the final SPL your combination of amp and cab(s) can produce.
 
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#118
I suppose the strangest thing about the underpowering myth is its inconsistency.
In its developed form, shorn of the real nonsense like square waves = DC, it seems to go something like
"If you have a 200W amp and 400W speakers and try to get 400Ws worth of volume then the distortion will cause speaker damage" ,
and there is a grain of truth in that, especially with HF drivers. But the alternate cases of trying to get 400Ws worth of volume out of 400W amp and 200W speakers and 400Ws of volume out of 200W amp and 200W speakers are far more likely to cause damage. And it also misses the point that trying to get 400W of volume out of 200W of amp and 2,000W of speaker isn't going to be an issue.
So actually the real lesson is that if you thrash the **** out of your kit it will suffer, which is scarcely a surprise.
 
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#119
Redbrangus said:
I think it's probably a bit more marketing than a unique design concept. The 'Master' control is still an amplifier input attenuator, but it's integration with the input limiting makes it possible to put some marks on the knob that correspond (more-or-less) to actual maximum output power. Still, I expect if you connected it to a 4-ohm load and drove the Line Input with a signal of (say) several volts, the amp would output it's rated 800 watts regardless of where the Master knob is set. At that point, I imagine the input limiting would be distorting the signal so badly that it wouldn't be usable, but the power would still be delivered to the load.
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I have been wondering about this on the Quilters. At bedroom levels (to be sure no limiting is happening) does turning it from say the noon 160W position to the 800W position change the volume of the system? If it doesn't then I suspect the "master" is really an output limiter control as opposed to a volume control. If it does, then it's a volume control that is still probably tied into the limiting circuits as you said in your post. I mean as far as I know, you can only control the potential output power of an amp by changing the rail voltages or by some sort of limiting scheme. I doubt Pat Quilter would be party to questionable marketing based on his long history of not doing that in pro audio.


JimChjones said:
I suppose the strangest thing about the underpowering myth is its inconsistency.
In its developed form, shorn of the real nonsense like square waves = DC, it seems to go something like
"If you have a 200W amp and 400W speakers and try to get 400Ws worth of volume then the distortion will cause speaker damage" ,
and there is a grain of truth in that, especially with HF drivers. But the alternate cases of trying to get 400Ws worth of volume out of 400W amp and 200W speakers and 400Ws of volume out of 200W amp and 200W speakers are far more likely to cause damage. And it also misses the point that trying to get 400W of volume out of 200W of amp and 2,000W of speaker isn't going to be an issue.
So actually the real lesson is that if you thrash the **** out of your kit it will suffer, which is scarcely a surprise.
Click to expand...

Even that square waves = DC myth has a kernel of truth to it, because mathmatically a square wave has the same power as a DC voltage of the same amplitude and the terms like "DC equivalent heating power" exist in electrical engineering. People who had just enough knowledge to be dangerous took that to mean that a square wave incident on a speaker would be equivalent to DC and would lock the cone in place, etc. In terms of the amount of heat dissipated in the driver, it is equivalent to DC but it's still very much an alternating signal.

A lot of the questions and debate on this subject is driven by oversimplification on both the amplifier and the speaker end. Trying to boil the interactions down to a single unifying number to make it easier to market leaves out all of the nuances like overdriven amplifiers can put out more than their rated power or that cabinet design affects speaker power handling and that at low frequencies drivers can reach their mechanical limit before they reach their electrical one.

And a 9V battery puts about 10W into an 8 Ohm load and about 20W into a 4 Ohm one.
 
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#120
Fortunately, the internal resistance of the 9V battery greatly limits the current (and thus power) it can deliver.
 

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