Under-powering cabs

[gstaats]

For sure. I like to remind myself that a 500w amp is only 3db louder than a 50w amp. (Double the volume). That is in the range of efficiency differences between a good and mediocre cab.

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.

 

[JeezyMcNuggles]

For sure. I like to remind myself that a 500w amp is only 3db louder than a 50w amp. (Double the volume). That is in the range of efficiency differences between a good and mediocre cab.

I'm pretty sure that 103 decibels isn’t double 100 decibels.

 

[TTA]

Check your math.

I was but you were faster. Time for public shaming.

 

[TTA]

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 by ten.

Yes! I better correct it soon…

 

[sawzalot]

This is the topic that just won’t die. I have been a member of this forum for fifteen years and I would lay odds that over that time there has been more outright false information posted about this topic than any other.

 

[gstaats]

This is the topic that just won’t die. I have been a member of this forum for fifteen years and I would lay odds that over that time there has been more outright false information posted about this topic than any other.

Heaven forbid someone from turning down their amp... they may underpower their cab...

 

[micguy]

I'm pretty sure that 103 decibels isn’t double 100 decibels.

It’s twice the power. It will sound louder; It just won’t sound like it’s twice as loud.

 

[pglaser01]

Imagine your mechanic telling you this….

“You know what your problem is? You’re underpowering your engine. To fix it you should press down harder on the accelerator”

EDIT: ok, maybe if your mechanic was an extra on the set of one of the Fast and Furious movies……

 

[Analogeezer]

I stopped reading around page 3 but there are actually two sides to this topic:

1. Driving a cabinet with an amp that puts out less than the rated power of the amp - this is generally a non-issue

2. Driving a cabinet with an underpowered amp that sends a harshly clipped signal to the cabinet. This might be a problem, especially if the cabinet has a horn but is more of a problem for older amps that did not have much power.

For about 12 years (1978 to 1990) I ran a rig with my Acoustic 220 and two single 15" cabs loaded with JBL E-140's. That amp in that configuration (4 ohm total load) put out 125 watts but that rating was at 10 percent THD. Nope you read that right, not point ten, not point zero ten, TEN PERCENT.

IIRC the JBL's were rated at 125 to 150 watts continuous RMS power, so with each speaker getting 60 watts or so they were not remotely overpowered.

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.

I switched to a QSC power amp that was 200 watts per side (rated at like point ten percent THD) and never burned another voice coil.

Now this all happened in the olden days, amps are way cleaner now and also even low powered amps are more powerful.

So while it is technically possible to kill a speaker with an amp that is "underpowered" the amps of today are a lot friendlier to cabinets and it's hard to find a head that is under 300 watts these days.

Analogeezer

 

[agedhorse]

I'm pretty sure that 103 decibels isn’t double 100 decibels.

It is double the power however

 

[aus_bass]

No jimmy, the cab is rated far higher than the amp. Therefore, you'll blow the amp because the amp cant handle the power of the cab. Duh

You guys are killin' me.

 

[agedhorse]

I stopped reading around page 3 but there are actually two sides to this topic:

1. Driving a cabinet with an amp that puts out less than the rated power of the amp - this is generally a non-issue

2. Driving a cabinet with an underpowered amp that sends a harshly clipped signal to the cabinet. This might be a problem, especially if the cabinet has a horn but is more of a problem for older amps that did not have much power.

For about 12 years (1978 to 1990) I ran a rig with my Acoustic 220 and two single 15" cabs loaded with JBL E-140's. That amp in that configuration (4 ohm total load) put out 125 watts but that rating was at 10 percent THD. Nope you read that right, not point ten, not point zero ten, TEN PERCENT.

IIRC the JBL's were rated at 125 to 150 watts continuous RMS power, so with each speaker getting 60 watts or so they were not remotely overpowered.

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.

I switched to a QSC power amp that was 200 watts per side (rated at like point ten percent THD) and never burned another voice coil.

Now this all happened in the olden days, amps are way cleaner now and also even low powered amps are more powerful.

So while it is technically possible to kill a speaker with an amp that is "underpowered" the amps of today are a lot friendlier to cabinets and it's hard to find a head that is under 300 watts these days.

Analogeezer

There are also HPF’s on most QSC amps.

 

[MultiScaleMale]

I don't think this is true...I believe all power amps have input level controls and I'm not aware of any in which the volume controls do anything other than attenuate the input signal. I could be wrong though... can you provide examples of the other type?

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.

 

[Analogeezer]

There are also HPF’s on most QSC amps.

This was a Series I, it did have the 30Hz filters which I engaged but it was the lack of power that was frying the voice coils, trust me.

You also told us some time back that ALL bass amps have some sort of HPF, even old ones from the 1970's to keep subsonics out of the output.

I was running it so hard it was audibly distorted, trying to keep up with guys with 100 Watt Marshall stacks (as in two cabinets and played loud).

Did you read the part about the rated power of the amp was at TEN PERCENT THD?

Analogeezer

 

[Stranger Danger]

Just thought I'd throw this one out there, and see what the general consensus is. I've heard many differing and often conflicting ideas on this topic, over the years. Do you see any issue in under-powering a cab ?

Example: Say I have Neo 212 cab, that's rated at 1000 watts RMS @ 4ohms, not "peak", "program" or "musical" watts, as often spun by marketers. Real continuous RMS watts.
Now say I match it with a class-D head, that does 300w @ 4ohms, or even 500w @ 4ohms. What's the downside ? Is there a downside ? Is it bad for the cab in some way ? Will the cab not perform optimally, unless it's driven hard ?

I see a lot of rigs, the other way round... a 500w head (or more) into a cab that's rated @ 300w RMS. Obviously, it just takes a little common sense, with the volume knob. I rarely see rigs where the cab capacity is significantly above the output of the amp.

What say ye ?

Underpowering a cab is absolute complete and utter nonsense

 

[bucephylus]

I stopped reading around page 3 but there are actually two sides to this topic:…..

2. Driving a cabinet with an underpowered amp that sends a harshly clipped signal to the cabinet. This might be a problem, especially if the cabinet has a horn but is more of a problem for older amps that did not have much power…..

+1
That’s basically my understanding of the issue.

 

[Nuage420]

I've got 30 watts at 8ohms pushing a 650watt 410 and sometimes I add a bergantino 350watt 210 and push 30watts at 4ohms. I also push a 1000watt 212 with a 400watt a/b.
All I see is plenty of volume and tons of headroom, but some of you are saying my cabs are in danger of blowing up and I should be looking to feed more power so I can push them to the max...?
I'll pass.

 

[agedhorse]

This was a Series I, it did have the 30Hz filters which I engaged but it was the lack of power that was frying the voice coils, trust me.

You also told us some time back that ALL bass amps have some sort of HPF, even old ones from the 1970's to keep subsonics out of the output.

I was running it so hard it was audibly distorted, trying to keep up with guys with 100 Watt Marshall stacks (as in two cabinets and played loud).

Did you read the part about the rated power of the amp was at TEN PERCENT THD?

Analogeezer

The HPF's on the QSC amps were 18dB/octave which are more effective (by far) than what was in the old Acoustic (6dB/octave).

10% THD isn't a big deal, anybody using an O/D or Distortion pedal on their pedalboard will be well beyond 10%... in fact most players struggle to hear 10% THD as distortion. They can tell that something is going on, but it does not sound distorted.

JBL's own MI applications notes say to de-rate their drivers by 50% to decrease the likelihood of driver damage under these conditions. Since bass drivers are (much) more vulnerable to mechanical damage (especially the 130/140 series due to their shallow gap construction and fairly soft suspension relative to overhang), the HPF is as important of a factor as power.

 

[daveman50]

If I play my upright bass really hard, will it explode?

 

[micguy]

Did you read the part about the rated power of the amp was at TEN PERCENT THD?

A lot of amplifiers have been rated that way over the years - the amp isn't necessarily one that's any less clean in normal operation, but by rating it's power when it's clipping some, you get a higher power number. Given that people don't perceive clipping on short transients until it's probably worse than that, there is some decent logic behind that philosophy.

I designed a power amp that had "clipping protection circuitry" in it. We experimented quite a bit to figure out how to set that circuit, and realized that we didn't want to prevent all clipping (that robbed the amplifier of a lot of its usable power) - we ended up with a circuit that acted when there was prolonged (more than a few milliseconds) of hard clipping, ad it didn't bring the gain back to completely clean - it just mitigated the worst of it. That was the best compromise - people don't really hear mild clipping on musical signals.