Has anyone blown any speakers by having too much clean power?

alexclaber · #1 07-03-2003, 07:00 AM

Continuing my research into using big power amps with lower rated cabinets, I'd like to know if anyone has managed to blow their speakers by driving them too hard with a big amp that was running within its limits (i.e. not clipping or on the limiter).

If you could specify what your lowest string is tuned to, if you were using a highpass filter (and what frequency it was at), what amp/cab setup you were using, how the speaker blew (i.e. over-excursion or voice coil overheating) and how loud a situation you were playing in, that would be great!

Just checking out the specs on the PLX amps, I've noticed that they quote Dynamic Headroom of 2dB @ 4ohms.

Does that mean that (with the 3002 for instance) when driving 900W RMS per channel (assuming it's a pure sine wave) into a 4 ohm cab (i.e. peaking at about 1250W) that the amp still has the capability to peak another 2dB higher (i.e. about 2000W peak)?

If this is correct that would give an peak amp power to continuous cabinet power ratio of about 5:1 (for a QSC PLX 3002 with my two Acme Low-B2IIs) Or am I barking up the wrong tree?

Alex

GreyBeard · #2 07-03-2003, 07:14 AM

I don't think any of us have done much research into "How to blow speakers with clean power". A good speaker rating is based on average continuous power over a specific period of time. My cab is rated at 400 watts. My amp is rated at 700 watts into 8 ohms. When I play normally the db lights hang around -20db with the peaks at -10db. My average continuous power is probably around 100 watts or less and my peaks are definitely less than 700 because it never clips. I play a 5 string bass and have been playing the rig this way for the past 6 months. All 4 speakers are still pumping air. In my 35 years of bass playing I've only blown 1 speaker. It was a 1x15 Peavey heavy duty driven with a Peavey Centurion. I also sort of remember something about FREE DRINKS ……..

bgavin · #3 07-03-2003, 09:24 AM

My two JBL E110 are rated at 75 watts each. I run them as a parallel load, 4 ohms total, from a 900 watt PLX 3002 channel. I turn the attenuator down on that channel because the JBL are much more efficient than my subs. They have been running 2x to 3x weekly for more than 2 years this way.

alexclaber · #4 07-03-2003, 12:41 PM

Come on own up, who's got too carried away with all that power? Don't be shy!

Alex

guyplaysbass · #5 07-03-2003, 02:22 PM

I had an old Peavey 260 Combo with a BW 15''. These particular speakers are known to be almost indestructible. On an out-door gig with no help from the PA, I managed to fry the voice coil by overdriving the preamp (I was doing this ON PURPOSE for distortion). I have since learned from this mistake. If I want to overdrive my preamp now, I have all the power I need (ie; Peavey DPC 1400X). There's one of my many stupid stories!!!

oddio · #6 07-03-2003, 02:48 PM

I have been able to crack stamped metal speaker baskets with high wattage, but these were mediocre speakers driven well beyond their Xmax.

Personally I maintain (IMHO) that, within reason(!), the majority of speakers are blown by underpowering them. Reproducing the square wave produced by an amplifier driven to clipping levels prevents the speaker from cooling itself properly, hence a melted voicecoil.

However, I will defer to Joris' opinion, especially if he steps in here and lambastes me.

guyplaysbass · #7 07-03-2003, 02:58 PM

I'll concur with oddio. That's exactly what happened to my BW. I under-powered it. I have never had a problem with more power then was needed/rated on cabs since then.

nashvillebill · #8 07-03-2003, 03:43 PM

Yes. Too much excursion, voice coil then started rattling.

alexclaber · #9 07-03-2003, 04:18 PM

Quote:

Originally posted by guyplaysbass
On an out-door gig with no help from the PA, I managed to fry the voice coil by overdriving the preamp (I was doing this ON PURPOSE for distortion

Did you mean you were overdriving the power amp to get distortion, not the preamp? (And nowadays you overdrive your preamp for distortion, not your power amp?)

Alex

guyplaysbass · #10 07-03-2003, 04:55 PM

The pre and post controls on a Peavey in question work as follows: The "Pre" controls the volume of the input, the "Post" controls the volume of the amplifier it's self (ie; output to the speaker). Turning the "Post" to it's maximum value will have the amplifier running at max power. The "Pre" can then be used to control volume. This gives you the most amount of headroom.

I was purposely running the "Pre" at max and the "Post" at somewhere around the 12o'clock position (don't really remember). This causes the front end (otherwise known as the pre-amp) section of the amplifier to overdrive on SS units such as the above stated Peavey. It gives one hell of a distorted tone, but as I learned, can fry one's speaker as well!!!

I now run seperate pre/power and can overdrive my pre at will (which I do, slightly). I have plenty of power running to the speaker and the only risk of damage is to the tube in my pre-amp. I'm using a Peavey Max Bass pre-amp with a Peavey DPC 1400X power amp into an Acme Low B-4. I overdrive the tube side slightly to warm up the top end of the signal and run the SS side slightly louder to keep everything solid.

Richard Lindsey · #11 07-03-2003, 06:23 PM

Quote:

Originally posted by oddio
I have been able to crack stamped metal speaker baskets with high wattage, but these were mediocre speakers driven well beyond their Xmax.

Personally I maintain (IMHO) that, within reason(!), the majority of speakers are blown by underpowering them. Reproducing the square wave produced by an amplifier driven to clipping levels prevents the speaker from cooling itself properly, hence a melted voicecoil.

However, I will defer to Joris' opinion, especially if he steps in here and lambastes me.

There's no such thing as underpowering a cab or a speaker. You can only be underpowered for your playing situation, which it sounds like you were.

Clipping a power amp doesn't result in square waves unless you do it to some insane level. It just takes something off the top of the wave.

Mark Reccord · #12 07-03-2003, 06:34 PM

This subject always seems to create lively debate 'round these parts.

It seems to me that a lot of the misinformation and arguments about the subject are related to semantics and not core concepts.

It is possible to damage speakers with too much clean power. It happens all the time. If the average amount of power being dissipated in the VC is in excess of the speaker's long term power handling capacity, thermal damage will eventually occur. No matter how clean the power is. Here's where semantics comes in: You can safely power a cabinet with an amp rated for more power than that cabinet. This is because under dynamic conditions (even if you hit the clip lights the odd time) the average power the amp is putting into the speaker is a fraction of the rated output power of the amp. Speakers can take transient peaks of much more than their long term rated power as well. You can't actually put more power in than the speaker is rated for though. So if you have a 600W driver and a 1200W amp and you set it up so that the amp is putting an average of say 750W into the speaker, you will cook the voice coil.

It is also possible to damage a speaker with perfectly clean power less than the speaker's rating, under certain conditions. For example, a speaker's power handling is greatly derated at the tuning frequency of a vented cabinet, because the suspension created by the air column greatly reduces cone movement (and thus cooling) at that frequency.

Finally, it is possible to damage a speaker by overdriving an amp rated for less power than the speaker. To a point. This is based on the fact that an amp will put out more than it's rated power when overdriven. This, in turn, is because square waves have more energy than sine waves at the same amplitude. So it's actually theoretically possible for an amp to put out twice its rated power under heavy clipping. This is where the 'underpowering' thing comes in. You can damage, say, that 600W speaker with a 500W amp, because the amp can put out more than 600W when clipped hard. You wouldn't damage that same speaker by clipping a 10W amp into it, though, because there wouldn't be anywhere near enough power disspated in the VC to cause any damage.

Debunking time. Some common arguments and my thoughts on them:

Myth 1: Clipped amps put out raw DC to speakers: Patently not true. If your amp is putting out DC, it is very sick. Square waves are in no way DC, assuming amps put out pure square waves under clipping (which they don't).

Myth 2: Square waves lock the cone into place for short periods of time, thus greatly decreasing cooling efficiency:

1) A square wave of a certain frequency will move the cone of a speaker in and out the same number of times per second as a sine wave of the same frequency.
2) The speaker cone will not just stop dead at the crest of a square wave. The cone's inertia will keep it moving. The amp does lose damping control during this time. While this doesn't sound very good, there isn't any evidence that it's damaging to speakers.
3) Speakers are on the order of about 1-2% efficient at converting electricity to sound, the other 98-99% is converted to heat. Decreasing cooling efficiency a bit isn't going to derate the speaker's power handling very much at all. Maybe by a few watts. At worst.

Bottom line: Be sensible with matching speakers and power and in using that power and everything will be cool. I've been plaing bass for 20 years and doing sound (arena level) for 10 and I can't recall ever melting a VC in a cone driver. I've seen lots of overexcursion damage though....
Phew, that's enough outta me....

Bard2dbone · #13 07-03-2003, 06:52 PM

I direct your attention to the sig above me. This is VERY true and I have been one of the stupid ones.

I blew a 2x15 with clean power. But when I opened it I discovered that the original speakers had been replaced by Radio Shack speakers. They were still 15"s but the cab was now rated at 100W. I had been consistently running it at 300-400W for a couple of months before the gig where it gave up on me. It actually sounded okay until that gig. I replaced the RS speakers with the specified JBL speakers and got the 400W I was supposed to have. And it went from sounding okay to sounding like the voice of God.

I recommend to anyone with roadies, A Randall 2x15 for lows, and an Acoustic powered 2x10 for highs, driven by an Acoustic B4. It is a seriously old-school sounding rig and it is waaaaay too big. But it sounded great.

alexclaber · #14 07-03-2003, 07:09 PM

Quote:

Originally posted by Mark Reccord
Bottom line: Be sensible with matching speakers and power and in using that power and everything will be cool. I've been plaing bass for 20 years and doing sound (arena level) for 10 and I can't recall ever melting a VC in a cone driver. I've seen lots of overexcursion damage though....
Phew, that's enough outta me....

Well said! Thanks for clarifying all those points (I did have pretty much that impression, regarding all the myths out there). Knowing that theory I've just been trying to see how that relates in practice to a bass playing situation. As you said, assuming the signal peaks do not cause over-excursion, as long as the average power into the speakers is less than the rated power everything will be fine.

I guess that leaves just one question - what is the typical ratio of peak power to average power for a bass signal (how long is a piece of string)?

In pro-audio (i.e. the program signal at a live gig) is that ratio about 4:1 (i.e. 6dB) because (for instance) a 500W cab will see peaks of about 2000W from a 1000W poweramp (following the 2:1 amp:cab power rule of thumb)?

Alex

oddio · #15 07-03-2003, 07:39 PM

Well put, Mark! Thanks for debunking a few myths.

Myth 2 was widely held when I worked in the pro audio field many years ago, and you've given a great (eg: technically accurate) explanation of what is happening there. Thanks for that!

alexclaber · #16 07-04-2003, 04:34 AM

Quote:

Originally posted by alexclaber
I guess that leaves just one question - what is the typical ratio of peak power to average power for a bass signal (how long is a piece of string)?

Just had a relatively bright idea regarding this -I'm going to record my bass into Soundforge and see what the waveform actually looks like. If I do that with all my different techniques and fx pedals I should be able to build up an accurate picture of how the average power compares to the peak power, which will give me a good yardstick as to how hard I can safely push the cabs.

Good idea?

Alex

arfur · #17 07-04-2003, 05:46 AM

I don't recall seeing a melted voice coil but I have seen damage where cones have failed particularly in old install systems where the same speaker has lived in a sweaty environment for a great many years. In the old paper cones the suspension around the edge would become brittle and finally fail. Floor monitors seem susceptable to this, probably because people keep pooring beer into them.
The other is melted crossovers in passive boxes. If you put double the rated power into a cab then the drivers may be able to take it but the crossover may give up the ghost.
I stick twice the rated power into my cabs and have never had a problem.

alexclaber · #18 07-04-2003, 05:57 AM

Just checking out the specs on the PLX amps, I've noticed that they quote Dynamic Headroom of 2dB @ 4ohms.

Does that mean that (with the 3002 for instance) when driving 900W RMS per channel (assuming it's a pure sine wave) into a 4 ohm cab (i.e. peaking at about 1250W) that the amp still has the capability to peak another 2dB higher (i.e. about 2000W peak)?

If this is correct that would give an peak amp power to continuous cabinet power ratio of about 5:1 (for a QSC PLX 3002 with my two Acme Low-B2IIs) Or am I barking up the wrong tree?

Alex

kurosawa · #19 07-04-2003, 08:56 AM

Blow speakers by overpowering? Got the t-shirt. Any speaker can be blown by overpowering.

Richard Lindsey · #20 07-04-2003, 10:43 AM

Quote:

Originally posted by Mark Reccord
This subject always seems to create lively debate 'round these parts. It seems to me that a lot of the misinformation and arguments about the subject are related to semantics and not core concepts.

It is possible to damage speakers with too much clean power. It happens all the time. If the average amount of power being dissipated in the VC is in excess of the speaker's long term power handling capacity, thermal damage will eventually occur. No matter how clean the power is. Here's where semantics comes in: You can safely power a cabinet with an amp rated for more power than that cabinet. This is because under dynamic conditions (even if you hit the clip lights the odd time) the average power the amp is putting into the speaker is a fraction of the rated output power of the amp. Speakers can take transient peaks of much more than their long term rated power as well. You can't actually put more power in than the speaker is rated for though. So if you have a 600W driver and a 1200W amp and you set it up so that the amp is putting an average of say 750W into the speaker, you will cook the voice coil.

It is also possible to damage a speaker with perfectly clean power less than the speaker's rating, under certain conditions. For example, a speaker's power handling is greatly derated at the tuning frequency of a vented cabinet, because the suspension created by the air column greatly reduces cone movement (and thus cooling) at that frequency.

Finally, it is possible to damage a speaker by overdriving an amp rated for less power than the speaker. To a point. This is based on the fact that an amp will put out more than it's rated power when overdriven. This, in turn, is because square waves have more energy than sine waves at the same amplitude. So it's actually theoretically possible for an amp to put out twice its rated power under heavy clipping. This is where the 'underpowering' thing comes in. You can damage, say, that 600W speaker with a 500W amp, because the amp can put out more than 600W when clipped hard. You wouldn't damage that same speaker by clipping a 10W amp into it, though, because there wouldn't be anywhere near enough power disspated in the VC to cause any damage.

Debunking time. Some common arguments and my thoughts on them:

Myth 1: Clipped amps put out raw DC to speakers: Patently not true. If your amp is putting out DC, it is very sick. Square waves are in no way DC, assuming amps put out pure square waves under clipping (which they don't).

Myth 2: Square waves lock the cone into place for short periods of time, thus greatly decreasing cooling efficiency:

1) A square wave of a certain frequency will move the cone of a speaker in and out the same number of times per second as a sine wave of the same frequency.
2) The speaker cone will not just stop dead at the crest of a square wave. The cone's inertia will keep it moving. The amp does lose damping control during this time. While this doesn't sound very good, there isn't any evidence that it's damaging to speakers.
3) Speakers are on the order of about 1-2% efficient at converting electricity to sound, the other 98-99% is converted to heat. Decreasing cooling efficiency a bit isn't going to derate the speaker's power handling very much at all. Maybe by a few watts. At worst.

Bottom line: Be sensible with matching speakers and power and in using that power and everything will be cool. I've been plaing bass for 20 years and doing sound (arena level) for 10 and I can't recall ever melting a VC in a cone driver. I've seen lots of overexcursion damage though....
Phew, that's enough outta me....

Well said, Mark. This really oughta go in a FAQ somewhere here!