Should amplifier output be restricted to the max power rating of the speaker?

[Dave TR3A]

Although this post is not directly related to an amplifier review, I did not see another forum that looked more suitable for it. If this doesn't belong here please direct me elsewhere and I will move it.

I've been working under a premise that may or may not be correct. I'm hoping that members of this forum might be able to help me in understanding whether one ought to restrict the output of an amplifier so that it doesn't exceed the maximum power rating of a speaker.

In my case, I am running a tri-amped 3-way system, where the tweeters are driven by one amp, the midranges are driven by another, and the woofers are driven by the third amp. Here is the info for each of the drivers in my system:

	Driver	Nominal impedance	Max. power rating (RMS)
Tweeter	B&C DE120	8 ohms	40W
Mid-range	Klipsch K-55	16 ohms	40W
Woofer	Klipsch K-33	4 ohms	100W continuous (400W peak)

• I know that the impedance rating is a nominal measure and that the impedance varies with frequency. From what I've been able to find on-line, the nominal impedance is close to the lowest impedance for the woofer and tweeter, but the mid-range impedance drops as low as 11 ohms (at ~5000 Hz).
• The max power ratings come either from on-line sources or from the manufacturer, but I do not know how they were determined.

I used ohms law to calculate what the maximum voltage and current would be at the maximum power. I came up with the following:

	Driver	Nominal impedance	Lowest impedance	Max. power rating (continuous) RMS	Volts @ max. power	Amps @ max. power
Tweeter	B&C DE120	8 ohms	8 ohms	40W	17.9 V	2.24 A
Mid-range	Klipsch K-55	16 ohms	11 ohms	40W	21.0 V	1.91 A
Woofer	Klipsch K-33	4 ohms	4 ohms	100W	20.0 V	5.00 A

Wanting to avoid burning out the drivers, I measured the voltage at different gain positions on each amp. I then set the gain (aka volume knob) on each amp so that the voltage produced could never exceed the limit for each driver (i.e., 17.9V, 21V, or 20V).

Should the amplifier output voltage be restricted so that it is cannot exceed the maximum power rating of the drivers?

If such a precaution is prudent, what is recommended when working with power amps that do not have a gain knob?

 

[tineared]

Limiting amplifier power is completely unnecessary. If you overdrive your speakers there will be plenty of audible distortion before you get to the point of damaging your speakers.
The real danger to your speakers is too much alcohol consumption during a party so you don't notice the distortion.
Ask me how I know

 

[Dave TR3A]

The real danger to your speakers is too much alcohol consumption during a party so you don't notice the distortion.
Ask me how I know

Or unsupervised children/teenagers who get their hands on the volume control.

 

[AndyMorris]

Or unsupervised children/teenagers who get their hands on the volume control.

Or who see what happens if you poke the paper bits.

 

[voodooless]

There aren’t really any standards for power rating anyway, and if there are, they are generally not used by consumer gear.

So take them with a pinch of salt and be careful.

 

[mps]

Depends if you are an ESL-57 owner

 

[Dave Bullet]

The power limits you are quoting are thermal limits. I.e the amount of heat generated by that power level, above which voice coil will be damaged through over heating

Excursion limit is frequency dependent. Above a certain frequency thermal limit takes over but below that less power than the thermal limit will actually cause the driver to exceed xmax. Dependent on driver the xlim may just be beyond or way beyond xmax. The spider usually limits travel but depending on backplate design the coil can smack the backplate.

For a 3: way speaker a rule of thumb is 10/30/60 % power spread across T/M/W drivers

It depends on your average volume level too. Allowing for 10-20dB headroom. Means 10 to 100 times reserve power. So an average 1w/1m at 86dB can be power limited at 106dB with a 200w amp

More is better imho. Just use your ears and listen for distortion

 

[Dave TR3A]

The power limits you are quoting are thermal limits. I.e the amount of heat generated by that power level, above which voice coil will be damaged through over heating

Excursion limit is frequency dependent. Above a certain frequency thermal limit takes over but below that less power than the thermal limit will actually cause the driver to exceed xmax. Dependent on driver the xlim may just be beyond or way beyond xmax. The spider usually limits travel but depending on backplate design the coil can smack the backplate.

For a 3: way speaker a rule of thumb is 10/30/60 % power spread across T/M/W drivers

It depends on your average volume level too. Allowing for 10-20dB headroom. Means 10 to 100 times reserve power. So an average 1w/1m at 86dB can be power limited at 106dB with a 200w amp

More is better imho. Just use your ears and listen for distortion

@Dave Bullet Thank-you.

I had wondered how manufacturers determined the power ratings for drivers. It’s helpful to know that exceeding the specified power limit will cause damage by overheating the voice coil.

My tweeters and midranges are compression drivers. Would the xmax/xlim concern apply to them?

 

[NTK]

@Dave Bullet Thank-you.

I had wondered how manufacturers determined the power ratings for drivers. It’s helpful to know that exceeding the specified power limit will cause damage by overheating the voice coil.

My tweeters and midranges are compression drivers. Would the xmax/xlim concern apply to them?

You can take a look at this Scan-Speak tech note. It may not be what you think.
https://www.scan-speak.dk/datasheet/tech/Scan-Speak_Technote02_Powertest.pdf

 

[Buckchester]

Limiting amplifier power is completely unnecessary. If you overdrive your speakers there will be plenty of audible distortion before you get to the point of damaging your speakers.
The real danger to your speakers is too much alcohol consumption during a party so you don't notice the distortion.
Ask me how I know

How do you know?!

 

[Buckchester]

Or who see what happens if you poke the paper bits.

Or you have a sub optimal system and you keep turning up the volume hoping to finally hear some bass. Ask me how I know.

 

[Dave TR3A]

You can take a look at this Scan-Speak tech note. It may not be what you think.
https://www.scan-speak.dk/datasheet/tech/Scan-Speak_Technote02_Powertest.pdf

@NTK Thanks for sharing that document.

I was hoping for clear guardrails so that I’d be able to know I wasn’t risking my drivers if I stayed below such-and-such a level. While the tests described in the document sound onerous, they don’t provide a clear safe/not safe boundary. I guess the situation is too complex to allow for the simple specification I’d been seeking.

Now I’m not clear on how useful the max power spec is for any driver.

 

[Jose Jimenez]

No. Too small an amplifier is worse than too large. When clipping, sending square waves to the tweeter(s) makes their life short. Depending on the linearity of the driver and output transistors, most (not all) BJT Class AB solid-state amplifiers sound their best at the lower levels anyway. It's what you like hearing and can afford. MOSFET and tube amps are a different flavor.

 

[Dave TR3A]

No. Too small an amplifier is worse than too large. When clipping, sending square waves to the tweeter(s) makes their life short. Depending on the linearity of the driver and output transistors, most (not all) BJT Class AB solid-state amplifiers sound their best at the lower levels anyway. It's what you like hearing and can afford. MOSFET and tube amps are a different flavor.

@Jose Jimenez I agree that clipping is a significant danger.

I’ve measured my amps to determine when they clip. What I found surprised me. My amps are supposed to have an input sensitivity of .775 Vrms. To my dismay, the actual input sensitivity was much lower — in the neighbourhood of 0.5 Vrms. A higher amplitude input signal would cause the amp to clip well before maximum gain.

Since some of my sources provide a higher amplitude signal (e.g., 2 Vrms) I needed to attenuate the inputs.

Whenever possible, I try to make my own measurements because I view the manufacturer-published specs with a jaundiced eye.

 

[four_strings_good]

There aren’t really any standards for power rating anyway, and if there are, they are generally not used by consumer gear.

So take them with a pinch of salt and be careful.

Living proof of this is the multitude of amplifier companies that use "watts RMS" as units of measurement for amplifier power. There are RMS measurements for AC voltage and current, but there is no such thing as watts RMS.

(Some people don't know how to pronounce parmesan or mascarpone either, which also makes me crazy...)

 

[DVDdoug]

I used ohms law to calculate what the maximum voltage and current would be at the maximum power.

Speakers are rated based on their nominal rated impedance so you don't have to calculate it at the actual minimum. And it's a "fuzzy" rating.

No. Too small an amplifier is worse than too large.

That's a common myth. You can damage a speaker either way, but more power is worse. The real problem is that as you turn it up past clipping, the average continues to go up even though the peaks are clipped and it's the short-term average that burns-out voice coils. The highest-harmonics are usually not that strong. If you clip audio badly you might notice a (relative) loss of highs because the lower-mid frequencies and their near-harmonics dominate.

Also, tweeters are often rated for the total power into the speaker system. A "40W" tweeter may not handle 40W on its own. The higher the specified crossover the smaller percentage of the power it has to handle so it can be rated for more power if they specify a higher crossover frequency.

 

[Everett T]

In my case, I am running a tri-amped 3-way system, where the tweeters are driven by one amp, the midranges are driven by another, and the woofers are driven by the third amp. Here is the info for each of the drivers in my system

Are you speakers active or passive?

 

[NTK]

@NTK Thanks for sharing that document.

I was hoping for clear guardrails so that I’d be able to know I wasn’t risking my drivers if I stayed below such-and-such a level. While the tests described in the document sound onerous, they don’t provide a clear safe/not safe boundary. I guess the situation is too complex to allow for the simple specification I’d been seeking.

Now I’m not clear on how useful the max power spec is for any driver.

In live sound applications when no sound from blown loudspeakers is strictly unacceptable, the amplifiers have limiters. Below is from the Crown LevelMax guide. You can see that Crown LevelMax has 3 different limiting functions, (1) peak limiter which limits the peak output voltage, (2) RMS limiter, and (3) thermal limiter. The speakers intended to be paired with the Crown amplifiers will each come with their set of parameters to be entered into the limiter for proper protection.

https://www.crownaudio.com/en-US/site_elements/levelmax-guide-for-i-tech-hd

 

[john61ct]

@NTK Thanks for sharing that document.

I was hoping for clear guardrails so that I’d be able to know I wasn’t risking my drivers if I stayed below such-and-such a level. While the tests described in the document sound onerous, they don’t provide a clear safe/not safe boundary. I guess the situation is too complex to allow for the simple specification I’d been seeking.

Now I’m not clear on how useful the max power spec is for any driver.

Test noisy material with your ears, make a note what level you start feeling uneasy, back it off a bit and set the amp to stop there.

If your amp even has that feature.

There are also limiter circuits you can place in the chain pre amp.

There are fuse types too but it's a fraught topic.

Anything in the chain impacts the SQ.

Now with skillz, an external dB sensor could turn iff the circuit...

 

[kemmler3D]

How do you know?!

Yep, reportedly Purifi 6.5" drivers will start smacking into themselves before the distortion gets bad. I guess if your speakers are truly fancy they might break without too much warning.