Underpowered Amplifiers

We all know that underpowered amps can lead to blown speakers, but is that really true?

Here's a post from another forum. I wanted to get the opinions here before responding...

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This is a myth that's been floating around for some time (most commonly used by hi-fi salespeople attempting to sell larger amps). An amp with ample power is a good idea for transients and distortion free playback, but content with harmonic distortion (within reason, of course) is not going to fry your speaker cones. If it would, then playing and loud music with distorted guitars, etc. would be equally injurious to your speakers. Also, while a lot of power might fry the coils on your monitors, if the actual cones started to fry, you'd be in trouble. Speakers on guitar amps, playing lots of harmonically distorted sound would be in real trouble.

My wife heard an interesting variant of this myth some time ago. A young salesperson at Best Buy explained to her that if your car stereo had lots of clean power, you could listen to music as loudly as you wanted without damaging your hearing. Hearing damage, according to him, occurred when you listened to an underpowered system turned way up.

Serious power amps are great to have in a studio situation, but you have to be careful. While experimenting with sounds, patching, etc., you can easily generate transients that will blow speakers. I speak from experience here.

BTW, for anyone who hasen't seen it, here's an interesting page of audiophile mythology...

http://www.geocities.com/CapeCanave...01/Audio_BS.htm

While you do need power to properly drive a loudspeaker, these days the question is how much power! Years ago, yes, you got the biggest amp you could get or afford. But years ago, the PSA-2 was a big amp!

I have played with a few amps, and while yes you do need proper power to drive the speaker in question, you can in fact have too much power also! I read where TWICE the RMS rating of the driver is advisable, but when I have done this I find I cannot drive the amp, and I dont quite get what Im looking for, and if you do drive an amp that legitimately has double the rated power its just as easy to blow your speaker!

If you have a woofer rated for 600 watts, I wouldnt use a 150 watt amp because you can severely overdrive the amp and severe distortion can burn voice coils, as well as you probably wont get sufficient output! But you can drive a 600w woofer with 450 watts and be happy if you like the sound, and have enough output!

Its a balancing act, and I have never really blown my tweeters ) JBL 2402 ) with ten watts per tweeter! But use too much power and it gets too loud and harsh for me!

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Originally posted by JonFairhurst
We all know that underpowered amps can lead to blown speakers, but is that really true?

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In my experience it is absolutely true!

In the past I have blown several speakers (all MF and HF units BTW) with 30 to 70 wpc amps. I have never damaged any drivers with large amplifiers. A 40wpc amp driven into hard clipping will over heat the HF voice coil very quickly...so will playing square waves for that matter!

If a speaker is rated at say 20 watts, just don't turn that 300wpc amp all the way up and you will be fine. Unless you are trying to play louder than a bunch of bumper cars:D , you will rarely use more than 20 watts with even the most inefficient speakers anyway. The idea that you should use 150 watt amps with 150 watt speakers is silly. You should use the best sounding amp you can find that plays the speakers at a level that pleases you. If you start to hear distortion, turn it down or get a bigger amp. Obviously you must be reasonable too. If you try to fill a stadium with a pair of AR3as powered by a McIntosh MC2500 you will blow them up if you turn it up past the point where the loudspeakers go into distortion, but that is just common sense!

On the other hand, if you listen to moderately efficient (93 dBw) speakers and rarely have peaks over 95 dB you probably don't need more than 25wpc. But heck that 100watt amp will sound fine too!

Widget

I recall on the JBL Pro www there is great article about this topic.

The reason for the expiry of mid cones and tweeters when an amplfier clips is the flattening of the sine wave till it resembles a square wave. The intense energy burns the coils if not damaging the suspension.

Clipping happens then the output voltage approahes the supply rails. In earlier power amps often the protection circuits would also cause damage due to spikes or chatter from their action.

This generally happens in domestic systems at parties and in live situations when the FOH is a DH.

For most of us common sense prevails and a few simple calculations can determine if your amps and speakers have sufficient dynamic headroom to avoid damage.

In studios at least John Eargle advises in his Hand book 6 db of system head room.

For example you need a maximum of 110 db (quite loud) at say 2 metres (2 yards approx) and your speaker is rated at 94 db @ 1 metre @ 1 watt and has 200 power rms rating.

Then we have 94 db for 1 watt @ 1 metre, so 200 watts = 23 db
and will give a net output of 117 db @ 1 metre. Since sound level drops 6 db for every doubling of distance at 2 metres the maximum output will be 111 db at 2 metres. This assumes normal acoustics and the 2nd channel will another 6 db for a total of 117 db at 2 metres.

In this case we have met the maximum sound level and will have the desired 6 db of headroom assuming you have a 200 watt per channel amplifier available.

If your speakers are 3 db more sensitive @97 db, 1/2 the power (100+100 watts) will be required for the above requirements.

However, if your speaker is 3 db less sensitve @91 db you not be able to satisfy the requirement as double the power (400+400 watts) will be needed which exceeds the power rating of the speakers.

Bi amping is a bit more complicated but at around 250-300 hertz it has the net effect of adding approximately 6 db of additional headroom due the added crest voltage drive and approx 50/50% power distribution across the spectrum.

So if you bi amped with 100+100 watts its like using a 400 watt amplifier(per channel).

You would need to assess that your mid and HF drivers were rated to handle this power and that your woofer has the thermal and displacement ratings.

I think we should make this a senior college subject so we don't have a generation of deaf people leading the world with their high powered auto systems!

Ian

I don't fully understand if turning down the level on power amps gives you any advantage over using a smaller amp with the output levels turned up a little.
This reminds me that I asked this question some time ago on this forum as "Where is the best place to get gain". I recall the answers were mixed.

I run my preamp at "normal" levels without getting clip LEDs while turning down the level adjustments on my power amps to give me my maximum tolerable listening levels. Am I gaining anything by having this "unused" power available? Would I be better off turning my power amps all the way up and turning down my preamp?

Tom

Provided your pre amp is not being overloaded I would turn the preamp up full and then adjust the power amps levels to the maximum level you are likely to listen at then reduce the pre amp gain.

In this arrangement you will get the best signal to noise ratio.

The concept of unused power headroom is important regardless of the preamp or power amp gain settings.

Ian

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Originally posted by Ian Mackenzie
Provided your pre amp is not being overloaded I would turn the preamp up full and then adjust the power amps levels to the maximum level you are likely to listen at then reduce the pre amp gain.

In this arrangement you will get the best signal to noise ratio.
Ian

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Hello Ian and Tom,

I totally disagree with this setting!

Turn up your amplifier at 100% and then adjust your preamp.

If you do not turn up at 100% your amplifier you will modify the input filter and you will loose some bandwidth and some harmonics by the same way.

If you can set the gain of your amplifier, set it or get it modified thus you have the less gain possible.

It will increase his signal to noise ratio and as the ratio is better will hear more micro details that were void by the hum of the amplifier.

By the same way the stereophonic image will be larger and deeper!

I made the test from 31 to 26 dB gain and I will keep a 26 dB gain, as it is now a standard.

I even plan to lower it at 20 dB for the studio amplifier I am still working on.

If you only listen at recorded music from a CD player, I will even suggest removing the preamplifier and either use the volume control from the CD player if there is one.

If there is none you must turn down the potentiometer of your amplifier. Not to loose some bandwidth, you must lower the value of the capacitor of the input filter.

I hope this will help!

I agree with Yuri. I have run my amps pulling the attenuators back, but I say it sounds best with the amps controls wide open, and adjust level from the active crossover!

Widget, I run 4 bullets ( 2402 ) per channel of D-75,s and have alot of level, never have to clip, never clip, never blow tweeters! Whenever Ive used a proper rated amp, according to manufacturers specs, 40 watts per tweeter, the sound is too hars, and uncontrollable! I have plenty of highs with ten little watts per tweeter, and they will get rather loud, way before clip! Another thing ive found throughout the years, is that bigger amps usually have somewhat higher input sensitivities, and up in the high frequency range I have always had to push a bigger amp to hard to get proper drive level, but an amp like the D-75 with its 3/4 volt input sensivity works perfectly for this range, and ten watts per tweeter works fine for me!

Bass. Low frequency needs power. But if your load is rated for 1200 watts do you really need 2400 watts? I have not had good results if I cant push the amp hard enough! Of course, I need enough power to reproduce cleanly, without square wave, or audible distortion, but I dont know about twice the rated output!

Neat thread!

Well I am underpowered looking at the driver rattings but overpowered for my application. I have 100 watts per driver but with the bandwidth limiting from using the active crossovers I don't think I could even get my E-145 amp to clip and still stay in the room. I cross at 50Hz 300Hx and 1.2K so the amps are loaffing except the subs which work the hardest with the lowest driver sensitivity. I am in the more power is a good thing camp. I also do not run all my amps wide open. I have the levels set-up with a balance between the active crossover and amp level controls. I found that to get the least amount of backround hisssssssssss/noise from the drivers using the available gain in the active crossover and dropping the level on the amps gave me less in room noise. Not sure if that was the best solution but it worked.

Rob:)

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Originally posted by Robh3606
Neat thread!

I found that to get the least amount of backround hisssssssssss/noise from the drivers using the available gain in the active crossover and dropping the level on the amps gave me less in room noise. Not sure if that was the best solution but it worked.

Rob:)

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Rob,

Ask Scott when he will have in his hands the PSS1200.

This amplifier has a 107 dB S/N ratio.

There is NO background hisssssssssss/noise, unless you stick your ear on the tweeter or the horn!

It as important as a fly flying ...

That is why I think that THE MOST important number is the signal to noise ratio and the CMRR if you have a balanced input.

Why more powerful amplifier have better signal to noise ration:

From one model to an other (PSS600-1200-2400) the noise is the same: -71 dB (measured at the output of the amplifier with no input signal).

a 300 Watts amplifier shows + 33 dB when it delivers its full power.

71+33=104 dB

As a 600W amplifiers delivers 3 dB more, then the S/N is 107 dB.

With a 1200W amplifier the signal to noise will be 110 dB.

Better the signal to noise ratio will be, more details will be heard as they are NOT covered by any internal noise.

If you lower the gain of the amplifier, you will lower his internal noise and the S/N ratio will increase!

One cannot imagine how many sounds can be on a CD!

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Originally posted by scott fitlin
Another thing ive found throughout the years, is that bigger amps usually have somewhat higher input sensitivities, and up in the high frequency range I have always had to push a bigger amp to hard to get proper drive level,

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Scott,

It is normal, it depends of the amplifier gain.

All our amplifiers have the same gain, now set at 26.

It means, whatever amplifier you take, with the same input signal you will have the same output power!

The PSS1200 has an input sensitivity of 2,3V

If you use a PSS2400 with this input signal it will deliver 1200W too!

And so on.

Why bigger amplifiers sound better on tweeters:

As the have a limited bandwidth the can offer a sound closer to a tube amplifier.

As they have a bigger transformer the internal resistance of the power supply is smaller and it changes the sound tonality!

Its the smaller, more sensitive amps that sound great on JBL tweeters. I have had big amps with 1.5v input sensitivity and it doesnt work well from 7K and up for me. The little amps give me a far more delicate sounding top end, and the tweeters sound sweet!

My subs and 15,s are what work the hardest in my system, and these ranges do require power. This is where I use big, high power heavy duty amps!

As for the highs and mid horns, well I am underpowered according to JBL specs, but with 22 tweeters and six 2395,s I have the room equally covered and dont think Ill ever need rated power to achieve clean, high enough SPL, sound!

Here's my conclusion regarding small amps...

The consensus is that it's not a myth, but still, you will only blow your speakers under fairly extreme conditions.

The reason that square waves are more damaging than sine waves is that when the speaker stops moving, the air stops moving and the heat builds up in the voice coils.

Most guitarists play through 12" speakers with no tweeters, so they tend not to blow their speakers under normal conditions. The guitar is usually "in the mix" by the time it gets to the full range speakers, so it's okay.

Here are some conditions:
300W speaker & 20W amp
- It'll sound bad but never blow
20W speaker & 300W amp
- Good bye!
300W speaker & 100W amp
- Works fine at moderate levels
- If pushed to clipping levels for extended time, bad news for the tweeter.
100W speaker & 300W amp
- Fine at moderate levels
- Fine loud
- If you hear your speakers hitting the end of their travel, you *will* turn it down. It's not like amp clipping. It *really* sounds bad. If you don't turn it down, you get what you deserve.

BTW, I use a 300W speaker with a 100W amp at moderate levels. If I were to DJ a block party, I'd rent a bigger amp.

Given that the original topic (on the other forum) was for near-field studio monitors rather than FOH and stadiums, none of them should ever blow their speakers, regardless of the amplifier used.

-Jon Fairhurst

G'Day All,
This links has an interesting view on HF failure.
http://www.st-andrews.ac.uk/~www_pa/...ing/page3.html

Regards Scott

Also, the term "clipping" is not exact, when the amp is overdriven, it does not "clip" the wave with a pair of scissors, that would be seen by the xover as DC, which will not pass thru a capacitor; What happens is that at every "clip", the amp oscillates very quickly at high amplitude, which is seen by the xover or cap as VHF, which goes thru at current 100 times or more than the tweeter is designed for, and of course it burns/breaks.

It is better to have reserve capacity on the amp to prevent the oscillation; of course if You still conciously overdrive it, more burning, etc., maybe a shrink would help :D

Alex.

Alex,

I know that with an electric guitar amp it does look like the signal is clipped off with scissors, but with slightly rounded edges. I would guess that a power amp into a speaker has more going on.

But a square wave, though flat at the top/bottom is not the same as DC. As I recall it's a series of frequency spikes. Let's say the signal is a 1kHz square wave. The lowest frequency component is still 1 kHz. After that you get the odd harmonics - 3 kHz, 5 kHz, 7 kHz etc... Each harmonic is weaker than the last, but for a perfect square wave the harmonics go on to infinity. (Which means that there are no perfect square waves in real life - that would account for the high frequency oscillations.)

BTW, tubes tend to distort in a way that produces even harmonics. That's why tube sound different (and better) than transistors when pushed hard.

But that's the frequency domain view (and that's how we know square waves will pass through the x-over). From the amplitude view the speaker sits there in a DC state between the transitions. It burns power and gets no cooling effect since it's not moving air. Not good.

But I think the shrink idea is excellent. I, like lots of people here, am obsessed with deep, clean bass. The only cure I've found so far is a pair of 2235Hs. :)

Jon:

We agree, also, my shrink made me get 4 2245H´s, it has helped.

Alex.

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Originally posted by Ian Mackenzie
I recall on the JBL Pro www there is great article about this topic.

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:yes:

JBL Technical Note: Danger: Low Power