Matching optimum power of amplifiers with those of speakers

Dear All,

I am a new member and I have therefore to apologize since I expect this question may be a widely overseen topic. I would need some help in a topic where in the past I have received totally opposite answers.

Question is: What is the optimum ratio in between the RMS amp power and the RMS speaker's power of a given system?

Since an amp may be clipping, the amp power should be double as the speaker's, to serve as a protection. Or otherwise, if the amp's power is too high, it can burn the speakers, therefore amp power should be half of the speaker power ¿?

And in between I've heard a series of answers providing non answers, blaming the infinite occurrences that may happen: if power has been defined incorrectly or following different standards, or the case of mismatched impedances, différent drivers' efficiencies, ..., or a miriad excuses used not to provide a real answer. Or the best answer of them all: you have to listen to the actual system to learn if you are happy with its sound ...

Any light into this simple question?
All the best, Alfredo

 

It is not a simple question by any means and the "non answers" you have already had are mainly right. There is no "correct" figure and not really even an optimum figure as so much depends on myriad factors, some of which it appears you have already been told about. The important factors are:

Speaker efficiency.

Room size.

The volume you listen at.

The sound absorption due to soft furnishings or even having many people in the room is another but rather less important factor.

It is almost always possible for any amplifier to damage any speaker!!

There is no "RMS" power handling rating for speakers for a start (unless a chart is given showing power handling at various frequencies). The figures given by manufacturers are a very rough estimate based on "typical programme material", which will usually be assumed to be a a good quality recording of acoustic instruments and of wide dynamic range.

A speaker rated by the manufacturer as "suitable for use with amplifiers between 50 and 200W" would probably have its tweeter blown by a continuous tone of only 20W (or less) at 15KHz, its midrange driver blown by maybe 30-40W of continuous 2KHz and the woofer could be damaged by only 30-40W (or less) at frequencies below around 30Hz by mechanical damage from excessive travel of the cone, especially if it is a reflex (vented) enclosure which have no damping on woofer travel below resonance.

Hence power handling figures are a VERY rough guesstimate based on "typical music" and therefore on the drive units taking "typical" amounts of power in their own frequency ranges and NOT continuous tones ie a speaker rated at 100W would often be fine with individual short term peaks of say 500W during a drum beat or as a piano chord is struck.

One of the most difficult aspects of all this is the highly non linear sensitivity of the human ear. You would be amazed just how loud 1 W per channel actually is! (so loud that many people probably wouldn't believe it was only 1WPC unless it was proved to them). However... and this is where it gets tricky.... to go twice as loud again needs not 2W as you may expect but around 10W... and twice as loud again around 100W.
This really is why so many speakers get damaged, either by a small amplifier clipping and blowing tweeters or a powerful amp blowing any of the drivers!

And no this doesn't mean there is an optimum figure between these limits because... well, see above!

Lets say you have a large expensive multiway speaker that the manufacturer claims "suitable for amplifiers between 50W and 350W" and is a fairly typical 90dB/W efficiency. Now if you play only very well recorded wide dynamic range large scale music, such as a live rock band recording or large scale orchestral works and you have a very large living room with a lot of soft furnishings, and you demand that it be at realistic live volumes then you would need at least the 350W maximum and in fact you could probably get away with 500WPC...and probably NEED the 500WPC to avoid clipping! I have in fact been at a demonstration where a completely uncompressed digital recording of a solo grand piano being played fortissimo very briefly lit the 500W LEDs on the amps power meter in a very large room!

On the other hand.... with exactly the same speakers in a fairly small living room and the user listens mainly to "a girl with an acoustic guitar" type music and doesn't play any louder than this even on the odd occasion he plays some rock music then only 10WPC would be perfectly sufficient even though the manufacturer says "50W to 350W". The 50W minimum is given as the manufacturer has to make some big guesses and he's allowing for someone having at least a medium sized room and likes to turn the wick up a bit now and then when the missus is out (still much quieter than realistic volume though) without blowing a tweeter due to clipping if the amp were only 20WPC.

So, as I hope you see from all that, it's very much "horses for courses" and there is no correct figure.

Your's, an amplifier designer who has worked for some well known companies

 

Dear Arkles

For what I see, the general wisdom to be the amp 50% more powerful than the speakers should be challenged as well for all the arguments that you say. And final decision should be based on the plan you expect to give to the speakers. If it was difficult before, now knowing it relies in so many factors, it can be an almost imposible job to answer the question rightly!!

Thanks Arkless!

 

Welcome!!!

 

Do we all live in concert halls? I think not, so there is little sense in domestic amplifiers with power ratings that the Albert Hall needs. But is is also true, one needs to have sufficient reserve power in an amplifier to deal with high transients - i.e., not cause the power rails to droop on crescendos. The continuous music rating power of an amplifier is not the same as RMS rating at a single tone - i.e., 1kHz. Early amplifiers seemed to be a bit short on power supply capacity, one notable exception being Sugden Class A amplifiers.

No one runs their system flat out - do they? A comfortable level ought to be one where there is sufficient headroom for transients, particularly given the dynamic range of some music, broadcast via streaming services. My Spendor BC1's were rated at 12 watts with their original drivers, yet were perfectly happy with 45 watts from my Quad 303 - the volume control of the 33 set somewhere between 1/4 and 1/2, when playing vinyl. The power reserve simply ensured that the power supplies never dipped on crescendos.

These same loudspeakers, in this case with new ferrite drivers which are rated higher, subsequently drove my 405, which is 100 watts per channel. Now, they are driven by my Quad 909, which is 140 watts per channel.

Needless to say, there is plenty of reserve in the power supplies. However, the important point here is, the power level used to drive my Spendors is, and always has been, dictated by the degree of delicacy I get from the music; vocal or classical.

So, I would never put a specific power value on any amplifier. The efficiency of the loudspeakers will dictate the reserve power needed, over and above what are comfortable listening levels. As for soft furnishings - one cant keep changing one's HiFi every time one's partner changes the décor. And I have happily listened to large transmission line speakers in some claustrophobic settings. Actually, most loudspeakers have been too large for the room sizes that I have seen. Both the volume control and the tone controls took care of most shortcomings. That, of course was before Tone - Controls became dirty words. And yet, the Filter and Tone Controls of my Quad 33 pre - amplifier overcame many problems associated with speaker location.

 

You should hear what BC1's can do with decent amplifiers!

I use mainly BC2's here BTW and usually driven by 16.5W RMS Class A monoblocks of my own design and build.

 

Nice, I can imagine. If I could have afforded a Class A Sugden, I would have. But even the second-hand prices are too much; particularly as with Class A there will be some heat degradation. I wouldn't mind my own decay, but I am not paying for someone else's.

I have a Sugden Bijou Headmaster which will ultimately be used when we down-size. However, it currently drives my 909 and my Stax SR - 3's, or alternatively my Spendors. Both my Headmaster and Quad 909 appear to be new - old stock, so I don't feel cheated. Indeed, I got them at tremendous prices. It is possible that too makes them sound better he! he!

 

There is a small square shaped class A power amp that Sugden made to go with the Headmaster. The Headmaster is a nice little unit. I rebuilt one with better capacitors etc a couple of years back for a customer. Very awkward to get apart to work on IIRC and due to the mounting of the headphone socket.

 

I looked at that, thinking it would be class A, but sadly not; A/B. The big beastie that goes with the Bijou rack is Class A, but an arm and a leg to buy. Plus, I won't entertain the kind of prices asked as after all these years; there is probably a bit of thermal decay. However, I'm happy with the 909, just as I am with my 405 and 303. Also, I have the Headmaster to look forward to when I end up with headphones only.

I appreciate the Spendors can easily be driven outside their comfort zone and I would not like to risk the new drivers which I recently fitted. There is an amazing 'Sweet - Spot' in their performance, but would not want to go beyond it. My Stax have no problem with power - well my ears would explode before I found their limit. So if I want to crank up the decibels, I just go over to them.

None of this was supposed to happen, being as we are moving and I am 77 years; not 75 as I said - tend to shave of a couple of years to pull the birds He! He! However, I think for once I got lucky with these two beauties. So while we still have a place big enough, I may just hunt down a pair of floor - standing monitors which can take a bit of punishment. My daughter's partner has a pair of B&W's, which I may get a chance to try. Would love a pair of Quad Electrostatics but that is a long standing pipe dream which will never materialize.

 

The Musicmaster is the one I'm thinking of and it is class A. Expensive new (£1200 ish) but no doubt available for much less S/H as I don't believe they sold very well due to being so low power (rated as 16WPC but IIRC hi fi world review reckoned only around 9W in practice). Horses for courses though.... my BC2's are a bit more efficient than the BC1 at 89dB/W and I have plenty of volume with either my class A monoblocks or my (modified) 10WPC Leak Stereo 20 in my smallish living room.

 

Rare as hens teeth, I should imagine. I can't find one anywhere. But one never knows! Thanks for the 'Heads - up', I never knew it existed.

 

I completely agree—having a massive amplifier for home use often isn't necessary. It's all about having enough reserve power to handle those dynamic peaks without distortion.