Another wattage question.

A common rule of thumb is selecting an amplifier is that the power rating should be 50% to 100% greater that the maximum power handling of the speaker if you are concerned about thermal overload (blowing out the speaker). The power rating of an amplifier is based on a connected load, usually 8-ohms. It is also based on a nice, clean sine wave signal. The music signal is not a sine wave but of one with dymanics (as Gregm notes above). This will cause more of a load on an amp to get the music signal as loud as the reference sine wave signal. That the reason for the higher amp power selection.

The low side of the recommended speaker power is to prevent the amp from clipping if you attempt to play at higher volumes. Clipping can damage a speaker driver just as easily as an overload - especially with digital sources which have very high dymanic swings.

So, you will be okay with 200 to 240 watts. But as Kchahoc states above, if you are reasonable with the volume control, going above 240 wpc will not be a problem. The best thing to do is to contact B&W as ask if the make and model amp has any problem they can foresee.

Bottomsup,

OK, no problem. I'll be very gentle.

You understand the basic relationship of W=V*A correctly. However, what you are thinking of is peak power rating of an amp, which is what is specified on an amps's data sheet. So, if an amp is rated at 100W & is connected to an 8 Ohm speaker, then the peak steady-state current delivered by the amp would be 3.53Amps. Steady-state implies that the input signal to the amp is not moving (Gs5556 has also state this in diff. words). To get this steady-state current, you'd have to feed the amp it's max. rated input signal. Now, who does this on a sustained basis? In home listening, no one! So, essentially, this much touted specification is quite useless when it comes to judging a power amp. It can, however, be used to *infer* what the amp *might* be capable of. Our brain tells us "if the amp has high max. Wattage, it is very likely that the designer installed a very robust power supply. Thus, it is very possible that this amp can deliver high current into a speaker load". NOT always true!! Some of the better power amps w.r.t. current delivery have wattage that doubles with each halving of speaker resistance. This trend (also listed on the spec. sheet) is a much more useful parameter than just the max. power rating. Doesn't tell the whole story by any means but gives a better insight into what the amp *might* be capable of.

If you re-read my earlier post, you'll see that the N805S puts out 85dB SPL @ 2m using 1W (82db SPL for 1 speaker but we never listen w/ just 1 speaker hence add 3dB to the above calculated #). Now, 85dB SPL at 2m is quite loud esp. if you are going to listen for 2-3 hrs straight! The point here is that you are using 1W of the amp. Well, this is 1/100th of the steady-state current vs. what was calculated above - 0.353Amps!

Let's just say that the amp has reached its max. delivering 0.353Amps. What happens when there is drum thwak or a sudden revving up of the orchestra as in many Beethoven works or when piano keys are suddenly hit as in many Chopin works? There is a sudden spike in the voltage delivered to the amp (from the pre). In terns of SPL, say, that the SPL level shoots to 102dB from 82dB. As long as this voltage spike is within the voltage rails of the power amp output devices, the amp will not clip. Since V=I*R & voltage increased, current must increase to make that equality sign hold true. R cannot change (FOR ARGUMENT'S SAKE ONLY). With this 20dB SPL increase, the amp has, suddenly, been asked to deliver 100X the power i.e. 100W while playing at a volume setting equivalent to 1W. It's within the spec of the amp but the amp must be designed to deliver 100X the power, which will equate to 100X the current - 3.53A from 0.353A. So, now, who will supply this current? Of course, the amp! But from where? From that honking xformer & power supply caps that should have been in its chassis. If the amp cannot supply this instantaneous power, the transient will be muted & the user will say - highs are not airy, bass lacks punch or is lean.

So, the volume setting on the pre. has not changed (you didn't touch it!) but yet the amp is asked to deliver more current. This is 'cuz music is a dynamic (forever changing) signal that requires different power output from the amp for each diff. bit of the music signal.

Notice too (FOR ARGUMENT'S SAKE) we have not reached the limit of the amp (i.e. it is not clipping) + the power output is NOT steady-state. So, we cannot use that simple W=V*A relationship at all.

There is another spec (often not written) called the dynamic headroom & it deals with power output from the amp once the amp's rated wattage has been exceeded. In this eg. if the 100W amp is asked to deliver 200W instantaneously (in terms of SPL level: it is 105dB i.e 3dB more than 102dB. 3dB in power is a 2X factor), could the amp do so? Well, if it had a 3dB dynamic headroom spec, it could. This will entain a might fine/robust power supply & the amp will be h-e-a-v-y with the additional heat-sinking! It'll most likely burn a hole in your pocketbook too! Once again, the xformer & power supply caps will be asked to provide this sudden burst of current.

I *suspect* that Jzzmn88's amp is suffering from this lack of current delivery. Hence, the need for a higher wattage amp. The higher wattage should have a better power supply & it should solve his problem but, as Gs5556 wrote, Jzzmn88 can easily get into a high listening to his music & crank it up & melt his N805S voice coils! 120W is more than sufficient for the N805S but ensure that this 120W amp can deliver some current. You'll find a lot of heat sinking (generally) on high current delivery amps as running high current thru the output devices creates a lot of heat, which could damage those devices if not dissipated rapidly.

Long-winded answer (hopefully not rambling!) & hope that it helps some.

bombaywalla: thank you for taking time to answer my question. Much clearer now. I always wondered about dynamic headroom spec but was afraid to ask. My "new" amp, an older Audio Research D-100, weighs twice as much as my old Hafler. Can I assume that it can handle these surges in power requirement more easily than the Hafler(48lbs compared to 24 lbs). Audio Research doesn't publish specs at 4 ohms? Or is that assumption invalid too?

Bottomsup,

IN GENERAL, I don't think that this assumption is valid. OTOH, ARC is a company with a long-standing reputation for good/excellent products. I have no personal experience w/ the D-100 so I cannot comment.

Nelson Pass once wrote an article on power supplies for power amps (It's on his website called "Power Supplies: Commentary for Consumers". Worth reading, IMHO). What he says there is that a toroidal xformer yields 30W/lb. From his paper I interpret that a 100W amp should be able to deliver 300W & that will force it to have a 900VA xformer, if the power supply is to be constructed correctly. Thus, this 900VA xformer will weigh 30lbs itself. Add heat-sinking, power supply caps, electronics, damping material & one could easily double the weight to 60lbs.
If a 100W amp can deliver 300W, it has atleast 3dB dynamic headroom.
Going by this white paper, 48lbs is on the low side. However, white papers are white papers! The proof of the pudding lies in listening to the amp. I doubt that the amp doubles its wattage as speaker imp. halves but that doesn't mean it's a bad amp! Listen to it, put it thru the ringer & you'll answer your question.

Bombaywalla, thanks, your reply above is one of the rare and priceless explanations that help people to get smarter in their audio quest... If there were Audiogon FAQ, this reply definitelly should be there.