Multi-Channel Amp questions

Hoping the A'Gon experts can educate me:

When calibrating a processor/amp or receiver, they say to use 75db as a reference point. If that's the case, why would anyone need an amp greater than 100W all channels driven?

If I have speakers that have a sensitivity rating of 85, 1 watt will get me to 85, 2 watts will get me to 88, 4 watts will get me to 91, 8 watts will get me to 94, 16 watts will get me to 97, etc.

What am I missing? I've read that higher watt'd amps give you better dynamics, etc. Is that really the case?

I have dynaudio audience speakers that have sensitivity ratings of 86. I'm running them through a B&K AVR317 (507 upgrade) with 150 - 185W per channel. They're 4ohm loads. Would I do better moving to separates with a higher powered amp? And what amps are recommended?
Forget about all those calculations. Listen to your system carefully and learn, if you haven't heard it before, to determine the sound of clipping. With 86dB-sensitivity speakers, I'd say you're at the edge of having NOT enough power for a home theater, which many of us love to run at higher sound levels than music systems.

More power gets you reserves to drive your speakers as loudly as you gets you a sense of ease...of never being strained.

I'd think a poweramp with at least 200 watts per channel into 8 and rated to double into 4 Ohms would sound better than your receiver, even tho 200 is just a fraction of a deciBel more than 185. (We'd need to be sure we're measuring exactly the same ways to determine just how much more power the poweramp has over that of your receiver.)

Here's another consideration--amps with fewer channels on one chassis tend to sound better than those with more, because there's more channel separation and also because each power supply has more headroom. That means that if you need 5 channels of amplification, 2 amps of 2 and 3 channels will sound better than one 5-channel amp, other things being equal (which, of course, they never are!). That's sort of why lots of us prefer the sounds of mono amps.

Consider the Proceed HPA-series amps--250wpc/8, 500wpc/4, in 2- and 3-channel versions. They were discontinued last year and are quite affordable.
First of all, 75 dB's is WAY quiet for most movies and is a useless figure based on theory and not reality.

Secondly, your power input vs spl output doesn't take into account thermal compression of the speakers. Now add the fact that you are probably going to be sitting measurably more than 1 meter away from the speakers and those spl's mean nothing at all. Taking that a step further, factor into this equation that dynamic peaks at low frequencies require BIG power and you might be getting closer to the truth. The fact that most ( there are obviously more than a few exceptions here) vented speakers present a very high impedance between between 25 Hz - 60 Hz and you need even more power than you think you do. That's because a typical SS or tube amp with a transformer can't transfer as much power as impedance climbs. Given that many large vented woofers produce impedance peaks of 60 - 120 ohms in the low frequency region, do the math. Your "plenty powerful" 100 wpc @ 8 ohm amp is now only capable of somewhere between 6 and 12 wpc right in the region where you need the most power.

Now factor in that most receivers are PITIFUL in terms of the amount of current ( power capacity at lower impedances ) that they are capable of producing and your "100 watt monster" starts looking like a yelping chihuaha.

As far as specifics go, Dynaudio's will benefit from as much power as you can give them. That is, if you use them in a decent sized room and expect good volume levels with clarity and impact. As far as your receiver goes, even B&K's amps have never been known to be "authoritative" into low impedances in stock form, so i'm quite certain that there is room to move in an upward fashion as far as one of their receivers go. Whether or not you really need more than what you have is strictly a matter of personal preference and desire. If it sounds good to you now, save your money. If you hear something that becomes a revelation and really opens your ears to what is out there, by all means, spend till you're happy again : ) Sean
If you had something like klipsh, I'd say that 100wpc is great. Not for a pair of 86db speakers. With your equations used, you would need at least 400 watts to even come close to what an efficient 92db speaker would produce. Most klipsch are even more efficient than that.

Secondly, THX requires 110db and THX ultra is 112b. I think it may be a little excessive but these were chosen for a reason.

Finally, one think not mentioned directly here is a sub. I agree with Sean that the bass will be robbing you the most. However, most people cross the bass over at 80hz. If you are doing that, 100wpc is more than enough. Keep in mind though that the sub needs to be up to the task. My guess is that you should be looking at a 400+ watt sub. As a general (very general) guide, producing bass usually uses up about 75% of the wattage required in an amp. The higher you cross the system over, the easier it is on your amp and the harder it is on the sub amp.............
Another thing to remember is that when you calibrate to 75db using the built-in test tone, it is a reduced level that is supposed to equate to the dolby or THX standard in normal listening.