2. amps-preamps
  3. 162140-151413

What is Floyd Toole saying about extra amplifier power and headroom?

I've been reading Floyd Toole's "Sound Reproduction The Acoustics and Psychoacoustics of Loudspeakers and Rooms" and came across a passage that I wish he went into further detail about. It has to do with whether having amplifier headroom has any noticeable improvement in sq. He happens to be talking about getting the bass right in small rooms, but in doing so, he also touches on the use of a larger amp for extra headroom: 

Remedies for unacceptable situations typically included spending more money on a loudspeaker with a “better” woofer (without useful technical specifications, that was a lottery of another kind) and a bigger amplifier (for useless headroom ...

It's the last part ("useless headroom") that I'm curious about. I have notoriously hard-to-drive speakers (Magico Mini IIs). Although the recommended amplification is 50w - 200w, in my experience, that's a bit of an underestimation. I'm driving the Minis with a Musical Fidelity M6PRX, which is rated at 230w @ 8ohms. (The Minis are 4ohm.) The combination sounds excellent to my ears at low to moderate listening levels, but I notice a slight compression in the soundstage at higher levels. My listening room, while small, is fairly well treated with DIY panels made from Rockwool, sound-absorbent curtains, and thick carpeting. So I don't think I'm overloading the room. But I have wondered if an amp with far more power than what's suggested (more headroom) would drive the speakers with a little less effort.

Those of you familiar with Toole or with driving speakers with power to spare, what are your experiences? If I went with, say, a pair of monoblocks that drive 600w @ 4ohm, would the extra headroom address the compression I'm hearing at higher levels? Or am I wasting my time and, potentially, funds that would be better spent elsewhere? 

Thanks!  


diamonddupree

Showing 4 responses by atmasphere

atmasphere's avatar
atmasphere
12,088 posts
 
I have come to believe, with limited data that the issue is not the amplifier’s power rating but how consistently it performs across the audio band, and this is a place where the math doesn’t quite live up to the audible effects.
The problem isn't the math, its how the amplifier is measured which is something else altogether. Most traditional amplifiers (tube and solid state) that employ feedback don't/can't use enough, so as frequency is increased distortion increases too. This results in brightness/harshness, and is fundamentally at the tubes/transistors debate.

To get around this problem, distortion is usually measured at 100Hz which is too low a frequency for the problems I described to show up. This has become a tradition, so there are those that do this and don't realize that its only done that way to sweep dirt under the carpet.


The reason not enough feedback is used is that you need in excess of 35dB of feedback to prevent it causing brightness (distortion) through its application. This is because traditional amplifiers lack the Gain Bandwidth Product to really allow them the proper amount of feedback at 7KHz and higher. In addition, phase margin is a problem so amps with this much feedback can be unstable and go into oscillation.


If you can run enough feedback, the amp will sound just as smooth as any tube amp running zero feedback. I think what you are describing is really just the amp showing off its limitations.
atmasphere's avatar
atmasphere
12,088 posts
 
DBAs add complexity and would also likely take more time and effort to get right initially as a result. If it were me I would solve one problem at a time....resolve the problem at hand first with existing sub then consider adding DBA after that is resolved, but only if worth it for you.
Actually in practice a DBA is easier to set up than a single sub, since you don't have to work so hard getting the bass right at the listening chair! When I installed my setup, I already had speakers that made deep bass (flat to 20Hz) so I only added two subs. I only placed them once. Only hooked them up once (IOW didn't play with phase). I used cheap speaker cable. I didn't mess with the amplifier crossover settings more than once. It worked perfectly the first time. By comparison I have a single sub in my bedroom, and moving that thing around so I finally got bass where I wanted it (and not all the bass is there, its a bit of a compromise) was a bit of a pain, plus the sub is sitting in an obvious space, but where it would be convenient doesn't work- no bass in that location.

Well its 45 amps per monoblock X 2 = 90 amps total.....nothing to sneeze at especially with a most efficient Class D design.
Uh- can we do a bit of math here? If 45 Amps, giving the amplifier the benefit of the doubt, so driving a 1 ohm load. The power formula is Power = Isquared x R where

power is in watts
I is amps (in this case 45) and squared

R is the resistance of the speaker

This means that if the amp makes 45 amps it also is capable of 2025 watts. If we are talking 4 ohms, then the output power is 8,100 watts. Clearly this isn't happening. A speaker that only dips to 4 ohms isn't going to need that sort of current in any event. The OP simply has no worries in this regard. The fact that the amp is class D is irrelevant in this regard and will behave as an ideal voltage source driving this speaker.

atmasphere's avatar
atmasphere
12,088 posts
 
Perhaps what @mapman was saying about increasing current flow could help this situation. I'll have to look into it. Thanks for your thoughts!
The thing is, when you have thermal compression there is a tendency to turn up the volume/throw more power at it (if there is increased current then there is increased power) but that does not help if there is thermal compression - in fact it makes it worse. But if you are planning to get some of the bass energy off of the woofer, then it will have less thermal compression. So Erik's suggestion should be looked into, as well as a Distributed Bass Array, since the shape of the room really isn't relevant (I'd talk to Duke at Audiokinesis about that since he is really one of the few selling a DBA-dedicated sub system). 
atmasphere's avatar
atmasphere
12,088 posts
 
If you're talking Floyd Toole, then expect to hear about a Distributed Bass Array too... but since that is off topic that's all I'm going to say about it.


But that isn't what the problem is with excess power. In a traditional push-pull solid state or tube amplifier, distortion decreases as you decrease power, to a point that is about 3-7% of total power, at which point the distortion goes back up. Below this point you are getting excess noise and distortion if your amplifier power is too high for your application, since most of the time you are only using a tiny amount of total amplifier power.


A different problem you've referred to is compression at high volume. This is a common complaint with lower efficiency loudspeakers, known as 'thermal compression'. Essentially the voice coils heat up in time with powerful bass notes and the like; this prevents the speaker diaphragm  from moving as far as it should. The only way to avoid this if you want the sound pressure is to go with a more efficient speaker, with which there will be less thermal compression.
More to discover