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!  


128x128diamonddupree
noble100's avatar
noble100
2,419 posts
 

     I mainly agree with everything erik_squires stated in his last post.  My only disagreement is that in my practical experience, the use of 4 subs, properly positioned and configured in a distributed bass array (DBA) concept system, results in exceptionally good bass performance in most any room.  Just as Dr. Earl Geddes scientifically proved with his PHD thesis over 4 decades ago.   
     There's also no need for any bass acoustic room treatments of any type when using this concept.  Mics, room analysis and room correction equipment are interesting and can be utilized if preferred, of course, but I'm certain none of this was required in my 23'x16'x8' room  to perform at a very high level. 

      On another topic that danvignau posted about, I'm failing to understand why having an abundance of amp headroom in an audio system can be accurately considered to be anything other than beneficial by Floyd Toole or anyone else.  
     As a practical but admittedly anecdotal matter, I drive fairly inefficient, 86db/1 watt, Magnepan 3.7i  4 ohm speakers with 2,400 watt/30 amp class D monoblocks with an abundance of beneficial results and no negative results that I've been able to discern.

Tim
noble100's avatar
noble100
2,419 posts
 

Correction, my monos are 1,200 watts each into 4 ohms

Tim
erik_squires
16,410 posts
 
 I mainly agree with everything erik_squires stated in his last post. My only disagreement is that in my practical experience, the use of 4 subs, properly positioned and configured in a distributed bass array (DBA) concept system, results in exceptionally good bass performance in most any room. Just as Dr. Earl Geddes scientifically proved with his PHD thesis over 4 decades ago.  


Sorry, was assuming 1 sub placement.  I don't have a problem with this either.
erik_squires
16,410 posts
 
I think one area we should talk about is not power, but sag. I don’t have a better word for it, but I’ve seen speakers with low impedance, sometimes deliberately low impedance, sometimes unavoidable, sometimes the result of ad hoc experimentation, become "discerning."  That is, they give off the impression that they are so revealing that different amplifiers now sound glaringly different.

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. I find that speakers with drooping impedance have this characteristic, and that amps which _should_ be quite stiff and sturdy, are still susceptible.

So, I don’t think 300 Watts is a lot better than 200, or maybe 100. It’s the output impedance in the location of the speaker’s impedance droop that matters a lot more. The mythical Krell 50 W Class A which doubles in power down to 1 Ohm is a great example of what I’m talking about. It’s also mroe than I would use, but it helps illustrate what I think is going on.

Yes, big amps tend to have more output transistors, and therefore, lower output impedance, but it’s not the power rating that makes them sound better with some speakers.

Best,

Erik
atmasphere's avatar
atmasphere
11,970 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.
More to discover