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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
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.
yyzsantabarbara
4,971 posts
 
Below is an email I just sent to the guy that did my ROON Convolution filter for my small room system.

Since you are sound guy I thought you would find this interesting.

I was curious how the Benchmark AHB2 in mono would sound with my Thiel CS3.7, which likes power at low impedance. So I took my single stereo AHB2 and switched it to mono and drove only 1 speaker. It sounded louder and with more details. It also was much more hard hitting. I listened to Soundgarden: A Sides completely on stereo. I then kept everything the same and played the CD again but in AHB2 mono and single speaker. Even with a single speaker I had more bass. In fact the bass was so much that I had to change filters to the first one you gave me where the bass was not enhanced. After I changed from filter 3 to filter 1 my irritation subsided.

I was always curious whether more power at low to medium volume gives you better sound. For me it seems obvious that it does. I believe in mono the AHB2 gain is boosted 6 db however, I am hearing more than increased loudness. It is a better sound.

I should add that the AHB2 SNR drops to 135 when the AHB2 is run in mono. That is not much different that the SNR 132 in stereo but I was hearing more details in mono. I think my Thiel gobbled up that extra power and made better sound and it was not the SNR that made the sound better. 

 - Filter 1 was a Convolution file that was very flat. Now using with the single speaker running a AHB2 in mono (700 watts @ 2 Ohm)

 - Filter 3 was a Convolution file that had a slight bump in the bass from 100 Hz down. I was using this filter with the stereo AHB2 which does not have much power into 2 Ohm (259 watts).

So yes, I am buying a second AHB2 to run in mono. Maybe Floyd was using a more efficient speaker.
diamonddupree's avatar
diamonddupree OP
75 posts
 
@yyzsantabarbara Thiel says those speakers, though rated at 4ohm, will go down to 2.8. With recommended power at 100w - 600w, I bet they spend a fair amount of time at or near 2.8. The AHB puts out 190 @ 4ohm, 240 @ 3ohm. I'm still learning here but I would say that's way underpowered for the Thiels. Have you considered a high output Class D? I believe @mapman is running Bel Canto Ref 1000 monoblocks after switching from a lower-powered Musical Fidelity and says the difference was night and day. I bet those would do wonders for your Thiels as well. 
yyzsantabarbara
4,971 posts
 
@diamonddupree Those numbers you listed was why I am still investigating more power. However, I really wanted make the AHB2 work because I like it more than other amps (I have heard a lot). For less than $3K I think I will improve my sound a lot by going mono. Even though the AHB2 is not officially rated at 2 Ohm because it cannot run a test tone for 30 minutes at 2 Ohm (the AHB2 in stereo can). I think that is inconsequential now after my test yesterday. Even if it clips the forward correction will stop that from getting to the speaker.

Since I have been investigating other amps. The 2 other lines I was considering are:

CODA #8
  • 150 | 300 | 600 (approx. first 18 watt class A but halved as impedance halved)
  • 250 | 500 | 1000 (approx. first 12 watt class A but halved as impedance halved)
  • 400 | 800 | 1600 (approx. first 8 watt class A but halved as impedance halved)
  • SNR 118

CODA #16 with a SNR 130 and first 100 watt Class A. I think it's power rating is as follows,  150 | 300 | 600.

The upcoming Class D Puriif amps with 1000+ watt in 2 Ohm. These ones maybe the closest sounding to the AHB2.

I was also considering the Luxman m900u which is similar in spec to the CODA #8 (150 | 300 | 600) but 3x the cost. It sounds great and the only one I have heard from this list.

I should be able to get a second AHB2 next month.
diamonddupree's avatar
diamonddupree OP
75 posts
 
@yyzsantabarbara a second AHB might not even do it. There's a pair of Belo Canto Ref 1000s available right here on Agon I think for less than $2k. 
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