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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
erik_squires
16,364 posts
 
So a couple of things.  If you don't have a high pass filter on your main amp, the speaker and amp will be forced to reproduce the lower octaves, even though you won't hear it.

Not only does this limit dynamic range, but it increases distortion heard from your mains, both harmonic and Doppler.

If you keep your mains at low volumes, this is not a problem, but if your goal is to use the sub to increase bass AND dynamic range, the high pass filter is a must.
atmasphere's avatar
atmasphere
11,936 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.
diamonddupree's avatar
diamonddupree OP
75 posts
 
@erik_squires I've considered an active crossover to accomplish what you suggest. My local shop didn't think it would have much of an impact but I'm still curious. I feel like there should be as little overlap of frequencies produced by both the sub and the mains as possible. It sounds like this is what you're saying too. I was considering this device:

https://www.jlaudio.com/products/cr-1-home-audio-subwoofer-crossovers-96020

Other commenters who know the Minis well have suggested getting all the bass out of them as they can produce, which is why I have the LP freq set to 38hz. I've played around with it in a few different places and it sounds good the way it is now. I listen to the Wood album by Brian Bromberg and can hear right down to the texture of the strings on his 300-year-old double bass. It sounds incredible. 
diamonddupree's avatar
diamonddupree OP
75 posts
 
@atmasphere , I just finished the section in the book on bass arrays and my room dimensions don't fit the use case that Toole recommends it for. My room is very irregularly shaped and small, so I wouldn't be able to arrange the subs the way he suggests. In addition, the frequency response he shows for my current sub configuration shows very flat response for the limited frequency range I'm using it for, which is basically 27hz (the lower limit of the sub) to 38hz (where I have the LP frequency set). It's a narrow range but it really fills out the bottom octave. 

I'm not encouraged by your observation about the low efficiency of my speakers since I have no desire to change that part of the system so I have no choice but to deal with it. Perhaps what @mapman was saying about increasing current flow could help this situation. I'll have to look into it. Thanks for your thoughts! 
erik_squires
16,364 posts
 
Hi OP:

The thing about a sub is that you aren't creating a speaker with sub, you are creating a brand new speaker system, of which the sub will handle the lowest octave.

So you are no longer just buying a new component, you are actively involved in speaker crossover design, and it's complicated. Crossovers are not absolute brick walls.  They have slopes and knee points and Q's, or knee sharpness. The idea that you can set your sub to 37 Hz and  your main speakers to 38 Hz is kind of funny. :)

I never said your speaker's were low efficiency.  I said they were going to be bass limited and with enough bass signal will show the original post was concerned with.  That is, they have limited dynamic range, but this range can be improved by limiting the bass the amp/speaker produces.

Do what you will.



Best,

Erik
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