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bout the Q for Amazing Woofers

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TNRabbit

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(pair of) SILVER-7 TUBE AMPs

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Post Thu May 27, 2010 6:07 am

bout the Q for Amazing Woofers

Found this posted on AudioKarma.com; don't know the author:

I created the Carver Amazing woofer system and Large Area Ribbon (co-authored with David Graebener) for Bob Carver in the mid 1980's as a consultant.

One question that is often asked was whether the Amazings had a high or low Qms. The Qms was quite high, we attempted to achieve a Qms of nearly 10, which is required if one is going to end up with a Qt of much greater than 2 and still maintain reasonable efficiency.
The trick is to be able to maintain 'linear' Qms. One of the big problems we found early in the development of the Hi-Q open baffle was that many of the Hi-Q drivers have Qms values that change depending on level. This non-linear Qms causes a non-linear Qts, which causes a non-linear frequency response with level...i.e. compression/distortion. This Qms modulation is one of the main reasons that some have found higher Qts drivers to sound bad. It’s is not the higher Qts, but the Qms -> Qts nonlinearity that causes the poor sound quality. Proper Qts match to baffle cut-off frequency will perform quite perfectly, even for higher Qt values if the resulting frequency response is flat and the Qt value is maintained at all listening levels.
The Amazing suspension components, spider and surround, took nearly 9 months to work through the design problems to chase out the non-linearities. No one had optimized high-Q woofers previous to the Amazings so the driver vendors were not able to help. We had to drive the component development ourselves. It was hard to get spiders with low enough damping (they actually call them dampers in Japan) to achieve high-Q and then it was even more difficult to develop spiders and surrounds that maintained constant damping.
Okay, that is probably more than what anyone wanted to know on that subject, but I felt it was important to start getting past this myth that “high-Q drivers are bad”. It is a systems approach. High-Q is not inherently bad, but ‘mismatched’-Q is. A high-Q driver is appropriate when matched to a ‘low-Q baffle’, just as a Low-Q driver is appropriate to match to a ‘high-Q’ Helmholtz enclosure.
The original Amazings had even higher Qts and lower Fs than the later Series. As is often suggested, lower Fs requires higher-Q, but this becomes problematic. For a fixed baffle cutoff frequency, with the standard 6 dB/octave high pass characteristic from cut-off down to the resonant frequency, the ideal high-Q gain would match that first order slope over the bandwidth from cutoff to fs. Unfortunately, the 12 dB of gain we needed in the original Amazings to achieve reference level at the Fs of 20 Hz required a Qts of about 4. This created a narrow band peak that doesn’t match the smooth, 6 dB roll-off slope of the open baffle. Because of this we had to use some shaping networks to smooth things out from 25 Hz to 100 Hz. We ended up with a fairly smooth response with a peak at 20 Hz and Fc at 17 or 18 Hz.
The later Amazings had a new lower mass paper cone woofer to replace the heavier honeycomb devices in the originals. We were able to get the moving mass down to less than 15 grams, which is quite low for a 12” woofer, but required for getting decent efficiency.
The Fs was raised to 30 Hz and the Qts was lowered to a value between 2.5 and 3. This worked much better in matching the Fs, Qts, and baffle Fc to get a smoother response and much higher efficiency.
TNRabbit
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Reese

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TFM-75

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Post Thu May 27, 2010 7:42 am

Re: bout the Q for Amazing Woofers

Interesting history and comparison. (I still have the honeycomb woofers going strong in my Originals upgraded to Platinum ribbons/x-overs) [Well, the x-overs aren't there any more either, since I'm using external active crossovers, but anyway, I'm still happy!]
Too often we enjoy the comfort of opinion without the discomfort of thought -- JFK

Sunfire Classic Tube Preamp; 2 Carver Silver 7ts, 2 Silver 9ts; bi-amped ALS (Originals upgraded to Platinum), Rane PE15's eq's and Rane AC22 xcrossover; Carver c-19 tube pre;
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