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Thread: JBL 4425: oiling the veneer, and bass boom

  1. #16
    Senior Seņor boputnam's Avatar
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    Damping Factor

    Norbert...

    You make a great point that has been touched on on another Thread here, of late. (http://www.audioheritage.org/vbullet...=&threadid=347 - an interesting mix of technical info and salesmanship...)

    This might be of interest to some: http://www.crownaudio.com/pdf/amps/damping%20factor.pdf

    Or, here's the .pdf for direct download.

    I have no relationship with, nor do I own or use any Crown gear (but they articulate this real well )

    I'm lately using QSC CX502 (LF) and CX302 (MF/HF) - each have damping factor "Greater than 500". The improvement over my prior rig-up is astonishing and largely attributable to (at least coincident with...) a materially increased damping factor.
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    Last edited by boputnam; 06-24-2003 at 09:00 AM.
    bo

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  2. #17
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    Re: Damping Factor

    Nice find Bo!

  3. #18
    Senior Seņor boputnam's Avatar
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    Hey, this one's cool, too!

    http://www.trueaudio.com/post_013.htm

    It validates Giskard's harping (oh NO!!) on using Monster Cable to reduce cable resistance, and another friend's imploring me to solder the speaker cables right to the network binding posts! Whoa...

    bo

    "Indeed, not!!"

  4. #19
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    Yes, and it has good points as well which perhaps indicate where PSS AUDIO is coming from.

    The earliest article I have on DF is from 1966 when George L. Augspurger (James B. Lansing Sound, Inc.) authored "The Damping Factor Debate". This particular article definitely reinforces PSS AUDIO's stance on published DF figures.

    In any case DF alone does not a good amplifier make.
    It that Yoda speak?

  5. #20
    Senior Seņor boputnam's Avatar
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    Completely!

    Got a copy of that Augspurger article to share? I can't imagine what I can offer as fair trade...
    bo

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  6. #21
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    I'll scan it at some point and send it via email to those interested. I think John Murphy expanded on the concept pretty well. His examples are definitely worth reading.

  7. #22
    Senior Seņor boputnam's Avatar
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    Thanks - keep me on the list!

    Yea, I'm glad I "re found" Murphy's site. I had used the PC-based RTA some years back, and thought it quite good. Re-looking at things, his site is very comprehensive.

    Cool.
    bo

    "Indeed, not!!"

  8. #23
    Paul Joppa
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    damping factor and speaker design

    OK, up front here: I'm an SET enthusiast. There have been several comments in this thread about the importance of high damping factor, which SET amps don't have. So it seems a good place to say my piece. Which is basically that the damping factor should be matched to the speaker.

    The total resistance - voice coil plus cable plus amplifier - is part of the cabinet design. The oldest JBL woofers (130A and its relatives) have an incredibly low Q, which provided great efficiency and worked well with amps of high output impedance, as was common before the Williamson amp made large amounts of negative feedback popular.

    A modern Theile-Small optimum design for a 2220 with a high damping factor amplifier is 0.75 cubic feet, and extends down to 130dB - not much of a woofer! In a 5 cubic foot cabinet (such as JBL designed and recommended in the late forties/early fifties) you could get down to 60Hz, with a 3dB ripple and a strong, narrow hump at 70Hz. Pretty poor. But double the electrical Q by driving it with an amp whose resistance equals the DC resistance, and suddenly you get a beautiful maximally-flat Butterworth alignment down to 50Hz.

    Now the 130A had a VC resistance of around 5-6 ohms, but was labeled 16 ohms in those days. An SET amp, or a triode push-pull such as was popular at the time, with no feedback, has a damping factor around 3 - i.e. an output impedance of about 5 ohms on the 16 ohm tap.

    Put a 2235 in the same box, and you can get deeper bass, very flat to 35Hz with a high fedback, high damping amp. But drive it with the high impedance amp, and it acquires a giant booming peak of 6dB or more around 60Hz, unless you put it into a huge 25 cubic foot box.

    So yes, an SET amp (or any amp with little or no feedback) will make a modern speaker design boom. But a high-feedback, high damping amp will generally make a well-designed vintage speaker boom too. Combine an SET (typical damping factor 2 to 3) with a speaker designed for that amount of amplifier damping, and it works just as well as a modern amp/speaker except the vintage setup is more efficient.

    Some amps from the fifties, including my own pair of Heathkit UA-1's, had a complex feedback scheme so that a unity damping factor could be generated even with high amounts of feedback. Presumably this was so they could drive the "original JBL/Altec/etc" kind of speakers correctly.

    And now, back to your regularly scheduled entertainment ...

  9. #24
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    Re: damping factor and speaker design

    "Which is basically that the damping factor should be matched to the speaker."

    How true! But pretty impractical for the person who is exhausted after programming a VCR.

    Your points are one reason why I really enjoy the computer software packages of today because all these factors can be taken into account if desired. It sure beats all the myriad formulas I used to have to work through!

  10. #25
    Senior Member GordonW's Avatar
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    Hm, 2214 12" deep-basket woofer, huh? So, would the 4425 sort of be like the "pro version" of the L100T? Because if so, the bass sbould NOT be boomy in any way!!

    Regards,
    Gordon.
    wish I could find an abandoned pair of L100Ts, or even better, L80Ts... that series seriously sounded GOOD...

  11. #26
    Norbert
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    Re: damping factor and speaker design

    Originally posted by Paul Joppa
    So yes, an SET amp (or any amp with little or no feedback) will make a modern speaker design boom. But a high-feedback, high damping amp will generally make a well-designed vintage speaker boom too. Combine an SET (typical damping factor 2 to 3) with a speaker designed for that amount of amplifier damping, and it works just as well as a modern amp/speaker except the vintage setup is more efficient.
    I think you're absolutely right Paul. It is the question of finding an amplifier with the right amount of damping for a speaker with a given amount of damping. Or vice versa ...
    As a lover of SET amps (I have a 300B Uchida amp which I don't use currently), I must admit that it was quite difficult to find a satisfying combination for my 4430s. I'm currently using a KT88 (triode) push-pull amp in my system. From time to time I swap with my Hafler 9505 (excellent MOSFET amp with almost no sonic contribution). I think the challenge is to design an amplifier (tube or SS or both) with SET sound, low output impedance and no feedback.
    I'll face that ...

    Best regards,

    Norbert

  12. #27
    Ken Schwarz
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    Bass boom can be caused by floor resonance. Is yours a suspended wood floor? If so, you could have bass problems which are not the speaker's fault.

  13. #28
    carolax
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    Bass driver damping

    A final followup (from me, at least): a different amplifier (a Bryston) sounded better than my Adcom, but it didn't alter the resonance. Time and Moby, however, seems to be doing the trick. I played some bass-heavy material at 'non-residential' volume for a few hours a day for several days, and the bass hump is leveling off. Thanks to everyone for their input.

  14. #29
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    Re: Bass driver damping

    Which Adcom?
    Which Bryston?

  15. #30
    carolax
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    Re: Bass driver damping

    Adcom GFA-5400
    Bryston 3B

    Although I only had it overnight, driving the 4425's it had better deep bass and less 'glare' in the upper midrange. I'll stop before I begin to sound like an audiophile [at this point carolax cracks open a can of Schlitz and spits into, or nearly into, a nearby waste basket].

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