The EPI-100 and Dynaco A-25 loudspeakers have 1st order x-over networks . I forget the website but this guy stated that these speakers can be made time coincident or coherent simply by putting the woofer in a separate box and mount the tweeter on top of the box . He said something about aligning the voice coils where the woofer and tweeters meet on an imaginary plane . Ever since reading that bit of info I've been curious about this . Has anyone ever experimented with this time coincident thing with their EPI or Dynaco speakers ?
Can EPI and Dynaco be made time coherent ?
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TerryO
Super Member
Aside from the fact that your friend is 25-30 years behind the times, yes I suppose you can. It might be interesting to know what he feels he's achieved when he's done this.
As far as aligning voice coils, it was a mistaken idea, that many (including myself) once thought to have some validity. The fact is that the acoustic center of a driver varies with frequency and may never be on the plane of the voice coil. Remember that with a two-way speaker you have two drivers whose acoustic centers are shifting with frequency and they are not the same. The best you can hope for is to have the drivers phase aligned at the crossover frequency for an orderly transition from one driver to the next.
I guess the question remains:
Are you really dissatisfied with your speakers the way they are?
Best Regards,
TerryO
johnda
Addicted Member
Sometimes a guy will lock onto a phrase and use it to show how MUCH more he knows that you do about a subject. Remember that great scene in a Woody Allen movie, where this guy is expounding on the details of this movie to his date and the movie director walks up to him and tells him to his face that he doesn't know the first thing about what he's talking about! Woody turns to the audience and says "Don't you wish this could happen?"
I like TerryO's question..I'll add to that by saying that the Dynaco A25's were among the best speakers I ever owned, time coherence be damned!
I like TerryO's question..I'll add to that by saying that the Dynaco A25's were among the best speakers I ever owned, time coherence be damned!
sealy
Addicted Member
I wouldn't discount time coherency designs out of hand. Technics used the technique(pardon the play on words) in their respected SB series. I had a pair of SB7000s, which sounded wonderful. KEF also used it with the renowned 105.
Nowadays, high-end manufacturers seem to be able to achieve the same results with advanced crossover design. The Spica TC 50, although not a current design, nevertheless has a sloped front baffle and uses a crossover that is purportedly time-coherent.
Nowadays, high-end manufacturers seem to be able to achieve the same results with advanced crossover design. The Spica TC 50, although not a current design, nevertheless has a sloped front baffle and uses a crossover that is purportedly time-coherent.
mhardy6647
Lunatic Member
Sometimes a guy will lock onto a phrase and use it to show how MUCH more he knows that you do about a subject. Remember that great scene in a Woody Allen movie, where this guy is expounding on the details of this movie to his date and the movie director walks up to him and tells him to his face that he doesn't know the first thing about what he's talking about! Woody turns to the audience and says "Don't you wish this could happen?"
I like TerryO's question..I'll add to that by saying that the Dynaco A25's were among the best speakers I ever owned, time coherence be damned!
Actually the Woody Allen vignette in question concerns not a movie director but the famous media critic/philosopher Marshall McLuhan (he who coined the phrase "Global Village" and "the medium is the message").
johnda
Addicted Member
Well... to get more to the point, of the OP...
Since the EPI and Dynaco speakers rely on the woofer roll-off (a second-order low-pass function) as part of their crossover function... they can NEVER be truly time-coherent at all frequencies. A second-order slope induces 90 degrees of phase shift at crossover. According to the theory, a true "time coherent" speaker can't have an effective crossover slope of HIGHER then first order... any higher-order crossovers (done other than by finite impulse response or other signal-processing means- in other words, OTHER than by passive components) will necessarily have "phase rotations" (where the phase goes through at least one polarity change, around crossover). These preclude time coherency... the time cannot be constant through a phase rotation.
So, other than some more exotic techniques using first-order crossovers (such as the elaborate scheme that Thiel uses to generate "quasi-first-order" crossovers- which technically still isn't time-coherent, in the strict sense of the word, or the "helper driver" concept used by B&O), there's no such thing as a time-coherent speaker, in the strict sense of the word. We can get close... so close, where the time delay is pretty negligible... but never truly flat time response, not even with full-range drivers (they have rolloffs at the ends of the spectrum- which themselves induce phase and time shifts)...
In short, the answer is- no, they can't be time coherent. Nothing can. Some can be made closer than others, though... how close is necessary for proper perception by the listener, is still a vigorously-debated source of argument...
Regards,
Gordon.
Since the EPI and Dynaco speakers rely on the woofer roll-off (a second-order low-pass function) as part of their crossover function... they can NEVER be truly time-coherent at all frequencies. A second-order slope induces 90 degrees of phase shift at crossover. According to the theory, a true "time coherent" speaker can't have an effective crossover slope of HIGHER then first order... any higher-order crossovers (done other than by finite impulse response or other signal-processing means- in other words, OTHER than by passive components) will necessarily have "phase rotations" (where the phase goes through at least one polarity change, around crossover). These preclude time coherency... the time cannot be constant through a phase rotation.
So, other than some more exotic techniques using first-order crossovers (such as the elaborate scheme that Thiel uses to generate "quasi-first-order" crossovers- which technically still isn't time-coherent, in the strict sense of the word, or the "helper driver" concept used by B&O), there's no such thing as a time-coherent speaker, in the strict sense of the word. We can get close... so close, where the time delay is pretty negligible... but never truly flat time response, not even with full-range drivers (they have rolloffs at the ends of the spectrum- which themselves induce phase and time shifts)...
In short, the answer is- no, they can't be time coherent. Nothing can. Some can be made closer than others, though... how close is necessary for proper perception by the listener, is still a vigorously-debated source of argument...
Regards,
Gordon.
KentTeffeteller
Gimpus Stereophilus!
They're classics, keep em stock. Both are excellent speakers when all is right. :tresbon:
nooboy
Active Member
Hem...
No. "The medium is the massage" is an afterthought coined in the title of a latter book that could or could not have sprung from a typeset error. Just check the wikipedia page or your original copy of The Gutenberg Galaxy. Which one should kind of always do before correcting someone, no?
Still love my A-25's, best sound for the money, methinks.
No. "The medium is the massage" is an afterthought coined in the title of a latter book that could or could not have sprung from a typeset error. Just check the wikipedia page or your original copy of The Gutenberg Galaxy. Which one should kind of always do before correcting someone, no?
Still love my A-25's, best sound for the money, methinks.
I'm glad you mentioned that.
Tilting them back slightly might help, and it would essentially accomplish all that redoing the enclosure would do, at a lot less work and without the reflections off the lip of the enclosure that would be part of the (moved back from the front) tweeter's total output. And it's authentically vintage -- everyone did this with their speakers in the mid late 70s.
I've got to say your friend seems determined to reinvent the wheel, and actually a wheel of uncertain utility. Everyone is in favor of time alignment, but perfection is probably not possible, the alignment has to be electrical and acoustical as well as mechanical, and the complexity of the solution may actually be a bigger problem than the original problem.
I've got to say your friend seems determined to reinvent the wheel, and actually a wheel of uncertain utility. Everyone is in favor of time alignment, but perfection is probably not possible, the alignment has to be electrical and acoustical as well as mechanical, and the complexity of the solution may actually be a bigger problem than the original problem.
mhardy6647
Lunatic Member
procure... or build... a pair of PolkStands 
http://www.polkaudio.com/forums/showthread.php?t=24335&highlight=polkstands
http://www.polkaudio.com/forums/showthread.php?t=24335&highlight=polkstands