1969-74 Dynaco A-25 speakers compared to modern designs?

[Dennis Murphy]

Thanks for the great post. I had completely forgotten about the aperiodic tuning slot, which absolutely didn't do anything. I think it was Dynaudio that used to sell a stuffed circular stand-alone version that I tried out a couple of times. It didn't work either.

 

[mhardy6647]

Dynaudio (or someone) indeed sold aperiodic vents (or whatever one might wish to call them) for DIY use at least into the 1990s -- I remember seeing them in the PartsExpress catalog.

I am not sure that they don't do anything...?

https://worldradiohistory.com/Archive-All-Audio/Archive-Audio/50s/Audio-Anthology-4-1957.pdf
(page 133)

 

[anmpr1]

The double Advent thing was also curious and never really made sense, for reasons that are well understand by anyone who has ever dabbled in speaker design.

It made total sense once you found out that the guy who was pushing that configuration eventually stuffed his smallish room with a four module Infinity IRS monster, and proclaimed it to be 'the Absolute Sound'. Those who knew called it Harry's IRS in a water closet set up.

In any case, it's clear from his review that he just wanted something that played loud, and four Advents were certainly louder than two.

 

[MrPeabody]

Dynaudio (or someone) indeed sold aperiodic vents (or whatever one might wish to call them) for DIY use at least into the 1990s -- I remember seeing them in the PartsExpress catalog.

I am not sure that they don't do anything...?

https://worldradiohistory.com/Archive-All-Audio/Archive-Audio/50s/Audio-Anthology-4-1957.pdf
(page 133)

View attachment 105494

My curiosity was piqued sufficiently for me to spend a little while reading the article. It is written by an electrical engineer for other electrical engineer, where the practice is to use electrical analogs for all mechanical aspects of a system, including acoustical aspects. My criticisms of the article would be much the same as with most amateurish articles encountered elsewhere, starting with the failure by the author to clearly state the goal at the beginning. Figure 8 on page 135 shows, at the right edge of the figure, a graph of "amplitude" vs. frequency. Except for the extremes it looks like an impedance graph. The text reads, "The resulting maxima of the induction voltage developed in the voice coil cause the well-known double hump of the voice coil impedance which is specific to the bass reflex system. The opposite polarities of the front and rear waves cause the lower resonance to be reduced in amplitude. A typical theoretical response is shown in Fig. 8." The author proceeds to discuss how his proposed enclosure design damps the double resonance. It was not clear to me whether he was talking about damping acoustic response peaks associated with the resonances, or damping the peaks in the impedance curve. Certainly the amplitude response curve in Fig. 8 is not representative of an acoustic response curve, even with the resonances. Possibly it would be for speakers that are driven by an amplifier having much higher output impedance than what is nowadays typical. He goes on to explain how the resonances that are evident in the electrical model can be suppressed with appropriate modifications to the analogous electrical circuit. After doing that, he then discusses the enclosure design by which the analogous electrical circuit is realizable mechanically. Crucially, it involves the enclosure being divided into two chambers, the smaller of the two housing the woofer and the slot opening with the acoustic impedance. The two chambers are separated by a cloth membrane that provides the optimal measure of acoustic impedance between the two chambers. At some point the author indicated that the front opening was large in order to improve the acoustic coupling. This in particular is bizarre, given that it is obvious that the acoustic wave exiting the opening will be 180 degrees out of phase with the direct acoustic wave from the woofer cone. The author certainly understood this. At the end of page 136, there is this: "The lower frequencies are somewhat attenuated by the cancelling action of the residual sound radiated by the slit. This defect is easily overcome by applying some bass boost in the amplifier." Given that he was well aware of this, why did he not realize that it wouldn't make sense to make the opening large toward increasing the acoustic coupling? I'm sorry but it just doesn't make sense, and the author ought to have realized that it didn't make sense.

The plain reason that the lower resonance is "attenuated" is that it does not exist because there is no Helmoltz radiator effect. As such, it is manifest that the two-chambered enclosure with the acoustic barrier between them is a means to damp the acoustic resonance that occurs in association with the driver's resonance Fs (shifted higher in frequency as a consequence of the stiffness of the air within the enclosure). Ditto for the ostensibly beneficial effect of the opening to the front. If the acoustic response peak associated with this specific resonance is dealt with in an optimal manner via either the well-established sealed enclosure principles or the well-established ported enclosure principles, then neither the extra internal chamber with the cloth barrier nor the front-facing opening will serve any real purpose. The only possible benefit of this opening is that it permits the enclosure to be small as compared to a sealed enclosure using the same driver. The penalty, which to me is not a small penalty, is that there won't be much bass to speak of, somewhere between a woofer properly mounted in a sealed enclosure and the same woofer suspended in free air.

 

[MrPeabody]

It made total sense once you found out that the guy who was pushing that configuration eventually stuffed his smallish room with a four module Infinity IRS monster, and proclaimed it to be 'the Absolute Sound'. Those who knew called it Harry's IRS in a water closet set up.

In any case, it's clear from his review that he just wanted something that played loud, and four Advents were certainly louder than two.

I was going to say that it still doesn't make technical sense, but then thanks to you it occurred to me that the real benefit may have been that if the pair was wired in parallel the halving of the impedance would mean that power would double, which would make the pair sound better for the same reason that turning up the volume control makes it sound better. But there would also be a shift in tonality toward bass, as I discussed.

 

[ModDIY]

A kit from Seas - A26

Seas A26 Kit

 

[Dennis Murphy]

I recently obtained a single Large Advent Loudspeaker for Amir to test. I'll be sending it off to him next week. The woofer surround has been replaced professionally, and the TS parameters measure close to spec. It sounds like what I remember from the pair i owned 500 years ago, although what I have isn't the first generation. It's the "new" Advent Loudspeaker, which has the same tweeter but with fluid in the voice coil. This allowed them to switch from a 2nd order electrical high pass to a single capacitor. It's a super cheap NPE which I'll replace simply to make sure the value it's operating properly. I think the Spins are going to surprise you a little.

 

[HiFidFan]

Daedalus speakers out of WA uses aperiodic enclosures. They are the only ones I know of that currently build this type.

daedalusaudio

 

[MrPeabody]

I recently obtained a single Large Advent Loudspeaker for Amir to test. I'll be sending it off to him next week. The woofer surround has been replaced professionally, and the TS parameters measure close to spec. It sounds like what I remember from the pair i owned 500 years ago, although what I have isn't the first generation. It's the "new" Advent Loudspeaker, which has the same tweeter but with fluid in the voice coil. This allowed them to switch from a 2nd order electrical high pass to a single capacitor. It's a super cheap NPE which I'll replace simply to make sure the value it's operating properly. I think the Spins are going to surprise you a little.

Thanks for doing this! I'm very much looking forward to it. Since the surround replacement was done professionally and the T/S numbers are close to spec, this is likely going to be as close to the original as you could hope to get, as far into the future as are now. Yogi Berra once remarked that the future ain't what it used to be. Replacing the capacitor will help to insure that it will perform as it should. I'll be especially interested in seeing whether distortion in mid-bass is any lower than what it is nowadays common with ported speakers. The sealed enclosure potentially makes the restoring force more linear than what it can be with the driver suspension. Also, the off-axis response will be very interesting, because I've always wondered whether part of the reason for the concentric ripples in the cone is to allow just the inner part of the cone to move with higher frequency. I'm also anxious to see how much distortion that tweeter produces. I won't be surprised if it is significantly worse than most modern tweeters.

 

[JRS]

My curiosity was piqued sufficiently for me to spend a little while reading the article. It is written by an electrical engineer for other electrical engineer, where the practice is to use electrical analogs for all mechanical aspects of a system, including acoustical aspects. My criticisms of the article would be much the same as with most amateurish articles encountered elsewhere, starting with the failure by the author to clearly state the goal at the beginning. Figure 8 on page 135 shows, at the right edge of the figure, a graph of "amplitude" vs. frequency. Except for the extremes it looks like an impedance graph. The text reads, "The resulting maxima of the induction voltage developed in the voice coil cause the well-known double hump of the voice coil impedance which is specific to the bass reflex system. The opposite polarities of the front and rear waves cause the lower resonance to be reduced in amplitude. A typical theoretical response is shown in Fig. 8." The author proceeds to discuss how his proposed enclosure design damps the double resonance. It was not clear to me whether he was talking about damping acoustic response peaks associated with the resonances, or damping the peaks in the impedance curve. Certainly the amplitude response curve in Fig. 8 is not representative of an acoustic response curve, even with the resonances. Possibly it would be for speakers that are driven by an amplifier having much higher output impedance than what is nowadays typical. He goes on to explain how the resonances that are evident in the electrical model can be suppressed with appropriate modifications to the analogous electrical circuit. After doing that, he then discusses the enclosure design by which the analogous electrical circuit is realizable mechanically. Crucially, it involves the enclosure being divided into two chambers, the smaller of the two housing the woofer and the slot opening with the acoustic impedance. The two chambers are separated by a cloth membrane that provides the optimal measure of acoustic impedance between the two chambers. At some point the author indicated that the front opening was large in order to improve the acoustic coupling. This in particular is bizarre, given that it is obvious that the acoustic wave exiting the opening will be 180 degrees out of phase with the direct acoustic wave from the woofer cone. The author certainly understood this. At the end of page 136, there is this: "The lower frequencies are somewhat attenuated by the cancelling action of the residual sound radiated by the slit. This defect is easily overcome by applying some bass boost in the amplifier." Given that he was well aware of this, why did he not realize that it wouldn't make sense to make the opening large toward increasing the acoustic coupling? I'm sorry but it just doesn't make sense, and the author ought to have realized that it didn't make sense.

The plain reason that the lower resonance is "attenuated" is that it does not exist because there is no Helmoltz radiator effect. As such, it is manifest that the two-chambered enclosure with the acoustic barrier between them is a means to damp the acoustic resonance that occurs in association with the driver's resonance Fs (shifted higher in frequency as a consequence of the stiffness of the air within the enclosure). Ditto for the ostensibly beneficial effect of the opening to the front. If the acoustic response peak associated with this specific resonance is dealt with in an optimal manner via either the well-established sealed enclosure principles or the well-established ported enclosure principles, then neither the extra internal chamber with the cloth barrier nor the front-facing opening will serve any real purpose. The only possible benefit of this opening is that it permits the enclosure to be small as compared to a sealed enclosure using the same driver. The penalty, which to me is not a small penalty, is that there won't be much bass to speak of, somewhere between a woofer properly mounted in a sealed enclosure and the same woofer suspended in free air.

I have wondered about this a bit. My understanding is a bit different--that you could take a woofer, put it in a too small box, get the expected peaky Chebychev alignment, and by adding the vent could bring the damping down to and get very well behaved transient response. This is pulled from the Master's thesis written back in 1988. Best I understand is that it's a slow leak, that allows some underdamping benefit in FR but by reducing back pressure, mitigates the ringing. Sort of makes sense, but I have never has a use case come up to make it worth testing.

And for those who might need a pair--Scan Speak still sells them. I'm curious whether that was part of the technology transfer when Seas stopped producing the A-25 drivers. Seems there was a bit of industrial espionage back in the day....

History of Scan-Speak

History of Scan-Speak

www.cfuttrup.com

 

[DanielT]

Could it be Seas 1.5 "dome Tweeter H-087 in Dynaco A-25 speakers?If so, for those who are interested, there is its data sheet.

Edit
Correct:

Dynaco A25 sound quality

Just built a version of this, but with Seas 27TFFFC instead of the original H087 tweeter. Filter is 4.7 uF and 2.2 ohm. I have never heard the original Dynaco 25, but I am stunned by the sound they make with the 27TFFC tweeter. It is detailed, and at the same time exremely relaxed. It can play...

www.diyaudio.com

 

[Flak]

I've always wondered whether part of the reason for the concentric ripples in the cone is to allow just the inner part of the cone to move with higher frequency.

You're correct, that's the reason of the "fried egg" tweeter

 

[AndreaT]

I had one single A-25 a long, long time ago. It was bought in an Italian RadioShack equivalent store named “Marcucci Hi-Fi” in Milan. It was driven by a push-pull mono tube amp home-built around an Acrosound transformer in a primitive metal enclosure (it is a mystery how no one was fatally electrocuted during home use for 20+ years, 1955-1976). As a preamp, a Dynaco PAS-1 did the job (it even had a TV input, a precursor of HT set-ups). It is difficult to remember the A-25 sound: it should suffice to say it did allow the enjoyment of Music in my developing brain, stirring emotion and desire, ambition and joy, and sealing (with its aperiodic non-sealed(!)) a lifelong passion for better sound and larger Music collection.

 

[fpitas]

I listened extensively to a friend's set. Very pleasant. They didn't do anything great, and the resolution was meh now that I've heard good speakers, but they were consistently fun to listen to. No idea why they are still a thing, but well, that's the vintage mystique for you.

BTW, as I think was pointed out above. The aperiodic cabinet allows using a woofer with a high Qts, without a huge bass hump resulting. The woofers had tiny magnets, hence a high electrical Q.

 

[fpitas]

Dynaudio (or someone) indeed sold aperiodic vents (or whatever one might wish to call them) for DIY use at least into the 1990s -- I remember seeing them in the PartsExpress catalog.

I am not sure that they don't do anything...?

https://worldradiohistory.com/Archive-All-Audio/Archive-Audio/50s/Audio-Anthology-4-1957.pdf
(page 133)

View attachment 105494

They certainly do something, although the tiny vents often sold are too small for some uses. The A25 packed damping material tightly in a large "vent" at the bottom to make an acoustic resistor. In that way they could use a woofer with a tiny magnet.

 

[mglobe]

Meh. I've owned them. Not much on top, not much on bottom, nothing particularly special about what's in-between. The "sleeper" of the era was the Large Advent, particularly stacked Advents.

I always favored the EPI’s over the Advents but honestly that might have just been brand preference. It’s been a looong time.

 

[tuvok10]

Dynaco A-25 loudspeaker

www.stereophile.com

 

[mhardy6647]

I always favored the EPI’s over the Advents but honestly that might have just been brand preference. It’s been a looong time.

Based on my impressions of the two -- naaah, you'd still prefer the EPI. The "Burhoe modules" were intrinsically better sounding than the OLA, and some EPI designs had enough oomph to run with (or, perhaps, even better) the OLAs in terms of LF output and extension.
I have never been too crazy about the large Advents, and I am still not.
... and, yes, there's a pair of Utility (vinyl) Advents in the basement. I need to re-surround 'em one of these days. I even already have a pair of the proper surrounds for the Masonite-ring woofers.

Heck, the later (Recoton-era, I think!) Advents (e.g., the 5012) sound better to me because the tweeters sound better. I had a pair of (dump-find) 5012 (walnut cabinet) and I thought they were not bad at all. I gave them to a hifi fellow-traveler who had a "thing" for Advents.

Even the later-still Advent "Legacy" models, with rather atrocious cabinetry, sound decent.


Also dump finds -- of course! Long gone, too -- I actually sold these (tho' not for much).
N.B. Those are $3 generic surrounds -- I couldn't rationalize investing much more in these big, rather flimsy boxes!

EDIT: I guess my point (if in fact I did have one ) is that either of the above, albeit rather cheesy, dome tweeters sound better (to me) than the OLA's "fried egg" tweeters.

(one of the aforementioned Utility Advents, solely to illustrate the Advent cone tweeter of old)

 

[mglobe]

Based on my impressions of the two -- naaah, you'd still prefer the EPI. The "Burhoe modules" were intrinsically better sounding than the OLA, and some EPI designs had enough oomph to run with (or, perhaps, even better) the OLAs in terms of LF output and extension.
I have never been too crazy about the large Advents, and I am still not.
... and, yes, there's a pair of Utility (vinyl) Advents in the basement. I need to re-surround 'em one of these days. I even already have a pair of the proper surrounds for the Masonite-ring woofers.

Heck, the later (Recoton-era, I think!) Advents (e.g., the 5012) sound better to me because the tweeters sound better. I had a pair of (dump-find) 5012 (walnut cabinet) and I thought they were not bad at all. I gave them to a hifi fellow-traveler who had a "thing" for Advents.

View attachment 318343

Even the later-still Advent "Legacy" models, with rather atrocious cabinetry, sound decent.


Also dump finds -- of course! Long gone, too -- I actually sold these (tho' not for much).
N.B. Those are $3 generic surrounds -- I couldn't rationalize investing much more in these big, rather flimsy boxes!

EDIT: I guess my point (if in fact I did have one ) is that either of the above, albeit rather cheesy, dome tweeters sound better (to me) than the OLA's "fried egg" tweeters.

View attachment 318346
(one of the aforementioned Utility Advents, solely to illustrate the Advent cone tweeter of old)

My first speakers, which I bought in HS were EPI 50’s. I later upgraded to the Epicure 20’s, which I had for a long time.

 

[Eurasian]

I still have a pair of EPI 400s somewhere in my loudspeaker museum. They are basically 4 of the 50s in each box and are (reasonably) omnidirectional. Also known as the poor man’s 1000s! They are pretty with subtle brass trim and very heavy.