AR-2ax: Early Midrange

[ReliaBill Engineer]

Anyone know why AR chose to put fiberglass padding over the paper cone midrange? Is the reason documented anywhere?

 

 

[ReliaBill Engineer]

Is it used to add dispersion? Tame a sizzling midrange? Acoustic (air) impedance matching? Or….add mystique?

[Aadams]

I knew the answer was deep in early CSP posts.  The results from a ChatGPT scan of CSP says the mid/tweet driver is allowed to operate unrestrained to its extreme upper limit and the fiberglass is used damp and muffle unwanted behavior when it reaches that point.  This approach made the crossover network simpler and less expensive.  Could be wrong but sounds reasonable. The main reason for the fiberglass is to smooth the power response.

[ReliaBill Engineer]

Thank you! Makes sense.

 

AR went through a lot of trouble to add the screen and pad, so I’m not going to mess with success. But I was curious. Having said that, I do crave a bit more mids detail than these offer. But that isn’t possible without molesting these and replacing the mid driver.

[Aadams]

1 hour ago, ReliaBill Engineer said:

Having said that, I do crave a bit more mids detail than these offer. But that isn’t possible without molesting these and replacing the mid driver.

It is not the mid driver but your span of control.  You have zero control over 3 1/2 octaves of mid range starting at 200 hz, while using the mid pot is a gross adjustment of the entire range from about 2000hz to  10khz. You probably need finer adjustment and may be able to get it with your preamp tone controls.  A 10 band graphic equalizer would probably give you the single nudge you crave.

Edited to reflect Early 2ax crossover frequency.

[RoyC]

17 hours ago, ReliaBill Engineer said:

Is it used to add dispersion? Tame a sizzling midrange? Acoustic (air) impedance matching? Or….add mystique?

AR mechanically contoured the response of all its proprietary drivers to achieve the desired outcome with very simple crossovers. As time went on manufacturers became much more reliant on more sophisticated crossovers.

Examples:

-The unusually tightly packed fiberglass under the phenolic dome of the early AR-2ax/AR-3 tweeter, AR-3 mid, 2a, 2ax mid cones, and AR-4/4x tweeter cones.

-The treatment applied to the 4x tweeter cone.

-The fiberglass pads on top of the AR-3/ 3a mids, AR-2ax mid, and AR-4 tweeter.

-The foam damping rings on the AR-10 and 12 inch woofers. Early woofers with these had much thinner/stiffer cones than the later foam surround woofers having no rings.

-These are in the same category as interesting items you have already encountered regarding the material suspending the tweeter domes.

-Other more typical items along the way were changes in the amount/type of cabinet damping material and crossover changes to accommodate changes in driver construction.

AR was conducting careful response measurements to achieve design goals. Tom Tyson provided photos at one point showing AR prototypes buried face-up in a field with microphones hanging over them to measure response with no reflections! Recordings of acoustic concert halls were also used as a reference. Much innovation and engineering went into achieving high quality sound from "small" speakers. AR was the first to do this.

Bear in mind that better speaker designers of the day were very aware of that era's limitations in recordings and radio broadcasts. To some extent they minimized the typical noise associated with them within the upper midrange, which is a very sensitive area of human hearing. I first saw this mentioned in an Advent publication when introducing the "New " Large Advent as being more responsive to cleaner "modern" recordings. Henry Kloss (co-founder of AR) also made comments in this regard along the way. Also, when these speakers were being made, lower powered amps (often mono), no equalizers, and (obviously) no digital effects were the norm. Two speakers were meant to fill a room, often from a (hopefully very sturdy) "bookshelf".

The above are probable answers to some of your questions. You are not the first person to ask them, nor would you be the first to desire a more modern sound. There is a "Mods and Tweaks" section of the forum for these discussions.

Roy

[Aadams]

OK here is the full three pages from 2012.  It is informative, entertaining and lots of words.

 

[ar_pro]

29 minutes ago, RoyC said:

The above are probable answers to some of your questions. You are not the first person to ask them, nor would you be the first to desire a more modern sound. There is a "Mods and Tweaks" section of the forum for these discussions.

Roy

Very clearly explained - thanks, Roy.

It's always seemed to me that if one wanted a more modern, slightly less first-generation sound from an AR speaker, the AR-91 and AR-92 are at their disposal.

[RoyC]

1 hour ago, ar_pro said:

It's always seemed to me that if one wanted a more modern, slightly less first-generation sound from an AR speaker, the AR-91 and AR-92 are at their disposal.

Agreed...and I would add the AR-11, 12, and 10pi to those options.

[ReliaBill Engineer]

4 hours ago, Aadams said:

It is not the mid driver but your span of control.  You have zero control over 3 1/2 octaves of mid range starting at 200 hz, while using the mid pot is a gross adjustment of the entire range from about 2000hz to  10khz. You probably need finer adjustment and may be able to get it with your preamp tone controls.  A 10 band graphic equalizer would probably give you the single nudge you crave.

Edited to reflect Early 2ax crossover frequency.

The XO to the midrange is at 1400 Hz, isn’t it? At 6 dB/ octave, the mid still has output at 700 Hz. So it carries output 700-8000 Hz. That’s where a great deal of detail is produced. The cartridges I’m using are known for their detail retrieval. That’s why I can hear a certain “numbness” of detail in these. The mid driver isn’t capable of reproducing it.  I’m used to hearing that detail. When I switch from “Speaker A” to “Speaker B”, I can plainly hear the difference.
 

Just an observation. I could correct it without making a visible difference in the speakers. But it’s a rabbit hole journey. 

[ReliaBill Engineer]

2 hours ago, Aadams said:

OK here is the full three pages from 2012.  It is informative, entertaining and lots of words.

 

Yikes! Volkswagens, Newton, “crude vs simple”, sealed vs ported, 1% caps, etc.! 
 

I’ll just accept that the fiberglass batting is used to damp “rough edges” at the roll off higher frequencies.

[Aadams]

39 minutes ago, ReliaBill Engineer said:

The XO to the midrange is at 1400 Hz, isn’t it? At 6 dB/ octave, the m

Yours may be 1400hz the early models with your woofer had 2000hz.  I doesn't matter.  Either way you are trying pick detail out of a fairly narrow band by using the pots to raise or lower, in mass , every frequency that is passed to the mid range driver.

The later model 2ax was 1400hz.  There were many thousands of 2axs purchased instead of AR5s because they sounded so much alike in the showroom for a lot less money.  All 2axs have the same mid.  Within a range of listening criteria and given identical placement,the AR5, 3a and 2ax could sound identical.  The dome mid of the 3a and 5 is potent, yet the total power response curves of all three speakers show they have a very similar sound.   If you read the thread I linked, user Soundminded discusses how difficult it is to make a 2ax sound "right" in a real room because of crossover points and the need for an equalizer.  That is my experience as well. 

All domed ARs from Roy Allison days to the last of the 9 series sound virtually the same when set to zero db attenuation before an equalizer is put to them.  The differences are power handling and ease of setup to get the smoothest response.  The 9 and 90 are the easiest to setup but they still sound a lot like a correctly operating 3a which sounds a lot like a correctly operating 2ax.  A 2ax, within limits, will sound as modern as an AR9.

You didn't say what your B speaker is

[ReliaBill Engineer]

I don’t mess with the pots. They stay at 85% tweeter, 75% mid. 
 

Detail is not power response. It’s the ability to resolve slight variations and frequency mixtures in the signal from the amplifier. That detail and ability to resolve small nuances won’t show up in a sine wave sweep. So the pot has no bearing in resolution, just in relative power output. 
 

My “other” hobby has been collecting vintage cartridges. I’m used to being able to swap cartridges and hear the differences. I have 160. I can hear differences in ADC styli, or Empire carts, or between the V15V-HE and V15-IV HE. Or an ADC 10E MKIV and MKII. But with these AR-2ax’s, that gets very difficult. I love the sound I’m getting, great for enjoying music, but not for enjoying different sounds from different transducers. Hence, why I said I “crave” more detail.

My main speakers are the 1989 Polk RTA-8Ts. Not big. But unique in the Polk line of that era. 2 midwoofers (MW) in parallel, with no crossover tricks to use both for bass, 1 for midrange duty. Ported, no PR. I installed the updated tweeters, but stock other than that. No XO rebuild, no recap. The parallel MW in D’Appolito vertical arrangement (MW-T-MW) resolves detail exceptionally well. Great imaging also.

[RoyC]

2 hours ago, ReliaBill Engineer said:

That’s why I can hear a certain “numbness” of detail in these. The mid driver isn’t capable of reproducing it.  I’m used to hearing that detail. When I switch from “Speaker A” to “Speaker B”, I can plainly hear the difference.
...it’s a rabbit hole journey. 

One person's "numbness"...Your preferences obviously lean to a more modern sound. It is pointless to get into a debate regarding preferences, especially in light of the comments above. There are many variables involved.

A rabbit hole indeed. You've already made the tweeter hotter than it was originally. :)

Roy

[Aadams]

1 hour ago, ReliaBill Engineer said:

 

Detail is not power response.

I agree.  I misunderstood what you are doing.   The good news is, now you have pair of speakers for listening to music and a pair of speakers for listening to cartridges.

[ReliaBill Engineer]

Great news!!

I have a pair of AR-3a dome midranges coming my way. They need repair work done.  I’ll document the repair. Then I’ll compare to my dome midrange “mod” for the 2ax. This will be my 3rd pair of 1965-1969 3a dome mid repairs. The other 2 pair got sent back to their owners locally (in state). This pair is mine.