KS12027 Horn

The KS12027 horn perched on top of my repurposed Altec 861 cabinet loaded with an Altec 414A woofer should actually be residing inside a ~ 4 cubic foot slanted cabinet with a Western Electric WE713C compression driver  bolted to its throat + a WE728B 12" woofer = Western Electric WE757A monitor speaker system.

I was honored to host this KS12027 horn and provide more modest accommodations for a couple of weeks courtesy of my audio preservationist buddy John Piro.

He rescued this KS12027 horn with a damaged 0.7" aperture and repaired/modified it so that mere mortals like myself can sample its delights using a more pedestrian 1" exit Altec 802 compression driver. 

Mono Hi-Fi set up: Neat P58H + Calrad SV16 + GE RPX or DL102, JEL SE2A3 mono integrated amp with Hashimoto H-203S OPT.

All of my listening was done with the above set up.

Altec 414A + mod'ed KS12027 + 802 + GPA34852

From the moment I mounted the KS12027 + Altec 802 combo above the Altec 414A woofer, the first adjective that came to my mind and stuck for the rest of its stay was, natural! Just like its WE/Altec 32 and KS12024 relatives, this is also a horn that doesn't sound like a stereotypical horn. In fact, the articulation (attack and decay of notes) and overall presentation of harmonic overtones via the KS12027 is even more natural and realistic. It made the Altec 32A/32C and Azzolina KS12024 sound merely like very high quality Hi-Fi which mainstream audiophiles may find more appealing. These findings were echoed by my friend Ding during his recent visit to the man cave.

Altec 32C, KS12027, Azzolina KS12024

Even if I loved the KS12027 so much, I didn't want to get attached to something I may never be able to own. So this past weekend I met up with John at a ham fest and handed back this valuable piece of audio history. After giving him my rave report, which is hardly representative of what most audiophiles like nowadays, he was not dissuaded. He is still seriously considering reproducing them. 😎

A visit to J-Rob's and the Silbatone 755A box by Dr. Stefano Bae

OB/755A

Despite the fact that Joe Roberts and I both revere the 755A, he never thought highly of my open baffle set-up. To this day, I'm still baffled as to why he featured my home-brewer article in Sound Practices. 😉 I enjoy the engulfing planar-like airiness of the 755A/OB combo which Joe finds lifeless, boring and lacking in snap. 

OB + 755A

Could the dip in this RTA trace be indicating a lack of snap? 😜

Custom-built Altec 618 + 755A

J-Rob custom 618 cabinet

  Back in the mid-90s, I owned a pair of original Altec 618s. Those sounded boxy and colored, perhaps due to their lightweight construction. So I traded them for a pair of early sand casted 811 horns...😧 Oh well, live and learn. 😁

When my friend Chong gave me a pair of well braced vintage DIY 618 boxes (allegedly built by an ex-WE engineer and used to house a pair of WE755As), I understood the snap that ticked the Joe-man.

ex-J-Rob Altec 618 + 755A

This is the nicest and most modern looking RTA trace in this survey! Loaded with Altec 755As, the sound is snappy and dynamic while retaining that warm and realistic sheen in the midrange, which evades most speakers marketed in the past 50 years. Although some listeners may still find that quality a vintage coloration...

A visit to J-Rob's

People don't always have to agree to remain friends

Case in point: Joe seems to prefer the Japanese 755 - Lafayette SK98/Hamlin/Pioneer PIM8L - over the ferrite 755C. However, if push comes to shove, I'd take the 755C over the various permutations of the PIM8/20L even if I played a role in reintroducing its virtues to the world. Maybe it's just my sentimental attachment to the 755Cs since they were my very first high-efficiency drivers?

J-Rob sez, "I shouldn't even be talking to you since you like 755Cs." 😆

When I visited Joe in the summer of 2018, he had a pair of 4 cubic foot WE753A look-alike cabs that house Western Electric WE728Bs driven by a spruced up Dyna SCA35 which was recently replaced by a J-Rob cloned Peerless A100 PP 6A5G amp built during the Sound Practices era. Signal source is a Mac Mini>Tidal>Audirvana>Wavelength Audio Brick DAC.

Despite the natural high frequency roll off of the 12" WE728B, it sounded very full-range because of its rich overtones. That is, overtones as musicians use it to describe musical instruments. A high quality violin, piano, or any musical instrument for that matter, possesses rich overtones and this has nothing to do with high frequency extension. Even FFT analyses have failed to identify why del Gesus and Strads sound harmonically richer and have greater carrying power in a good acoustic space compared to a painstakingly crafted modern violin.

I was looking forward to hearing the Silbatone SE300B amp on the floor but Joe hasn't gotten around to re-wiring it for 120V, and also to LPs, but the Gates CB500 was waiting for a resurfaced idler wheel. Then he showed me his next project - Klangfilm horn repros (center background) + Altec and JBL large format compression drivers.

Finally, I was introduced to the pair of Altec 755As perched on top of the WE753C-like cabs. I was quite impressed with the sounds emanating from the 755As!

This 755A/box combo was featured in the Silbatone exhibit at the Munich High-End 2018 Show. Prior to that, Joe spoke fondly about the box and gave DIY hints in this Audiokarma thread. I was hoping that he'd be inspired to update his Junkyard Jukebox blog for a formal presentation of this cabinet design before he went on a sabbatical from the internet.

Since there's no sign that he'll be back anytime soon, with his permission, I took it upon myself to write about this 755A cabinet breakthrough.

Silbatone/Dr. Bae WE/Altec 755A box plan
Copyright Silbatone Acoustics

I re-drew the box plan based on a quick sketch Joe uploaded in the Audiokarma thread. Please note that this cabinet plan is published solely for the Audio/DIY community as a public service of Silbatone Acoustics. 

Under no circumstances should this be used for commercial purposes! 

Let's see if that works...truth be told, I am well aware that there have been a few enterprising individuals who have benefitted commercially from projects published in this blog. So I'm not holding my breath.

John Piro Clone Build
Interior walls + back lined with two layers of Kimpak

I sent the drawing to my buddy John Piro shortly after my Altec 753C cabinets were delivered.

Can anyone distinguish JP's clone from the original Silbatone box? 

Dr. Bae designed Silbatone box + 755A

J-Rob comments:

"The cabinets were designed by genius Silbatone engineer Dr. Stefano Bae as a group project for the Silbatone-sponsored DIY/Western Electric forum in South Korea. 100 pairs were quickly sold out, immediately driving up the world price for 755As. The original uses Silbatone's custom silver litz wire, which is said to be an essential ingredient of the original recipe, and top shelf WBT connectors.

Over the years Silbatone designed and made literally dozens of experimental boxes, including back horns for up to four (755A) drivers per side. This was probably the most successful, although rather modest.

My analysis of this untuned little cabinet is that it falls under the 'small box with a hole in it' design paradigm."

Unlike before when I felt the need for at least SE300B power to make the OB/755A sing, now I'm enjoying near-field sounds from my JP cloned Silbatone/Dr. Bae boxes + Altec 755As driven by an SE46 amp with tiny Noguchi FM6WS Finemet OPTs.

The SE2A3dx which normally drives the Altec 753Cs in the main listening area had no trouble driving the Altec 755As in Silbatone boxes to moderately high SPL levels.

 Even without the silver litz wire and high tech WBT connectors, I heard all the virtues of the slant 618 box + open baffle combined = detail, snap, macro and micro dynamics from the 618 box + airiness and lack of vintage boxy colorations from the open baffle.

The Dr. Bae/Silbatone cabinet transported the 70 year old 755A design to the 21st century in a monitor-sized package!

Altec 753C

After many years of raving about the Altec 32 horn, the only stone left unturned was using it as it was originally designed, that is, with the compression driver oriented to radiate upwards.

Last year, I received a message from John Piro, whose beautifully crafted preamp project was previously featured in this blog. He sent me pictures of his interpretation of the legendary WE753C using an Altec 414-16B woofer + Altec 32A + Faital Pro driver. 

Altec 414A + 32C + 802D + 20275 

I was very impressed and requested if he could build a pair for me. We finally met at the Kutztown Radio Show last September. He and his wife had dinner here at home when he delivered my beautiful cabinets!

A Western Electric 753C used as a monitor at WABF in the early 50s

As an homage to the legendary Western Electric 753C, which was a major inspiration for this project, I henceforth christen this speaker system as the Altec 753C.

Altec 2-way XO redux

I've settled on running the 414A woofers in full range mode (ala J-Rob) and replaced all the crossover resistors with non-inductive 12W Mills. This took away the last vestiges of grain in the upper frequencies.

RTA Graphs

Altec 2-way

Altec 614 + 414A + 32C + 802 with GPA34258

The above RTA was taken from this blog entry

VS

Altec 753

Altec 753 + 414A + 32C + 802D with 20275

I could've lived happily with my Altec 614 or repurposed 861 (aka Madrid) cabs + the inverted 32 horn situated on top. However, with proper horn orientation, the sound was taken to another level of refinement. Now the blend between the 414A and horn is practically seamless and I bask in the airiness of the top end, which I attribute to significantly improved high frequency dispersion, precisely as envisioned by Western Electric engineers.

In spite of the stellar midrange performance of the Open Baffle mounted Altec 755As, I started the Altec 2-way project in the late 90s to address the following issues:

1. More efficiency
2. Extend the frequency extremes
3. Less congestion in dynamic peaks with complex musical passages

After 20-some odd years, those goals have been realized. The Altec 753C is pretty much end game!

Thanks to my buddy Joe Roberts, who explained the intricacies of proper 32 horn orientation. The idea was carefully stored in my mind. I just had to wait for the right opportunity to implement it. And of course, a big thanks to my friend John Piro (aka RCA NUT), craftsman extraordinaire!

Let's hear some music. 😊

Altec 32 Horn Nomenclature

This Altec 32 horn update was prompted by this must read article from Electravolt tracing how the Altec 32 horn evolved from the Western Electric 32A.

Altec 32A

Cast aluminum + longer throat

Altec 32B

Plastic + shorter throat

pics borrowed from sound-impression

Probably a very short run, rarely seen in the market

Note: no ribs

Altec 32C

Same as 32B but with re-enforcing ribs

Note: re-enforcing ribs

Azzolina Audio KS12024

Western Electric KS12024

Western Electric KS12024
Note: ~ 140° throat bend

The Western Electric KS12024 is a 2-piece cast aluminum 4-cell sectoral high frequency (800-1000Hz cut-off) horn with a 50°H x 40°V dispersion pattern. It was implemented in the Western Electric L-8  and L-9 speaker systems.

Western Electric L-8
from the Silbatone Collection

Western Electric L-8  = 1 x WE754A + 1 x KS12024 + 1 x WE713B (phenolic diaphragm)

Western Electric L-9

Western Electric L-9 = 2 x WE754A + 2 x KS12024 + 2 x WE713B

Western Electric L9
(Note: 2 x WE713B)

Images were taken from J-Rob's KS12024/Western Electric L-8 post at Hifi Haven. If you want to learn more about the Western Electric L-8 and L-9, you can download the pdf file at the bottom of Joe's post.

Read about J-Rob's musings (post #125) about the original KS12024 and Azzolina repro at Hifi Haven.

Late 40s Western Electric Brochure
from www.hifilit.com

The KS12024 horn was from the same era as the WE32A horn that included the venerable WE757A studio monitor, WE713x compression drivers, WE755A, WE756A and WE728B direct-radiator speakers.

Having been a long-time user of the Altec 755A, Altec 32A/C and more recently an Altec 756B, I consider this period as the golden age of design and manufacture of permanent magnet transducers. So when J-Rob offered to loan his personal pair of Azzolina Audio 3-D printed KS12024 horns for me to blog about, it was a privilege and opportunity difficult to pass up.

Azzolina Audio KS12024

I've been a frequent visitor to the ElectraVolt blog but never imagined I would be listening to, let alone reviewing, Charles Michlin's KS12024 reproductions. 

The original Western Electric KS12024 has a 0.7" round opening* but this Azzolina remake was optimized for 1" compression drivers like the Altec 802 that I am using for this review. As relayed by J-Rob, Charles claims the oval (rather than round) aperture sounds better. 

*All the WE horns from this era (including the 32A) were designed for use with the WE713x compression drivers and shared the same aperture size

Just like the original, this is also a true 4-cell compound sectoral horn. AFAIK, this is the model built from an amorphous plastic material. Everything else you need to know about the Azzolina KS12024 can be found here.

Testing

Altec 414A/614 + Azzolina KS12024/802 + 3000H

The Azzolina KS12024 horns were tested in the Altec 2-way system. 

Altec 414A in 614 cab 

Altec 414A woofers in 614 bass reflex enclosures running in full range mode

High-pass XO w/HF eq

Altec 802-8D compression drivers (below) with 16 ohm GPA 34852 diaphragms were mounted to the horns using the above high-pass crossover

Amorphous Plastic Azzolina KS12024 vs. Plastic Altec 32C

Altec 32C + Altec 802

This horn presents a back row perspective and may sound too laid back to those who prefer a closer to the stage presentation. The mid-range is juicy, full and vivid without a hint of shout. While the highs are feathery smooth, silky and airy, which I've only heard from its smaller sibling, the Altec 3000H. Tish (my wife) describes this horn as sublime. I think the 90°H x 40°V dispersion helps provide that holographic and lush atmosphere. 

I've written so much about this horn. Just use the search button on the right hand corner to find past entries.

Azzolina KS12024 + Altec 802

I was initially lured into the 3D-like midrange this horn produced and that lingered on. There's an uncanny realism to male and female vocals as well as musical instruments that renders an "in your room" presence. Compared to the 32C, this horn offers a front row perspective. Yet there isn't a trace of harshness or shout. Definitely a horn that will appeal to those who find the 32C too laid back yet need a horn that doesn't sound like a horn.

It is superior to the 32C in terms of articulation and detail without ever sounding analytical. There's a lot of snap, if I may borrow a term from its owner 😉, just like its stable mate - the WE/Altec 755A.

Azzolina KS12024 + Altec 802 + Altec 3000H

Tish thinks it is as good as but not quite as lush as the Altec 32C. I agree. This is primarily a tonal balance issue. Due to the prominent midrange, the extreme treble frequencies are slightly subdued giving a less airy atmosphere. Adding the Altec 3000H to fill in the uppermost octave helped in approximating the lushness of the 32C. But I still found myself wanting more "hall sound." I speculate that the KS12024's narrower 50° horizontal dispersion contributes to the comparatively austere background. Perhaps I am belaboring this point since my personal preference when I attend concerts is to sit towards the back row so I can relish equal amounts of direct and reflected sound.

Summed up

2 x KS12024 = 100° H x 40° V

These two horns share the same sonic DNA but are genetically modified for different applications. The more up-front and involving Azzolina KS12024 betrays its theatre heritage versus the mellower 32C, which was intended for near-field monitoring. I can't help but wonder if a horn design combining the virtues of both designs is a possibility.