Field Coils for Audio Systems

[Zeotrope]

May I ask where you got the turnbuckles made? I also use turnbuckles but don't have the best implementation currently.

 

[ArtisanalAudio]

May I ask where you got the turnbuckles made?

I assembled the turnbuckles: Bodies from Johnson Marine, right and left hand stainless all-thread, locknuts and pushrod ends from Industrial supply like McMaster-Carr. Also, because of limited rotation of the pushrod ends all connection points need to be set at angles to keep things from binding. Modelling in CAD is important for this.

 

[Zeotrope]

Thank you, that's very helpful. I was using something like you see below, which broke as I was adjusting one of the midrange horns. I'm now using like what I am holding, but they are not the best aesthetically.

 

[ArtisanalAudio]

The design of the racks is based on mounting the midbass and midrange horns at their balance points while also physically aligning the voice coils on the drivers. The midrange is adjustable several degrees up or down for focus. The midbass can adjust up and down, but also forward and backward to maintain alignment relative the the mid range depending on the focus points.

Adjustment for the tweeters works in xy and z axis so the entire array can focus and align for any listening position. These adjustments are critical for optimal performance. The tweeter mounting hardware was sourced from optical test equipment and modified for the heavier loads of the alnico horn tweeters.

The rack design was also working towards being minimal and reducing the visual mass of everything while also being fairly textbook "form follows function". One side of the racks distributes the audio signal while the opposite side carries the field supplies. The cost of the racks, metal finishing and hardware is significant but it pulls everything together.

 

[Zeotrope]

Yes, my rack as well physically time aligns all drivers.
Where did you align the Fostex tweeter in relation to the midrange horn? Did you align the start of the brass 'horn' of the tweeter to the throat of the midrange? Of course, the path travelled by the sound is not quite identical; but assuming it is, I'm not 100% where the tweeter should be relative the the midrange, for example.

I was also careful to place the tweeters at listening ear height; not easy when you are working with horns and you want to minimize the distance among drivers.
The fact that there are 4 drivers is completely imperceptible to the ears, it's really amazing.

 

[dcathro]

Interesting thread.

My first exposure to field coil drivers was with RCA 1428B drivers on RCA multi-cell horns around 2000. Even in a mismatched system kludged together the RCA drivers produced the most effortless and natural midrange I had ever heard. This was at the home of the late Steve Steve Schell who was an encyclopedia of audio history and became the instigating force behind the limited production Cogent field coil drivers. During the '00s and teens I worked with Cogent to produce a few systems.

But still so many people are stuck with typical 15" midbass–usually ported–and a compression midrange and some tweeter. While there will be significant gains over permanent magnet drivers such systems still have many longstanding limitations built into the designs.

Where Cogent went one better was the design of their DS-1448 midbass drivers. With compression midbass giving high output down into the 70-80Hz range the transparency and transient response dramatically improves. Distortion is greatly reduced and the presentation becomes even more life-like.

And not content with just that in my own installation I built in bass horns. Originally 30Hz flares that put out ~106db w/m, when I reconfigured my space for the Cogent arrays I added so eq for a solid 20hz.

The main arrays are Cogent DS 1448 and 1428 field coils and Fostex T-925a tweeters driven with Western Electric 300B amps tweaked for me by Cy Brenneman. The bass horns are driven with VTL MB-125 mono amps. So it's all horn loaded and yet when listening there is no sensation of the mass of equipment producing the sound.

I got pretty lucky with the implementation. Oh, and I cannot overstate the impact of effective room treatment on the system: a good room can make mediocre equipment sound far better while a poorly treated room can make the best equipment unlistenable.

eso

Hi Artisanal Audio,

Thanks for sharing your system and your involvement with Cogent.

I remember Steve sharing on Audio Heritage his love for field coils and the development of the Cogent midrange. He was a top guy.

@denimhunter - your type of system?

 

[dcathro]

(It’s surprising and embarrassing how few very high end speaker manufacturers use field coils…but that’s a different topic.)

Never mind field coils, how many manufacturers are even using Alnico? Wilson even made it a major marketing point as if that were a new invention!

 

[dcathro]

The base design is from Bill Woods. Bill provided the basic design used by both Cogent on the West Coast and ΩMA on the East Coast.

Cogent principal Rich Drysdale and I devised what we felt was a cleaner connection between the drivers and horns that eliminated Bill's throat castings. Those castings limit the thickness of the wood sides and I wanted thicker material for a better visual balance. After experimenting with the basic design of Bill's I shortened the mid horns a couple inches while maintaining the mouth diameter. This increased the internal angle of the horns and makes a better match with the internal phase plug geometry.

The 1448 are on the truncated midbass as designed by Bill Woods. The networks use a 1st low pass at ~350Hz if I recall. The Mids are a simple 1st order high pass as are the tweeters. In my system the networks use charge-coupled capacitors and the inductor for the low pass is a custom wound 10Ga coil with negligible insertion losses due to the large gauge wire.

Wow, what a great looking room and system!

 

[Zeotrope]

I wonder why Cogent didn’t succeed in the marketplace?
The only other company that made an RCA1428/1443 replica is Wolf von Langa, as far as I know. They seem to have had more success with it in their own speakers.
There was also an LM(?) Chinese replica.

 

[ArtisanalAudio]

Ultimately the Cogent drivers were extremely expensive to make which required a hefty price tag for raw drivers. But once someone has those drivers, good implementation requires a great deal of knowledge most people don't have and so they get poorly implemented and then fault the drivers since their own lack of understanding could not possibly be the issue

I can't tell you how many times I've been contacted by someone with horns bought from Dr, Bruce Edgar, or JBL/FC conversions from others who are upset that people aren't holding their hands to assemble a system when they only paid for raw drivers. People surf the internet and get big ideas but very few of them are willing to put in the time and effort to really understand what is required to build a really top-notch system.

The alternative is to build a turn-key system, but the issues there become funding and marketing. The level of expertise and time required to bring a full system to an appropriate level of fit and finish pushes the retail pricing into a very limited market. And that market takes a lot of money to market to.

Rich still provides a few drivers to ΩMA as needed, but that's about it for the original Cogent drivers. I'm really thrilled to have a personalized set of drivers.

And Rich and I keep collaborating on other ideas like turntables and tone arms. Plus I continue to rebuild and rework various series of Urei studio monitors to keep me amused.

 

[Zeotrope]

I love how they copied the RCA plaque as well!
I am thrilled to have the original RCA 1443, with a spare pair of NOS in box diaphragms as well... they will outlive me for sure.

It took me 8 years to have my custom horns designed, built, and tested... I'm glad I did it, but I would never do it again.

 

[ArtisanalAudio]

This is very interesting and a beautiful build, @ArtisanalAudio, congratulations!
Are those Bill Woods Conical horns?
I would love to see more photos of the speakers, especially the frame and how the drivers are attached to it.
Looks like you are using an active crossover?
Can I ask how you have implemented the 1448 and 1428 drivers? What are the differences?

“My first exposure to field coil drivers was with RCA 1428B drivers on RCA multi-cell horns around 2000. Even in a mismatched system kludged together the RCA drivers produced the most effortless and natural midrange I had ever heard.”

I am biased, but I agree that the RCA1428/1443 on a conical horn has unmatched midrange tone and realism. I auditioned the Wilson Chronosonic XVX (with the intent to purchase for a second system) and they did not achieve the same level of realism in the midrange.
(It’s surprising and embarrassing how few very high end speaker manufacturers use field coils…but that’s a different topic.)

I got quite lucky with my setup, using the Fostex T500aMkII and Supravox FC woofers in the upper bass, and a transmission line active sub, I have flat (+/-5db) response in my room from 20Hz to 20K. Not that frequency response defines a world-class system, but it’s a good foundation.
The Duelund CAST crossover has made a big difference, in addition to room treatment and vibration control and AC and ground filtering for the FC field coil power supply. The PSU itself also makes a tremendous difference - the magnetic field is in the signal path, after all.

I've been seeing your speakers posted in several groups on Facebook during the past week.

Kind of a curious coincidence. It looks like this page posted the picture and several others shared it:

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[Zeotrope]

I've been seeing your speakers posted in several groups on Facebook during the past week.

Kind of a curious coincidence. It looks like this page posted the picture and several others shared it:

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Interesting... thanks for sending. One would think they should have asked me for permission to repost!