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KEF's Breakthrough VECO Speaker Technology to Lower Distortion–Explained by Its Inventors

Hi Jack,

Outstanding work! You are a model for young engineers. I feel you may end up at a University like Wolfgang. You have a lot to give and your demeaner is admirable.

Question: Why did Velodyne select "Mid-engine" transducer topology rather than the typical Rice-Kellogg cantilever voice coil for their subwoofers? Here's a drawing of an exact copy of the Velodyne 12 that P.Audio marketed as their TM12 minus the cone/surround assembly and dustcap. P.Audio's sister company, A-Ton sold the voice coils to Velodyne back then. https://patentimages.storage.googleapis.com/1b/52/2e/b224c5507cbe19/US4727584.pdf

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Oh. here's 18 Sound new Tetracoil motor from the Aug. 2025 Voice Coil review but the voice coil is cantilever.
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olieb said:
Yes, but aside from looking like a tinkering solution not ready for industrial scale this is not exactly what you want for a coax driver ;-)
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Today, the very ingenious B&M velolicty pickup principle -- humbucking planar coils partially dipping into a horse-shoe magnet perpendicular to the drive coil -- could be easily adapted for cheap and simple mass production, using rigid-flex multilayer PCB, neodymium magnets, 3D printed stuff (including metal) etc.

For bulllet-proof full motion control notably for woofers/subwoofers, one would need a (helper) excursion sensor to capture and correct dynamic DC offset (and perhaps overexcursion).
 
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smowry said:
Hi Jack,

Outstanding work! You are a model for young engineers. I feel you may end up at a University like Wolfgang. You have a lot to give and your demeaner is admirable.

Question: Why did Velodyne select "Mid-engine" transducer topology rather than the typical Rice-Kellogg cantilever voice coil for their subwoofers? Here's a drawing of an exact copy of the Velodyne 12 that P.Audio marketed as their TM12 minus the cone/surround assembly and dustcap. P.Audio's sister company, A-Ton sold the voice coils to Velodyne back then. https://patentimages.storage.googleapis.com/1b/52/2e/b224c5507cbe19/US4727584.pdf

View attachment 468446
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Hi Steve,

Thanks for the message! By mid-engine I presume you mean 2nd suspension at the back of the driver? That's how I'd also do it with a dual gap motor I think. It gives as much space between the suspension elements as possible and means that there's higher "turning" stiffness to resist any turning moment of the soft parts. This would generally result in the rocking mode being higher and less rocking motion in general.

Would you agree?
 
Hi Jack,

Thank you for your quick reply.

Of course I agree. You are the only person that has even mentioned "rocking" to me since Dr. Roger Mark (Brown University), the Transducer Group Scientist at BOSE, almost 30 years ago. So now with gaps to spare do you think my concept assembly shown below is workable?

screenshot.401.jpg
 
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jackocleebrown said:
Not least that we can use a current output amplifier which avoids power compression in the midrange and gains a small amount of inductance related distortion reduction.
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The midrange for the UniQ from the top-of-the-Line KEF Blade and Reference META (KEF transducer model SP1815) has a copper sleeve for the tweeter, and
a copper ring and two aluminum rings for the midrange:

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Exploded diagram:
1754695177912.png


Here is that driver, with a simulated current drive:

1754706225132.png

The 3rd harmonic distortion (green line) has been lowered by at least 10dB.​


But with a full copper sleeve, as proposed earlier there would be no need for a current output amplifier... here is a Purifi PTT6.5XNFA:


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Current output amplifiers have no effect on the PTT6.5X04NFA. I suspect the massive copper sleeve and variable winding voice coil is responsible for this:

1754695952105.png



One could say that the Purifi is already fully optimized for a conventional amplifier.

The next advancement, IMHO, is to implement sensors with a control loop in the digital domain.
Robert Schmidt did this a couple of years ago:
audioxpress.com

Interview: An Afternoon with Robert Munnig Schmidt

Robert Munnig Schmidt is intrinsically connected with advanced motion systems and motional feedback applications. He is now retired from his own consultancy business, following an outstanding career that includes key R&D positions at Philips, being part of the ASML founding team, and teaching...
audioxpress.com audioxpress.com

On a side note, reviewers, not being loudspeaker designers, are not always intimately familiar of how to measure (bass) distortions with accuracy.
With a standard microphone, bass distortion should be done outdoors via ground plane measurement as to 1) not be influenced by reflections and 2)not overpower the microphone. If bass distortion is measured in the nearfield eg <10 cm, or mic-in-the-box method, then a low noise, high SPL capable microphone should be used- otherwise what we are measuring distortion of the mic. Unfortunately the mic specification that Klippel Sales Engineers will tend to sell is not suitable for near field bass distortion measurements .

So reviewers have to opt for a middle ground eg. amirm at 1/3m

But then, they cannot verify any claims made by the designers. Sometimes the speaker designer have to politely step in.
 
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tktran303 said:
But with a full copper sleeve, as proposed earlier there would be no need for a current output amplifier... here is a Purifi PTT6.5XNFA:

Current output amplifiers have no effect on the PTT6.5X04NFA. I suspect the massive copper sleeve and variable winding voice coil is responsible for this
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Hi @tktran303,
Thanks for the comments. I've seen you mention this quite a few times. However, I don't agree that the full copper sleeve will make the difference you anticipate. We use copper sleeves typically for tweeters but we also had one on the previous Blade Uni-Q MF. The reason they're used on tweeters is that at high frequency the conductive rings need to be as close as possible to the voice coil in order to magnetically couple due to the skin effect. For lower frequencies it's not necessary to get the conductive ring as close and you will still get good coupling. But the performance of a copper sleeve is severely limited by the resistance, which is comparatively high versus a ring because the sleeve is very thin. I believe the effect of adding a sleeve to the Reference Uni-Q would be minimal. I put the superb performance of the Purifi in the midrange down to the very smart use of neo in the pole. This gives the entire magnetic circuit higher reluctance and drops inductance significantly. They also stabilise the inductance versus coil position extremely well with their configuration. Hats off to them, they have done a brilliant job.

Thanks for sharing the distortion measurements of the Reference Uni-Q. Nice data! It's hard to measure distortion down to these levels. You must have an excellent test setup.

A couple of things to also consider. A current amplifier is no more expensive than a voltage amplifier, whereas the distortion mitigation features in the KEF Reference Uni-Q are relatively expensive. Current amplifiers also avoid power compression. The beneficial effect of the velocity feedback on the area around fc also needs to be considered too.
 
smowry said:
Hi Jack,

Thank you for your quick reply.

Of course I agree. You are the only person that has even mentioned "rocking" to me since Dr. Roger Mark (Brown University), the Transducer Group Scientist at BOSE, almost 30 years ago. So now with gaps to spare do you think my concept assembly shown below is workable?

View attachment 468787

Where I am trying to take the concept is to implement VECO but also to Commutate the Voice Coil to realize a Class B Moving Coil Transducer. Is this possible?
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Hi Steve,

I think that you updated the drawing. The voice coil is in the middle and uses an iron free magnetic gap? And then you have the steel on the two return paths to keep the reluctance of the magnetic path low? It looks like a nice configuration for low inductance distortion. Did you already simulate to see what gap flux is achievable?

The sensor coils would be conventional wound coils? They'd certainly have some magnetic coupling to the voice coil but it may not be too much because of the distance and that there's not much steel around the voice coil. This can be simulated to check. You really want to have a high xmax on the sensing coils (higher than the voice coil) to keep the velocity signal as linear as possible so the dimensions will need tweaking compared to the drawing.

I think that the absorbative curtain probably will have too much flow resistance and reduce the Qms of the driver so the open basket version is probably better.

Are you proposing mirrored mechanical parts for kms symmetry? That's nice but the additional rear diaphragm is adding mass that might offset this benefit.
 
Thanks Jack.

The concept is new. It get's updated about every 20 minutes. I hope to begin simulation soon. I will let you know how things are going as I move along. KEF is emerging as an industry leader and you need to get credit for that. Your staff must love you. The way you explain things and your calm demeaner are excellent from my view and you recognize others for their achievements too. Your technical knowledge is at the highest standard but there is no sense of arrogance. Outstanding!
 
Jack, Yes the suspension is the 2 x surrounds and yes the anti-diaphragm will add mass. However, there is no spider and there is no dust cap but still that topology will add mass.

So okay, I remove the absorptive curtains but what about the constant pressure on the diaphragms' backsides and the related modes and anti-modes of the diaphragms. What will that do? The basket can be designed to reduce the volume between the diaphragms. My drawing is just to illustrate the concept for discussion. The two diaphragms with air between is a sandwich. I wonder if the sandwich would allow me to reduce individual diaphragm mass when using two with closed basket vs. using one or two with an open basket. What does the sandwich assembly do? I just don't know but I must figure out how to simulate. Any comments from you and/or any ASR members are welcome and encouraged.
 
sigbergaudio said:
(From the Youtube description)
KEF's latest innovation in speaker technology, VECO (Velocity Control), is revolutionizing active speaker performance. In this in-depth interview, KEF's VP of Technology Jack O'Clee Brown and R&D engineer Prathmesh Thakkar reveal how combining a current-drive amplifier with a custom-designed velocity sensor leads to unprecedented control over driver motion, slashing distortion and enhancing clarity.

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I am a proud owner of KEF Reference 1.
Thank you.
 
I recently discovered the The SoundStage! Network videos on KEF technology with Jack Oclee-Brown and enjoyed it a lot. It took me some time to understand where the magnetic field (orthogonal to the driver magnetic field) for the sensor coils comes from. I took 2 pictures from the video and added some description to illustrates it:

veco2.jpg


As already pointed out earlier using motion sensors for feedback in active speakers has a long history. In the 1980s I personally heard a Philips MFB (I think they used a piezo electric sensor) and was not very impressed. But I also heard a little Backes & Müller BM6 and was very impressed by the bass quality of that little speaker. Backes & Müller also uses a dynamic sensor in their bass drivers (located under the dust cap). Old B&M models additionally used a capacitive sensor for midrange and tweeter, but recent models just use DSP in this frequency range. LowBeats has a nice article on the B&M history: https://www.lowbeats.de/en/50-years-of-backes-mueller-always-active-against-the-current/, which includes a link to the (renewed?) B&M sensor patent https://patentimages.storage.googleapis.com/0f/a0/5b/5823fab71255cf/DE102015102643A1.pdf. KEF’s VECO looks more elegant to me. Looking forward to see/hear some HIFI speakers with VECO.
 
Torin Krell said:
The powered Philips MFB RH544 with motional feedback (and other interesting features) was used in a few recording studios at the time and was apparently used in the mixing of at least one Pink Floyd album.
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I though the Philips 22RH545 was the studio version..
 
Hayabusa said:
I though the Philips 22RH545 was the studio version..
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Yes, iirc it was partly meant (and a few times used) for that. But @Torin Krell is correct:

The Story Of The Philips RH544

One of the best-selling albums of all time was mixed on a long-forgotten experimental hi-fi speaker. Forty years later, are Philips' innovations finally catching on?
www.soundonsound.com www.soundonsound.com

From that article:

"Well, what if I told you that a number of classic Pink Floyd albums, including The Wall, were mixed on a pair of Philips RH544s? Andy Jackson, David Gilmour's long-time engineering collaborator, takes over the story:

"I didn't work on The Wall project but we were still using the 544s after that on The Final Cut, The Division Bell and on Roger [Waters]'s Pros & Cons Of Hitchhiking album."
 
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