Post by Owen Y on Jul 23, 2019 22:42:30 GMT 12
'Field Coil' loudspeakers is another name for Electromagnet loudspeaker drivers - as opposed to the 'Permanent magnet' loudspeakers drivers used by almost all cone loudspeakers today. Cone loudspeakers in (valve) radios back in the 1920s-40s were driven my an electromagnet powered by electric current. Later, home audio & commercial cone drivers too (eg Voigt, RCA, Lansing, Western Electric)- before 'permanent magnets' became economically widely available. In recent times & today, permanent magnets are the norm - AlNiCo, neodymium, ferrit/ceramic, etc. But modern Field Coil (FC) drivers and/or loudspeakers are being offered my a number of niche manufacturers - eg. Voxativ, Shindo, Feastrex, Rullit, Line Magnetic, G.I.P., Supravox, EMS, Wolf von Langa, Cogent.... ie. quite a few! What advantages do Field Coils offer, if any, over Permanent Magnet (PM) loudspeakers? - They say that the magnetic flux field generated by an electric power supply, is less effected (modulated) by music signal - ie, the music produced sounds more dynamic, more resolution & 'real'. - The flux strength of a FC driver can, to some extent, be varied by altering supply voltage & thus the character of the driver too. However, as said, you need a PSU to 'energise' FC drivers. And the driver tends to be bigger & heavier - due to the big wire coil of the electromagnet. And they tend to be costly to make. 
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Post by Owen Y on Jul 23, 2019 22:48:17 GMT 12
Voxativ FC 'full-range' driver:  Feastrex FC driver:  Wolf von Langa 'Kilimanjaro' series FC woofer: 
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Post by Owen Y on Jul 23, 2019 23:03:53 GMT 12
Around 2010, I dabbled in FC drivers using these units made by a pal overseas.  They are branded 'Old Brown Dog' (OBD):  They feature paper diaphragms built on Lowther cone frames & embossed for stiffness, with double-wound voice coils (a la Paul Voigt): 
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Post by Owen Y on Jul 23, 2019 23:17:45 GMT 12
However, I didn't pursue this experiment to completion at the time, because i did not have a suitably sensitive tweeter to match & also after discovering the ridiculously inexpensive (~NZ$147 at the time) SEAS FA22RCZ 'full-range' (PM) driver.  (SEAS FA22RCZ in the foreground, OBD FC driver behind.)
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Post by cartridgeguyonline on Jul 24, 2019 8:31:48 GMT 12
Very interesting, I remember reading something about these making a comeback somewhere a couple of years back. Its basically using an electromagnet instead of the usual magnet isnt it Owen ? Is there a part 2 to this story where they get mounted in a cabinet and ride off into the sunset to live happily ever after ?
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Post by Owen Y on Jul 24, 2019 12:14:50 GMT 12
Is there a part 2 to this story where they get mounted in a cabinet and ride off into the sunset to live happily ever after ? - not yet, but I'm working on a sequel to the story right now  When the SEAS drivers arrived, I liked the warmth & robustness of those drivers so much, in the horn speakers & I have lived with the SEAS for several years. But whilst these are excellent regular 'full-range' drivers & I went ahead & added a crossover & tweeter,... ultimately they are not optimal for backloading with a bass-horn - as shown here.
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Post by Owen Y on Jul 24, 2019 12:55:57 GMT 12
The recent arrival of a pair new OBD cone diaphragm units from Sth Africa, has activated this project again. The new cones have 'whizzers' to extend HFs. Also different paper material  
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Post by Owen Y on Jul 24, 2019 12:58:13 GMT 12
Unbolting the existing Diaphragm Units: 
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Post by Owen Y on Jul 24, 2019 13:06:19 GMT 12
A close-up of the double-wound (inner & outer) Voice Coil. This is, in concept, the same as Paul Voigt's 1930s driver design for horn speaker drivers - to maximise flux strength with a very short & narrow magnet gap, lightweight cone & 'underhung' voice coil, with a strong 'motor' & very short cone movement (xmax). All designed for high sensitivity, fast response & at the same time to be able to drive a big bass-horn. 
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Post by Owen Y on Jul 24, 2019 19:00:42 GMT 12
This is the magnet gap, into which the above voice coil slides - the gap width is only around 1.5mm wide.  
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Post by colinf on Jul 24, 2019 22:49:33 GMT 12
Nice! I heard Feastrex drivers briefly at Munich (don't know which) and was struck by how alive and fresh they sounded on operatic voice. So how is the flux in the voice coil gap different to that from an ordinary magnet system, given similar flux strength and size of each unit?
AMR-iFi R&D
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Post by Owen Y on Jul 27, 2019 10:46:26 GMT 12
Hi colinf - I am unsure if the gap flux is any different from that of a Permanent Magnet - flux is probably dependent more on the pole-gap design & magnet strength. However, 'they' say that electromagnet / field coil flux field is less affected / modulated by the voice coil signal (from the amplifier). There is also the belief that higher voltage field coil drivers are preferable to low voltage versions - eg with the vintage RCA FC drivers which came in different voltage versions.
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Post by Owen Y on Jul 27, 2019 11:16:30 GMT 12
These are the PSUs I built to power the FC drivers - just a simple valve-rectified CLC filtered, 145-150vdc at 200mA. Rectifers are 5Z3. If I used say 83 mercury-vapour rectifiers, impedance would be lower but (i) I'd probably need to trim V down a bit & (ii) it's always good to have pre-warm switching for these MV tubes. 
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Pundit
Post by belbo on Aug 24, 2019 6:03:45 GMT 12
Amazing how more and more it feels like older technologies do something special in audio, everything old is new again in a way.
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Post by colinf on Aug 25, 2019 0:44:30 GMT 12
The laws of physics don’t change. Some great technologies were worked out years ago. The bean counters then got in and the rest is history! 😀 Seriously though, the understanding of physics is best today and old technology is being highly refined.
AMR-iFi R&D
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Member
Post by lancaster on Mar 5, 2020 15:05:13 GMT 12
Really interesting topic, thanks for sharing. A few years ago when I built some full range speakers I was checking out other designs and came across a few companies offering Field coil drivers (Shindo, Line Magentic)...I've not heard any, but would like to learn more. Awareness seems to be on the up and I like the thinking colinf that modern technology allows the original designs to get better.
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Pundit
Post by Neil on Mar 5, 2020 22:37:52 GMT 12
All the field coils I have heard have been sublime. I really wanted to get the field coil versions of my speakers but couldn't stretch to it. I've heard all the various Shindos, all the A23s, and a pair of Feastrexs and they all share that amazing immediacy and presence and 'aliveness' that normal magnet drivers just can't do. Sooner or later I will have some. I have been contemplating changing my speakers our for a pair of the Shindo FC 604s, but we shall see...
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Post by colinf on Mar 6, 2020 2:35:00 GMT 12
Hi colinf - I am unsure if the gap flux is any different from that of a Permanent Magnet - flux is probably dependent more on the pole-gap design & magnet strength. However, 'they' say that electromagnet / field coil flux field is less affected / modulated by the voice coil signal (from the amplifier). There is also the belief that higher voltage field coil drivers are preferable to low voltage versions - eg with the vintage RCA FC drivers which came in different voltage versions. Interesting, I didn’t know the magnetism can be changed by the voice coil signal. I guess if you can hear the difference between different permanent magnet materials on a ‘normal’ driver, having an electromagnet might allow for even more adjustability. Was also meant to say modern manufacturing techniques can make for more consistent and precise machining, so modern versions of these speakers should be even better than their ancestors. Owen, did you go any further with the valve rectified power supply?
AMR-iFi R&D
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Post by Owen Y on Apr 2, 2020 13:10:57 GMT 12
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Pundit
Post by beeman on Apr 3, 2020 7:51:31 GMT 12
How are the new cones Owen?
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Post by Owen Y on Apr 3, 2020 12:43:26 GMT 12
beeman - I am working on rebuilding the PSUs right now. But somehow, despite this lockdown, seem to be v busy with other 'incidental' non-audio issues - eg. income & financial rejigging for the inevitable coming downtown
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Pundit
Post by beeman on Apr 4, 2020 12:20:57 GMT 12
Yeah sucks how that stuff keeps getting in the way of the real work
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