Goodbye Permanent Magnet Speakers

[Hari Iyer]

I have only listened to speakers with permanent magnets till date and have very briefly listened to Alnico magnets speakers at FM @shafic and @prem JBL speakers that use an Alnico midrange driver. There use to be one more type of speakers in the early 1900 and continued for almost 50 years till Alnico magnets were discovered. These speakers used a field coil which were powered by DC voltage to generate flux density when powered on. The flux density can be modifed by the amount of current flowing through the field coil and can be made mid-band or broad band by varying the gauss/Tesla. I was looking for these type of speakers since long - may be around 3 to 4 years but all of them were out of my budget - because they are rare to find and hence quite boutique in that sense.

Suddenly during a lazy Saturday search the one i bought surfaced and i ordered them immediately as they were within my reach budget wise. Of-course they are not a Western Electric or RCA but a pair of 8" field coil speakers from a radio maker in the early 1930s named "Sachsenwerk" which were quite popular for their "Olympia" brand of radios. My pairs are actually salvaged from one of the radios which were lying in stock unused for too long in a antique shop. The paper cones looked quite new and unused and there were no physical damage to the cone or the frame for a driver that old but had some age related paint scarrying - that too not noticeable.

I listened to some youtube videos from this radio and some OB speakers mad out of them and liked what i heard. The mids were quite liquid and heavy and the tone is what i was looking for - very open and no compression of sound. Field coils have their reputation to have extreme low distortion in their pass band and would be to some extend band limited based on the driver size and the magnetic field strength. I have a plan for the DC supply for the field coil suggested by the seller which i shall implement while i am waiting for the drivers to arrive. Its for me to discover whether i will require a sub-woofer + tweeter to support the extreme ends but i shall be open for them too based on my initial listen and setups. Pasting some YT videos of some radios from where this driver would have been salvaged for me, enjoy!

Thanks for looking.

Thanks for listen

 

[arunkvivek]

I have only listened to speakers with permanent magnets till date and have very briefly listened to Alnico magnets speakers at FM @shafic and @prem JBL speakers that use an Alnico midrange driver. There use to be one more type of speakers in the early 1900 and continued for almost 50 years till Alnico magnets were discovered. These speakers used a field coil which were powered by DC voltage to generate flux density when powered on. The flux density can be modifed by the amount of current flowing through the field coil and can be made mid-band or broad band by varying the gauss/Tesla. I was looking for these type of speakers since long - may be around 3 to 4 years but all of them were out of my budget - because they are rare to find and hence quite boutique in that sense.

Suddenly during a lazy Saturday search the one i bought surfaced and i ordered them immediately as they were within my reach budget wise. Of-course they are not a Western Electric or RCA but a pair of 8" field coil speakers from a radio maker in the early 1930s named "Sachsenwerk" which were quite popular for their "Olympia" brand of radios. My pairs are actually salvaged from one of the radios which were lying in stock unused for too long in a antique shop. The paper cones looked quite new and unused and there were no physical damage to the cone or the frame for a driver that old but had some age related paint scarrying - that too not noticeable.

I listened to some youtube videos from this radio and some OB speakers mad out of them and liked what i heard. The mids were quite liquid and heavy and the tone is what i was looking for - very open and no compression of sound. Field coils have their reputation to have extreme low distortion in their pass band and would be to some extend band limited based on the driver size and the magnetic field strength. I have a plan for the DC supply for the field coil suggested by the seller which i shall implement while i am waiting for the drivers to arrive. Its for me to discover whether i will require a sub-woofer + tweeter to support the extreme ends but i shall be open for them too based on my initial listen and setups. Pasting some YT videos of some radios from where this driver would have been salvaged for me, enjoy!

Thanks for looking.

Thanks for listen

i followed you to O/B subwoofer and you up the game to F/C speakers

 

[Hari Iyer]

i followed you to O/B subwoofer and you up the game to F/C speakers

I don't know if i am saying goodbye to current traditional speakers, but that will be for me to discover after i plug them and i am okay with what i listen, but i am expecting it to be good hopefully. As per some other forums, these cones are a bit stiff and heavy, so i am expecting the compliance to be from low to medium, that means they will be suitable on a OB loading and can pump some good bass too. The highs can be delivered by increasing the field coil current a bit but within safe limits of the field coil which is rated at 6 watts. I read that the flux density can be varied from 1T to 2.16T with this driver.
Some interesting read about FC drivers, https://www.tweekgeek.com/blog-tweek-geek-speek/field-coils-are-the-future/

 

[shafic]

The virtue of a Field Coil

audio, tube, horn loudspeaker, high efficiency, loudspeaker, turntables, vinyl, field coil,

electravolt.blogspot.com

Congratulations Hari on your new purchase, fieldcoil drivers. These are made in East Germany. I have the same brand tweeters. Excellent ones.

In my listening experience fieldcoils have very low distortion than alnico magnets.

Just like tubes, TTs, vinyl have made a comeback, in the west the fieldcoils also gaining popularity.
GIP of Japan made some excellent replica of Western electric fieldcoils. Companies like ems and few others are also making very good fieldcoils. But this is an expensive n complex hobby.

I have a small collection of vintage fieldcoils ( Jensen, Western electric, RCA, Altec etc etc) also a modern one, EMS ( Electro magnet speakers, France).

Looks like 2020 may bring more discussions and explorations of fieldcoils in the forum.

My resonant cabinets for ems fieldcoil drivers are almost near completion. Will start a thread on them soon.
Cheers!!

https://imgur.com/Pl1eroh

 

[Hari Iyer]

Great Shafic. I never new you had a field coil collection too. My FC speaker are designed to be loaded in an OB frame. They have phenolic spider and I have never seen them in real action. Looks very compliant though.

 

[alpha1]

What are the advantages, and why did the world move to permanent magnets?
Why should the distortion be low in field coil vs Alnico / rare earths?

 

[mpw]

The world moves at a commercially driven level .

Not always what is " better " gets selected

I suspect part of the answer lies here somewhere

 

[Hari Iyer]

Yogibear, you have addressed the benefits quite well. In permanent magnets (Alnico, ceramic, neodymium ) the flux density (Gauss / Tesla) is fixed and so is the magnetic induction (BI). With field coil the flux density is generated by the strength of the field coil. As we all know that a coil mounted in a iron core gets immediately saturated when a DC voltage is applied to it and they get magnetized. The same principle is used in the FC speaker. The intensity of the flux density can be changed by altering the voltage / current passing through the field coil. For my particular driver they need around 80 volts and 75mA current which approximates to around 6 watts maximum. With this its supposed to have a flux density of around 2.15T which is very very high level for any permanent magnet speaker to achieve (For comparison the Beryllium tweeter which i recently purchased have a neodymium magnet of 1.2T). So if i lower the voltage fed to the FC the flux density will reduce. Lower flux density will make the driver midrange centric. For extended highs and lows the flux density needs to be increased within safe limits of the field coil so that they don't get too hot and open.

Typically the DC supply needs a filter choke and probably a current limiting circuit and DC needs to be as pure as possible, else there will be a risk of hum getting into the sound. The filter choke prevents any back emf generated by the FC and the current limiting component ( i plan to use a 15 Watt incandescent lamp in series with the field coil as suggested by the seller) will limit the current for the FC.

The world moves at a commercially driven level .

Not always what is " better " gets selected

I suspect part of the answer lies here somewhere

mpw, The FC drivers were phased out because of the high cost involved to make / run them. Alnicos became wildly available and were much cheaper than using a FC. Also additional investments for a DC power supply having a choke filter, foil capacitors, transformers were additional cost over the field coil. So i shifting to Alnico were the most sensible thing to do. FC speakers are now a days completely belong to the DIY domain for people who know what they are doing and can take calculated risk with testing and experimenting and learn in the process. But again FC speakers are available for purchase too at unheard of weird price points shunning a normal person from considering them. For me the journey is more exciting than the final destination

one more radio video

What are the advantages, and why did the world move to permanent magnets?
Why should the distortion be low in field coil vs Alnico / rare earths?

FC are expensive to make and even more expensive to implement and a bit unwieldy too. You will need another pair of wires supplying high DC voltage to the FC speakers.
Distortion is low because the magnetic flux density is constant and aligned in the field and does not vary with the music signal. Also there is no suckout of magnetic induction during peak load.

I actually did some power supply testing yesterday and discovered the FC current can be very easily controlled / regulated by just using the incandescent lamp. I tested yesterday with 15 watt lamp and found for a 200V source DC voltage i get around 38V DC and 38mA current with a 1Kohm load (which is same as my FC DC resistance) which is around 1.5 watts power dissipation. So i did some math with a 25 watt and 40 watt bulb and got 3.2 watts and 5.8 watts power which is the safe limit for the FC. So looks like it will be easy to control the tone of the driver by just replacing the bulb which is very easy if mounted on a socket. Looking ahead to test again with these two bulbs this weekend and report.

 

[alpha1]

FC are expensive to make and even more expensive to implement and a bit unwieldy too. You will need another pair of wires supplying high DC voltage to the FC speakers.
Distortion is low because the magnetic flux density is constant and aligned in the field and does not vary with the music signal. Also there is no suckout of magnetic induction during peak load.

1) But is there a variation in flux density in a permanent magnet? And by alignment point, I interpret that in a permanent magnet not all the crystals will be pointing in same direction. But does that cause additional distortion in speakers, because it is the net effect of magnetic field that moves the speaker coil?
2) Why would a permanent magnet's flux vary with music signals?
3) Again, why would there be a suckout of magnetic flux during peak load? I can understand the issues at the voice coil end, but that would be same for FC as well as PM

 

[Hari Iyer]

1) But is there a variation in flux density in a permanent magnet? And by alignment point, I interpret that in a permanent magnet not all the crystals will be pointing in same direction. But does that cause additional distortion in speakers, because it is the net effect of magnetic field that moves the speaker coil?
2) Why would a permanent magnet's flux vary with music signals?
3) Again, why would there be a suckout of magnetic flux during peak load? I can understand the issues at the voice coil end, but that would be same for FC as well as PM

alpha1, I am no expert in FC speakers and just quoting from what I read elsewhere,
- parmanently magnets, especially woofers during peak demand will face suckout of magnetism temprorially as the flux density is fixed and crystals aligned in a fixed geometry and magnetism is generated by electron spin.
- FC magnet generates flux density by voltage and current and these alone determines the field strength. The field strength can be as high as 2.5 T and even though there could be some suckout but they still have plenty of reserve magnetic field/ flux.

Analogy is using a 1000uF Vs 30,000uF capacitor in your power supply for the amplifier. A smaller capacitor may not be able to provide drive for your woofer, but the large capacitor easily can. Hope that makes sense.

Voxativ founder describes field coil magnet to produce warming up and the warming up magnetic flux density gives a different sound signature close to real life.

 

[ISO]

Yogibear, you have addressed the benefits quite well. In permanent magnets (Alnico, ceramic, neodymium ) the flux density (Gauss / Tesla) is fixed and so is the magnetic induction (BI). With field coil the flux density is generated by the strength of the field coil. As we all know that a coil mounted in a iron core gets immediately saturated when a DC voltage is applied to it and they get magnetized. The same principle is used in the FC speaker. The intensity of the flux density can be changed by altering the voltage / current passing through the field coil. For my particular driver they need around 80 volts and 75mA current which approximates to around 6 watts maximum. With this its supposed to have a flux density of around 2.15T which is very very high level for any permanent magnet speaker to achieve (For comparison the Beryllium tweeter which i recently purchased have a neodymium magnet of 1.2T). So if i lower the voltage fed to the FC the flux density will reduce. Lower flux density will make the driver midrange centric. For extended highs and lows the flux density needs to be increased within safe limits of the field coil so that they don't get too hot and open.

Typically the DC supply needs a filter choke and probably a current limiting circuit and DC needs to be as pure as possible, else there will be a risk of hum getting into the sound. The filter choke prevents any back emf generated by the FC and the current limiting component ( i plan to use a 15 Watt incandescent lamp in series with the field coil as suggested by the seller) will limit the current for the FC.

I suggest variable voltage regulated power supply . Since coil resistance is constant, by varying voltage, current can be controlled precisely over wide range for the testing. I don't see need of choke in series. Pl. note regulated supply removes almost all the ripple and hence there is no risk of hum from the speaker.

 

[Hari Iyer]

I had checked some SMPS supply that have variable voltage and current but they are quite pricy

 

[ISO]

I had checked some SMPS supply that have variable voltage and current but they are quite pricy

I built 0 to 30V regulated power supply using. IC LM 317. At 30 V current will be 30mA in the coil . Cost should be less than 1000 INR. If you are in Pune, we can do that experiment using my power supply.

 

[Hari Iyer]

I received the drivers yesterday and I measured the FC resistance to be 1030 and 970 ohms. They require around 70v to 80 volts and around 70 mA to max 80 mA to sing well. I have now connected to unregulated power supply and I am using a 25 watts bulb + 2 Philips tube light choke ( DCR 75ohms and L-0.82H) in a B,L,C, L,C configuration. The raw DC is 280v DC and output DC is 70 v with load. I am continuously monitoring DC and it's rock steady. AC voltage is 20 v at output, but they don't give any hum( zero hum).

Received the FC drivers on friday morning. I had already built a prototype of the power supply and wanted to finalize for the other channel. This took most of the time on the friday late evening and saturday morning. I had to call the carpenter for minor modifications of this driver on the same OB which was used by my earlier russian driver. Some images to feast your eyes,













If you notice the FC SPL they are qqquite band limited only from 100Hz till around 7KHz. But there is where you should stop further analysis and just listen. They sound a bit band limited at the bottom octave which i believe is more due to the OB frame and structure than the driver themselves. But i dont have the option to modify the baffle unless i build something new. At the top-end they dont sound restricted and sounds more extended till around 15KHz easily as i did not miss any high frequency sound or harmonics content in any of the music i listened.

If you are a music lover you will just add a sub and enjoy this setup. But if you are a measuring freak like me you will wonder why they measure band limited but sound quite extended. I then checked distortion and found it to be less than 0.2% from 300Hz till 7KHz. At 100Hz it rose to around 3% which was more 2nd harmonics. I now need to research if the low distortion of this driver makes up for the extended listening response? Also if you notice the cone breakup after the roll-off is very minimal compared to any midrange driver you might have seen. The satori midrange driver which i recently used were pathetic in this area and you need heavy filters to keep the cone resonance down.

I am now currently using them along with my H-frame subwoofer (Bolton vintage woofer) and am still analyzing how they perform on other genres of music. Till now, i did not notice any need for tweeters - this could also be because my ears are band limited to anything above 13KHz.

Its a popular belief that mics can measure what the ears cant't listen. The FC drivers now make me think the opposite - the mic CANNOT measure what the ears can listen. The mic cannot measure timber, harmonics, liquid mids, holographic sound stage, bass tightness, transient response etc. JA of stereophile magazine believes that none of the subjective parameters have any corresponding objective correlation. How true.

 

[shafic]

Its a popular belief that mics can measure what the ears cant't listen. The FC drivers now make me think the opposite - the mic CANNOT measure what the ears can listen. The mic cannot measure timber, harmonics, liquid mids, holographic sound stage, bass tightness, transient response etc. JA of stereophile magazine believes that none of the subjective parameters have any corresponding objective correlation. How true.

 

[Hari Iyer]

Echos my own experience with some rare vintage FRs I have listened to and experienced. Yours is one level up journey. Itches me to try one.

For others, why FC drivers ? Here:

Field Coils Are The Future!!!

Anyone who's been an audiophile for a long time knows the irony in the title of this blog. Nonetheless, I believe it's true. But, what is a field coil? Why do they sound so damn good, and why the hell aren't there more speaker manufacturers using them?I've been obsessed. Obsessed with my first...

www.tweekgeek.com

They are rare to find at low price, try your luck if you can land on some good deal. They will up your quest for FR drivers.

 

[jls001]

- the mic CANNOT measure what the ears can listen.

Amen to that!

 

[spirovious]

Its a popular belief that mics can measure what the ears cant't listen. The FC drivers now make me think the opposite - the mic CANNOT measure what the ears can listen. The mic cannot measure timber, harmonics, liquid mids, holographic sound stage, bass tightness, transient response etc. JA of stereophile magazine believes that none of the subjective parameters have any corresponding objective correlation. How true.

That what many FMs have been writing here, the end result I.e. the sound what they hear are the real parameters to judge a system.Any technically better system (speaker,amps) can sound poor in reality.Factors can be many like cables, room etc.
Good that you discovered it.
Cheers.

 

[Hari Iyer]

That what many FMs have been writing here, the end result I.e. the sound what they hear are the real parameters to judge a system.Any technically better system (speaker,amps) can sound poor in reality.Factors can be many like cables, room etc.
Good that you discovered it.
Cheers.

I can't say fully that I have discovered that, but with FC drivers it looks so. But again in my recent 3 way TL speakers without measurements nothing can be build. So to say that discard all measurements and only listen and tune will also be wrong imo. Also this discovery about the FC being band limited was possible only by measuring them, or else I would be under illusion that they are true full-range

Most speakers beyond 100Hz and 10 kHz has more harmonic content than real music and these frequency are porne to distortion and fatigue. Maybe because the FC drivers are very low in distortion my ears are tolerating them well beyond its bandwidth. It will be even more interesting to discover if our ears also work similar to our eyes. For instance, our pupil gets dilated and automatically shrink in presence of bright light. Do the ears also try to block itself from similar distorted sound. If yes, then I can understand the extended frequency response and all our quest will be to purchase drivers with extreme low distortion.

Below link of voxativ horn speakers

Voxativ Ampeggio loudspeaker Measurements

Sidebar 3: Measurements

www.stereophile.com

These speakers are also band limited from 100 Hz to 9 kHz and retail 30k$+

Past 3 days I operated the FC drivers at 4.9 watts dissipation which is around 81% of its max capacity. This was done so that I can burn in the coils ( first 30 minutes of usage in day 1 gave me enamel burning smell). Now that the coils have settled it's time for some adventure. I shall operate them today first at 33% capacity ( 15 watts bulb) and 60% capacity (20 watt bulb). This will reduce the flux density to reasonable level I think. Maybe the extended highs which I am listening will also be tamed a bit. After that I can consider using a tweeter for the upper octaves. Shall report outcome later.

After a break of almost 3 months, this week i had started working on my own OB speaker. In the meanwhile i had tested the OB earlier with a Bolton cone tweeter which was with me past 30 years. Though they sounded good, they were low on level and could not match the field coils. In the meanwhile i had procured a Isophon Telefunken cone tweeter from FM @yogibear and tried paring them with the field coils. The Isophon had a dip (or rather a notch) in its response at 5KHz on both the drivers. I think it was design like this as both the drivers exhibit this notch. So integrating with the field coil had its own challenge and you will have to live with the deep notch at 5Khz (though on listening i could not identify this notch)

Initially i had added just a 4.7uF CTR capacitor and a 7.2 ohm resistor for level matching the tweeter with the field coil and listened for a day or two which was quite good. I later thought of using a boutique capacitor to replace the CTR. Mundorf , Jantez and Audyn came to mind. I was over-whelmed by reading this article about how using different capacitors can influence the overall sound signature of your setup. Some of these capacitors cost a fortune and could be more than the price of my drivers which drove me crazy.

Humble Homemade Hifi - Cap Test

Humble Homemade Hifi - Very high quality loudspeaker kits, components, upgrades, modifications and custom solutions.

www.humblehomemadehifi.com

After reading the article fully i thought if capacitor is going to influence so much on the overall sound they what could be done. I had earlier come across an article by 'Acoustic Reality' about using capacitor-less crossovers using a series topology. I then simulated some designs and came up with just a single inductor for both the tweeter and woofer and am using this since yesterday. Agreed this is not a flat response or rather a well measured response (so to speak) but my ears are liking what i am listening and the sound is not compressed (rather its quite open). After listening this for a day i now agree with the above link that using different brand and type of capacitors can influence sound as i have used none and i am finding it way better than using one. Here is the link of the AR article for DIYers to try with benefit.



And here is what i have implemented,

 

[Kamal]

I have only listened to speakers with permanent magnets till date and have very briefly listened to Alnico magnets speakers at FM @shafic and @prem JBL speakers that use an Alnico midrange driver. There use to be one more type of speakers in the early 1900 and continued for almost 50 years till Alnico magnets were discovered. These speakers used a field coil which were powered by DC voltage to generate flux density when powered on. The flux density can be modifed by the amount of current flowing through the field coil and can be made mid-band or broad band by varying the gauss/Tesla. I was looking for these type of speakers since long - may be around 3 to 4 years but all of them were out of my budget - because they are rare to find and hence quite boutique in that sense.

Suddenly during a lazy Saturday search the one i bought surfaced and i ordered them immediately as they were within my reach budget wise. Of-course they are not a Western Electric or RCA but a pair of 8" field coil speakers from a radio maker in the early 1930s named "Sachsenwerk" which were quite popular for their "Olympia" brand of radios. My pairs are actually salvaged from one of the radios which were lying in stock unused for too long in a antique shop. The paper cones looked quite new and unused and there were no physical damage to the cone or the frame for a driver that old but had some age related paint scarrying - that too not noticeable.

I listened to some youtube videos from this radio and some OB speakers mad out of them and liked what i heard. The mids were quite liquid and heavy and the tone is what i was looking for - very open and no compression of sound. Field coils have their reputation to have extreme low distortion in their pass band and would be to some extend band limited based on the driver size and the magnetic field strength. I have a plan for the DC supply for the field coil suggested by the seller which i shall implement while i am waiting for the drivers to arrive. Its for me to discover whether i will require a sub-woofer + tweeter to support the extreme ends but i shall be open for them too based on my initial listen and setups. Pasting some YT videos of some radios from where this driver would have been salvaged for me, enjoy!

Thanks for looking.

Thanks for listen

Fascinating!
Never heard of such drivers before, thanks for sharing.
Thanks also for the trip down nostalgia lane... old fashioned radio with the "magic eye" tuning indicator.
Would love to read what you do with your pair of drivers.