My first impressions a about the “differences” of field-coiled derived form the time when I ran the Telefunkens war time drivers but at that time I had no reference how the same driver would act with permanent magnet. Also, the Telefunkens are the regular MF 8” drivers - the compression driver is totally different story. The only more or less illustrative indication about the compression field-coil driver I got from listening the Klangfilm Bionor with permanent manger vs. the set of it’s own electromagnet drivers. Still, since I do not like the results I heard from any of Bionors it was hard to say anything defiantly then.

This morning I spent another few hors this morning with field-coiled S2.  The entire speaker sounds horrible and the S2 field-coil sounds “strange” and “different”. I did have a chance to reconcile a little my feeling about it.

I have a constant sensation that the field-coiled S2’s sound remarkably reminds me the sound or OTL amplifiers. It has the same ability to super precise to follow the very minute dynamic fluctuation of signal and superb transient but those phenomenal qualities are not connected with anything else, primary it has no correct harmonic backup for the sonic events.  The instantaneous feeling of those “absentminded sounds” is very impressive and many people ignorantly fall for it  ythe long run or generally correct sound.

I sense that the field-coil, at least the one that I made, has a lot of common with OTL sound. It is very clean, it is electrostatic-like free from grain, it is very softly and very naturally enters the sonic stresses but it always, I mean always, always, always too thin, meaning it has no acoustic-like harmonic stricture. As the result it sound remarkably sonically imprecise and insultingly musically hollow and worthless.

I do like SOME deferential effects of the field-coil has but the biggest revelation I got this morning when I un-screw the S2 field-coil and put back the S2 with it’s default permanent magnet. 

Well #1:  the perfectly tonally and harmonically balanced sound went back to its normal state and the horrible honk that I got using the electromagnet completely despairs.

Well #2: to my big surprise the softness that very much impressed me with field-coil did not become an anti- softness with permanent magnet. The regular S2 did hold it’s own. It did not demonstrate “hardness”. Instead the S2 demonstrate it’s ability to be “stressed” to deeper level of impassiveness then the S2 field-coil. I find that S2 field-coil can not be aggressive or bold if the music calls for aggressiveness and the S2 field-coil mellows down the stress of impressionism aka act like electrostatics. Why the Mravinsky orchestra’s strings sound like Vienna Philharmonic and why Mravinsky orchestra woodwinds sound like Prague Philharmonic? Since then Russian brass sounds elastic and fertile but not nasal and bruttal?

Well #3: Harmonics, harmonics, harmonics … here is where all music lives. The very first note from the S2 permanent magnet immediately set the things straight.  Using adjectives I would characterize is as “rich” vs. “thin”. The S2 permanent magnet has large tone with beautifully-connected notes.   Each note is not juts a note itself but has a referenced via harmonics to what was before and after, It has sot of harmonic ambiance noise that dose slightly masks the appearance of the fundamental pitchers but without that harmonic noise the pitchers sound too contrived and artificial.  The S2 field-coil is less loaded with ambiance and the core note are more naked. Also, the tone is less grained and more hygienic.  It is very positive for sake of observation and dissection but it is slightly destructive as it makes listening less fluent but more intellectual.

Are the harmonics or distortions? I do not know. Is it possible to make the S2 field-coil to sound harmonically correct by injecting a long tail of upper bass channel. Possible.  Is it possible that using larger magnet the harmonics pattern of the field-coiled driver might be changed? Possible.

I do not know where I go from here, as well that I do not know how final my observations are. So, far I see a LOT of problems with field-coil and what I learned make me do not take seriously the experience of other folks who try to make the field-coils. For instance they are some Japanese, German and French speaker makers who try to make two channels field-coil speakers. Now I know that it is foolishness. The field-coils can not work well across a wide range and should operare only within a limited amount of octaves. The higher frequency is the more flux the driver want and the less frequency is the medium or low flux sound much more useful.

So, what will I end up using? Hm… I do not know yet.  The S2 field-coil “as is” is not correct direction and to get the most out of the concept the entire magnetic structure of the driver should be rebuilt to take care bout the heat dissipation. So far the conversions cost me ~$700. I have an idea how the S2 driver might be changed to run off the field coil. I would estimate that I would cost around $2500K. Then I need to change the entire Macondo configuration trying to make the field coil to sound harmonically correct.  I doubt that I would go for it, as I frankly speaking have no needs. I would like to inject into the regular S2 some softness but I am not wiling to change anything else in the sound of this driver. So far, at the level in whish I did it, a field-coil magnet does not sound to me like a useable solution to me. I will give more time to the field-coiled over the next few weeks but so far you have heard my preliminary observations…

cv wrote:

I presume this is with the new production diaphragms? Just wondering if the old ones might restore some of the youthfullness which was diminshed by the field coil?

I gave another run to Vitavox S2 with electromagnet and while I was listening I come across to an interesting observation. I screw the S3 in 330Hz horn and was trying to run it with 500Hz crossover point. It is pretty standard configuration, the way in which Altec, JBL, Vitavox and many others. This setting never produces good results (due to multiple reasons) but I did not search for result but was interested how the S2 with electromagnet would act.

It was kind of predicable. At 3 amperes (at full blow) the electromagnet did OK above 3kHz region and at 1.2A it sounded Ok around 500-700Hz. (Remind you that the electromagnet produces at 2.4A the same flux in the gap as the default Alnico magnet does). However, what was the most fascinating to observe is HOW the S2 with electromagnet sounded at it lower knee.

When I heard the bass of the S2 with electromagnet I suddenly realized that I heard something that remotely resemble the Lowther driver. Let me to explain. Let forget that people out there are using the Lowthers with stupid back-loaded horns and to lets to abstract out the sad contribution of the back-loaded concept (particularly for this driver). So, if disregard the back-horn sound and to pay attention at the lowers Lowther region (200-500) then it is very annoying that Lowther upper bass never free, never has that “fog sound” and always insultingly dry. I always attributed it to always too low crossover point that usually used with Lowther but now, after the play with electromagnet, I have another opinion.

I feel that Lowther is juts have too strong magnetic field for proper bass reproduction. The Lowther people scream that this driver have huge magnet, huge amount of BL, huge flux, super small gap, low excursion and so on…. This is all fine and it does allows to make a very effective magnetic saturation in the gap but it …. horribly affects the lower knee performance of this driver. The Lowther just has too strong and too stiff magnet system that does not allow the driver to play soft…. Apparently it DOES exist a reasonable balance of the flux density projected to the rest qualities of the driver (cone, suspension and so on…) The Lowther is obviously is on the HF side and should not used for any bass. Whatever it does below 800 is very inappropriate… Who knows… If the Lowther would have somewhere around 1.6T in the gap then it might have some lower midrange and perhaps it’s upper mid would not be so barbarianly-resolute.

Rgs,
Romy the caT

cv wrote:

But the real reason for my post is to ask: I was thinking about the results you reported and am really scratching my head as to why the FC S2 should sound honky, given that its acoustic geometry is completely unchanged. Any ideas as to why??

Yes, Chris, this is the question that I also ask myself and I have no explanation. The acoustic geometry was not changed, the crossover and response was not changed. What was the problem? I would in a way understand it if the S2 with electromagnet would give up more harmonics (primary second) then it might “overwhelm the crossover point”. (Now it is one octave from the horn rate but with “fatter” driver it might need to go further). However, the electromagnet gave up opposite result and I would characterize its sound as harmonically “thinner”. I really have no explanation of this phenomenon. Also, it would be worth to mention that honk generally more inclined to show up in mono horn installations then properly phase-alighted stereo installations. I do not know why but stereo does minimized honk. When I play with the field-coil I did run juts one channel. From another prospective my single channel with permanent magnet has no honk qualities and the same channel with permanent magnet has no honk as well. Go figure!

cv wrote:

I don't suppose the 330 horn was any different in this respect? It's the 4 sector vitavox right?

Yes, it was.

Here is the similar approach undertaken with Altec 288C to the one that I took with my Vitavox

http://chaudioroom.com/CH288287.html

Some critiques. A minor mistake in the text is that the author says that “magnetic fields generated will go down to zero when the power to the field coil is removed”. If it not as the core still get some residual magnetism but it is not so important. I would not also agree that it is possible to moderate sensitivity of the driver with change of electromagnet magnetizing force. The biggest question I would have about the heat dissipation design. They look like did a good job to have a magnetizing coil’s filler to propagate the heat from the coil to the outer wall of the driver. I do not know if it is sufficient as the coil’s filler does not touches the wall of the driver – this is VERY big mistake in my view.  I would like to see the temperature of this thing after 2 hours running at 2A. The core mass is also seems a little too small and I would like to see another coil at the outskirt of the coil’s filler, magnetizing the return pass. That would force to turn a new driver’s can though….

So, I generally feel that such a driver might be a good prototype to learn about a character of field-coil (the same as I did with my S2) but it is not a final solution.

The good part is that the author makes a sonic claim that is not as idiotic and the claims of other who play with field-coils (“more flux” etc…). The author says:

“A permanent magnet driver sounds somehow louder due to the high pitch distortions, the field-coil driver will sound slightly lower due to lack of distortions and with the most natural and sweetest sound.”

I would agree that field-coil is in a softer side but I am not convinced that it is due to “lack of distortions”. I do not say that the guy is wrong I just do not feel that what author said exhausts all differences between the field-coil and perm magnet.  There is much more than just that. I might understand how he deals with the fact that field-coil less loud (perceptual) but how he deals with field-coil lost of transient characteristics? The guys do not talk about it at all.

Here is how I see the game: he needs to load the amp that drives the electromagnet driver much idler.  This would give some transients back at lover distortion but it would also eat output. The increase of current in field-coil with attempt to gain more sensitivity would not work as it will screw the bottom knee of the driver’s response, not to mention the alleged heat buildup in the field-coil. So, as I see it we have a need to have a dedicated amp with higher gain to drive electromagnet driver. It might work and it might not as we will essentially send more voltage to the voice coil…

I have no judgment how it all work all together but the simple generalizing statement that the “permanent magnet have high pitch distortions” does not address the whole picture in my view.

Listening the argument of the field-coil’s people I note that they constantly stress the demagnetization subject of the driver’s magnet, advocating that with electromagnet “the field coil will always produce the same strength now and 100 years in the future.” It is true but let to see how important it is. The myth of demagnetization was born from the fact that all compression drivers derive from professional sound application where max sound pressure is the key  and where the channel are driven by powerful, mostly SS, mostly PP direct-coupled amplification. Yes, in those cases the Alnico magnets are vulnerable for demagnetization and you might find a lot of used partially demagnetized compression driver from movie theaters and sound reinforcement fields. However, how “dangers” the demagnetization fear in the really of home use.

First of all the contemporary rare-earth magnets do not care (care much-much less) about demagnetization. Even of the old Alnico-based magnets are use then it is also I feel is not big deal. The home used compression drivers do not exposed to heat or to shaking – something that demagnetizes drivers very aggressively in pro outside use. The home compression driven by low power SET amps with output transformer- so, there is not DC component in there and the voltages are very low. My Melquiades amp outputs to Vitavox under 3V and 110dB sensitive S2 screams at 3V like wounded in ass hippopotamus. The normal operation of my driver is at 0.4-0.9V with this voltage and with no LF submitted to driver is it imposable to demagnetize it. Even if the demagnetization does take place then to recharge a driver costs $20 and you do it just one what you buys the driver (as it might come from unknown source and it might be used by some cretin who drives his playback with 500W Crest amp.

There is another subject why I personally do not feel threaten by demagnetization and do not feel that is it a serious argument. There are two types of compression drivers: high flux and medium flux. Some drivers run 2.2T-2.4T in gap and have cone’s suspension to operate the in the environment where the diaphragm is over damped by flux force.  I do not use those drivers and I prefer the moderately flux drivers (1.5T-1.8 T in gap) as I feel that they have higher compliance of diaphragm and the diaphragm more hams by suspension instead of the intrinsic stiffness of the flux. So, those moderately flux drivers get demagnetizes much slower (of the same magnet was used). In fact it is very difficult to fully magnetize the 2.4T driver as when the magnetizing machine of off then the driver is losing right the way around 20% of charge. With 1.7T in gap (Vitavox, Altec, JBL etc…) there is a room to step back in flux density…

Anyhow, I do not feel that demagnetization is a factor that needs to be considered. In fact I do not see any utilization advantages of the field-coil beside that the driver become much simpler and easy to assemble. If electromagnets have any sensible advantage then it should be in the realm of sonic results. However, so far I do not see anybody (beside me?) trying to talk about the actual SOUND of the electromagnets.

be wrote:

I just spoke with a guy who have made both permanent magnets and electro magnets for loudspeakers. He said that the reason that electro magnetic speakers have a smoother and more grain free sound, is due to the temperature rise in the mechanical parts of the magnet, this sligtly changes the mechanical resonance characteristics of the magnet. He even experimented with heating up a permanent magnet of a loudspeaker to the working temperature of the fieldcoil version and was thereby able to emulate the benefits of the latter. So gentlemen, put a lit candle under your compression drivers.

 Eventually the people not just blobbing the imaginary idiocy about electromagnets but taking business and demonstrate sane thinking. Vey very good! If we presume that he is right (and it is very much might be so) then the following thoughts might derive from it:

1)    Is it the “slightly changes the mechanical resonance characteristics of the magnet” or rather it is softening the suspension of the cone?

2)    It would be interesting to measure the primary resonance of the cold electromagnet driver and “hot” one.

3)    The “heating up a permanent magnet of a loudspeaker” does simulate the working temperature of the field coil but it also demagnetizes permanent magnet. The different magnets react differently but still: was it softening sound due to temperature or due to redaction of flux in perm driver?

4)    How your friend (who is he, can you bring him in?) compensates the softening of sound with warm electromagnets? Less loading?

5)    If your friend is correct then I presume that the different cores of the electromagnet might react differently to temperature change.

6)    If your friend is correct and flux acts differently in different temperature then can we use this effect by putting output transformers into a soft heating chamber and impact sound this way?

Hi.

1)  He was very certain that it was not a result of a softening of the suspension or the speech coil for that matter.
2)  Probably it it is more significant to look at the harmonic structure and decay of the vibrational spectrum of the magnet when excited.
3)  It was definetly not a matter of weakening of the field, the temperatures used where way to low to have any effect on this.
4)  He has not pursued electromagnets since they have sonic ill effects as well and the small advantage of grain freeness over permanent magnets can be elliminated with the said heating anyway.
"He" is Filip Keller / AER
5)  Probably
6)  I am shure of that, you could just mount a power resistor on transformer core and put some current in it.

Regards
be

Somebody at AA asked about the difference between electromagnets and perm magnet. Thankfully the AA idiots who pick the pieces of brain at my site and already learned that the idiotic blabbering about “the electromagnets have more flux” is completely out of picture. Now they come up with more refines explanations. Still, I feel that it is off the mark.

Somebody Al Sekela posted a reply that I found worth to comment upon:

http://www.audioasylum.com/forums/hug/messages/14/143034.html

 I composed a reply, that of was deleted from AA, but I did not truly intend to post it in the AA sewers to rather to advanced the concept at my site. Her is the deleted post:

***************************************

I am glad that after the years of foolish drooling and proclaiming faulty idiotic statements the AA people gained some more or less common sense and look at the electromagnets more rational then just brainless glorification. Still, there is a lot of wrong in what people sell to themselves about electromagnets. In what Al Sekela said I see some inconsistencies that I would like to comment upon. I am glad that you do not sound as a Moronic industry whore, (there are some voices in this thread that do), so I would extend to you some credibility and integrity.

Al Sekela wrote:

Permanent magnets are not ideal. When used for speaker motors, their magnetic fields may respond to the fields created by the signal currents in the voice coils. This response may have some granularity, depending on the magnetic domain properties of the motor. The result is a subtle distortion that increases in fraction of the signal as the signal decreases in level. This kind of distortion gives a mechanical character to the sound, especially in well-recorded natural music, where the decay into silence has musical significance. This kind of distortion limits the resolving ability of the speaker.

To the problems flux modulations the permanent magnets driver and electromagnets are exposed equally, In fact if you look more carefully then you might observe that electromagnets sink flux faster then permanent magnets of the same magnetic density.  So, if you explain the less granular decay by electromagnet driver (and I partially agree with you) then it might not be explained by “fields created by the signal currents in the voice coils”.  It shall be something else. Also, let have some perspective about it. If we deal with Cogent-like MF drivers that are 500Hz and up and if it has 110dB sensitively then to develop let say 90dB it needed 50mV. If the driver runs at 1.8T then the 50mV in the voice coil generate the field that is absolutely negligible it relation to 18.000 Ga. So, the explanation of the flux modulations and contra-modulations I feel are very inappropriate. It is not bass driver but FM.  BTW, if you did experiments with use of  bass drivers and electromagnets then you might know that  electromagnets have very questionable advantage in lower frequency.

Al Sekela wrote:

Two other areas in audio have similar problems. Push-pull output stages suffer from what is called "crossover distortion." The tube or transistor amplifier characteristics change as the signal approaches zero. A band-aid is to increase the bias, so most power amplifiers are called "Class A-B," with both sides of the push-pull topology conducting current at zero signal. A true Class B amplifier would allow each side to go into cutoff (no current) exactly at zero signal. A true Class A amplifier never cuts the output devices off, and is very inefficient. However, many audiophiles prefer this type because it avoids crossover distortion. Single-Ended Triode (SET) amplifiers avoid the problem completely because the output device does not get close to zero except at maximum signal.

And how the electromagnet driver deal with crossover distortions better then permanent magnets? It is not to mention that people who run MF with PP amps are a bit … wrong, unless it is a ceramic magnet with hard suspension (that would be totally different subject)

Al Sekela wrote:

Tube amplifier output transformers also have grain that depends on the core material. Different choices for core material give different tradeoffs in distortion verses ability to handle large bass levels, efficiency, and cost. The tradeoffs are so severe that some folks use so-called Output Transformer-Less (OTL) tube power amps, such as the Atma-Sphere amps. These amps are entirely free of the transformer grain problem and are capable of superb resolution.

Sorry, it is absolutely wrong and you might to educate youssef a bit deeper about the harmonic problems of OTL amps. BTW, do not use AtmaSphere as it is a very bad example of OTL amps. Also, there is no conflict in SET between “grain” and ability to handle large bass levels – you looks like confided about it.  Still, if to accept that what you propose is accurate then what it has to do with the subject of electromagnets drivers and their alleged advantage over perm magnet?

Al Sekela wrote:

Good reproduction requires the audio equipment to work well at the extremes of maximum and minimum signal as well as at average signal levels. Field coil speakers have an advantage over permanent magnet motor speakers at low signal levels. Whether they perform well at high levels depends on other design choices that are common with permanent magnet motor speakers.

Ok, you feel that a permanent magnet motor has an advantage at minimum signal then can you propose an explanation why you feel so and how can one verify it? I am not sure that I agree with it, I do not disagree either. I am not sure that you have a right explanation and I am not sure that it leads to a right concussion. I have mine explanation but I would hold my view until you indicate that we at least talking about the same thing.

Despite my generally-negative experience of driving Vitavox S2 from field-coil I might consider to try in the coming year the same experiment one more time, since I am driving my MF driver now by a different DHT amp. I would like to see how the DHT-neess of amp would work with electromagnet.

Well, it is not that I had too negative experience with electromagnet; I just did not find it advantageous… BTW, what I am considering to do if I move to new home is to build a small and vey rudimental magnet charger devise. It is very simple but I have an idea how to “exit” the re-charge, the idea that I would like experiment with…

be wrote:

This time also make shure that the cavities in the two versions, the alnico and the fieldcoil, have the same size and volume of cavities behind the coil in the magnet structure. In this way you make shure that it is the two magnet types you are listening to and not some irellevant cavity resonance and reverbaration chamber.

Be, I am not sure what you mean. The volume of fieldcoil and core is slightly different then the volume on perm magnet but the cavities behind the coil are not the cavities where pressure passes, therefore I do not think that any resonances or reverberation in magnetizing chamber has any affect to sonic out of the driver.

be wrote:

Maybe the easiest would be to put a fieldcoil into the alnico version also, this would also open up to some experimentations by shorting the coil or even to try the electro-alnico magnet you sugested some time back.

Yes, I had this idea but I do not think that would go into it. Above someone told about an European guy who heated up his permanent magnets and was able to simulate by this the sonic attributes of fieldcoil. In my view it was a sanest thing I heard from fieldcoil crowd. I very much would like to try it myself, but I need a way to recharge the perm magnets as with Alnico the driver would lose magnet force with heating. There is a key in all of it – I do not feel that fieldcoil has any sonic advantages (I feel it is opposite – the fieldcoil sounds inferior) and I do not feel a need to change anything from how my S2 sounds now. However, if the heating up of the magnet would make the difference as expected then it might be a good direction to use “hard” drivers with “loose” cone and “soft” magnetizing force.

be wrote:

"the cavities behind the coil are not the cavities where pressure passes" Yes, but the speach coil and diphragm can "see" the air volumes behind the magnetic gap and will be loaded by the resonanses created, and the radiated sound might be affected also. Think of a 8 inch woofer put in a 10 litre or a 100 litre closed enclosure, no question that that is audible.

"the chamber where the magnet is (would it be perm magnet of electromagnet) is absolutely decoupled from any paths where sound flows"
The situation is the same as the backside of a woofer looking into a closed enclosure, and this relationship is audible.

Also, actually in a compression driver, there will be a narow opening between the inside of the speech coil and the central polepiece in the magnetic gap thru witch the throat and the phaseplug slits are connected to the inside of the magnet!! Audible? I dont know.

"In fact some drivers has no chambers for magnets and keep the space wide open"
The question is if they would sound different if this space where closed?

I think that now that you will make some experiments it would be a pitty if your results would be determinned by something else than what you are exammining, especially since it would be so easy to avoid (by putting something into the perm magnet)  Also it could open up for some new possibillities voicing.

be

Be,

this is an interesting thinking and if you are correct and if there is an opening from front chamber into magnet assembly then the volume of the magnet chamber would certainly affect the sound of the driver. The point that I argue is that such an opening shall not exist. The whole point of a properly made compression driver shall have a very smooth and no ripples transition from the diameter of diaphragm to the diameter of the throat. Any sudden change, any step in the transition is absolutely not acceptable, not to mention an opening into another cavity. Now, saying that I do not insist that such as an opening do not exist. I was trying to recall it but it is not in my mind now. Let look at the common design of a compression driver – where do you see this opening would be?

Anyhow, if such an opening into magnetizing cavity exists then I think it would be with the driver that have non-radial phase plug, like Altec after 808, RCA and many others. The drivers with radial phase have the last layer of the plug covers the diaphragm’s output, at least it is what I think.

Be, if you look at the picture of the S2 then you will see that between the exit of the phase plug and beginning of throat there is a circular channel that violates my view of proper transition in compression driver. This channel is on the side of the throat exit and it more looks like an added volume to post-compression front chamber. The pressure from throat in the way forward toward to the exit does not pass this channel as the channel is almost behind. I never liked this channel, I wrote about it before and I meant to patch it. How it affects sound I do not know. In context of this there I think it is worth to note that this channel is a few mm deep and it does not got all the way across the poll and is not connected with the chamber where the magnet is. So, regardless what kind magnet it would be permanent or electromagnet it would not be relevant from THIS perspective. Again, I do not insist that there is no vent from sound cavities to magnet cavities but I would not like it is. If it was true then the magnet chamber size would be certainly the factor.

BTW, I read recently a feedback for one guy who invested a lot of time in the electromagnets and he has many of them (I think he is associated with Rulit). I do not particular care about his views one of his comments I find interesting. He claims that his electromagnets drivers, being driver from all warned up amp, begin to sound in the way he likes only after 1 hour of operation. This would perfectly comply with the idea of the electromagnets to sound different due to heated magnet structure. So, hypothetically, if we a medium magnet that do not loose magnetic force with temperature and if you wind a heating coil over it with a thermostat, then we would get the same effect. Now, the bigger question would be if the effect is positive in any way. One of the thing that I would like to repeat with my electromagnet is to use non-plastic cones. I had worse sound with electromagnet but was it due to the fact that plastic suspension of the S2 diaphragm got warmed up?

I was under impression that neodymium is more resistant against temperature, mechanical shock and flux modulation, it turned out that I was wrong about temperature:

http://www.intemag.com/magnetic_properties.html

It turn out that Alnico is the best material to heat it up. I just need to have my own magnetizing devise and I would try it.