altec 806

Somebody from AA was asking about electromagnet. The subject pup up here from time to time but this time Bruce Edgar uploaded an interesting comment. Bruce is not a typical AA Moron and if he says “inaccurate things” then it is not due to ignorance but rather because of business interests. Well, he is with his constitutional rights to say whatever he wish but I think  some commentary about his deposition need to be made:

http://www.audioasylum.com/forums/hug/messages/15/156766.html

Bruce Edgar wrote:

The first experience was that FC drivers out trump many horn problems. In my Subwoofer horn, the bass sometimes was lacking any response below 35, depending whether you had a wood or concrete floor or lacked the proper boundary setup. My living room wood floor was a real problem for me compared to the concrete floor in my sound rooms at my industrial site. But when the 18" FC driver was installed instead of the JBL 2241 ferrite driver that I formally used, the bass low end went all the way to 20-25 Hz instead of wimping out at 35-40Hz with the ferrite driver. The low end of my midrange horns was also improved with FC compression divers.

Ok, let not to be overly excited here. JBL 2241 is a driver with primary resonance of 35Hz.  Bruce’s Subwoofer horn is 1/8 hyperbolic 35Hz horn(it might get lower in a given room by the fact the it loaded to boundary). Loaded to the horn the 2241 probably drops to mid 20Ha and Bruce driver it back to 35Hz with back chamber. I do not argue that implementation of electromagnet might drop the resonance frequency. Warn you that the resonance frequency change NOT because of use electromagnet but become they built the driver differently. I did make the experiment and I can testify that with the same driver (no other modifications) and the identical amount of flux the resonance frequency was slightly HIGHER with electromagnet. To have identical Fs with Alnico and electromagnet I was forces to drive my filled coil with lower current that created lower flux in the gap, lower then permanent magnet did. I think Bruce does disservice to true explaining the drop of Fs by use of different magnet type. If the primary resonance was dropped then it happen because other reasons, not because the contribution of electromagnet. The most important part: the Bruce’s Subwoofer horn is 35Hz horn, how the drop of Fs was able to benefit it if the month high pass 35Hz. Sorry, Bruce, what you are reporting violates whatever you taught us and what I experienced. I do insist that driving over any horn the frequency that is lower then what a horn can pass unavoidably worsen Sound, making it boomy, homogenized and mudding the entire horn articulation. That is why I ALWAYS high-pass any of my horns just below the horn rate, always.

Bruce Edgar wrote:

The explanation for the differences in low end response characteristics is that horn low end response is limited by horn mouth size, flare frequency, horn flare type (i.e. hyperbolic, exponential, conical or tractrix) and reflections. Given that your design is designed well, reflections are the big restriction. Mouth or floor reflections can come back to the cone and try to push the cone back in spite of the input audio trying to push the cone forward, resulting in VC interactions and cancelations. The FC speaker has better control of the VC in the gap because of the stiff power supply and thus can resist the push back of refected waves.

Sorry, I do not buy the whole reflections idea in THIS case. The Bruce’s Subwoofer horn is floor firing design with very low clearing from the floor. It has of cause huge acoustic reflection back to the driver but this reflection is permanent and this reflection has been factored into the sizing of back chamber. This type of horn does not pick new room reflections and if it does then it happen to a degree that is negligible to mass of diagrams of a bass driver. This is why Bruce’s Subwoofer horn does not need the back chamber tuning in respect to a different room – the clearing between the mouth and floor is permanent and the reflectors are permanent.

Bruce Edgar wrote:

Steve Schell gave me the genesis of the following explanation. A ferrite or alnico speaker magnet can be thought of a storage battery of magnetic energy. When a battery reaches the limit of its current capacity, it just gives up. In the case of a FC, a stiff DC power supply will look like an infinite capacity storage battery, i.e. a flashlight cell compared to a large marine storage battery. In the case of FC magnets the magnetic field in the gap is always in control of the VC and thus reflections can not cancel out the low end response.

Hmmmm. It is not a secret that Steve Schell loves electromagnets but his love of it shall not blur the reality. There is no “infinite capacity” and there is no “current capacity”. There is no continuing discharge in magnets of loudspeaker driver. The only sagging of magnetizing force is possible due to the flux modulation from voice coil. If voice coil cares a high current transient then it creates a transient magnetic field that sags the flux created by magnet. So, the claim is that electromagnet while creating the same magnetic force is able to buffer the flux modulation better. Possible BUT there are a few problems with it. First: not one ever published data of flux modulation in the SAME driver with perm and electro magnets. It is not hard to do and if I am manufacture who would like people to buy my electromagnet driver then by measuring the flux modulation differences I would objectively closed the subject. Did you wonder why no one did it? By presumption is that because there is no difference. In horns we use compression driver, even the bass drivers we use as they are compression drivers. Compression drivers do not take a lot of current and they are driver very gently. That 18” driver in the Bruce’s Subwoofer horn practically does not move and has in operational mode a few mm of excursion, it take very little current from amp. If Bruce make high excursion bass speakers for stadiums with 1000W power handling and 30Hz response, and particularly ported enclosures then of cause the flux modulations would be a huge subject but I do not think that we are near to that. I am sure there are plenty of Bruce subwoofer horn users who drive then with 2A3 or 300B, come of what king current they drive across that voice coil? Second: are you sure that the polls for electromagnet and perm magnet are the same sixe and the same material? It highly unlikely that they are the same as with electromagnet there is a need to dissipate heat and therefore the core under the field coil is always much larger. So we basically are talking not about the electro vs. perm magnet about about the driver with different magnetic stricture. If you in Alnico driver change the type of Alnico magnet then it changes sound. So, in electromagnet you very much change the magnetic stricture by incorporation a core from different material.

Anyhow,  I can go on and on. I do not insist that flux modulation is not the subject in home use horns BUT if someone repost that change the bass driver from perm to electromagnet did make ”improvement” and if this ”improvement” is not imaginary (which is the case most of the time) then it happened because it was factually different driver, not because it used electromagnet. I am not against better driver or against electromagnets. I am against the methodological dishonesty.

Bruce Edgar wrote:

You could improve the performance of regular loudspeakers with additional magnets, but manufacturers generally won't do it because magnets cost money and add to material costs. But I have observed exceptions. I have tried the early JBL Neo compression driver (I don't remember the model number) which used a stack of Ne0 slugs in its design. I didn't notice any difference in performance between it and my favorite 2441 driver. Several horn customers wanted to use their new JBL small "hockey puck) Neo compression drivers. I was able compare them to my standard 2441 driver in listening tests. The 2441 was clearly better. Part of the difference may be due to the minimal Neo magnet (which actually measured better than the 2441 in response tests).

Well, if anybody compares Altec 806 and 808 then they have seen how “could improve the performance of regular loudspeakers with additional magnets”.  I do not believe that by adding more magnet performance might be improved. A properly designed driver use as much magnets as necessary and it’s suspension, cone damping and many other parameters are based upon the given amount of magnetic force.

decoud wrote:

If a magnetic field is static, who cares how it is created? Isn't the main potential advantage of electromagnets one that no one has explored at all - the possibility of modulating the field strength in direct response to the audio signal? So, one extracts some parameter of the signal in parallel - say instantaneous frequency power - and adjusts the field strength on-line.

Too lazy to research it and, rather than mentally invent new technology, I keep thinking, "cutter heads", like the old, analog "lathes" that cut the metal masters.  This has to take some real force, perhaps analagous to a speaker coil, and I dimly recall that the cutter is not allowed to "fly off the handle" but it is somehow "controlled", coming and going, and according to signal strength, during the process.  I would not be surprised if someone reading here is prepared to illuminate this matter.  Or, someone who makes the rounds might put it to someone like Steve Hoffman and then get back, post a link, or...

Paul S