One last thing, I just noticed there is a guy converting Altec permanent magnet drivers to field coils on ebay for big bucks. Here:
Permanent Magnet to Field Coil Conversion Altec 288 B C - eBay (item 290412361082 end time Apr-10-10 06:16:13 PDT)
Better start winding those coils
This guy does this as well, and also makes his own drivers:
Field-Coil Speaker(s) | Kilimanjaro-Series | Wolf von Langa
So most of the sonic improvements of which we read, especially the improved resolution, are attributed to the integrity and uniformity of the magnetic field?
And hot they can get... i designed an aperiodic mid box for a Feastrex FC and it got so hot the bezel on the outside of the box was hot enuff to be uncomforatble to hold your finger too.
dave
I've been reading this blog for a bit now.
If they display it at the Lonestar Audiofest in a month and a half I'll likely be there to hear it.. bearing in mind that its a prototype by an extreme perfectionist (..Dave).
It will be interesting to see if Mark can get production to a reasonable price-point.
I honestly don't know.. it's a theory.
It would also be really tough to properly compare UNLESS you had identical drivers (excepting the magnets). (..the voltage would have to result in identical T/S param.s as well.)
Without the phase-plug and venting to the front of the driver it might not work well (..at all).
It's something I'm concerned with when I look at Dave's Mark-Audio modification. ..i.e. no front venting on the Alpair 12, and IF it's a box (..particularly closed or aperiodic) then heat is almost certainly going to be a problem for the VC.
All Daves are..
..and yup, I'm sure Mark is as well.. what I'm not so sure is at what price and how long before customers can have final product in-hand. It may be unfeasible (..perhaps requiring a different driver rather than a motor alteration).
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I honestly don't know.. it's a theory.
It would also be really tough to properly compare UNLESS you had identical drivers (excepting the magnets). (..the voltage would have to result in identical T/S param.s as well.)
hmm that sounds like a doable and worthwhile experiment, a pair of already converted drivers (new motors only) and a pair of originals, with colliberated TS Params, in similar enclosures. Any takers?
I bought a pair of his 15" JBL A150WVL bass drivers to work under my Supravox 215s. He says that they are based on the JBL 150-4C, but they use the (genuine) k145 cones. I have to say that that they are as good as I could have hoped for and very reasonably priced.
Classic Audio Reproductions in the States make a similar driver which is double the money, and does not look to be as well engineered.
If I had not just spent so much money on my current project, I would shell out for a pair of his compression drivers and Altec Basses to make some horns.
So could you explain what happens when you use less than optimal current to get higher Qes? Do you know how your pole pieces behave under that particular condition??
Sorry, but it is not disadvantage. Increasing temperature of the magnetic structure (to some degree) always has some positive sonic effects.
Take a look at this topic GoodSoundClub - Romy the Cat's Audio Site
That's because Feastrex use very mistaken concepts regarding FCs- low voltage and high currents...
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Both my field coild drivers: the Wolf Von Langa's and the Supravox's only get warm to the touch!
Hi,
< 45deg.
Ambient temp is 20 to 30 deg.
Don't understand what you mean by the deltaT casing-ambient question.
deltaT is the temperature difference, in this question from casing to ambient.
for Ta=25degC and Tcasing =40degC, deltaT=15Cdegrees. Note that 15degC is very different from 15C degrees. 15degC is 15Cdegrees above zero degC (melting point of water at NTP) 15C degrees is any temperature difference between two bodies.
As said earlier the inside of a sealed box could have a high ambient temperature.
The combined power lost in Voice coil + magnetic coil could raise the ambient to ? How high?
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This should be no more of an issue than with say an inefficient subwoofer running at relatively high spls for a couple of hours at a time in a sealed box. (In many cases a mediocre quality transistorized class AB or class B amplifier is included and also partially heats the inside of the box.) I have worked with powered speakers systems where the woofer magnets were so hot after a couple of hours of operation that you could not touch them safely > 65 degrees C IIRC, (not rise over ambient) and these units were not anywhere close to failing. (Time spent at a nameless mid-fi company)
In the case of a lot of FC the woofers are commonly used in vented alignments (reflex, Onken [large vent reflex]) or horns so they do get some limited cooling that way. Most FC horns I have seen are mounted in the open and are more than adequately cooled. I suspect there are at least a few FC drivers out there with high enough QTS to be suitable for OB where cooling would be a none issue.
Incidentally there are a fair number of vintage RCA and WE FC drivers that are 70 - 80yrs old and are still original. The typical low voltage RCA FC whether woofer or HF horn is only dissipating about 17W and the magnet structure is sufficiently large that even after many hours of operation they are barely warm.
Many of these drivers are also extremely efficient so the average power dissipated in the VC will be relatively low - and this is generally the component in any driver that has the biggest problem dissipating heat.
I have no experience with any of the newer FC drivers like the Feastrex, but even if it runs somewhat warm I would doubt that would constitute a serious reliability issue.
And they do sound good.. IMHO and YMMV
Just my
worth...
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Tom Danley's Labhorn had two aluminium plate access panels. They were dissipators for the horn even though it approached 50% efficiency.
2000W amp into 3ohm with average signal @ <=-10dB will dissipate <100W in the Voice Coil.
Now take a 500W amp driving a 500W speaker that is 10% efficient and average power -10dB.
The average input power is 50W, the Voice Coil dissipates 45W.
If you seal the box and then add on another 10W to 50W of magnetic circuit energy dissipation the speaker ambient could be extremely high.
For us normal listeners this SPL will never be required and temp is not a problem, but that does not mean we always ignore the dissipation and ambient temperatures. Sometimes they can become a problem.
2000W amp into 3ohm with average signal @ <=-10dB will dissipate <100W in the Voice Coil.
Now take a 500W amp driving a 500W speaker that is 10% efficient and average power -10dB.
The average input power is 50W, the Voice Coil dissipates 45W.
If you seal the box and then add on another 10W to 50W of magnetic circuit energy dissipation the speaker ambient could be extremely high.
For us normal listeners this SPL will never be required and temp is not a problem, but that does not mean we always ignore the dissipation and ambient temperatures. Sometimes they can become a problem.
While I don't disagree with what you are saying who uses a FC driver in a sealed box where the heat generated would be difficult to dissipate? The only FC drivers I have seen in use were all used in vented or horn type enclosures or in the open where cooling is not really an issue.
Yes, you are quite right...the type of cabinet is important... I have used the Feastrex FCs in ported cabinets and BLHs ... it does get warm but not as hot as you would feel uncomfortable. Never tried it a a sealed box but I would expect the heat build up to be greater than say the ported cabinets over time.. Also if you use a lower voltage setting .. the heat generated is definitely much less.. so there's quite a bit of room for tweaking and fine adjustments..
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