Pete B

I've been suspicious that the SD data used in the measurements of Bret's drivers might be in error. I mentioned this to Ken Kantor and he posted this reply, thank you by the way, which I'm posting here since it impacts the measurements that were the basis of much of this discussion. Fortunately, the larger error was in the 303 woofer, which was not the main focus of the discussion. >Pete, >Per your email, I have measured the cone diameter of a 303a woofer >and an NHT 1259. This was just a ruler/eyeball measurement, made >from halfway across the surround roll. As such, it will differ >somewhat from effective acoustical radiating area, and is also >probably not reliable down to the mm. But, I did measure both at >the same time, and so my guestimation error is probably close in >both cases. 303a woofer: diameter = 253mm Sd = 502.7cm^2 NHT 1259 woofer: diameter = 251mm Sd = 494.8cm^2 >This conforms pretty well to the published data on the 1259, which >I did not look at before I did the measurement. On the other hand, >my recent measurement of the 303a Sd is much higher than what is >shown in the Klippel report I have on my website. Though I did the > response measurements and basic SSP's in this data package myself, >I did not do the Klippel work. I will ask the tech who did the >Klippeling how the Sd number was derived. >-k >Ken Kantor

Thanks Vern, nice to hear from you. I'm just copying these old posts over: The original woofer sounds much better, more low end extension (not that it has much to start with) and probably less top end on the woofer so that it transitions better to the tweeter. The ceramic woofer seems to be correct for the A-25XL which had a different tweeter, and perhaps crossover so it's just not right here. I might try a compensation network on the woofer to get a better match, but I'll probably look for the correct woofer on e-bay. SEAS suggests the 27TFFC as a replacement tweeter acording to Madisound and I have a feeling it should work well given what I've seen from measurements. Pete B.

I wrote this to another group: I've been curious about compliance shift over time in rubber surrounds in the hope of finding trends in materials, manufacturers, or the environmental exposure. Some surprising information. I have a pair of Dynaco A-25s that I bought recently purely for the purposes of reverse engineering. The woofer used in the early production model was a SEAS 25 TV-EW with an ALNICO magnet. SEAS has a suggested ceramic magnet replacement 25 F-EW which is what was used in one of the systems. It is interesting that both are well within specification going by the free air resonance one from 1971 and the other from 1990. SEAS: 25 TV-EW date code 1971 measured Fs = 23.6 Spec = 20-25 Hz 25 F-EW date code 1990 measured Fs = 27.4 Spec = 26 Hz I noticed in listening to these systems that the one with the ceramic magnet was significantly more efficient and measured it midband as 3dB more. The data below shows only 2dB but it is a theoretical midband value. I believe that the cone behavior is slightly different providing a bit more output midband. This is probably the driver used in the A-25XL model from Dynaco which is claimed to be 3dB more efficient than the original. Measured T&S parameters: SEAS 25 TV-EW UNIT SAMPLE: PLB#1 1/12/06 UNIT DATE: 25 week of 1971 Rubber surround part number: SR 231/1 Effective cone diameter = 21 cm measured Effective cone area = 285 cm^2 old SEAS spec, 350 cm^2 new (25F-EW) seems the old spec of 285 cm^2 was an error and might explain the difference in moving mass below: Measured SEAS Delta M 15.75 Spec Fshift -16% Fs 23.6 20-25 Vas 205 Re 5.7 Qe .48 Qm 4.2 Mms 37.5 30 no .36 SPLref 87.6 88 Bl 6.6 .8 Qts .43 Cms . ==================================================================== SEAS 25F-EW(H250) UNIT SAMPLE: PLB#2 1/13/06 UNIT DATE: 48 week of 1990 Rubber surround part number: SR 231/1 Effective cone diameter = 21 cm measured SEAS Spec effective cone area = 350 cm^2 Measured SEAS Delta M 15.75 Spec Fshift -15% Fs 27.4 26 Vas 144 175 Re 5.5 5.8 Qe .49 .39 Qm 3.9 3.8 Mms 39.6 33 no .58 SPLref 89.6 89 Bl 8.8 Qts .44 .35 Cms .85

Hi Vern, Yes I also like Dynaco equipment and agree that we should have a dedicated section here. A few nice links: http://home.indy.net/~gregdunn/dynaco/components/speakers/ Another, Note that the A-50 XO schematic seems to be incorrect: http://www.t-linespeakers.org/classics/dynaco.html I'm reverse engineering a pair of A-25s just because I always thought they had interesting qualities. I wrote this about them some time ago: Compared one A-25 with the 25F-EW (claimed to be the modern replacement woofer) to another with the original 25TV-EW and they are significantly different. The 25F-EW is more efficient making the speaker sound shouty in the 1 kHz region, and less well integrated with the tweeter. The stronger magnet provides more damping and there is less deep bass output with the 25F-EW. Checked the efficiency difference with warble tones and the difference is right on 3dB at 250, 314, 400 Hz, then it varies between 1 to 3 dB for the rest of the bands up to 2 kHz. I believe this makes the 25F-EW a replacement for the A25XL which was 3 dB more efficient and used a ceramic magnet woofer. See the A25XL reference here. http://www.t-linespeakers.org/classics/dynaco.html Pete B. >Hi there; > >I know that there is at least one other member who is very >interested in classic Dynaco. > >If there is enough interest from the members, perhaps, Mark, >may create a Dynaco discusion group.

>I believe that the tweeter is another one of THE most popular >tweeters of all time, the Peerless KO10DT (it went under >another number for a while) used in Cizek, Polk, some studio >monitors and many others. Does this look like the tweeter in >question: > http://www.classicspeakerpages.net/dc/user_files/883.jpg >It was a fully exposed, 1" fabric dome, simple driver no >vent or rear chamber. Should be easy to mod a modern driver >as a substitute. >The KO10DT was listed within the last few years at >PartExpress. I have the specs and one sample from the 70s. > >Pete B. I found a picture of the Type A on the net and that was the system that I saw/heard, however the one in the picture has what looks like an Audax tweeter with a rectangular face plate similar to the Peerless. I'm fairly certain that the one I heard had a Peerless, but either system could have had the tweeter replaced or Snell might have changed at some point in production. I'd be interested to hear confirmation one way or the other from owners. Pete B.

Here's a picture of the AD5060: http://www.hupse.nl/radio/speaker_images/AD5060Sq8.jpg Link: http://www.hupse.nl/radio/speakers/AD5060.htm Pete B.

Thanks for the new Dynaco section Mark, are you going to move the A-25 posts here or should I copy them? Pete B.

>Peter, I own a pairof the Snell B's (nice speakers though >even more massive than the A's) and every individual speaker >is a Vifa and labled with the Vifa labels. All common stock. > >I guess we can credit Snell for maybe matching these in pairs >that test close. But I have never read that anywhere. And >certainly not in the Snell replacements I would think since >they do not claim this and they will sell tweeters / mids as >singles. > >The Snell replacement for the Type A tweeter is a common Vifa >tweeter. I do not know the recommended Type A midrange >replacement and that'd be the best news to find. The woofers >are long throw and I can't imagine ever having to replace them >(refoam but not replace). > >I am actually amazed how well Snells make the Vifas sing. I >have built speakers with upper level Seas and Scan Speak >tweeters and mid-woofers and they are really much finer >speakers than the Vifa components. I suppose this is why they >appear in the most expensive speakers we see today. > >BTW, the Snell A mids are the same mids from the Dahlquist >10's. An easy thing to tell once you have seen both. Still, >the mid itself is impossible to source with its minimal >markings. Belgium is all you can really tell. > >That's why I would like to know what Snell now sells and >recomends as a mid replacement. > >Thanks! Ah, OK you've jarred my memory, I heard vintage Snells recently and I think they were Type A's based on what you say here. That octagon shaped mid is the Phillips/Norelco AD5060 probably the SQ8 version and not the woofer version. Does it have a plastic rear chamber? This is a low cost true midrange, short voice coil, and a light shallow cone. It was used in Infinities, DQ10s and many others. I have both the SQ8 version and the woofer version. They do not handle much power. I also have the specifications from Phillips/Norelco. I believe that the tweeter is another one of THE most popular tweeters of all time, the Peerless KO10DT (it went under another number for a while) used in Cizek, Polk, some studio monitors and many others. Does this look like the tweeter in question: http://www.classicspeakerpages.net/dc/user_files/883.jpg It was a fully exposed, 1" fabric dome, simple driver no vent or rear chamber. Should be easy to mod a modern driver as a substitute. The KO10DT was listed within the last few years at PartExpress. I have the specs and one sample from the 70s. The AD5060 has been out of production for some time, but you might find old stock somewhere. It should be fairly easy to set up a SEAS or Vifa mid as a replacement. These are all simple drivers, if you select a smooth driver, and match Fs, Qtc, voltage sensitivity, and input impedance then the piston band and crossover response will match. Would you like to tell us those Vifa part numbers for Type B replacements? Pete B.

> >What's missing here is for Snell discussion boards to share >the speaker replacement models for Snells. That gives the >owner a choice of going for the Snell "matched" >pairs at apremium price, or the same speaker set for a lesser >value "unmatched." > >Older Snell models, like the J's and K's. the C's, the Type >B's and others all have Vifa components and at that time Snell >left the Vifa sticker with speaker number on them. Wonderful >gift. > >Does anyone know the replacement speaker for the Snell A >midranges? As I have mentioned, repair, don't replace the >woofers. But the mids are both oddly shaped, oddly old and >stiff feeling and I would love to hear how well they sound >with the reocmmended current Snell mid replacements. I am sure >these are Vifas. The tweeters are Vifas. > >Anyone do the replacement and can share the model number? > >Finally, I am going to not only make a new Type A clone of my >own but make it with regards to Peter's original A's. I >believe it can be done and that the use of modern cabling, >caps and coils will make a difference. Same values, same >connections. I also think that the bass unit can remain about >the same, but that the upper module can be re-worked to a more >acceptable modern look. This is worth checking and is always a >thrill to do. So imagine the bass unit on its side. Then the >upper unit of simlar shape keeping the same midrange box size >(it is sealed in a small wood box in the upper part) and the >same relationship witht he tweeter. Just a slightly smaller >but still curved wood baffle. > >I believe, and I am not an audio engineer if that's a premise >that will help your deny or entertain my read on this, Peter >had this right and if he had the ingredients we have today >(like Scan Speak components and OFC) this speaker would sound >even better. > >I can see a Snell A clone that has a top portion that remains >true to the original but used fine veneers and exposes the top >speakers as a thing of beauty. > >I am keeping two sets unchanged. But my third is going to be >my map for a Type A clone. > >Anything can be built. Especially if you have enough love for >the product that you are willing to do the work to figure it >all out.I have difficulty believing the A's couldn't be a kit >by a major company. > >A Snell fan. Hi, I don't follow Snells much, but I do have a copy of a review of the Type B from Audio 1992 in my files. I remember reading this review, and noticing the very common D25AG tweeter from Vifa in the pictures. Snell didn't even use a custom face plate like most others, looks like the stock version. Good move keeps cost down and allows owners to use stock replacements. This was probably THE most popular tweeter of all time, used often with a custom face plate by PSB, Vandersteen, Paradigm and many, many others. I remember looking at the mids and they also looked like a 5-1/4" Vifa P13 type. This and the P17, P21 were also very common drivers, but I've found that they're often custom variations on the stock unit. I don't know the Snell Type A but if you tell us about the mid I'll try to help, is it 5-1/4", poly or paper cone? Is the voice coil 1", what is the DCR, what are the magnet dimensions, does it have a cast frame? There are short and long throw versions of the p13 so you might want to watch out for that. Word out is that the stock classic Vifa units are going out of production. Some claim that it's impossible to "clone" a commercial speaker but I don't know where they get this idea as a sweeping generalization. Certainly if a system uses an unusual driver it might be the case, but many systems use fairly common drivers. I've done it several times with excellent results and I believe these Snells are a good choice given the use of Vifa drivers. Drivers are often selected by manufacturers and you might want to keep this in mind. Pete B.

I don't expect you to open them if it wasn't in your plan, the rest of the info would help to determine what you've got. >Can you tell us for each pair, do they have the masonite >woofers, or all steel? >Do they have the orange tweeter with masonite, or are they the >orange flush mount? >If you open them up, you should find: >Two 16 uF caps, early Large Advent, my Rev1 >One 8 and one 16 later Large Advent, my Rev2 >One 13uF cap, The New Advent Loudspeaker, my Rev3 >Do any of them have "Designed by Henry Kloss" >printed on the back? > >Pete B.

Can you tell us for each pair, do they have the masonite woofers, or all steel? Do they have the orange tweeter with masonite, or are they the orange flush mount? If you open them up, you should find: Two 16 uF caps, early Large Advent, my Rev1 One 8 and one 16 later Large Advent, my Rev2 One 13uF cap, The New Advent Loudspeaker, my Rev3 Do any of them have "Designed by Henry Kloss" printed on the back? Pete B.

I'm fairly familiar with AVA but not by model number how many watts is that unit? I don't follow your U4, U3 designation? Masonite would be the early model, do you know if you have the 16/16 or the 16/8? Newer style, do you mean The New Advent Loudspeaker ,what I call a Rev3 or 13uF? I've given a strong recommendation above for Advents with BSC, it is easy to build and try seems the choice is obvious. However, you'd be wise to go over your Advents and at least replace the old caps. Pete B. >My amp is the AVA OmegaStar 170EX. The speakers are wired in >parallel. I haven't gotten around to trying anything along the >lines of a BSC or crossover upgrades (still digesting the >science in this thread). I have three pairs (my original 'New' >U4 that I bought in 1978, a U3 masonite ring and another U3 in >the newer style).

Dave (Daveshel) writes: >Pete, your baffle mod sounds interesting, and I my have to >experiment with that. My preamp has an EPL that needs >something to do. Although I may have accidentally achieved >good baffle with my setup. My Advents (4 larges is 2 stacks) >sit on stands that are 6" off the floor in the front and >sloping down toward the back so that the speakers are at a few >degrees off from vertical. And they are placed roughly >20" from the wall. I have always loved their sound and >was amazed at the synergistic effect when I added the second >pair. Not boxy at all. > >Thanks, guys. Yes there should be some synergy, the larger total baffle area should reduce the amount of required baffle step. There's also an efficiency gain when pairing up drivers. Anyway, I'm wondering how your driving them, are they in parallel driven by a high current amp? Have you tried the BSC circuit, I believe that it will help anyway and I believe Russ runs doubles with BSC and likes it. You could try 3 to 4 dB if you prefer less or place them closer to the wall. What version are your Advents, 16/16uf, 16/8 uf, or the New Adevent with a 13uF? Have you tried any crossover upgrades, that we've talked about here? Pete B.

Hi Frank, Thank you for commenting, and you probably know that if we were on a photography site the tables would be turned with you being the professional expert. No matter the case I do believe that it is good to keep in perspective that this is just HiFi after all, and your humor is welcome to lighten things up. We all need a laugh from time to time and I did enjoy your humerous posts. That's quite a rig you've got there with stacked LSTs and Phase Linears. I enjoy hearing about your listening sessions and impressions. We've talked about our common auto, photography, electronics, and sound interestests, and there are several others here that also have many of these same interests. Thanks again for your kind words Frank, and take care! Pete B. >Dear CSP members, I just read for about an hour or more some >of the threads on this site about the differences of the AR >12" woofer through out its history that I never read >before, I had to stop reading as it was overwhelming. I am >almost completely drained to read the vast amount of knowledge >and expertise from such site notables as Pete B., Tom Tyson, >Vern and countless others(respects to all) that I feel like a >child commenting about world affairs in the UN General >Assembly when I post. I can't believe the amount of knowledge >and experience that some of this site's members have. I had to >stop as I was being drained and getting a headache! >Although I'm beyond impressed, I also feel belittled by such a >range of helpful and interesting people on this site. I also >feel if nothing else, maybe I'll just try to be the court >jester here and painfully try to slightly amuse some people >here if nothing else, if that's at all be possible. Or >probably a better suggestion would be to just sit back quitely >and learn from people who know so much more than I do. I'll >listen to my system and stay away from the site as being >liable of making a complete ass of myself sounds like a better >idea. >I want thank all of the wonderful contributors for sharing >your vast individual experiences and 'know-how'. Reading this >page has made me feel that I am very grateful and lucky to >have found this site in the first place less than 2 years ago. >I feel secure in knowing that I have the privilege of being in >such powerful company. >May I extend my deepest thanks and personal happiness I feel >to read all of your opinions. >Respectfully and thankfully, frankmarsi@verizon.net

Test post.

Russ recently asked me a good question in private e-mail as to why the original values for the 4.7K version are not the same as the values in the new tables that I posted. This was my answer: Basically, I did a back of the envelope calculation the first time and remember I said that the step would be slightly less than 6 dB. The chart values are derived from simulation where I take into account source and load impedance. If we look at the chart and do a rough interpolation we see that 4.7K for R2 falls between 5 and 6 dB just as we expected. The cap value from the chart would be about .062 uF and I mentioned that the new values offer a slightly lower center for the step, 400 Hz vs. about 500 Hz IIRC, which explains why .047 uF was used in the original circuit. You should be fine if you like the step size as it is. You could experiment with .047, .056, .068 etc. if you like, to move the center of the step, I expect that the change will be very small since it is a gradual slope. I'll just mention that I built the BSC for using AR-2ax's a few feet out from the wall behind them and elevated for better image height. It was a first cut and I was going to adjust as needed. There are no correct values since it is placement dependent. I should probably revisit this since I used early Advents with the 16/16 uF crossover and the extended setting was required with BSC. I didn't know that the Large Advent went through several revisions. Based on what I know, I've not tested this yet, I'd build a new BSC from the tables as 5 dB for 16/16 uF Rev1 Advents used in the Extended position, and 4 dB with 16/8 uF Rev2 Advents also in the Extended position. Use the cap value in the chart but feel free to experiment with different values. The "correct" value will depend on placement and many other factors, however any one mentioned above will be far better than the stock speaker, IMO. Pete B.

Thank you Russ for offering to help jackfish. jackfish, it's not too difficult to build, I think it is a good soldering exercise if you like this sort of relaxing work. Let us know if you need more help or prefer to have someone else build it. Pete B. >I don't know if this question has been asked yet or not, but >is there someone that will make these circuits for the >electronically challenged? Time, effort and materials would >gladly be compensated. Thanks. Or are they are fairly easily >made with a soldering gun? I have two pair of New Large >Advents that I want to stack.

Hi Steve, The 2.2 ohm reading is just fine, mine measured 2.35 and Advent says approximately 2 ohms. If your curious it is good to also measure each of the drivers to see if they match and far off readings can indicate a problem. One thing to note is that the tweeter connections, on those old tweeters go through screws that you might want to make sure are clean and making good connections. You might have also noticed my comments about the switch: http://www.classicspeakerpages.net/dc/dcbo...g_id=&page=#707 Remember that cleaning all these connections will bring the tweeter level up if there was corrosion and high resistance. The most likely issue is that the ESR of the electrolytics has gone up with age and it is likely that the polys make the system sound more like it did when new. First I suggest that with the speakers as currently wired you put your system in mono, put the speakers side by side, and use your balance control to switch between the two. Do they sound the same? If they do we can leave one as is and wire the poly back up but with a 1 ohm resistor in series to simulate the ESR that you prefer. It would be best to bring out the connections so that you can try different resistor values in order to match the reference speaker. Use music that your familiar with, interstation FM noise, or test CDs. Try the poly alone but this time compare against the other speaker with the electrolytic still in place - do you still hear a large difference? New electrolytics typically have about .2 to .3 ohms of ESR and this is just about 10% of the 3 ohm decrease resistor, so it is not that significant but probably is barely audible. I'd be willing to measure the electrical frequency response of the network with the electrolytic, and the poly in order to determine the best way to address your question, are you in CT or nearby by any chance? Let me comment that you now have a hybrid of Rev1 and Rev2, to complete the Rev2 upgrade you need a low DCR, .45 mH inductor in place of the old 2.2 ohm red dot type. It is true that the difference is only in the extended position and even the hybrid is better than the original. Pete B. >Hi Pete, > >I replaced the capacitors in my large Advents with Bennic >polys from Madisound: 16uF for the C1 capacitor and 8uF for >the C2 “increase” capacitor that was previously 16uF. While I >was in there I disconnected the tweeter and measured the DCR >of the L2 shunt inductor. It read 2.2 ohms in each speaker. >Note: I don’t have the most accurate DMM so that could be off, >but L2 is definitely not shorted. > >I replaced the caps in one speaker first and thought I liked >the sound so I did the other speaker. After that is when my >ears started to hurt. The speakers were incredibly harsh in >all 3 switch positions. This setup truly sounded like a >transistor radio with a subwoofer. And, if I was listening to >a weak FM station, i.e., one with a little background static, >it was unbearable. > >Undaunted, I left the caps in place over the weekend to see if >it was just me being overly sensitive. By last night my >soldering iron couldn’t heat up fast enough. I put my original >16uF electrolytics back in. I left the 8uF caps in place for >now since I usually listen to these speakers in the “normal” >position. > >Anyway, I’m at a loss. I don’t have a capacitance tester so I >can’t verify the Bennics but they are marked as 16uF and >physically they are about twice the size of the 8uFs. Further, >after heating up the electrolytics twice now to >desolder/solder them I’m really concerned about their >reliability. > >Ideas and comments are appreciated. > >Thanks, >Steve > > >

Hi Steve, I've added some comments about the woofer inductor at the thread below since you seem to be open to mods. I also comment about some minor differences between the 16/16 and 16/8 not noted above in our discussion: http://www.classicspeakerpages.net/dc/dcbo...d=689&page=#700 Pete B.

I've put a lot of thought into this and considering that it is established that electrolytic caps do not age well, and after measuring several that did not meet their voltage specs and in one case had highly increased capacitance, I now suggest always replacing electrolytic caps that are more than 10 or 20 years old. The exception to be considered on a case by case basis is the large computer grade caps found in some systems, and perhaps some of the more exotic types. But always the electrolytic caps to the mid and tweeters which are more likely to fail due to a leaky cap: http://www.classicspeakerpages.net/dc/dcbo..._id=7195&page=4 http://www.classicspeakerpages.net/dc/dcbo..._id=&page=#7710 http://www.classicspeakerpages.net/dc/dcbo..._id=&page=#7728 http://www.classicspeakerpages.net/dc/dcbo..._id=&page=#7713 I measured the electrical transfer function of the large Advent tweeter crossover and the Q is on the high side, somewhat over 1 if I remember correctly. I was surprised to find this and it does make the circuit more sensitive to ESR, however the shunt inductor has a high resistance, around 2 ohms from memory, and this should lessen the sensitivity to ESR. I suggest replacing the caps with reasonably priced film caps, Mylar or poly are fine. I'd go with 15 uF which is within the 10% tolerance of the original cap but will lower the Q of the circuit just a bit. You might also want to lower the cap across the increase switch to 8 uF since this was a good refinement to the design. I plan to look at this a bit more but I don't know how soon that will be. It's also suggested that you measure the DC resistance of the shunt inductor as it was mentioned earlier in the thread that it cannot handle much power and turns shorted in one poster's system. I'd actually replace this inductor with a low cost air core type, then add a resistor in series to provide a total DCR equal to the original. This will lower the distortion at high levels in the tweeter circuit. Speaking of this distortion, I always noticed distortion in the mids/highs when turning up the Large Advents, harshess or a sort of crying quality, it adds to our perception of loudness and tells us to turn it down. I tend to wonder if this was deliberate by Henry Kloss to save drivers from overload. I'm not sure if it's due to the very small iron core inductor, or the tweeter having zero Xmax. It is worst at fairly high levels and not much of an issue for normal listening but does take away from the transparency of the system at moderate-high levels. Thanks again for your feedback Steve, it's always nice to hear. All the best to you in the New Year! Pete B. >Pete, > >Mine have two 16uF caps in each crossover. I was considering >replacing these with poly caps but I'm concerned about >changing the voice. I figured these were voiced with the >electrolytics so I'd just stick with them. Any thoughts you >have on this changeout would be appreciated. > >Happy New Year, >Steve > >PS: My Advents are the walnut veneer versions. >

Thanks for the feedback Steve and the email. I'm going to set up the A/B again soon to demo it to some friends. Did you happen to notice if your Advents are the early ones with two 16 uF caps in the crossover? Later ones had 16 and 8 uF, and even later had a 13 uF IIRC. Pete B.

> >This is why I previously asked about the orientation of the >driver, it would be interesting to tilt the enclosure so that >a bias is introduced into the rest position to bring the peak >of the Bl curve to x=0, then retest the distortion. > >Pete B. > An obvious solution to bring the peak in the Bl curve to x=0 is to build the driver with the voice coil slightly offset. It is interesting that this is exactly what Bowers and Wilkins does in their midrange drivers. From page four of their white paper: "For these drivers, a different solution was used, based on the simple recognition that it is not the flux distribution itself that matters, so much as the force factor (Bl). Simply offsetting the rest postion of the coil significantly improves force symmetry (figure 7)." B&W 700 series white paper, in .pdf format: http://www.bwspeakers.com/downloadFile/tec...sWhitepaper.pdf Pete B.

Another modern driver: http://www.northcreekmusic.com/Image25.gif From: http://www.northcreekmusic.com/NorthCreekKits.html Note that while the Scan Speak motor is patented, there were similar patents much earlier than Scan Speak's. "The SCAN-SPEAK 18W/8545SC is a woofer of remarkable design. First and foremost, the driver is designed for enormous linear excursion. This is achieved through SCAN-SPEAK's development of the patent-protected SD-1 motor structure, diagramed below right. Close inspection of the conventional motor (above) and SD-1 system (below) reveals the following: The conventional motor structure voice coil length of 12mm or less is simply too short for reasonable output at low frequencies. Conventional top plate thicknesses of 6mm yields a peak-to-peak linear excursion of only 6mm (voice coil height minus air gap height = peak-to-peak linear excursion). The SCAN-SPEAK voice coil length is 19mm, yielding a p-p linear excursion of a full 13mm, more than twice that of conventional woofers. The conventional motor structure suffers from high second harmonic distortion at low frequencies due to an asymmetric magnetic field about the top plate; that is, there is more flux below the top plate than above. The SCAN-SPEAK pole is extended well beyond the top plate, creating a symmetric magnetic field which greatly reduces low frequency second harmonic distortion. In a conventional woofer, high amplitude excursion modulates the voice coil inductance: outward travel decreases the amount of pole surrounded by the voice coil, reducing its inductance, where as inward travel increases the amount of pole surrounded by the voice coil, increasing its inductance. This is one reason why most woofer impedance curves do not look like a simple "resistor plus inductor" at high frequencies. This displacement-dependent voice coil inductance causes intermodulation distortion between high excursion low frequencies and higher frequency information, as well as creating frequency response aberrations by perpetually misterminating the crossover network. In addition, the inductance variations create a solenoid-type force between the voice coil and pole which introduces an offset to the coil rest position, increasing second harmonic distortion. The SCAN-SPEAK voice coil always surrounds an equal amount of pole, eliminating this source of distortion. The SCAN-SPEAK impedance curve is retraceable independent of drive level. In a conventional motor structure, current within the voice coil creates dynamic magnetic flux which modulates the motor field by partially demagnetizing and remagnetizing the section of the pole surrounded by the voice coil. This phenomenon creates both third and intermodulation distortion throughout the midrange, and is the other reason why conventional woofer impedance curves are nonlinear. The SCAN-SPEAK motor structure is constructed with copper rings, called "Faraday rings," bonded to the pole. The Faraday rings create a sympathetic flux which exactly matches the voice coil flux, eliminating this source of distortion. The concave cone-shaped, vented pole top eliminates reflections between the dust cap and pole while cooling the magnetic structure. The SCAN-SPEAK 18W/8545 cone is constructed of carbon fiber loaded paper and coated with damping compound. Damped carbon fiber is an exceptional cone material because it combines low mass with incredible strength, therefore it is extremely rigid at low frequencies yet can be easily controlled by the damping compound at mid and high frequencies. The 18W/8545SC exhibits nearly theoretically perfect performance for a 7" driver - including the rising response and gentle turnover before roll off. One can find far more detailed information about the causes and solutions of motor-induced distortions the AES publications Loudspeakers, Vol. 1 and Vol. 2 (60 E 42nd St, NY, NY 10165). Suffice to say that the engineers at SCAN-SPEAK have used every resource to develop and manufacture a motor structure which minimizes or eliminates the common second, third, and intermodulation distortions found in virtually all conventional loudspeaker drivers, resulting in a 7" woofer with exceptional low end performance even at very high volume levels while providing midrange purity rivaling many smaller drivers. The SCAN-SPEAK motor structure is unique and patent protected."

>The force factor or Bl(x) curve is not so pretty because this >is a very primitive driver. The Bl peaks at almost 10 but at >x= -3 mm, that's nearly half way through it's "linear" >excursion. It has no small signal linear excursion due to >this offset in the peak and this probably explains why the >distortion does not continually decrease with decreased >excursion. This is why I previously asked about the orientation of the driver, it would be interesting to tilt the enclosure so that a bias is introduced into the rest position to bring the peak of the Bl curve to x=0, then retest the distortion. Pete B.

It is interesting to take a look at a modern driver such as the Peerless XLS: http://www.d-s-t.com/link/main/tech/xls.jpg from: http://www.d-s-t.com/link/main/tech/xls_intro.htm Key from the DST site: A. Aluminum Spacer The aluminum spacer serves as heat sink for the coil to reduce power compression. B. Black Anodized Voice Coil The 2-inch 4-layer voice coil is wound on thick black anodized aluminum for improved heat dissipation. C. Stacked Magnet System The twin stacked magnetic system is optimized by FEA (Finite Element Analysis), to create a symmetric powerful magnetic field in the air gap, and provide space for the 44 mm max excursion of the voice coil. D. Distortion Reduced Motor A long 4 layer voice coil normally results in high self induction and impedance varying with excursion. Its many ampere turns react on the magnetic field in the air gap. These two main factors causing motor distortion in subwoofers are practically eliminated by the combined impact of the Aluminum Short Circuiting Ring (D) and the Aluminum Spacer (A) on the pole piece. At the same time they both contribute as heat sinks for the voice coil, reducing power compression. The result is unbelievable clean bass reproduction. E. Vented Cone To eliminate compression under the dust cap the cone is vented by 8 large holes. This way the coil is cooled and there is no need for a bore in the pole piece. F. Nomex® Spider The spider is made of high tech Nomex® material. This material is chosen for its high rigidity and long term stability. The suspension will stay in shape for a very long time under heavy load. G. Rubber Surround The surround is made from SBR rubber because of the wide operating temperature, low creep and long term reliability. H. Fiber CompositeCone The cone is molded from a propriety air dried wood free pulp with a blend of Nomex®, Kevlar® and glass fibers bonded together by deep impregnation with polymers. This creates an ultra stiff and relatively light cone that will stay stable even under very large sound pressures. I. Rigid Cast Aluminum Basket The rigid cast basket with an aerodynamic profile provides the necessary sturdy base for the magnet structure and suspension and allows for the 44 mm max excursion of the cone. The spider is ventilated to achieve the lowest possible compression and allow air to flow freely to create a cooling effect for the voice coil.