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  1. #1

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    Default SDA X-over inductors

    I've been scouring the threads over the past couple days since I'm laid up after some surgery.

    I've read about all the different cap and resistor combinations and also looked at the tolerance's on those pieces. Anywhere from 1-5% or more variation in accuracy.

    I'm curious to find out why the inductors are seldom looked at and the tolerance values not improved upon.

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    I've been curious about that as well:o

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    There is a thread where Stu and Matt commented on replacing inductors (probably one of Darqueknight's threads)..........it's not necessary unless you have the thorough knowledge to implement an air core vs. iron core. Leave them alone the iron core stock inductors do the job just fine and don't wear out like caps, etc.

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    Depending on location in the circuit and their actual quality ,replacing small guage cored inductors with air cores could prove benificial as they are less prone to saturation.However it is important to keep the DC resistance values of any replacement coil close to that of the stock units.The big coil found in SDA's has a high DCR out of necessity to keep the load shown to the amp reasonable,so should'nt be altered.

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    This has been brought up before and I posted the following responses before, but here they are again. The first one is from Stu Lumsden, head of engineering.

    Resistance is not always futile. The DCR of our inductors is accounted for in the crossover design. Changing it / lowering it will affect a different balance than the design intent. We typically use large enough wire that the DCR of the woofer inductors is in tenths of an ohm range. As to switching inductor for reasons of other qualities, there can be benefits if the designer has not done his homework. Weve used air-core inductors for many designs and steel or iron cores for others. We base our decisions on the perceived use of the product, cost and size. Iron or steel cores increase the value of a given inductor by focusing the magnetic field created by the windings. This has the advantage of reducing the size and number of winds needed to achieve an inductance value. This also means that larger wire or fewer turns can be used to achieve a lower DCR. The concern is that the core (iron, steel laminate, ferrite) wil saturate a some maximum field intensity and pushing current above that level into the inductor will make it become non-linear. The inductor is actually reverting to acting like an air-core as it is over-driven but only for the overdriven portion of the signal. So the signal becomes distorted, bad noises, scratchy, etc.. We have to choose the wire gauge and core material so as to provide head-room for the largest signals (including transients) that we expect the speaker to reproduce linearly. Air-core inductors do not saturate as more current is passed through them unless the current is so great that the wire begins to heat. It is not necessary, however, for the inductors to have such high limits because there are plenty of other practical limitations on loudspeaker output. Many of these are simply physical, like the maximum possible cone excursion of the drive units. Go beyond this and once again distortion. Typically we can design our speakers with steel-laminate inductors and easily reach undistorted levels well in excess of 100 dB in the case of appropriately sized units. The down side of air-core inductors is as mentioned earlier that they will require more wire and hence have higher DCR. So more power from the amp will go into heating up this inductor and less into making sound. In this case, resistance is indeed futile. "
    The next two are from Matt Polk.

    Usually, in a typical low pass crossover, one would not want to swap out an inductor for another one with much different characteristics because it would alter the voicing of the speaker. However, in this case there could be a performance benefit if the amp being used doesn't mind seeing a lower impedance. The inductor I believe they are talking about is part of the "Full Complement Sub-Bass Drive" circuit. That's a term that Sandy coined to describe the system we used to allow the SDA drivers to work in parallel with the stereo drivers in the bass while producing the SDA signal at higher frequencies.

    In this generation of the SDA's the stereo drivers were nominally 6.5 ohms and the SDA drivers were nominally 3.5 ohms. The plus terminals of the SDA drivers on each side were connected to the plus amp terminal on that side via their cross-over network. Then, the negative terminals of the SDA drivers on one side were connected to the negative terminals of the SDA drivers on the other side via the interconnect cable. This causes the SDA drivers on the right to produce an R-L signal and the ones on the left to produce L-R. Since bass is pretty much mono in most recordings, if the SDA drivers both get full-range R and L signals they would cancel at low frequencies and the SDA drivers would just sit there acting like badly tuned passive radiators. So, we added an inductor in each speaker between the SDA driver negative terminal and the amp negative terminal on that side. At low frequencies that diverts the signal back to ground instead of through the interconnect to the SDA drivers in the other speaker. As a result, at very low fequencies the Right SDA drivers produce only right channel bass and vice versa, while still producing the R-L and L-R signals at higher frequencies. The transition occurs between about 50Hz and 150Hz.

    However, because of the DC resistance of the inductor, the system isn't perfect. I don't recall the DC resistance of these coils but it was high, at least several ohms depending on the model. This means that the SDA drivers will continue to produce some SDA signal even at very low frequencies. Decreasing the DC resistance will definitely improve the bass response of the system both qualitatively and quantitatively. However, before you rush out to buy those Hi-Q replacement inductors be aware of some concerns.

    We chose the higher DC air core coils for a couple of good reasons. First, we were always on the edge of acceptability with the impedance of the SDA's. The DCR of these coils kept the minimum impedance high enough for the amps available in those days. Depending on the model, reducing the DC resistance of these coils may take the minimum impedance down to around 2 ohms. If your amp doesn't mind, you shouldn't either. The other concern is saturation of the inductor core. Air core inductors don't saturate. Given the cost of adequate ferrite or laminated core inductors at the time, plus the need for a higher DCR, the air core choice was obvious. So, when switching to a ferrite or laminated core inductor make sure it will handle at least 5 amps without saturation. That's equivalent to 100 watts of low frequency power through the SDA driver.
    As an addition to the thoughts I gave earlier on inductor quality you should advise that only laminated steel is acceptable for most any high quality system. Ferrite saturates at current levels much too low. Its high permeability allows one to make very high inductance values with less wire (so lower DCR) but the current range is poor at best. For all I know this may be the reason that all quality / high current transformers use laminated steel.
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    Thanks for the read Jesse. A bit above me, but it does seem as if MP was hedging his bets? I wouldn't know where to begin except for replacing an inductor with same DCR, albeit with a lower tolerance level perhaps 2%.

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    "Decreasing the DC resistance will definitely improve the bass response of the system both qualitatively and quantitatively. "

    "The DCR of these coils kept the minimum impedance high enough for the amps available in those days. Depending on the model, reducing the DC resistance of these coils may take the minimum impedance down to around 2 ohms. If your amp doesn't mind, you shouldn't either."
    It would be interesting to get some advice for some replacement inductors. There seems at least to be an opening in MP's comment for some improved bass response. Maybe something with amorphous core materials.
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    Quote Originally Posted by Janne View Post
    It would be interesting to get some advice for some replacement inductors. There seems at least to be an opening in MP's comment for some improved bass response. Maybe something with amorphous core materials.
    The links show the type of inductors he was alluding to, and using in place of the big 16mh air core.The lowered resistance in series with midbass's(DA section) will give the amp better control over the drivers but as he warns unless you have an arc welder for an amp the load could become to difficult.
    http://www.erseaudio.com/Products/SuperQCoils
    http://www.madisound.com/manufacture...edgehammer.php

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    I always replace the inductors in my xover upgrades, one because I can and two because I'm not going to waste my time doing all that other work and not do it all. Is it neccesary? depends on who you talk to I guess but I work with North Creek not Matt Polk.

    I rebuild mine on wafer board and remount it. This is pretty much a must in the LSi series(LSi15) since space is at a premium and relocation is needed.

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    George Short knows a thing or two about loudspeakers.

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    What would the existing DCR be in the 16mH coil in the 1.2TL? 16mH also seems to be a non standard size. 15mH is the closest or have one special made? I saw the North Creek people could handle this. My amp is fairly powerful so I don't think extra power requirements would be a problem. What DCR should I aim for? Would it make sense to change the other inductors to larger AWG and reduced DCR?
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    Quote Originally Posted by Janne View Post
    What would the existing DCR be in the 16mH coil in the 1.2TL?
    Not sure exactly but since it's wound with only 18 guage wire it will be several ohms.
    16mH also seems to be a non standard size. 15mH is the closest or have one special made?
    Madisound might accomodate special order values or maybe unwind an 18mh to get 16.
    I saw the North Creek people could handle this.
    I think NC only wind air cores.Copper is expensive so getting a custom wound low DCR 16mh air core will be very cost prohibitive,not to mention big.
    Would it make sense to change the other inductors to larger AWG and reduced DCR?
    No as pointed out in Stu L's quote the DCR of the other coils should stay the same as their resitances have been accounted for in the design. The quotes also seem to indicate that they used steel laminate cores with high saturation points so changing to an air core of the same DCR likely won't be worth the cost and effort.

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    "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche

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    Just to add to the wealth of knowledge in here.

    Earlier it was quoted:
    "As an addition to the thoughts I gave earlier on inductor quality you should advise that only laminated steel is acceptable for most any high quality system. Ferrite saturates at current levels much too low. It’s high permeability allows one to make very high inductance values with less wire (so lower DCR) but the current range is poor at best. For all I know this may be the reason that all quality / high current transformers use laminated steel. "

    It is my belief that lamination is a means to reduce eddy currents in the 'metal' core. Any material will benefit from lamination.

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    Janne, have you upgraded your crossovers (caps and resistors) yet. If you haven't, you should do so before messing around with the inductor. If so and you still think the bass response is lacking, I would suggest you look upstream before messing around with the inductor.

    My amp is fairly powerful so I don't think extra power requirements would be a problem.
    Powerful is one thing, being able to handle 2 ohm and lower loads is another.

    IMO, there is no reason to change the inductors, any of them.
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    Quote Originally Posted by Janne View Post
    . My amp is fairly powerful so I don't think extra power requirements would be a problem.
    Your MC500 is a beast and 2 ohm stable.

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    Quote Originally Posted by FTGV View Post
    Your MC500 is a beast and 2 ohm stable.
    That is what I thought when I looked in the manual. It should be able to deliver rated power down to 2 ohm. I have not tried welding yet as I did not buy the optional wire feeder. :o

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    I thought upgrading the inductors while the x overs were being done would have been a good idea. A first blush it may be but, if one ever elected to sell the upgraded speakers, the new owner may have a difficult time if they did not have an arc welder for an amp. I'm fairly certain my 501's would hold the load all day at 2 Ohms, unfortunately the next might not. I'll have to think this over for a bit before I make a decision.

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    Quote Originally Posted by F1nut View Post
    Janne, have you upgraded your crossovers (caps and resistors) yet. If you haven't, you should do so before messing around with the inductor. If so and you still think the bass response is lacking, I would suggest you look upstream before messing around with the inductor.



    Powerful is one thing, being able to handle 2 ohm and lower loads is another.

    IMO, there is no reason to change the inductors, any of them.
    XO's are all done. The way I read MP's reply was that the choice of inductor was a compromise to accommodate for lower powered amplifiers. Lowering the DCR would give you better bass response if your amp could take it. I have 2 ohm connections on the amp so I don't think it would be a problem.

    Just curious why everything on the 1.2tl's XO's seems upgradeable but the inductors are a no no

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    Allow me to repeat, there is no reason to change the inductors, any of them.


    If your bass is still lacking, it's not the speakers.
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    Quote Originally Posted by Janne View Post

    Just curious why everything on the 1.2tl's XO's seems upgradeable but the inductors are a no no
    Diminishing return's.As has been covered replacing the stock steel laminates inductors with air cores is not likely to make any worthwhile improvements.As for replacing the 16mh with a lower DCR type,this might well prove benificial but it's hard to quantify with out actually trying it.It might be an interesting experiment if one has an amp with the current capability to drive the lowered impedance.

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    Changing inductors is scary to the folks that haven't done it since it would require them to actually build a xover instead of doing a Lego block installation. It's ok guys, you've come a long way baby, no offense.

    It is expensive and I certainly agree less important than the other 2 components but I like to be different and follow the other camps advice instead of this camp for the most part. lol.
    Last edited by dorokusai; 06-23-2010 at 07:44 AM.

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    Just want to throw in my second hand, heresay experience.

    I've seen this mild debate here before. I was going to change out my inductors since I completely upgraded/updated my SDAs however, when Ben was working on his SDAs and went to change out the inductors, I followed his progress all along the procedure. He told me that if you don't get the specs on the inductors down pat, it makes the SDA sound unnatural and imparts IIRC a hollow sound to the music.

    As stated above there is a lot more to consider when it comes to inductors than just getting the mh ratings correct so be careful!

    Mark, in the CRS's that I have that you and Jesse worked on; did you change out the inductors? I'm just curious.

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    I don't recall ever working on loudspeakers for you Joe. I've never heard an unnatural or hollow sound from a modified SDA although I'm sure it's possible if you don't know what you're doing. I digress, the dual SDA tweeter exhibits an unnatural SDA effect at times but that's easy to remedy.

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    Quote Originally Posted by dorokusai View Post
    Ilike to be different and follow the other camps advice instead of this camp for the most part. lol.
    We're crushed.:p

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    Quote Originally Posted by F1nut View Post
    This has been brought up before and I posted the following responses before, but here they are again. The first one is from Stu Lumsden, head of engineering.



    The next two are from Matt Polk.

    Very good info, Jesse. Thanks for posting it.:)

    I've wondered this same thing for a while, and that cleared everything up for me.
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    Quote Originally Posted by dorokusai View Post
    I don't recall ever working on loudspeakers for you Joe. I've never heard an unnatural or hollow sound from a modified SDA although I'm sure it's possible if you don't know what you're doing. I digress, the dual SDA tweeter exhibits an unnatural SDA effect at times but that's easy to remedy.
    No, no Mark. These are the CRSs I bought from Antny. You put the Vampire posts on them, Jesse did the woodwork, I don't know who changed out the tweeters to 194s and I thought you had done that too. That's why I'm asking if you changed out any inductors.

    Now do you recall which CRSs I'm talking about? From what I understand these have made their way around the club.

    Yep the dimentional tweeters can give the effect of instruments and vocals flying overhead and coming from behind you but that is what make them so much fun to listen to. I've lent them to my older son and he uses them in his HT rig and loves them . . . but he knows that at any given moment he could get a phone call me telling him to bring them back!!!:D

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