Placement of Inductor Coils

[Guest baumgrenze]

I believe I remember reading a posting that looked well informed that said one should be very careful about the placement of inductor coils relative to each other. As I recall, it also said that if they needed to be close because of space constraints, their axes should be at right angles to one another.

I went searching for it again tonight, but I could not find it with the terms I put into the search engine.

The question came up for me when I saw the images in this posting:

5738, RE: Subjective testing of AR-11 speakers

Posted by r_laski, Mon Mar-21-05 03:13 PM

Two AR11 crossovers.

In the attached photo, the inductors appear to be very close to one another.

Is my memory playing tricks on me regarding inductor placement?

Thanks,

baumgrenze

[Guest hilltroll67]

Those pronouncement look like the ones stated by the guy at Human Speakers. A google search should find him. I have never seen a commercial xover that is constructed to that concept.

Bob

[soundminded]

>I believe I remember reading a posting that looked well

>informed that said one should be very careful about the

>placement of inductor coils relative to each other. As I

>recall, it also said that if they needed to be close because

>of space constraints, their axes should be at right angles to

>one another.

>

>I went searching for it again tonight, but I could not find it

>with the terms I put into the search engine.

>

>The question came up for me when I saw the images in this

>posting:

>

>5738, RE: Subjective testing of AR-11 speakers

>Posted by r_laski, Mon Mar-21-05 03:13 PM

>Two AR11 crossovers.

>

>In the attached photo, the inductors appear to be very close

>to one another.

>

>Is my memory playing tricks on me regarding inductor

>placement?

>

>Thanks,

>

>baumgrenze

Theoretically this advice was correct. A transformer is nothing more than two or more coils in close proximity. The steel core of a transformer merely serves to concenrate and focus the magnetic fields around wires making coupling the induction from one coil the primary to the other coil the secondary more efficient. Air core transformers do exist and can be made inadvertently by an unfortunate placement of coils in close proximity. The efficiency of this "mutual transconductance" as it is known is increased by placing the coils physically close together and in the right (wrong in this case) orientation. In practical terms for loudspeaker crossover networks it may not be as serious a problem in the real world as the theory would at first lead us to believe. Because the actual currents in the coils are relatively low, often no more than a few amps at most and more typically in the few milliamps to few hundred milliamps range, the magnetic fields around them are rather weak. They also fall off quickly with distance. Even a few inches may be enough. Each individual case is different. A good speaker designer will check the final configuration to be certain he has not made this mistake. BTW, if your speaker is bi wireable, an easy if incomplete test is to disconnect the jumpers and power only one section at a time. If you hear sound coming from the speakers which are not being powered, you may have a problem. Usually you won't though.

[Steve F]

This can be a real-world problem under the right circumstances. Seldom, but it does happen.

When Boston Acoustics introduced the VR12 in 1994, it was the industry's first 3-way center channel speaker. It used two 6 1/2" woofers, which flanked a vertically-arrayed 4 1/2" midrange and 1" dome tweeter.

The woofers crossed over very low--about 400 or 500 Hz, well below the point at which they'd become directional--and the M-T array took over above that. Since the M-T was vertically laid out and they handled the range from 400-20,000hz, the VR12's horizontal dispersion was abosolutely uniform and lobe-free over a very wide horizontal angle.

It crushed the puny dual 5 1/4" horizontally-arrayed MTM centers that were out at that time, and inspired a whole host of center competitors to rush their 3-way products to the market.

But BA found out the hard way about chokes and magnetic interference. Since the woofs crossed over very low, their chokes were rather large. During pre-ship testing, BA found that there was visible interference when the VR12 was sitting on a typical 27" or 32" CRT (that's all there was in 1994) TV set, despite all the drivers being completely magnetically shielded.

They found that it was the large woofer chokes that were the culprit. They were literally "pulsing" magnetic energy in response to the input signal. On a TV's blue test screen, it was like a bad light show, in time to the music.

So BA used two half-value chokes together, mounted in "bucking" style (like bucking magnets on a woofer) on the PCB, and that completely cured the problem. BA even went on to apply for and receive a patent on the bucking choke.

You never know.

Steve F.

[Carlspeak]

I use the rule of thumb to keep the inductors as far away from each other as possible. When space is at a premium, they should be mounted a right angles to one another which usually will mitigate the issue under discussion.

It's all about the music

Carl

Carl's Custom Loudspeakers

[soundminded]

>This can be a real-world problem under the right

>circumstances. Seldom, but it does happen.

>

>When Boston Acoustics introduced the VR12 in 1994, it was the

>industry's first 3-way center channel speaker. It used two 6

>1/2" woofers, which flanked a vertically-arrayed 4

>1/2" midrange and 1" dome tweeter.

>

>The woofers crossed over very low--about 400 or 500 Hz, well

>below the point at which they'd become directional--and the

>M-T array took over above that. Since the M-T was vertically

>laid out and they handled the range from 400-20,000hz, the

>VR12's horizontal dispersion was abosolutely uniform and

>lobe-free over a very wide horizontal angle.

>

>It crushed the puny dual 5 1/4" horizontally-arrayed MTM

>centers that were out at that time, and inspired a whole host

>of center competitors to rush their 3-way products to the

>market.

>

>But BA found out the hard way about chokes and magnetic

>interference. Since the woofs crossed over very low, their

>chokes were rather large. During pre-ship testing, BA found

>that there was visible interference when the VR12 was sitting

>on a typical 27" or 32" CRT (that's all there was in

>1994) TV set, despite all the drivers being completely

>magnetically shielded.

>

>They found that it was the large woofer chokes that were the

>culprit. They were literally "pulsing" magnetic

>energy in response to the input signal. On a TV's blue test

>screen, it was like a bad light show, in time to the music.

>

>So BA used two half-value chokes together, mounted in

>"bucking" style (like bucking magnets on a woofer)

>on the PCB, and that completely cured the problem. BA even

>went on to apply for and receive a patent on the bucking

>choke.

>

>You never know.

>

>Steve F.

AR9 was a real problem for Sony KV2781R, a 27" CRT set circa 1986. I had to drag the speakers halfway across the room to satisfy myself that there was nothing wrong with the picture tube. Very noticable color fringing was visible in the corners of the CRT. This was due to the woofer magnets, not the crossover coils or voice coils since the problem was evident even with the amplifier off. No such problem exists with Sony KV-36XBR250 a 36" set circa 2000. Evidently that model's CRT is much better shielded. Since the magnetic fields due to the coil will be modulated, problems with a crt due to them should not be evident when the speakers have no signal going through them. However, if it's the magnets which causes the problem if one exists, evidence of it will always be there. Interestingly, KV-36XBR250 is also not subject to stray magnetic fields from Empire 9000M either. This speaker manufactured in the mid 1960s long before this type of interaction was considered a problem has a magnet which was advertised to exceed one million lines of flux. Judging from the size of the magnet which made the AR9's seem puny by comparison, it's an advertising claim which was probably justified.

[Guest baumgrenze]

Bob, you are right. Here's the pronouncement:

Air Core Choke

Value given in millihenries, followed by the specified wire gauge, if available. Although this is not always done, choke coils should be mounted with their axes at ninety degrees to any others that are within six inches or so of them.

And the URL:

http://www.humanspeakers.com/xverkey.htm

For what it is worth.

As I worked on my AR-2AX's and AR-5's I noticed that the inductors are held down by aluminum rivets and spring steel quick nuts.

I gather that a steel or other ferromagnetic hold down cannot be tolerated because the inductor would no longer be an 'air core' inductor. Is this correct? Just how big an effect would an 1/8" bolt down the center of the inductor have? "Humanspeakers" addresses this topic, too, qualitatively.

Thanks,

baumgrenze

If so, is it acceptable to use a brass screw and washer to hold down the inductor?

[johnieo]

>If so, is it acceptable to use a brass screw and washer to

>hold down the inductor?

yes, brass or aluminum screws are fine; please don't use a steel screw. Some x-o use hot glue and or cable ties.

>As I worked on my AR-2AX's and AR-5's I noticed that the

>inductors are held down by aluminum rivets and spring steel

>quick nuts.

Attached is a word file that gives the value of mutual inductance between two coils as a function of spacing. I used two AR coils as examples. Made these measurements for a couple of friends who asked this same question. If you have an inducance meter and a couple of spare coils, you can do this yourself.

If you look at the illustrations, you will see that you must literally stand the coils on top of each other to observe a large M. However, M is only part of the issue. The voltage induced in one coil is equal to M times the rate of change of current in the other coil. (E1 =M di2/dt and E2=M di1/dt) So it is conceivable that a some voltage could be induced in one coil if the current in the other were changing rapidly and M were significant. This might possibly be a practical issue if the coils were literally touching, and the drivers in question could reproduce the coupled signals.

Regarding the little spring steel nut-- I just measured the inductance of an AR #9 coil used in the AR-3a with / without the steel nut; L increased from 3.22 to 3.23 mH. Ditto for a second coil: 2.34 to 2.35 mH. Practically speaking, read this carefully--the AR #9 coil is 2.85 mH!!! Roy C. has seen this coil range from 2.35 to 3.2. So a 0.01 mH increase in a large coil is "noise" compared to the design value variation. I didn't have a 0.044 mH coil free to test. Someone else might have fun doing that.

2117.doc

[Carlspeak]

A quick survey of the air and iron core inductors offered by Parts Express shows a CLAIMED tolerance range of 1-3% of stated value regardles of the mH value.

A check of Tom Tyson's 1974 coil table shows coil mH tolerances increasing with decreasing mH values. Higher values like the #9 were about 3%. Low values

Interestingly, number of winds tolerance was only 1 or 2 over the entire range of coils. I wonder then why the lower mH valued coils were allowed to vary more. Just a guess here >>> because they were typically wired in parallel with drivers?

I'm not nit picking here - just curious.

It's all about the music

Carl

Carl's Custom Loudspeakers

[Guest baumgrenze]

Once again, thanks for the further input. I appreciate your patience with my 'novice' questions.

For future reference, is there a reason why seasoned participants seem to avoid talking about the "Human Speakers" guy except to do so obliquely? I'll be glad to conform without explanation if that is the norm.

baumgrenze

[johnieo]

Carl:

The inductance of a coil is determined as you know by the number of turns, the wire diam, insulation thickness, and bobbin geometry. An inductance calculator will yield 52 turns for the 0.044-mH, AR-#1 coil. Inductances of 0.047 and 0.041 mH yield 53 and 51 turns, respectively. So for starters the inductance tolerance appears to be tied to the turns spec. Inductance and number of turns don't increase linearly, so a 1-turn spec will translate to a smaller inductance variation in a larger coil. Also, final value will depend on winding density, uniformity, and so on, so same number of turns might not always yield exactly the same inductance.

[Carlspeak]

>Carl:

>

>The inductance of a coil is determined as you know by the

>number of turns, the wire diam, insulation thickness, and

>bobbin geometry. An inductance calculator will yield 52 turns

>for the 0.044-mH, AR-#1 coil. Inductances of 0.047 and 0.041

>mH yield 53 and 51 turns, respectively. So for starters the

>inductance tolerance appears to be tied to the turns spec.

>Inductance and number of turns don't increase linearly, so a

>1-turn spec will translate to a smaller inductance variation

>in a larger coil. Also, final value will depend on winding

>density, uniformity, and so on, so same number of turns might

>not always yield exactly the same inductance.

I was also wondering if the winding density had an effect but dismissed it thinking inductance was primarily a function of wire variation and length.

Based on what you've said then, perhaps Parts Express *claimed* tolerances may not be true?

It's all about the music

Carl

Carl's Custom Loudspeakers

[RoyC]

As John mentioned, I have found several out of whack #9 (2.85mh) coils, but I should add that I have never found any other AR coils to be out of line with stated tolerances.

I have used a fair number of Jantzen 18ga air core coils sold by Parts Express. ALL measured right on the money.

Roy

[RoyC]

>For future reference, is there a reason why seasoned

>participants seem to avoid talking about the "Human

>Speakers" guy except to do so obliquely? I'll be glad to

>conform without explanation if that is the norm.

I doubt any of us know much about him other than his unimpressive "AR-3a Rebuild" described on his website.

Roy

[johnieo]

Carl:

I would assume PE could spec coils to the tolerance they do with no problems. I'll bet coffee modern winding tools don't count turns, but measure L on the fly and make very accurate coils.

The AR bobbin had a slot on one side from in which the first turn started and a hole on the other side from which the last turn exited. That arrangement permitted winding errors of +/- whole numbers of turns! I am sure today's equipment is far more sophisticated than that used 30-50 years ago!

To get a large inductance in a small space, one wants to pack the turns very close for good flux linkage. That is why the insulation thickness on magnet wire is a bit less than a half mil. The Janzen coils from PE looked well made; tightly wound and dipped in shellac--measurement difference likely less than repeatablity of my fifty-buck LC meter!

[Guest hilltroll67]

>>For future reference, is there a reason why seasoned

>>participants seem to avoid talking about the "Human

>>Speakers" guy except to do so obliquely? I'll be glad

>to

>>conform without explanation if that is the norm.

>

>I doubt any of us know much about him other than his

>unimpressive "AR-3a Rebuild" described on his

>website.

>

>Roy

>

I mentioned him "obliquely" because the inductor spacing/orientation thing was all I remember from his site - other than the general impression that he is nearly as opinionated as some of the "seasoned participants" here. Reminded me of my Dad.

Bob

[dynaco_dan]

Reminded me of my Dad.

Ditto!