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Old 5th March 2004, 03:52 PM   #1
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Jason Hubbard's Avatar
Join Date: Apr 2001
Location: Poole, England
Default Air core inductors

I finally bit the bullet and got some inductors made up for me.

They worked out at 12 each for a 2.4mH inductor wound using 1.5mm wire which is tightly bound, impregnated with varnish and then baked hard for 8 hours.

I bought 4 of them to build my first monoblock power-amplifier - they weigh 750g each. The quality is excellent, i'm very pleased and looking forward to using them.

If anybody has interest in these or similar inductors then you can contact Dave Stephenson direct on +44 (0) 1622 670338.

First time i attached a picture - hope it works!
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File Type: jpg inductorsml.jpg (12.4 KB, 756 views)
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Old 5th March 2004, 03:57 PM   #2
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Bill Fitzpatrick's Avatar
Join Date: Jun 2001
Location: Eugene, OR
40+ years in audio and I've never seen an inductor like that. Do you mind telling me what they do in the amp?
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Old 5th March 2004, 06:33 PM   #3
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Jason Hubbard's Avatar
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Location: Poole, England
Default What do they do?

They are to be the L element in a Pi filter.

I'm sure they could be more compact, but the size is not an issue to me as i have pretty much as much space within the monoblock enclosure as i need.

I intend to use a CLCLC arrangement with these inductors and 58,000uf capacitors - probably overkill, but this is DIY so no problem.

Got the inductor dimension details from here...

...and then had the coils wound to the spec suggested from the website.

Measured on an LCR meter at 2.4mh and DCR 0f 0.45ohms so pretty close to calculated values.
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Old 2nd April 2004, 07:26 PM   #4
SineEra is offline SineEra  United Kingdom
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Join Date: Nov 2003
Location: Manchester. UK
Wondered how the CLCLC PSU worked out? As I am considering a choke regulated supply with my Aleph X.
Not sure from your picture where the connections for the inductor are.
Any chance of more pictures of your PSU?
Did you try the cap across the transformer secondaries as per thread Choke for Pi Filter ?

thanks Simon
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Old 2nd April 2004, 08:51 PM   #5
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Jason Hubbard's Avatar
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Location: Poole, England
Default not built my power supply yet....

Sorry can't give details of how things have worked out on these inductors as i've not put the supply together yet (still awaiting capacitor delivery from the group buy, but that will happen anytime now).

The leads are hidden beneath the inductor - they are about 300mm long and will allow me to stack the inductors in such a way as to create mutual inductance (i'll be using a CLCLC filter) and increase the realised inductance values.
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Old 3rd April 2004, 12:31 AM   #6
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Location: Los Angeles, CA
Default The coil calculator website doesn't seem to work...

Hi Jason,

I came across that website last night while trying to design some inductors myself, but I could never get it to work. I enter the desired inductance and it alsways comes back with a server error.

Have you used it lately?

Here's one that is just as good, although you have to supple the coil diameter and width (called "length").

Coil Calculator
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Old 3rd April 2004, 04:45 AM   #7
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Default Jason, Can you post a close up picture?

Of that coil inductor. I'm thinking of winding my own and was wondering just how perfectly commercial coils are wound.

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Old 3rd April 2004, 09:18 PM   #8
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Location: Poole, England
Default How coils are wound

Bear in mind i'm drawing on my own very limited experience here, but it seems to make some sense....

The first thing you need is a former - the width of it should be a multiple of the diameter of the wire that you are using.

Feed the width and the height into an inductor calculator website (the link i gave is definately not working properly, but the one that is mentioned later in the thread is better anyway as it allows more control over the parameters used to calculate the answers).

Using the answers generated from the website as your guide wind the coils as tightly as you can onto your former. You could use cable ties to hold the coils in place when you remove the windings from the former but i'd not keep them there any longer than necessary - replace them with cotton tape spiral bound around the coils and tied off tightly, then remove the cable ties.

My coils were then immersed in varnish for 10 minutes and baked for 8 hours - there is absolutely no chance i can see of the coils resonating against each other or breaking free for the others
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Old 4th April 2004, 10:49 AM   #9
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Default I've been reading up on coil making(long)...

the last few nights because I want to upgrade the x-over's in my speakers. I have Hammer Dynamics Super-12 high efficiency single-drivers.

They feature a 12" driver which handles 45-10kHz with a super tweeter crossed in above that. John Wyckoff, the designer, wanted the crossover point well beyond the range of critical hearing where normal crossovers work ~ 2k-4kHz. The tweeter is mounted directly infront of the main driver to maintain time-coherence much like a coaxial (see picture below).

The tough thing for me is that I have two coils in series on the main driver(0.65 mH & 0.24mH) that have to handle a very wide range of frequencies well.

Common belief is that heavier gauge wires handle low frequencies far better than thinner gauges at the expense of high frequency performance and vice versa.

I don't have a lot of money to experiment with different coils, so I thought that I would go with a known winner for my speakers - Solen Hepta-Litz multi-stranded since the maker of my speakers said that they were the single most impressive upgrade they've ever heard.

I talked to a Solen technician who actually suggested that I use their $5 16 gauge Litz coils for my 12" and their standard (and cheap) $1.75 20 gauge coils for my tweeter. Now, that is a value buy!!!

But, having said all that, I'm still intriqued by Northcreek coils which are reputed to be some of the best made for deep, detailed, bass. Northcreek swears by the belief that the larger the gauge the better. In fact, their top coil is a whopping 8 gauge!!! They even use 10 gauge coils on the tweeters in their top of the line speakers.

This kind of flies in the face of all that talk about "skin effect" and poor high frequency response in heavy gauge wires.

Of course, Northcreek coils, especially their heavy gauge ones, are way out of my price range. There's no way I can afford to spend $200 on coils as an experiment.

**BTW, the Solen technician told me that their 10/12/14/16 ga. standard single wire coils exhibit "non-linearities" above 5kHz. The 18 & 20 ga. extend to 26kHz before problems.

More surprising was that their Hepta-Litz has problems at:

10 ga. >300Hz
12 ga. >500-1kHz
14 ga. >8kHz
16 ga. >25kHz yet extends up 100kHz

I can't afford to risk big money(for me) on the unknown like heavy gauge coils like Northcreek, but I still want to try them out. I have heard really great things about their big coils.

So, that's what got me thinking about making my own. It's no where near as complex as transformers. Although, one supposed problem with winding large gauge coils is that the high tension necessary to bend the wire while winding causes micro-fissures and irregularities in the crystalline structure of the copper(I said supposedly). Northcreek claims to use a "low-tension" method of winding which avoids this.

I was thinking about how many "low-tension" ways there are to bend wire and the only thing that I can come up with is to heat it. Most large magnet wire is rated to 200C (391 F), so it can handle pretty high tempetures. Why not slowly heat the spool of wire in a pot of boiling water to get it up to 100C(212 F) so that it is plyable enough to unroll and roll easily without too much tension or stress.

Northcreek also talks about a DIY method of "goop-wound" winding which is done by adding an adhesive like hot-melt glue while the coil is wound.

Goop-winding is the process of winding an inductor inside a bobbin or former, while a bonding agent (usually hot melt adhesive) is applied directly to all of the windings throughout the entire process. Goop-winding makes a very unattractive inductor, but they sound remarkably good. This is the only method recommended for those that wish to "wind their own".

So, all you need to do is spread hot-melt glue to each layer and around the outside when it's done. Allow the coil to slow cool and your done.

The way to make sure that you get the exact value needed is to use a multimeter with one of the wire penetrating points and test at different lengths of the last layer until it reads the proper value., then cut.

An 0.65 mH 9 AWG coil like I need would have:
4.0" inner diameter
1.5"x1.5" winding
76 ft of wire
81 turns
6.18 layers
13.11 windings per layer
3.01 lbs weight

And a DCR of 0.05 ohms. just a little over Northcreeks 0.03 ohms rating. Of course, in the real world it probably won't rate that low, but who cares in that region.

Now, I don't expect it to be anywhere as easy as I've discribed it, but I can get 350 ft. of 9 AWG copper magnet wire for $25 from a local surplus store. That's enough to do all 6 coils I need. And it won't hurt to give up if I find out it's too much hassle.

What do you think?

Click the image to open in full size.
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Old 4th April 2004, 05:05 PM   #10
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Default Goop winding

This sounds like a very good idea - as you said it won't look pretty but then that's not what the exercise is about is it?

If you can buy the magnet wire surplus then you have very little risk, so go for it!

My only comment would be that sometimes the crossover network relies on the DCR of the coil being as intended by the designer - if you change the coil's DCR then you risk upsetting things - I remember reading from the B&W website where they advised to simply incorporate a high quality resistor in series with the inductor to retain the DCR behaviour of the original as closely as possible. May not be necessary in your case but might be worth checking out just in case....
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