Ferrite Core Inductors for Crossovers

[r_s_dhar]

1. Could someone suggest a nomogram for designing a DIY ferrite cored inductor to be used in passive Xover?

2. The ferrite rod found in older AM radio sets -- can we use this as a core? A friend of mine tells me that these rods are unsuitable as they get easily saturated at audio frequencies.?????

 

[kelticwizard]

I think the way it works is that ferrite core inductors get saturated at high frequencies, not low. Anything suitable for radio work will easily be OK for audio frequencies.

The major purpose of adding ferrite is to cut down on the number of turns required for air-core inductors. The more turns, the higher the DC resistance. The higher the DC resistance, the less desirable the inductor. However, ferrite adds undesirable distortion effects, so it is a balancing act. Air core inductors give superior performance, assuming that you can get one with a low enough DC resistance.

The basic approach is to use air core inductors unless the DC resistance simply gets too high, then switch to ferrite core.

 

[r_s_dhar]

Thanks Mr. kelticwizard for that piece of information

 

[Lisandro_P]

My speakers use a passive LCR filter to feed the tweeter, and the inductors use a ferrite core. They're SMALL, and i haven't messed with those yet (they look handmade and somehow fragile, the coils are very tidy and covered with shrink tubing), but these speakers sound to me, and everyone else, amazing. It's a compromise; i'd love to switch the inductors to air cored ones, and perhaps there'd be an audible diference, but for me it's just not worth it.

 

[Thatch_Ear]

Lisandro,
Inductors limit the highs. If you have small ones in the section with your tweeter it is simply to limit how high it will go. It could be your tweeters need to operate under a certain level to avoid dammage. If that is the purpose I can see little advantage to changing them out as where they are cutting the frequency off is more than likely above the range of human hearing. Just a guess.

 

[Lisandro_P]

Exactly. I tried to not to mess too much with the crosover (only replaced a bipolar electrolytic with a polyester cap), because it has quite an impact in the sound and it's suposedly optimized for the drivers and the enclosure.

 

[planet10]

Inductors limit the highs. If you have small ones in the section with your tweeter it is simply to limit how high it will go.

Are the LCR in series or is the L shunting the tweeter?

dave

 

[planet10]

The major purpose of adding ferrite is to cut down on the number of turns required for air-core inductors. The more turns, the higher the DC resistance. The higher the DC resistance, the less desirable the inductor. However, ferrite adds undesirable distortion effects, so it is a balancing act. Air core inductors give superior performance, assuming that you can get one with a low enough DC resistance.

A middle ground is to use a gapped inductor -- similar to what is done in SE Output transformers.

Some paper wrapped around the core before you start laying on the windings would probably do the trick (Note: i've never actually done this myself)

dave

 

[Lisandro_P]

The R & C is in series with the tweeter and the L is conected to the C-tweeter node and ground.

 

[planet10]

The R & C is in series with the tweeter and the L is conected to the C-tweeter node and ground.

That makes it a 2nd order filter (with a bit of padding -- the R).

You can ignore what Dave Thatch_Ear said about the L rolling off the top.

dave

 

[Lisandro_P]

Damn, you're rite... still, i'm not touching that inductor. Yet, at least

 

[r_s_dhar]

Calculations for Ferrite Core Inductors

When I started this string I was also looking for help in calculating values for the construciton of Ferrite Core Inductors. Are there any nomograms available? Please can the elders help?

 

[Lisandro_P]

Heh... building inductors is much an art as it is science. I once built a large one for an onboard tone control for my guitar, and i easily run into 4 or 5 different equations for a same shape (toroid).

Try a web search on google, that got me started.

 

[f4ier]

dhar,

CALSOD has a free design program for air-cored inductors. You should avoid ferrite cores as much as possible to get the best sound out of your filters.

If you really want to use ferrite cores for your inductors, you'd have to measure the inductance (with an inductance meter) after every several turns of wire until you get the desired value.

good luck

 

[kelticwizard]

Well, it's a little late, but I could not help but post this real easy air core inductor calculation program I came across.

I am sure the CAL SOD program is good, but this is one more option.

If you have decided to go air core, this is one more program to help you. Fill in the desired value in a box-bing!-get the number of turns you need.

Say dhar, just how big an inductor do you need? Alternatively, where are you trying to cross over? If you are looking to crossover at 100 Hz, then air core inductors might be simply too big. If you plan on crossing over at 500 Hz or above, then air core inductors are a reasonable way to go.

I might be able to dig up something for a ferrite core inductor. However, there is one thing-there are different kinds of ferrite. Some ferrite cores add a lot of distortion but gives you a lot of inductance for a comparatively few turns. Other forms of ferrite require more turns of wire-less than air core inductors, of course-but give low distortion that begins to approach air core inductors. You would have to know
A) the permeability of the ferrite you are working with
or
B) the inductance of a given ferrite inductor and just calculate how many more turns are required to change it.

Let us know how you are making out on the project!

 

[kelticwizard]

PS: There are places, such as Parts Express and Madisound, where you can get audio quality ferrite core inductors at a very large range of values, if it comes to that. I am sure there are other places that carry them as well.

 

[r_s_dhar]

Ferrite Core Inductors

Thanks Keltikwizard.

I would like to crossover at 500 and 5000 -- I dont think one has much choice here.

Grain and permeability -- those, precisely, are the points. Would 10cm ferrite rods found in pocket AM/FM radio sets of the 60s and 70s do?

As for Air Core design, I have enough material. I came across some Elektor designs in one of their loudspeaker specials and every one of them was based on Ferrite core. That set me thinking. May be, I thought, things have changed a lot in the West and they must have come up with some magic ferrite compound that gives the best of both worlds. Well, is there such a magic compound or are people going back to the dear old known ferrite rods?

Thanks again.

 

[Vivek]

Shashidhar, try www.lalena.com. They have online inductor calculators.

 

[kelticwizard]

RS:

I can't believe what I just did. Yesterday I posted a long message telling you I was going to to give you the link to an air core inductor calculator. Then, I didn't give you the link!

The men in the white coats should be coming for me any moment now.

Here, at last, is the link:
http://www.colomar.com/Shavano/inductor_info.html

If you are using an 8 ohm speaker and a 12dB/octave crossover, you want the inductor to be about 12 ohms at the crossover frequency. That would indicate an inductor of 4 mH.

Using that program, here are wire guages, forms, etc. to wind your own inductor of 4 mH.

Note: I believe the wire gauges are somewhat different in America and India. Note that the wire diameter, in Metric, is included so you can get the right gauge. I believe the chart is in American gauge wire.

I would post the chart, but it dies not fit with the posting parameters of this BB program. Suffice it to say, American gauge 14 thru 18 are the best choices. The 18 gauge wire has about a third of the cross-sectional area of the 14 gauge, and shoud cost commensurately less-but have higher DC resistance. If possible, try to keep the DC resistance below 1 ohm, even 1/2 ohm if possible.

If forms that fit the model are not available, you can interpolate by the forms that are larger or smaller. Things are not that exact with this.

 

[r_s_dhar]

Ferrite Core Inductors

Thanks Vivek.

For air-core coils there is another interesting programme that you can dowload. This one gives you values even for square or rectangular bobbins -- which means that you can make use of plastic bobbins generally used in power transformers.

you can download this programme at: [/email][email protected]