Transformers for tweeter amps on ferrite donuts

[Wavebourn]

Hi!

I have some common mode input chokes from power supplies. They are wound on donuts with 20 mm internal diameter, 40 mm external diameter, 12 mm thick.
Each winding has 41 turn, 18.2 milli Henry.

The question is, what material can it be? What is saturation induction, and how much power I can pass through them for a power amp from 3 kHz frequency?

[nigelwright7557]

Cores can be air, iron powder, ferrire or iron.

Usually the wire thickness determines max current but also saturation can too.
Too much saturation can cook the inductor.

[Wavebourn]

It looks like a ferrite with relatively high permeability.

[wrenchone]

Well, common mode chokes are often wound on higher permeability ferrites (mu ~5000 or more) to allow a large amount of common mode inductance with relatively few turns. For the higher perm ferrites, saturation flux is usually less than that for power materials. Losses are higher, too, but that will hardly be a concern for an audio transformer. I back calculated and got an inductance coefficient of 10,000 for the core, meaning that it probably is made with high permeability ferrite. To get a ball-park idea of what you're dealing with, you can take the dimensions of the core and compare with the specs of other cores with similar dimensions. The Ferroxcube and Magnetics, Inc. web pages are probably a good place to start. A picture of the core here would help, as I might have a EWAG as the the maker, judging from the color and overall shape. They may even be marked.... Also, toroid core shapes/dimensions (at least for ferrites) are less standardized among manufacturers than other shapes like E cores, so often the exact dimensions of a core can offer some clue as to who made it.

These could possibly work to make a high frequency push-pull transformer to drive tweeters. You basically have two concerns, the first that you have enough turns to support the required volt-seconds, in order to avoid saturation. The second is that you have enough primary inductance so that you are not stealing a lot of power that should be going to the load in order to drive the magnetizing inductance.

The power you can pass through the cores will depend on the volt-seconds and the amount of copper you can load on. I don't think the core losses will be of any great consequence at audio frequencies. If you wind for a max peak flux density of 2500 gauss or so each way at max drive level, minimum operating frequency, you're likely going to be safe, as that's a very conservative level even for wimpy high-perm ferrite materials. Winding for this flux density also will most likely ensure enough primary inductance to get the job done.

[smoking-amp]

L = Mu (4 pi N^2 A 10^-9)/le

L in Henries
A core area in cm^2
N turns
le magnetic path length in cm
pi = 3.14159
Can figure the Mu from this and your data

Usual power ferrite material will be from 2000 to 5000 Mu
Bsat around .35 Tesla (thats useable Bmax, the max spec maybe more like .45 to .50)

10^4 Gauss = 1 Tesla

for design:
B = (V 10^4 )/(4.44 f N A)

B in Tesla
V in Volts
f in Hertz
N turns
A core area in cm^2

You can figure on 4 to 5 times larger area core needed in ferrite than in steel (toroid) to do the same job.

edit: at 10,000 Mu it would be about twice the losses of the usual power ferrites, maybe .4 Bmax Tesla (spec) and .3 Tesla Bmax useable. For comparison, Bmax spec for Ni permalloy 80 is around .7T and steel is around 1.8T I think. 1.5T Bmax useable is what I see often mentioned for grain oriented toroids for audio. For EI core its more like .7T for real HiFi OT and 1.8T for a guitar OT. Virually all of the low cost OTs around are rated as guitar OTs (Wattage rating).

[Wavebourn]

Thank you gentlemen!

It has FDK stamped on it, the core itself looks shiny, almost like a hard ferrite, so 10,000 may be a good guess.

Looks like probably I can pass 100W through it easily.

[smoking-amp]

100 Watt! Maybe at 50 KHz.

You need 4 or 5 of these (attached) 6 inch ferrite toroids stacked up to do a 100 Watt standard audio OT. Like $50 each unless you get samples.

[Wavebourn]

I am going to torture one using generator, resistor and oscilloscope...

Originally Posted by smoking-amp

100 Watt! Maybe at 70 KHz.

You need 4 or 5 of these (attached) 6 inch ferrite toroids stacked up to do a 100 Watt standard audio OT. Like $50 each unless you get samples.

I will check... The idea is, to use it's primaries in series, secondaries in parallel with 8 Ohm outputs, with Edcor's 10K P-P 100W transformer to equalize it's response on high end.

[wrenchone]

Smoking amp - you really need to take the Bsat data for ferrites with a grain of salt. Most of the Bsat numbers given are at 10 Oe or so MMF where the material is really in deep saturation. The real numbers are something like about 3500-4000 gauss at room temperature, about 1000 G less at 100C. That's for power material. The high perm materials have a lower saturation flux density, so actually, the 2500G number I gave may be a little bit on the wild side, with 2000G being a more sensible number. You also want to accomodate an asymmetric waveform without going into saturation.

The FDK designation comes from Fuji, so the cores are on the old side. It doesn't appear that Fuji is in the ferrite business any more, but I may have some old data squirreled away (no promises, though). Trying to do a 20-20k transformer with these cores would be insane, but a transformer working from 5k-up would be doable. The equations given by Smoking-amp are a good guide, especially the second, which is the classic "transformer" equation. Rearrange it so you can figure out the turns needed for a given voltage and flux swing. Once you get the volt-seconds issue dealt with, the throughput power will be determined by the copper losses, keeping in mind that the saturation flux density of the ferrite goes down with increasing temperature, so it is inadvisable to let the core get too hot (it will get heated up by the windings).

I've been thinking for quite some time of doing a dedicated tweeter amp with a ferrite output transformer, or augmenting a transformer with limited bandwidth with a small, HF transformer.

[smoking-amp]

"The real numbers are something like about 3500-4000 gauss at room temperature, about 1000 G less at 100C. That's for power material. "

That's what I meant earlier by "useable" versus "spec" Bmax. In the final analysis, one has to work out the magnetizing current one can live with. Hopefully a tweeter OT is not going to get hot. We are in basic agreement here. In any case, on Wavebourn's 10K Mu material, he should find the Fuji datasheet to check the relevent curves for certain. I just took a quick look at Fair-Rite's #76 material, which is a similar 10K Mu matl.

"I've been thinking for quite some time of doing a dedicated tweeter amp with a ferrite output transformer, or augmenting a transformer with limited bandwidth with a small, HF transformer. "

I thought about this once also, RDH4 has a frequency splitter/combiner circuit for a dual freq. band OT. (page 888) Maybe for use with a dedicated speaker setup.

Trying to do the LF or the whole band (ie 20 Hz) looks impractical to me with ferrite. (5 six inch cores!) Huge cores. Winding capacitance issues at the top end. I've heard some negative comments on line before about some attempt at a ferrite OT, ie sounded bad, but I doubt if they really used sufficient ferrite.

No reason to expect much difference really from a steel toroid except maybe bandwidth and initial permeability. Might be interesting to do a hybrid version with a steel toroid and a ferrite toroid in the same windings. The ferrite should give better initial Mu at low power and the steel should handle the flux for high power. Sort of like pin-striping with permalloy.

I'm still turned off by the idea of 1000s of turns thru a toroid though (at least without a toroid winder machine). If I use ferrite it will be for a Berning type OT. Or a switched capacitor OT. There was a thread here a few years back where these got worked out conceptually. Once one gets the PC board for the SS stuff worked out, it should be easy to do any OT that way. Even does the HV B+ for free.