Found Ferrite cores - any good for audio?

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jhaible

Well-known member
Joined
Jun 24, 2004
Messages
530
Location
Germany
I found some _huge_ ferrite cores in a dumpster at work.
Before I grab them and take them home, I'd like to know if they are any good for audio.

here's what I know:

3C 85 printed on them.

Farnell label on the bag, reading:

N2-5B
PHILIPS 431202037350 E-Kern 49x25

Farnell order code 177-779 (not valid anymore)

I did a quick, rough measurement of AL (with a single
turn), giving something in the 3500nH range.
They are so huge, it should be no problem to wind
500 ... 600 turns to get Inductors in the 1H range.

Big question: Will it be worth the effort? Will they sound
good in a passive EQ or the like?

Any hints and opinions welcome!

JH.
 
[quote author="gyraf"]I'd say any high-AL-value ferrite core is worth having..[/quote]

I salvaged them.
I found another bag with slightly smaller cores, almost the same AL.
And the bobbins and clips to match.

JH.
 
Pultec inductors, maybe?

You would need to do a sweep test.
Those darn ferrites come in so many flavors, you never know what is gonna work for audio.
Most were designed for hi-freq work.
Wind an inductor, put a cap and resistor in series. wind it up and plot the bell.
If one side of the bell is missing, it will not be good for Pultec.

Ferrite pwr xfmr?
Never heard of it.

Strip wound 29 ga M6 core in the shape of a torroid, yes.
 
There were some comments about cores a while back, cant remember who made them. Interesting tho'....

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Does someone know what material the core of the 1073 locut inductors are made of?


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These are ferrite "pot core" type inductors. The core can come in a number of shapes and sizes. The material itself would be a "green ferrite". There are a number of formulations that were made that each had different characteristics. Then there are bobbins and various mounting systems and clamps that hold these things together.

Every manufacturer has it's own formulations and they produce datasheets that should help in selection. A typical material you can look at would be 3B7 which is produced by Ferroxcube...

http://www.ferroxcube.com/prod/assets/3b7.pdf

Ferroxcube lists a number of other materials as applicable in audio LC filter applications i.e. 3D3, 4A11, 4B1, 4C65, 4D2, 4E1...

you would be looking at Quality Factor vs Frequency curves to see which material will yield higher Q in the operating band and you would want to look at the B-H curves... Flux Density (B) vs Coercive Force (H) or Hysteresis Loop... to see if the loop is reasonably closed and flat over a broad range... or not. The B-H curves are where a lot of the "color" is... as to what Neve actually used... Geoff Tanner MAY know who the original manufacturer was... Robin Porter at Neve would know since he is still have the original 1081 inductors made. Neither will tell you

Here is an overview of the various materials Ferroxcube produces...

http://www.ferroxcube.com/prod/assets/sfmatgra.pdf

other links

http://www.ferroxcube.com/

http://www.fair-rite.com/

http://www.mag-inc.com/

I have an application sheet for Ferroxcube core materials that the above data came from, it is a .pdf, so I can email it to you if you wish.

Peter
 
Ok, I found a data sheet for this specific material:

http://www.alg.demon.co.uk/radio/136/3C85.pdf

It's still hard to understand what it's telling me.
For instance, I know the area in the hysteresis loop means
the power loss - but it doesn't tell me how it will sound.
(Or does it? Maybe someone experienced could tell me it's
no good, or worth a try, after a short glance.)

Chris - your method of actually measuring a bell courve
sounds like the right thing, but this means I have to wind the
whole thing first.

Is there any analytical/theoretical way to decide if a material is
"interesting" (i.e. worth winding an inductor and then listening
to it in-circuit for the final decision) ?

JH.
 
Oh, I forgot: There's one thing I did, of course:
Measuered the L of one turn at different frequencies.
It's pretty consistent at 100Hz, 10kHz and 100kHz.
(I used a HP 4191A analyser.)

JH.
 
[quote author="peterc"]

I have an application sheet for Ferroxcube core materials that the above data came from, it is a .pdf, so I can email it to you if you wish.

Peter[/quote]

Thanks for the links, Peter. This application sheet, does it have 3C85 material listed?

JH.

jhaible(at)debitel.net
 
Just put a hundread turns on there, measure the inductance, find a cap that will form a res circuit at a fairly high frequency, say 20,000 cps, then do a bell curve.

Plot the ac voltage vs frequency across a series resistor, say 100 ohms, and you be in like Flint.
 
[quote author="peterc"]
Thanks for the links, Peter. This application sheet, does it have 3C85 material listed?

No it does not. Goes from 3C81 to 3C90, none of the Ferroxcube data I've found seems to include that formulation.

Peter[/quote]

It seems it's more for power applications than for audio. Given the place where I found them, it's a lot more likely they have been bought for switched power supplies, than for audio.

When I find the time, some time, I'll give Chris' method a try, nevertheless.

Thanks to all who replied!

JH.
 
If they are for power applications, they will nearly always have an air gap in the core to linearize Al-values and reduce saturation. This will greatly reduce Al-value though. But your relatively high Al-value suggests that these are ungapped cores - which imo will make them good candidates for audio work. If you can manage to get something like 2x5 or 2x10H into them, try winding a signal transformer - that'll show a lot about what the core behaves like. Our friend CJ posted a test-setup for transformer saturation and B-H-curves here - should be in the trafo meta.

Jakob E.
 
[quote author="gyraf"]If they are for power applications, they will nearly always have an air gap in the core to linearize Al-values and reduce saturation. This will greatly reduce Al-value though. But your relatively high Al-value suggests that these are ungapped cores - which imo will make them good candidates for audio work. [/quote]

Good point!

If you can manage to get something like 2x5 or 2x10H into them, try winding a signal transformer - Jakob E.

Now there's an idea!
I never thought of that - on the tiny pot cores I've used before I was aiming into the 100mH ... 1H range. But with this huge geometry, 10H would certainly be possible!

JH.
 
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