Ferrite cores for audio transformers?

[gyraf]

Hi Group,

And Mr. Hacksaw in particular..

I have been researching for equalizer-inductors for my G14 lately - and it seems that there's some extremely efficient ferrite core material around.

E.g. Epcos makes ferrite core sets with an Al-value of 28.000nH/T2. That is a staggering 2.8mH for ten turns..! :shock:

Which means relatively few turns to obtain desired inductance. Which again means controllable winding capacitance. I use up to some 20H inductors, and these are actually possible to get working without too much trouble, using these modern ferrite materials..

So my question is:

:?: Can ferrite cores be used for audio transformers?

Has anyone tried? Is there any industrial examples of this?

Any guesses on possible bad side-effects?

We known that ferrite are used for tape heads, so it may not be an all-together bad material for audio frequencies.. Or..?


Jakob E.

 

[Bo Hansen]

Hi Jakob,

I remember that the Kongsberg from Norway use ferite core for all transformers in the old NRK console series made for the Nordic broadcast corporations.
Also Nagra use ferite cores as transformers in the taperecorders under 1960/70`s (maybe I have some information about core types and coil turns, I will take a look in my Nagra manuals)

I think ferite cores must be big because they saturate more easy compare with a normal transformer core.
Maybe a filar winding structure can be suitable fore a ferite core with a single bobine ????

-Bo

 

[clintrubber]

At least I saw it being mentioned this morning in
the .pdf PRR kindly provided:

http://headfonz.rutgers.edu/zero-Z-tranny.pdf 2.2Meg PDF file.

It's on page 337:
'A more economical version, using a ferrite transformer core,
is being used by Quad Electroaucoustics..."

Bye,

Peter

 

[mik]

[quote author="gyraf"]E.g. Epcos makes ferrite core sets with an Al-value of 28.000nH/T2. That is a staggering 2.8mH for ten turns..! :shock:
[/quote]

:shock: where to find those core ?
I also looking for core for inductors EQ. till now I've only experimentated with PM series and inductor cames are too big...

 

[gyraf]

where to find those core

www.schuricht.de - they ship internationally (fast!)

Look for Epcos EP- and P-core..

Remember to get coil former and clamps when you order.

 

[PRR]

> Can ferrite cores be used for audio transformers?

Of course.

Historically, ferrites gave much too low inductance for good bass response with low winding resistance. Sort of halfway between air and iron.

But there is a LOT of research for switcher power supplies which need extremely low copper and core loss. And some of these new ferrites are getting up around the inductance of iron.

Watch your saturation. I was astonished when I saw audio power output transformers on "amorphous" cores: historical ferrites would never carry that much flux, but I guess they do now.

"Ferrite" is an old word and I don't think any of the new materials are strictly ferrites. But we have air, iron, and "other stuff", so everything else tends to be called ferrite.

 

[gyraf]

..this is beginning to look good..!

Historically, ferrites gave much too low inductance for good bass response with low winding resistance

I'm getting some 15H:15H at 100+100 Ohms copper resistance. Self-resonance still way out of audio band, so stray reactance dosen't seem to be a serious limiting factor here..

 

[mik]

thank you J.
M. :thumb:

 

[tk@halmi]

As usual I get half (or less) of the story right most of the time, but I have read it somewhere that ferrite pot cores are self shielding. It kind of makes sense as the core encases the windings. Could pot cores be used for audio or I am missing something?

Thank You,
Tamas

 

[gyraf]

Could pot cores be used for audio or I am missing something?

This is exactly the topic of this enquiry.. :grin:

Jakob E.

 

[tk@halmi]

OK, I missed the "pot" part. Wow, this would be really cool.

 

[Guest]

[quote author="PRR"]
Historically, ferrites gave much too low inductance for good bass response with low winding resistance. Sort of halfway between air and iron.
[/quote]
What do you mean Iron???

Initial permeability (1)/saturation induction (T)

Air 1 / infinity
Iron (Fe-Si) 200-500 /1.5
Feritte 5000 /0.2
Mumetall (FeNi76Mo) 20000/0.8

I mean, that good ferite is somewhat between Iron and mumetall,
Some like FeNi50 (Py50) , but with saturation order of worse.

For audio chokes, O.K, but for transformers :-/ ,
and magnetophone heads - only erease.

xvlk

 

[CJ]

One experiment worth a 1000 theories.
Try one out Jakob!

Sounds interesting.

 

[Consul]

Okay, when you say "ferrite" core, is that referring to those powder cores I see used in a lot of RF stuff?

Just seeking a little guidance... :thumb:

 

[Guest]

[quote author="Consul"]Okay, when you say "ferrite" core, is that referring to those powder cores I see used in a lot of RF stuff?
[/quote]
ferric ???
Carbonyl Fe powder core ?
The cheepest and the worst. It is used for power supply
RF reject filter. They are so bad, that damp itself.
Without perspectives in audio...
There are really between Fe and air, mu about 20.

But there are better powders to audio,
sendust and permalloy powders? They was widely used
for pupination coils in telephone cables.
They are scrapheaped for around the world...
mu about 200, toroid, I use this in speaker crossover )).

xvlk

 

[CJ]

Jakob, you can put a resistor in series with the primary of your inductor and see where it saturates. Put your scope across the resistor. Use a big enough resistor to get your signal off the noise florr. (above fuzzy signal)
Then note the input voltage at saturation. This will tell you how many db it will take. This curve tells you when it is satutated:

Also, if you can get the charts on the core material, you can calculate how many kilogauss at 20 cycles with:

B=RMS Sine input x 10^8/(89 x Number of turns x Cross sectional area in square centimeters)

Once you get the gauss, look at the hysterisis charts for that core material and see if you are below the knee or not.
cj

 

[peterc]

Does anybody remember this info posted (AFAIK) by Geoff Tanner?


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 found this very informative. :thumb:
Regards
Peter

 

[Guest]

[quote author="peterc"] would be 3B7 which is produced by Ferroxcube...
http://www.ferroxcube.com/prod/assets/3b7.pdf
[/quote]
For someone OKSIFER or SONOX can be in the drawer...
xvlk

 

[gyraf]

Thanks, guys!

I'll go take some measurements..

edit: At 20Hz sine, I get some 3.6VAC through the winding before something like CJ's picture develops across a 47R series resistor. This shot is taken with a 200 Ohms generator source impedance, primary inductance 15H, no secondary load:

Jakob E.

 

[ijr]

One listen test will be better then 100 theoryes.
Elsewhere I've read OEP's sounded better than Jensens to some people...
Why not? Maybe this sound is what they are searching and
let's leave all this stuff about quality & etc...
Well, maybe ferrites will be new page in history of audio?
:roll:
In audio transformer iron working below 500 Hz
(most cases), allmost all above 500 Hz is how do you wind the trafo;
there's some tricks allowing to make low level 600:50K trafos with
very good HF freqresp. from relatively cheap iron, say,
-1dB @50KHz...but good clean LF resp. from ferrites is new thing.
They have different saturation curve and too much distortion at low freqs.
(another time, sorry for my bad english).
I know russian folks who tried to use tens of types of ferrites
for interstage and input trafos in tubamps...
and they were not limited in time or money...
they spent too much time to proove old theory:
ferrites not very good for LF.
I am happy with homebrewed out trafos wound on
0.006' 50% Ni EI625 Arnolds Magnetic Company
old iron and UTC/Jensens for inptuts. )
But...who knows???? Maybe new ferrrite trafos will be better than
Jensens.... :roll: :grin:
EDIT: Yakob, you are GREAT!
RadioDesignersHandbook, p.888, Loudspeaker Divider Networks,
fig.21.9 schem. with two output trafos!!!!