Winding audio x-over inductors, core questions

[James Lerch]

Greetings All,

I'm finally building a set of surround sound speakers for the house.
I'm currently working on fabricating the cross overs for the midrange
drivers.

The question is, how would you measure if an iron cored inductor is
going to saturate and cause a problem?

The golden rule for 'audiophiles' is air cored inductors only in the
signal path, but I need 5x 0.6mH inductors and that turns out to be
ALOT of magnet wire. (approx 270 feet of 20 gauge wire for 5 air
cored inductors.)

Being a pack rat, I've salvaged a bunch of toroidal cores out of
scrape 360watt SMPS. The cores were originally used as inductors on
the line input section. I've test wound one of these cores with ~8
feet of 20ga wire, and achieved my goal of 0.6mH. Here's a picture of
a bare core and my home made inductor if it helps:

http://lerch.no-ip.com/atm/Ind_Core.jpg (170KB)

At the moment, I am at the decision point of deciding between abiding
by the 'audiophile law' or just using the iron cored inductor.

If it helps, this inductor will be used as part of a Butterworth 3rd
order low pass filter, with a crossover frequency of 3Khz, driving an
8 ohm midrange speaker at no more than 50 watts rms.

Also, I can measure the impediance vs frequency over the audio range,
can I test the iron cored inductor at various amplitudes settings and
get an idea on the likely hood of core saturation becoming a problem?

Thanks in advance,


Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)
http://lerch.no-ip.com/ChangFa_Gen (My 15Kw generator project)
Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent.
Calvin Coolidge

 

[Phil Allison]

"James Lerch"

The question is, how would you measure if an iron cored inductor is
going to saturate and cause a problem?

The golden rule for 'audiophiles' is air cored inductors only in the
signal path, but I need 5x 0.6mH inductors and that turns out to be
ALOT of magnet wire. (approx 270 feet of 20 gauge wire for 5 air
cored inductors.)

** A 0.6mH air core inductor is quite small.

You grossly exaggerate.

Being a pack rat, I've salvaged a bunch of toroidal cores out of
scrape 360watt SMPS. The cores were originally used as inductors on
the line input section. I've test wound one of these cores with ~8
feet of 20ga wire, and achieved my goal of 0.6mH. Here's a picture of
a bare core and my home made inductor if it helps:

http://lerch.no-ip.com/atm/Ind_Core.jpg (170KB)

At the moment, I am at the decision point of deciding between abiding
by the 'audiophile law' or just using the iron cored inductor.

** How do you know it is iron ?

More likely ferrite or a powdered iron composite.

Core saturation ( and consequent loss of inductance) occurs when a
particular current level in the winding is approached or exceeded. To find
out, just put your part in series with a 4 ohm resistive dummy load and
drive the combination from an audio amp with a sine wave at say 1000Hz.

With one end of the inductor grounded, monitor the waveform across the part
for distortion - when you see some the game is up.

I sat = I pk through the 4 ohms at the onset of distortion.

A goal of I sat = 7 amps or 20 volts rms across the 4 ohms would be good.




........ Phil

 

[James Lerch]

"James Lerch"



** A 0.6mH air core inductor is quite small.

You grossly exaggerate.

I haven't built a 0.6mH air cored inductor yet, but inductor.exe
provides the following output using 20 gauge wire.

+--------- Results For Circular Bobbin ----------+
¦ ¦
¦ Inductance = 0.6000 mH ¦
¦ Wire diameter = 0.8100 mm ¦
¦ Coil height = 25.4000 mm ¦
¦ Bobbin inside diameter = 12.7000 mm ¦
¦ ¦
¦ Resistance (copper wire) = 0.5690 ohms ¦
¦ Required coil build-up = 9.0810 mm ¦
¦ Required length of wire = 17.0103 m ¦
¦ Number of turns = 248.5900 ¦
¦ Number of coil layers = 7.9275 ¦
¦ ¦
+------------ Press ESC to continue -------------+

17 meters of wire = 55 feet per inductor, and I need 5 of them.

I'm sure with some optimization I can get that number somewhat lower,
but not be very much. Of course, I may have made a mistake with the
inputs to the indutor.exe program.

** How do you know it is iron ?

I don't, I could cut one in half, but I don't know if I could tell
even then.

More likely ferrite or a powdered iron composite.

Core saturation ( and consequent loss of inductance) occurs when a
particular current level in the winding is approached or exceeded. To find
out, just put your part in series with a 4 ohm resistive dummy load and
drive the combination from an audio amp with a sine wave at say 1000Hz.

With one end of the inductor grounded, monitor the waveform across the part
for distortion - when you see some the game is up.

I sat = I pk through the 4 ohms at the onset of distortion.

A goal of I sat = 7 amps or 20 volts rms across the 4 ohms would be good.

Now this I can do, thanks for the input!



Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)
http://lerch.no-ip.com/ChangFa_Gen (My 15Kw generator project)
Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent.
Calvin Coolidge

 

[Phil Allison]

"James Lerch"
"Phil Allison"

I haven't built a 0.6mH air cored inductor yet, but inductor.exe
provides the following output using 20 gauge wire.

+--------- Results For Circular Bobbin ----------+
¦ ¦
¦ Inductance = 0.6000 mH ¦
¦ Wire diameter = 0.8100 mm ¦
¦ Coil height = 25.4000 mm ¦
¦ Bobbin inside diameter = 12.7000 mm ¦
¦ ¦
¦ Resistance (copper wire) = 0.5690 ohms ¦
¦ Required coil build-up = 9.0810 mm ¦
¦ Required length of wire = 17.0103 m ¦
¦ Number of turns = 248.5900 ¦
¦ Number of coil layers = 7.9275 ¦
¦ ¦
+------------ Press ESC to continue -------------+

** Errr - a 25mm high x 25mm diameter coil is tiny !!!!!!

17 meters of wire = 55 feet per inductor, and I need 5 of them.

** Boo hoo .............



......... Phil

 

[John Popelish]

Greetings All,

I'm finally building a set of surround sound speakers for the house.
I'm currently working on fabricating the cross overs for the midrange
drivers.

The question is, how would you measure if an iron cored inductor is
going to saturate and cause a problem?

The golden rule for 'audiophiles' is air cored inductors only in the
signal path, but I need 5x 0.6mH inductors and that turns out to be
ALOT of magnet wire. (approx 270 feet of 20 gauge wire for 5 air
cored inductors.)

Being a pack rat, I've salvaged a bunch of toroidal cores out of
scrape 360watt SMPS. The cores were originally used as inductors on
the line input section. I've test wound one of these cores with ~8
feet of 20ga wire, and achieved my goal of 0.6mH. Here's a picture of
a bare core and my home made inductor if it helps:

http://lerch.no-ip.com/atm/Ind_Core.jpg (170KB)

At the moment, I am at the decision point of deciding between abiding
by the 'audiophile law' or just using the iron cored inductor.

If it helps, this inductor will be used as part of a Butterworth 3rd
order low pass filter, with a crossover frequency of 3Khz, driving an
8 ohm midrange speaker at no more than 50 watts rms.

Also, I can measure the impediance vs frequency over the audio range,
can I test the iron cored inductor at various amplitudes settings and
get an idea on the likely hood of core saturation becoming a problem?

First of all, there is no guarantee that similar looking cores are
actually the same material, so you may need to test each inductor.

Then you need to roughly quantify how much peak current these
inductors must pass and still be fairly linear in your application.

You can approximately identify what core you are dealing with by
measuring its dimensions, calculating its magnetic cross sectional
area and its effective magnetic path length, and its AL value
(inductance per turns-squared). I think those 3 values and the peak
current are enough to calculate the peak flux the core must pass
without getting into saturation. The physical dimensions and AL value
will also allow you to find approximately the equivalent core from a
catalog,
http://www.mag-inc.com/powder/2006a_Powder_Core_Data.pdf
though it will not tell you the kind of metal the core is made of, so
you will have to either measure or make a conservative guess of the
saturation flux.

The math for these calculations is described starting on page 8 of:
http://www.mag-inc.com/pdf/2005_General_Information.pdf

 

[GregS]

It seems a torroid would have more influence at saturation, and
standard audio coils just use a bar inside the coil
which I don't think is going to saturate nearly as fast. I have even
slid the bar in and out to fine tune the inductance. I
had to use several mH coils for woofer setups. It not desirable to not use iron
for these.

greg

 

[Chris Jones]

Greetings All,

I'm finally building a set of surround sound speakers for the house.
I'm currently working on fabricating the cross overs for the midrange
drivers.

The question is, how would you measure if an iron cored inductor is
going to saturate and cause a problem?

The golden rule for 'audiophiles' is air cored inductors only in the
signal path, but I need 5x 0.6mH inductors and that turns out to be
ALOT of magnet wire. (approx 270 feet of 20 gauge wire for 5 air
cored inductors.)

Being a pack rat, I've salvaged a bunch of toroidal cores out of
scrape 360watt SMPS. The cores were originally used as inductors on
the line input section. I've test wound one of these cores with ~8
feet of 20ga wire, and achieved my goal of 0.6mH. Here's a picture of
a bare core and my home made inductor if it helps:

http://lerch.no-ip.com/atm/Ind_Core.jpg (170KB)

At the moment, I am at the decision point of deciding between abiding
by the 'audiophile law' or just using the iron cored inductor.

If it helps, this inductor will be used as part of a Butterworth 3rd
order low pass filter, with a crossover frequency of 3Khz, driving an
8 ohm midrange speaker at no more than 50 watts rms.

Also, I can measure the impediance vs frequency over the audio range,
can I test the iron cored inductor at various amplitudes settings and
get an idea on the likely hood of core saturation becoming a problem?

Thanks in advance,


Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction, Testing, and
Coating site) http://lerch.no-ip.com/ChangFa_Gen (My 15Kw generator
project) Press on: nothing in the world can take the place of
perseverance. Talent will not; nothing is more common than unsuccessful
men with talent. Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent.
Calvin Coolidge

I highly recommend building separate amplifiers for each driver and using an
active filter crossover which can probably have lower distortion than most
passive crossovers and can be more easily designed to give any particular
desired frequency and phase response. You will be likely to achieve better
damping of the bass driver that way too. Even very cheap IC amplifiers will
give far better distortion than very good loudspeakers, especially if the
powerful bass signals are not passing through the same amp that amplifies
the midrange signals to which the ear is more sensitive. Audiophools will
laugh at me but the joke is on them.

Chris

 

[Paul Mathews]

If winding those turns is too much trouble:
surplus pre-wound air-core coils are frequently on offer on ebay.

get something close to your needs and modify it..adding or removing
turns

inductors can be used in series and in parallel

Simple bobbin-wound coils are much easier than anything on a toroidal
core, anyway. It takes a lot of skill and knowledge to design and
fabricate a reasonably good quality audio filter inductor with anything
but an air core.

Paul Mathews

 

[Jim Thompson]

If winding those turns is too much trouble:
surplus pre-wound air-core coils are frequently on offer on ebay.

get something close to your needs and modify it..adding or removing
turns

inductors can be used in series and in parallel

Simple bobbin-wound coils are much easier than anything on a toroidal
core, anyway. It takes a lot of skill and knowledge to design and
fabricate a reasonably good quality audio filter inductor with anything
but an air core.

Paul Mathews

Bobbins? Use a Du-Ta-Du... aka toilet paper cardboard tube ;-)

...Jim Thompson

 

[RobS]

Starting with the number of turns I'd work back to get an AL factor.
Then correlate the AL factor and the toroid dimensions with manufactures
data. It's also possible to do the calculation to get the permeability.
It may be possible to correlate these figures with color of the core.

Once you get this info it shouldn't be hard to work out the B field at
the given current, then it will be obvious if the core is suitable for
the given inductance/core combination.