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MoFo inductor options for use above 1kHz only
MoFo inductor options for use above 1kHz only
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Old 19th November 2017, 10:52 AM   #21
Attila I is offline Attila I  Germany
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sorry ZM, I wasn't advertising any tv-trash
but what the heck is SOA?? sun over alabama?
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Old 19th November 2017, 11:05 AM   #22
Zen Mod is offline Zen Mod  Serbia
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MoFo inductor options for use above 1kHz only
naah .... I was pullin' your leg

of course it is Sons of Anarchy

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Old 20th November 2017, 05:03 PM   #23
PRR is online now PRR  United States
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On this forum, "SOA" is Safe Operating Area, the combinations of Power, Voltage, and Current that a transistor is safe to work at.

Asking an average power transistor to hold 90 Watts is like asking you to hold 90 kiloGrams all day long.
____________

I would think an air-core could be fine for the upper half of the audio band, especially for a compression driver which, in home use, would hardly-ever get over 1 Watt. (If you did peak to 20 Watts, ear-distortion would exceed amplifier distortion.) (Large PA systems are different; I have run way over 100W in large venues and outdoor concerts.)

As a bare minimum, the choke value could be similar to the value you would use in a simple passive crossover. 1mH at 1KHz is 6.3 Ohms, so you can work out 800Hz, 1.6KHz, 8, or 16 Ohms from there.

1mH air-core is a couple pounds of copper wire. Not an economy part, but cheap compared to any good compression driver.

If you use this value the load-line is very oval. Since the amplifier has no reserve signal current on the down-swing, power will be down. You want a larger value than bare-minimum.

PartsExpress lists several heavy air-cores. 7 mH 0.98 ohms DCR is 52 bucks, comparable to the iron-core for bass. For DIY without math, "multilayer air core inductor calculator" search brings up the tools. (Try to go near ID and length about the same, OD about 3X larger, but 2X either way is near optimum.)

"Infinite" seems good, but is not in stock. Any practical very-very-large choke will have high copper resistance. (Silver not much better, and that much Silver in private hands invites legal/banking investigation.)

DCR is not a vital parameter here as long as it is small compared to load. If much less than 1/4 of the load impedance, we can approximate the effect as a need for a slightly higher supply voltage, which is easily done. (If a perfect coil with 16V supply and 8 Ohm load gives 16 Watts, then a 2 Ohm DCR needs another 4V of supply or 20V, or essentially a common 19V laptop supply.)

Air-cores throw some field around. In the MoFo, I would not be too worried about left-right coupling because the MOSFET's impedance is so very low that it will absorb induced energy. Crossover builders do not have this luxury but do fine putting coils cross-ways to each other.
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Old 20th November 2017, 05:34 PM   #24
eduard is offline eduard  Netherlands
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Hello,
SOA is sexueel overdraagbare aandoening in the Netherlands. The English crowd calls it sexual transmitted disease.
Being diy audiophile is pretty close to a disease too.
Greetings, Eduard
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Old 20th November 2017, 06:43 PM   #25
CharlieLaub is offline CharlieLaub  United States
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@PRR: what about the suitability of a powdered core type inductor like I liked to earlier? See:
Jantzen Audio 3.3mH 15 AWG P-Core Inductor Crossover Coil
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Old 21st November 2017, 01:18 AM   #26
Pascal Nguyen is offline Pascal Nguyen  Australia
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Default Interpretation

Common belief, due to large application of ferrite core transformer in sub MHz’s SMPS, that ferrite is inappropriate for audio spectrum.
In fact it is not, as long as the core is unsaturated :

Air core transformer L=2.8mH
Ferrite transfo L=2.73mH

1) Bias Iq for best THD+N at 1KHz
Air core 2.5A
Ferrite 1A

2) THD+N for 0.3Vrms 1KHz input
Air core 0.054% @ -11.1dB output/8 Ohm dummy.
Ferrite 0.30% @ -11.2dB

Clearly those numbers indicated that my FX2243 ferrite core 2.73mH is inappropriate to build a MoFo.
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Old 21st November 2017, 01:23 AM   #27
Pascal Nguyen is offline Pascal Nguyen  Australia
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Quote:
Originally Posted by CharlieLaub View Post
@PRR: what about the suitability of a powdered core type inductor like I liked to earlier? See:
Jantzen Audio 3.3mH 15 AWG P-Core Inductor Crossover Coil
Sorry to jump in

If Jantzen releases the “permite ” B-H spec, one can based on BH loop to work out the Le or “effective length” of an A/m or Ampere per meter to calculate core saturation point.
With a saturated core, the 3.3mH Jantzen is a pure DCR, within non saturated region it is a 3.3mH choke like any body else.
My case, the FX2243 is saturated at 1A @ 1kHz.
I connected this amp (3A limited power supply) to an Android music player:
Coil inductance went down with frequency, creating a pure DCR load to mosfet’s source.
Vicious circle: low frequency -> low inductance -> high Ids.
At any drum beat from my music, the PS will drop down to keep a safe preset current limit, hah hah a free built in Wah Wah pedal.

If Jantzen can guarantee a 2A flow @ 1KHz, i would buy it with a fine print “don’t less any drum beats in” (you are the in DSP)
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Old 21st November 2017, 01:44 AM   #28
Pascal Nguyen is offline Pascal Nguyen  Australia
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Quote:
Originally Posted by PRR View Post
On this forum, "SOA" is Safe Operating Area, the combinations of Power, Voltage, and Current that a transistor is safe to work at.

Asking an average power transistor to hold 90 Watts is like asking you to hold 90 kiloGrams all day long
...
Hi PRR,
Hope you had a nice week-end, mine was bounded indoor.
The 90 Watt was selected for best THD+N to that specified iron core and IRFP240, no thought about similarity in M.K.S.A.. (abréviation de mètre, kilogramme, seconde, ampère)

Average power dissipation in my chook shed
Attached Images
File Type: jpg 5EE419F6-EB52-4675-88B4-2EBDD377A7A3.jpg (1.01 MB, 152 views)

Last edited by Pascal Nguyen; 21st November 2017 at 01:52 AM. Reason: Vocabulary correction
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Old 21st November 2017, 02:08 AM   #29
woody is offline woody
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After playing with this calculator a moment it looks like a decent inductor won't take all that much wire after all.

For a 1mh coil using # 16 wire on a 1" form .5" long would only use .46 lb of wire and have a resistance of 24ohm.

Or for a 3.1 mh coil wound on a 1" form .5" long using # 18 wire would only use .5 lb of wire and have a resistance of .67 ohms

Air Core Inductor Designer / Calculator
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Old 21st November 2017, 02:30 PM   #30
Michael Rothacher is offline Michael Rothacher  United States
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MoFo inductor options for use above 1kHz only
Default Tweeter Amp

Here are a couple of small inductors I had in the lab, both 4mH, one air, one iron.

And here are the promised measurements: (aqua - air, green - iron) both in a 19V 1.7A circuit.
Attached Images
File Type: jpg coils.jpg (598.1 KB, 139 views)
File Type: png frair.PNG (15.8 KB, 361 views)
File Type: png friron.PNG (15.8 KB, 145 views)
File Type: png thdvfrair.PNG (11.3 KB, 146 views)
File Type: png thdvfriron.PNG (11.3 KB, 90 views)
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