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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 16th November 2017, 05:40 PM   #1
CharlieLaub is offline CharlieLaub  United States
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Default MoFo inductor options for use above 1kHz only

I'm interested in building the Big MoFo and using it to drive a compression tweeter in an upcoming 2-way active loudspeaker project. I'm still a little bit unsure (or clueless if you prefer!) about the tradeoffs when using various inductors in the MoFo circuit when and if I only need to reproduce frequencies above about 1kHz, as well as the inductance that is necessary to support that frequency band. Can a lower inductance air core be used? Air core inductors are not typically available in large inductances like 50mH, so if I use another type (e.g. powder core, iron core) are there problems with saturation, etc. of the inductor under the steady DC current that is used? How might distortion be effected by inductor type? Unfortunately I really do not have any insight into these issues at all or how I could investigate them on my own. My primary motivation in exploring options is to try and make the amp lighter and cheaper to build for this particular application. If the best option remains one of the Hammond inductors then that is fine, but I wanted to throw this out there and hopefully learn something from the responses that I receive. Thanks.
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Old 16th November 2017, 07:12 PM   #2
ErikdeBest is offline ErikdeBest  Switzerland
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MoFo inductor options for use above 1kHz only
The inductor is the load for the mosfet: the higher, the lower distortion. The speaker will be connected in parallel to this load, so you will never get a load higher than your speaker impedance.
To calculate the reactance of the inductor, formula is 2 pi f l. So for the 100mH, the reactance would be about 630 ohms : much higher than the speaker load. So you can go with mayber 10mH. The air core wont saturate, I dont know about the others.
Last week I read your tutorial on the DSP with the raspberry - I am very tempted to try it out and hopefully learn some linux along the way.
Cheers, Erik
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Old 16th November 2017, 07:18 PM   #3
Michael Rothacher is offline Michael Rothacher  United States
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MoFo inductor options for use above 1kHz only
Hi Charlie,

The cutoff frequency of the filter defined by the output of the amp can be approximated as:

.2/6.28*L

So, even a .5 or 1mH air core should be workable with your requirements.

If I get the chance this weekend I'll try one and get some THD figures for you.
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Old 16th November 2017, 07:33 PM   #4
Michael Rothacher is offline Michael Rothacher  United States
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MoFo inductor options for use above 1kHz only
Likewise, you could get away with smaller caps too.
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Old 16th November 2017, 07:56 PM   #5
CharlieLaub is offline CharlieLaub  United States
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Originally Posted by Michael Rothacher View Post
Hi Charlie,

The cutoff frequency of the filter defined by the output of the amp can be approximated as:

.2/6.28*L

So, even a .5 or 1mH air core should be workable with your requirements.

If I get the chance this weekend I'll try one and get some THD figures for you.
Awesome! I was hoping to pick your brain at Burning Amp about this, but that was not in the cards this year. Thanks for the info. I will look for a post about the THD implications in a few days. Thanks for the help, Mike!
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Old 16th November 2017, 08:10 PM   #6
CharlieLaub is offline CharlieLaub  United States
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Originally Posted by ErikdeBest View Post
The inductor is the load for the mosfet: the higher, the lower distortion. The speaker will be connected in parallel to this load, so you will never get a load higher than your speaker impedance.
Ah, that makes sense. I can see that from the schematic: L1, R5, and the load are all in parallel with C2 providing AC coupling above some cutoff frequency.

While I am looking at the MoFo schematic, what is the purpose of R5? Is this just a "drain" for any accumulated charge on the output cap (C2) or does it help linearize the load impedance seen by Q1?
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Old 16th November 2017, 08:19 PM   #7
Michael Rothacher is offline Michael Rothacher  United States
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MoFo inductor options for use above 1kHz only
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Old 16th November 2017, 08:40 PM   #8
CharlieLaub is offline CharlieLaub  United States
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Maybe for my application a powder core inductor like this would be a possibility?
Jantzen Audio 3.3mH 15 AWG P-Core Inductor Crossover Coil
Low DCR and can probably dissipate heat well. Not sure how these would perform when fed almost 2A of DC current, however...

Will the lower inductance reduce the amount of ripple rejection at the PS switching frequency? The low noise is one of the main reasons why this amp is so attractive to me when the load is a 105dB/W compression driver.
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Last edited by CharlieLaub; 16th November 2017 at 08:43 PM.
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Old 16th November 2017, 09:22 PM   #9
Zen Mod is offline Zen Mod  Serbia
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MoFo inductor options for use above 1kHz only
you can go with lesser Iq , also

you'll not need so much power , working so up
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Old 17th November 2017, 10:28 AM   #10
Pascal Nguyen is offline Pascal Nguyen  Australia
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Hi Charlie,
Do not worry much about noise with SMPS, my unshielded output transformer can pick up 50Hz interference from domestic appliances nearby, but the level is less than -90dB, at the far end of 20kHz, grass line is quite elevated about 5 dB above the -120dB threshold.
I used a trifilar transmission line air core transformer, straight out of a 100m drum of 3 core electrical cable, no more no less than two 3 way connectors attached at both ends.
The result is stunning, not easy to tell the difference between my SIT amp and my “Reflow MoFo”
(don’t trust me, due to my old age hearing and speakers quality, )
Well, the coil resonated at 1650Hz with 3.3uF cap, “on line calculator” told me that I have a 2.8mH inductor, that is the way for one who knows only +-x/.

Cheers from Pascal
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