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Using Air core inductors (instead of original iron core)

elnaldo

elnaldo

Lunatic Member
Hello. I need to wind the coils for a pair of Infinity Qb (bought the drivers some years ago, built the cabinets last year, I'm building the crossover to factory specs), and it uses a 1.5mH iron core (1st order xover) for the woofer.

Doing some math, an air core inductor for that application using AWG 16 (similar to the original) would have 0.3 ohm DC resistance, which doesn't look too much (woofer nominal impedance is 4 ohm). Winding an air core would be easier to me, and I don't need to buy the iron core (the original looks like an E-I square core).

Do you think that the tenths of an ohm difference I will have using the air core instead of iron core will make such a difference? The mid-range driver has an attenuator to adjust to the desired level.
 
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My Two Cents Worth: I'm not one to argue with Dr. Audio BUT speaker manufacturers usually used iron core inductors primarily to save money as iron has always been a whole lot cheaper than copper. Iron core inductors typically saturate at much lower current levels than air core inductors and are more prone to heating issues based upon their design, but these issues typically manifest themselves only at higher drive levels,

The Infinity Q series line of speakers were designed for the mass market in high production volumes so I'm not sure the Infinity designers were all that concerned about the "Q" of the cross-over inductor and instead just wanted the target millihenry value at the lowest reasonable cost while still maintaining a minimum level of quality. If this were a high end esoteric design, I'd say copy the original exactly, but in this case I really doubt you would be able to hear a difference between the two types of inductors.

You could always wind one of each type and see if you can hear a difference (although individual driver tolerances would probably make the comparison between the two speakers not really reliable).
 
I bought the copper wire and iron cores today, both things were cheap, I see money is not an issue in this 2 small inductors (approx $20 on materials for 4 inductors, 2 air core 1mH, 2 iron core 1.5mH). Actually, I think that dealing with the iron core will be more expensive to me than just winding an air core coil (time is money).

I could make a 1.5mH air core inductor before cutting the wire and test it. Anyway I think at the end I'll stay with the original design.
 
If it's only 1.5mH, then that's an easy thing to do as an air core.

What's the DC resistance of the original coil? Does anyone have access to one to measure? I'd bet it is probably not that much lower in resistance.



Regards,.
Gordon.
 
GordonW said:
If it's only 1.5mH, then that's an easy thing to do as an air core.

What's the DC resistance of the original coil? Does anyone have access to one to measure? I'd bet it is probably not that much lower in resistance.



Regards,.
Gordon.
Click to expand...

I own a pair of original Qb but I don't want to open the cabinets again (children around, and I don't have a place or tools to do this at home)

I was using this calculator to estimate the wire length and buy approx the quantity I need.
http://tesla-institute.com/!app/sim/acic.php

I see a 1.5 mH is not too different from the 1mH air core. I could try one and do some measurements and tests.

I've bought already the cores. I could always save them for a future project, they were cheap anyway.
 
stickman said:
The Infinity Q series line of speakers were designed for the mass market in high production volumes so I'm not sure the Infinity designers were all that concerned about the "Q" of the cross-over inductor and instead just wanted the target millihenry value at the lowest reasonable cost while still maintaining a minimum level of quality.
Click to expand...
The Qb had a list price of $192 each in 1978. Calculating for inflation, that's a 2020 price of about $1500/pair, a not-inconsequential sum. While I don't doubt that cost played a factor in their design, don't make the mistake of thinking that these were "budget" speakers. They weren't cheap.
 
I know the Qa, Qe, Qb, were the bottom line models from an expensive brand. Not cheap but for sure the factory had a budget on them.

I'm building a pair from scratch for a friend (new cabinets, new crossovers, original drivers bought from parted out speakers). I'm not making money with this, I just want my friend having a good speaker to replace the Marantz Model 400 (not HD-400) he is borrowing now (he needs to give them back), a 3 way speaker with a large dome tweeter. I'm sure the QB's won't disappoint him after some years listening to the Marantz.
 
elnaldo said:
I know the Qa, Qe, Qb, were the bottom line models from an expensive brand. Not cheap but for sure the factory had a budget on them.
Click to expand...
I believe they skimped where they felt the sound wouldn't be affected significantly. For example, the cabinets use really nasty-looking and thin oak vinyl veneer, but they're made of reasonably thick particle board (which was the standard of the day for speaker building, MDF not yet being widely available; I don't even know if it had been invented then). Crossovers used pretty cheap components as well. But the drivers are of excellent quality.
 
Well, finally managed to open my Qbs (had to do it, no other way to know the original inductor values), and the woofer inductor in my speaker measures 0.85mH (not 1.5mH) , 1.4 ohm DC resistance. The air core for the mid-range measures 0.96mH 2.0 ohm.

Wire gauges are approx AWG 16 for the woofer, 18 or 19 for the mid-range.

I'm using AWG16 for both inductors, and the 1mH air core is getting large!

I see also that the online inductor calculator is accurate about wire lenght- inductance, but not accurate about DC resistance of the coil.

Infinity Qb, Woofer Iron core inductor, 1.4 Ohm, 0.85mH:
20210104_111835.jpg

Infinity Qb, mid-range air core inductor, 2 Ohm, 0.96mH:

20210104_112432.jpg
 
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Cored inductors do potentially face saturation and thus distortions. However, that's at currents approaching their design limits. And by the time you get to that level of current, the speakers themselves are likely producing comparable distortions, and for similar reasons.

I personally prefer air core, as it's one less thing to worry about. But I've got no problem with cored inductors(or any other compromises) where they make sense.
 
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