Inductor tolerance 10%, why not less?

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j.frad

Well-known member
Joined
Nov 12, 2008
Messages
236
Location
Brussels
Hi there!

I was just wondering why manufacturers usually have +/-10% tolerance on their inductor values and not less?
In the specific casse of someone wanting to build a matched pair of passive EQs, can it be a problem or do we just don't care much?

Thanks a lot!
 
It comes down to money. 

It costs a lot more to "sort" the cores to less percentage than that. Assume that they make 1000 cores at +/- 10%, and you want them to be 2%, so they sort them to +/-2%, +/-4%, +/-6, +/-8% , +/-10%. Assuming an even distribution for simplicity, you would only be interested in 20% of the manufactured cores, so they are stuck with 800 garbage cores ( in reality these get pushed to other people with looser tolerances).  So you have to pay a premium for your batch of inductor cores.  There are ways to get close while winding, or maybe add series resistance to the inductor with less DCR, but if you want a really matched set, IMO you have to start with the core.

FWIW, I've been planning on doing some "real" pultec inductors for months now,  toroidal cores and all, and I'm currently waiting on a quote for +/- 2% cores... I was "warned" by the sales dept that they are going to be at least three times the price of standard cores..  But I'd rather have the assurance.  I already have the winding quote, so I should have a feeler thread up this week, (check back if you're interested) since we'd need to get about 100 or so to meet minimums, etc.     

(btw, which mfgs are you referring to?  Are these "made-for-audio" inductors or plain old inductors from mouser etc?)
 
I'm not referring to anyone in particular, I remember seeing similar numbers around audio dedicated sites and others, usually more around the 20% range. It's not an accurate statement.
I'll look out for your thread, even though I'm currently going for another passive EQ...
Cheers!
 
Well, most caps are also 20% tolerance, you have to pay more for 10%,5%,2% or even less, or buy tons and sort like Jacob said, though with inductors for a passive EQ you would have to sort L and DCR if wanting to build a stereo unit. In most applications (pro audio uses a very small part of the component market) these tolerances don't matter, so that's why mfgs don't bother with them (or us). Unless we pay more. If you stick to someone who makes inductors for audio (Cinemag, Sowter, Altran, as well as any of the guys making them here) you should be able to get better tolerances).

Which EQ are you building btw?
 
I think this is quite a complicated issue which basically boils down to the magnetic parameters of the core. It is hard for the core manufacturer to control these and so as others have said the only way to get close tolerance parts is by measurement and selection.

Unfortunately this is itself fraught with problems simply because the inductor is not a linear device. This means that the inductance you measure depends on the signal level and the frequency at which you measure it. So at one level and frequency you may well select two inductors to be within say 2% of each other but this is no guarantee this will also be the case at other levels and frequencies.

I suspect it is these properties that contribute to the unique sound of inductors.

And then, when you get your 2% inductors you need 2% capacitors too except when you get to the high frequencies you find the self capacitance of the inductor becomes significant and so it goes on.

Cheers

Ian
 
mitsos said:
Well, most caps are also 20% tolerance, you have to pay more for 10%,5%,2%

Wima do many 10% caps and Panasonic do 5% film caps that are just as inexpensive. But you are right, when it comes to 2% capacitors there is a steep premium.

Cheers

Ian
 
I have read of that freq-dependant phenom and been warned against certain materials. I wonder if the move to ferrite was due to this? I mean, pretty much everyone uses ferrite inductors nowadays, and I'm sure they are fine, but I've also heard some people say they are not so great. I'm not a purist in any sense of the word, though I am leaning away from ferrite more out of nostalgia than anything else, for I am not old enough to remember any of the things ebay calls vintage..

I have been very happy with Wima polypros. I havent tried panasonics in a while, is it those red-purpley dipped ones you are talking about? 
 
mitsos said:
I have been very happy with Wima polypros. I havent tried panasonics in a while, is it those red-purpley dipped ones you are talking about? 

Those are the ones. I am using them right now in prototype Pultec and Helios Type 69  EQs.

Cheers

Ian
 
Thanks for the great info guys, I guess I was looking for some king of reassurance.
I'll be building a NYD stereo eq and I want it to be made right.
 
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