EQ design-- Multitap Ind.+Caps or Singletap Inductor+caps?

[Gwaggin390]

Been looking into this as i want to play with some Quad8 modules i have.
The Quad8 uses an Inductor tap for each frequency. looking at modules like Neve and Helios, they use a singletap,(or a couple) and then vary the cap values to get different frequencies.

Is there a method to the madness? or as long as you are achieving the
desired frequency, an inductor+cap is an "inductor+cap" and will sound the same?

If i had to guess i'd say they used single tap inductors to save money and board space?

 

[SSLtech]

There's always impedance to consider...

Keith

 

[NewYorkDave]

To put it very generally, they are not the same.

If using a single inductor and switching caps, the bandwidth of boost or cut will change as you switch frequencies. To maintain the same "Q" as you switch between frequencies requires a different value of capacitance and inductance for each position.

EDIT: Kubi beat me to it.

 

[NewYorkDave]

Whoops... Replace "bandwidth" with "Q" in my earlier reply.

Naturally, even if all positions have a Q of, say, 1.5, the actual bandwidth would change as you change frequencies. I hope I didn't cause any confusion.

Variable Q is acceptable in some "non precision" applications, and in those applications you may find a single L with switched Cs.

 

[ioaudio]

i find this (free) software quite useful http://aade.com/filter.htm

Gwaggin390, you already emailed me about this, no? maybe we should post the schematics, since they look interesting.

-max

 

[Gwaggin390]

Hi Max,
yes it is me, and i posted a question before, but it was another issue.

I did have a question for you, so i'll ask it here.
The Q8 stuff uses 2 inductors for the low frequency band,
one 2 tap inductor for 50hz and 140hz, which uses NO cap, and one
2 tap inductor for 250 and 400 which uses a cap for each frequency.

In the other post i was trying to figure out how it gets 50hz with a 3.25H inductor tap and it was mentioned that it was probably a shelf.
My technical knowledge is limited, but from what i read on the site, it seemed like an inductor used like this was more a high pass, but again, i'm learning here.
If anyone could help, i'll post the schematic(well, half of it)
The inductor in question is L1.

For Max-> if the the inductance was no higher than say, 3H(ish), could you wind 2 coils (3 taps each) on your inductors?

To anyone--> The above inductor would be used for only the Low Freq. so if engaged, the Low would use one OR the other coil, never both.
Would that be a problem having 2 coils on one bobbin?
(but only ever using one at a time?)

 

[ioaudio]

[quote author="Gwaggin390"]
For Max-> if the the inductance was no higher than say, 3H(ish), could you wind 2 coils (3 taps each) on your inductors?[/quote]

i would have to try that, but it should work

 

[syn]

To anyone--> The above inductor would be used for only the Low Freq. so if engaged, the Low would use one OR the other coil, never both.
Would that be a problem having 2 coils on one bobbin?
(but only ever using one at a time?)

Why would you want to do this? I've read your post a few times but i still can't see the point in doing this...

sorry if I'm missing the obvious....

 

[Gwaggin390]

well, if i can figure out the PCB design software, and ever get to it,
it would save a lot of space and save me from buying an additional inductor.

I wouldn't imagine you could do this , in general, with the inductors i've usually seen but it looks like it would work with the size and shape Max uses.

I doubt it would also work if you were using the same bobbin for 2 different bands at the same time. The Q8 Low Freq. band design would use either/or in this case,
so my hope is to be able to get away with it.

 

[CJ]

OK, this is all real simple, leakage.

The closer you have the copper to the iron, the less loss.
Winding a "sponge" coil, or trying to go cheap with two on one core, forget it, well, real men do not do it.


Don't sweat the Q because
a) it moves all over the place
b) people like the boost/shelf curves best, peak curves are just not as musical, at least not to my ears. Mostly a brute force EQ idea, not a tracking tweak.

Also, as mentioned before, when you wind a toroid unevenly, the core gets uneven distribution of the flux, the metal warps unevenly, due to magneto-restrictive force, which is a buzz word that means the molecules expand and contract durind a changing field.
Probably not a big deal at audio levels, and especially with a steel core, but these guys try and squeeze max performance from each component in order to build a better product and get a raise.

Kind of like a ten speed bike, you save a gram here, a gram trher, pretty soon you have a bike that folds up like a pretzel on the first downhill right.

 

[ioaudio]

[quote author="CJ"]OK, this is all real simple, leakage.[/quote]

yes, thought about this too - this might be the reason for seperate inductors.

 

[CJ]

OK, I finally got the right answer.
Does the engineer have a large family?
Then he will wind individual inductors.
Read on:

It is easier to fine tune the inductance of an individual coil.

Tapped Inductor:

When you tweak one tap to fine tune your peak curve, the other peak curve might shift a bit.

Say you have 100 turns for a 100 mh inductor.
If you wind 100 more turns, you do not get 200 mH.

lets see if we can set up a cheap equation to breadboard this online,

Henrys =k (T^2) , where k is a constant.

Using 100 mH from our example above, T stands for turns, some winders use small t, text books use N for turns...

Henrys =k (T^2)
0.1 mH = k (100^2)
0.1 = k (10,000)
k = 0.1/10,000

k = 0.00001

So plug in 200 turns to see that the relationship is quadratic in T,

Henrys = (0.00001) (200^2)
Henrys = (0.00001) (40,000)

Henrys = 400 mH

So
100 T = 100 mh
200 T = 400 mH.

In theory.

But....and this is a big butt, you have libraries full of books that tell us this ain't gonna happen, because typically, we usually get a lower number than this, since there is no such thing as an ideal inductor.

OK, so now you go back and add a few turns to make up for core loss, leakage inductance, whatever, so that you get 200 mH exactly.

Well guess what, now your 100 mH tap is off.
Why?We did not even touch the 100 turns, the 100 mH should still be there?
More library books.
Self inductance, Lenz's law, (hey, are not those the same thing?), gremlins like that will step out of the brush.

So now, you have screwed up a good inductor by trying to go cheap and get two for one, so the engineer has to go back and request a new coil from the winding dept, see where I am going with this?

Much easier to wind a non tapped version, but once you get volume going, maybe better to contract the engineering time to develop a mass produce coil that has every tap on one core, ie Pultec.

So in the 40 hour (80 if large family at home) a week world of the pro engineer, sometimes they do not want to waste the time to develop a precise tapped inductor, better to just get it over with and wind individuals, and much easier to tweak if you get a ship load of caps that are 20 percent off, or your supplier can no longer get 0.0068 in by next week, you can tweak your individual coil without screwing up the other band, which would be a complete pain.

So your more precise eq's will have individual inductors, like your Tube Techs and so forth, your WE phone line antique eq's will have mass produced, 20 percent tolerance inductors. And all points in between, ie Neve.

 

[Gwaggin390]

Cj,
thank you for all of the effort,
i do get what you are saying.
To even think of trying the project would put cost at a big factor,
with the 500 series eq's that are coming out(the purple is supposed to be $600ish)
other than the challenge of the DIY, if it becomes that expensive, one might opt for the finished product with warranty/etc.
I talked with Dave Garen at Cinemag and he was saying they wouldn't even think of making the stuff at less than 25 each, and while they have some info, they don't have resistance or core material info so it would be all but a complete re-engineering job.

I was just trying to balance keeping it simple with getting the most out of it.

I think the description of it being a "vibe" eq was pretty right on.
I've literally heard it referred to again and again as, "not surgical, but very musical" and it pretty much is.
I'm not opposed to continuing in the same vein.
I liked the idea that it was a pretty simple eq, could be reproduced with pc mount rotary switches, not a ton of parts, and sounds pretty good.
(i actually use the Q8 405 which is very similar but has a hi/low pass as well........i'll get to that at some other point.....)
So when i was trying to decipher the inductor and the concept i was just thinking,
"hey, Max can wind up to 7 taps, what could i do with that? Hmmmmmmm..."
I guess the accuracy of the taps depends on how good Max is at what he does..................Max?

 

[Sorr]

Thank you CJ for your information on inductors!
Sorr

 

[CJ]

No problem.
I hope everything I said is actually right.

OT: A book by Van der Veen, toroidal output transformers and amps, just came into the Stanford Library, I took a quck look at it last night, and it is great, real easy to read, big print, on what leakage does, what C does, I will have to snap some jpgs.

 

[CJ]

No problem.
I hope everything I said is actually right.

OT: A book by Van der Veen, toroidal output transformers and amps, just came into the Stanford Library, been waiting a year, every time I check the card catalog it sats "due in Oct 24" whatever, well its finallty there.
I took a quick look at it last night, and it is great, real easy to read, big print, on what leakage does, what C does, I will have to snap some jpgs.

Hey look, more transformers!
Click on Signal Transformers, never heard these, anybody?

http://www.amplimo.nl/index.html?lang=en-uk&target=d34.html

this is interesting:

"With these transformers you make or keep signal leads symmetrically.
Compared to asymmetrical leads this causes a big advantage: much less sensitive to hum, interferences and oscillations."

Wow. Somebody actually warns against thisd, this is a first:

type number TM2P

Never try to check the windings with an ordinary multimeter. The DC-current of the meter will magnetixe the core!

Plenty of threads on this, bottom line:

Most DMM's are OK, but check Ohms on your meter by running the leads directly into another meters ammeter input, you do not want more than about 1/2 a mill, or else you need to degauss, either by runnung stong signal, or running strong signal, as a de mag will not pentereate the can.

Interesting, the warning only is on the high ratio xfmr's, so maybe all those turns amplify the ohmeter current.

 

[ioaudio]

[quote author="CJ"]
100 T = 100 mh
200 T = 400 mH.

In theory.

But....and this is a big butt, you have libraries full of books that tell us this ain't gonna happen, because typically, we usually get a lower number than this, since there is no such thing as an ideal inductor.
[/quote]

cj, i think these kind of problems only happen if the inductor core material is not consistent along it´s magnetic path.

 

[ioaudio]

i the "a" half of the core has a higher Al value than the "b" half, than the green winding will have a higher inductance than the red counterpart, regardless it´s identical turns.
so if you dont spread the windings evenly across the hole window, variations will occour.

 

[CJ]

you get better coupling if the windings are evenly spaced on a toroid core.

also, as you layer a toroid, the outer most windings will be further than the inner, leakage goes up, so you get slight variations of the inductance formula.
But you are right, if the windings are tightly coupled, then they do match the formula pretty closely.

I am engineering a CT for a new GFI at work, I just read this morning in the Magnetics design literature, it says to space the turns evenly, so it was good to see something that was a gut feel turn into fact.
cj

 

[CJ]

ok, here we go,

a lot here, but i am in info overload mode right now, but knock yourself out:

http://www.mag-inc.com/library.asp