Adjustable inductors

Discussions and experiences with user adjustable gaps.
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VinylSavor
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Joined: Fri Jan 28, 2005 10:38 am

Adjustable inductors

Post by VinylSavor »

Hi guys,

nice site and YAF (yet another forum) ;-)

Since I'm currently working on a line level crossover for my active system again:

How about introducing some adjustable inductors, with nice cores and low DCR ?

values for such filter applications range from 10mH to 2-3 H
This would also cover the values needed for a 600 Ohm LCR RIAA

Ciao ... Thomas
dave slagle
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Post by dave slagle »

How crazy do you want to get with this idea???

how about an inductor you can adjust vaule electrically?

If we operate under the assumption that a saturated domain behaves as air, then all we need to do is introduce a cross field to locally saturate a section of the coil.

if you want to remain a bit more sane... i think i found just the ticket for exactly what you need (and a huge plus for the adjust a gap crowd in general.)

Its called shim stock and is color coded mylar shims in assorted thicknesses. it eliminates the need for the postit note standard, and allows for ease of predictable gaps.

which direction do you want to go (be a man and lets try the cross field:-)
Attachments
its called shimstock, a kit of 5X20 inches is about $30 and has 1 of each.
its called shimstock, a kit of 5X20 inches is about $30 and has 1 of each.
shim.gif (15.54 KiB) Viewed 21012 times
VinylSavor
Posts: 7
Joined: Fri Jan 28, 2005 10:38 am

Post by VinylSavor »

Hi Dave,

I guess I would go for the sane approach here !

Are you sure that cross field thing would yield an inductance which would be halfway constant with a varying signal ?

Thomas
dave slagle
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Post by dave slagle »

VinylSavor wrote:Hi Dave,

I guess I would go for the sane approach here !
I don't blame you... toss out some desired numbers so we have a place to start.
Are you sure that cross field thing would yield an inductance which would be halfway constant with a varying signal ?
of course not... i don't have a clue but i'm still trying to get my head around what would happen. oh well... we can save that idea for another day.

dave
VinylSavor
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Joined: Fri Jan 28, 2005 10:38 am

Post by VinylSavor »

Hi!

good values which are mostly needed are something like 50, 100 and 200 mHy. The 50 could be made to be 45 than it also covers the needs for the 75uS timeconstant in a LCR RIAA.

Then I'm currently using 10mHy for the tweeter crossover and 2-3 Hy for the bass.

I currently adapt the imüpedance of the filters to the inductors which are available. So I'm using Lundahl 45mH coils for bass/mid xover and 1.8Hy for the bass xover.

IN which steps could the adjustments be made for a coil centered at 100mH ?
Any idea of the DCR ? Should be below 10 Ohms.

Thomas
dave slagle
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Post by dave slagle »

should be easy to do... the shim kits will make adjustment easy and repeatable.

what voltages and frequency... keeping the AC flux down will be a concern.
(max AC and min HZ)

the tradeoff will be DCR vs. linearity. It looks like 2 coils on an amorphous core that measures 200mhy with a small gap will be under 10 ohms. then increasing the gap will drop the inductance to whatever value is needed.

maybe doing it so a butt gap with the coils in parallel will give you 200mhy??? While i generally object to the series parallel option, in this case i think it can help us out a bit since you will be able to get the same inductance two ways and hear the DCR vs. linearity tradeoff.

i'll run some numbers and see what we get. then wind one to compare to the math. After than the rest can be interpolated. Early next week when i get the shimstock, i can play with some gaps and report back.

dave
VinylSavor
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Joined: Fri Jan 28, 2005 10:38 am

voltages and frequency

Post by VinylSavor »

Hi Dave,

voltage would be typical line level, say 2V RMS. Taking into account some headroom, maybe 5V ? In the mid/bass xover, frequency would go all the way down, say 10 Hz.

Thomas
dave slagle
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Post by dave slagle »

they say a picture is worth a thousand words.

in a perfect world, the lines would be straight and parallel (i think)

not to far off :-)

dave
Attachments
When i varied the voltage and frequency the results didn't change terribly much so I am somewhat comforted by the measurements.
When i varied the voltage and frequency the results didn't change terribly much so I am somewhat comforted by the measurements.
mayer.gif (10.21 KiB) Viewed 20990 times
VinylSavor
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Post by VinylSavor »

Looks great!

With a few turns more that could even cover both values for the 600 Ohm LCR EQ! DCR is nicely low!

About how big are the changes with freq. and voltage ?

Thomas
dave slagle
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Post by dave slagle »

here are some measurements... take them for what they are. They were made with the series connection with my B&K 885 which can measure at 100, 1000, and 10000hz and 50mv 250mv and 1V. I simply pushed the buttons and recorded the numbers. There are a few suprises, but for the most part they measured as expected. I would trust the 100hz numbers as most accurate, since they are away from the capacitive issues. And for 100hz, the butt gap increases in L 40% from .050V to 1V but with a .08 gap, this is reduced to 10%. The .064, half core, and air cored all look quite linear.

the butt gap represents no intentional gap (least linear) and the air core is just that, (coils were placed apart form each other) The 1/2 core is with 1/2 of the C-core only and represents a very large airgap.

i attached the numbers in excel format in case anybody can come up with a way to plot them that makes any sense.
Attachments
linearity.gif
linearity.gif (6.08 KiB) Viewed 20980 times
inductor linearity.xls
(8 KiB) Downloaded 901 times
VinylSavor
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Post by VinylSavor »

Hi!

looks good! Especially in the region of few 100 mH!

This would cover most of my needs. In the moment I'm using Lundahl 1.8Hy RIAA coils for the subbass xover at 50Hz.

It would make sense to have a different inductor to cober the higher inductance range. It could be made such that the adjustment range overlaps with this one.

Looking forward to give this a try!

Thomas
dave slagle
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Post by dave slagle »

i agree that a one size fits all approach isn't the best way to go, so here are a few more pictures with a few more measurements.

I adjusted the perm and brought fringing flux into the math to make the lines match, then ran the numbers for two sizes smaller and the seemed to line up.

I will suggest that the end user should measure as they gap, to account for minor errors. (more on this later)
Attachments
two wire gauges smaller corrected for fringing and perm with actual measurements.
two wire gauges smaller corrected for fringing and perm with actual measurements.
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the plots from above with adjusted initial perm and consideration of fringing.
the plots from above with adjusted initial perm and consideration of fringing.
image003.gif (11.65 KiB) Viewed 20967 times
estimate for three wire sizes smaller (note 3 wire gauges=double the size) so the parallel connection of this should be the same as the series connection first one.
estimate for three wire sizes smaller (note 3 wire gauges=double the size) so the parallel connection of this should be the same as the series connection first one.
image001.gif (11.52 KiB) Viewed 20970 times
dave slagle
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Post by dave slagle »

thomas wrote:For the full xover (4 ways) I need in total 8 coils.
Most important is the xover between upper bass and mids which will be
around 500 - 1k Since I'm still playing with the xover frequency the adjustability comes
in handy. For this I need a value around 100-200mHy. I need 4 of them and they
should all be the same and will be set for the same airgap. I need 4 (2
per chanel) since I'm doing LCR type filters
I think the first one will do nicely for that, it will let you play with the series connection (higher DCR, larger gap, and more linearity) and the parallel connection (lower DCR, smaller gap, less linear) to see if there is a problem.

I'll do 4 of those.
For the 1.8Hy a DCR of 16Ohm ist still fine.
I think you might need to go to a bigger core or a higher dcr to keep it reasonably linear. If the 25 ohm dcr is too high, going to a bigger core will get some more linearity. These are all (core type construction) so there are two bobbins and a single core, so maybe the best approach is to arrange it so the bobbins in parallel and a small gap give 1.8hy at 15 ohms, then in series, you will be able to increase the gap to get a nicely linear device at 60 ohms... If 60 ohms is too high, give me an absolute maximum DCR and I'll do that for the series connection and see where the parallel one puts us.

And for the tweeter I need something in the
range few mHy up to 10mHy.
here is where i would expect the amorphous cores (or high nickel) to help out, while the resonant frequency of the others is still well below 10K, when you get to the tweeter, you need to be sure your choke is behaving as one. an air core seems wrong to me since they will have more capacitance.
What I use now (the Lundahl coil) has 25 Ohms so this would be noimprovement with respect to DCR.
The issue with DCR is less critical in the subbass xover. All it does is some gain loss due to the resistance which forms a voltage divider with the terminating resistor. But still it's preferable to loose as little gain as possible.

I'm currently playing with the impedance of the LCR networks, and am going lower with it. In the moment I have networks with impedances around 300 Ohms and below. In this case the impact of the DCR becomes more critical.

The woofers which I use in the subbass are the least efficient compared to upper bass,mids and treble. So I already need more gain in the respective amps aqnd want to loose as little as possible.

The subbass just kicks in with a slope of 6dB below 50 Hz. The drivers covering upper bass/mids are not rolled off in that region. So the DCR really only looses gain and does not alter the frequency response.

In the mids region I'm doing a 'real' crossover and the DCR has more side effects.

so for the pair of 1.8hy i'll stick with the 12-15 ohm range.
then i'll look at a bigger core, i really think you want a gap of .01 or more. I'll still keep the DCR at 12-16 ohms and see what the math suggests. and then give them a go on the winder. I'll attach the graph of what we should get from 2 core sizes bigger.

it would be nice to get a butt gap to give you the needed 1.8hy since maybe DCR is really more important than linearity.
Attachments
this is a bigger core estmate with a dcr of 12-15 ohms
this is a bigger core estmate with a dcr of 12-15 ohms
image001.gif (8.68 KiB) Viewed 20961 times
Steve Kaufman
Posts: 35
Joined: Wed Jan 26, 2005 6:00 am
Location: Fort Collins, Colorado

Shimstock

Post by Steve Kaufman »

Hi Dave

Glad to hear the big box o' iron is on its way out west. Since we will be adjusting lot's o' gaps it would be nice to accomplish this in a more reproducible way than with post-it notes. The shimstock kit you mentioned above would be a great solution. Do you sell these or can you recommend a source?

Thanks

Steve
Gary P
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Joined: Sun Feb 06, 2005 5:20 pm

Post by Gary P »

Hi Dave,

This thread on inductors is very interesting. I'll probably be interested in some too!

After listening to Steve's LR phono pre I'v been inspired to try LR EQ for myself. Doing a fully balanced design with LR EQ would give me the opertunity to hear a complete system that has no capacitors anywhere in the signal path, and I'm talking all aspects of the signal path current loops, not just the direct path.

I've already been mentaly working on a fully balanced design using the all Teflon RC EQ network from my SE phono pre but after hearing LR EQ the plans have changed...

Gary
IslandPink
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Joined: Thu May 12, 2005 8:01 am
Location: Denbigh, North Wales

Himmel

Post by IslandPink »

Thomas,

Interesting, I agree with your comment that Wolfgang's LCR phono was good - I voted for it at least 4 times - it always showed well .
I was only disappointed that I didn't get a good opportunity to compare either of the Bench LR phonos against Wolfgang's in detail at any stage .

I'm close to doing something with my own phono ( after I get the amp for the Slagle outputs finally finished !! ) but I think my first step will be to try a lower-impedance RIAA network . Currently I have a 105K RC network driven by a D3a in triode mode . I want to try D3a pentode into a 21K network instead. I suspect ( as does Chris Vry ) that some of the LCR advantage may be due to the lower impedance . Worth a try, before buying more coils !

Mark
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