Audio Inductors with more than 6-7 taps

[warpie]

hi ppl,

One of the designs I'm working at the moment requires an audio tapped inductor with 8 taps (10 leads).
However it's pretty hard to find so many taps on a single inductor nowadays and I'm wondering why...

From my little experience I believe I could use 2 separate tapped inductors with (say) 4 taps on each and connect them in series.
However, this would increase the total DCR since it'll be the sum of the 2 DCRs, right?

soooo, is there any important reason for not using a single tapped inductor?
          is it better (and why) to use 2 tapped inductors in series instead?

please share your opinion/experience and help a liitle poor diy guy...  ;D

many thanks
w.

 

[abbey road d enfer]

From my little experience I believe I could use 2 separate tapped inductors with (say) 4 taps on each and connect them in series.
However, this would increase the total DCR since it'll be the sum of the 2 DCRs, right?

Correct, but is it really a problem? Very often, inductors are used in conjunction with resistors that reduce the Q (increase the overall DCR of the resonant circuit).
In addition, you may find that each inductor has a smaller DCR so when put in series, the equivalent DCR is not much higher. In fact if you make two inductors with a core as big as you would use for one complete inductor, you can use a larger gauge of wire, so the overall DCR may be less. In addition parasitic capacitance shall be smaller.

 

[ioaudio]

i don´t know which cores you use but epcos coil formers with 12 pins are available e.g. here
http://www.spulen.com/shop/product_info.php?products_id=1179

-max

 

[warpie]

i don´t know which cores you use but epcos coil formers with 12 pins are available e.g. here
http://www.spulen.com/shop/product_info.php?products_id=1179

-max

thanks for the link ioaudio.
However, at the moment I'm thinking of buying one instead of DIY my own 

From my little experience I believe I could use 2 separate tapped inductors with (say) 4 taps on each and connect them in series.
However, this would increase the total DCR since it'll be the sum of the 2 DCRs, right?

Correct, but is it really a problem? Very often, inductors are used in conjunction with resistors that reduce the Q (increase the overall DCR of the resonant circuit).
In addition, you may find that each inductor has a smaller DCR so when put in series, the equivalent DCR is not much higher. In fact if you make two inductors with a core as big as you would use for one complete inductor, you can use a larger gauge of wire, so the overall DCR may be less. In addition parasitic capacitance shall be smaller.

very good info here. thanks.
So, from what I can understand here you are saying that it doesn't make a big difference using either one (tapped) inductor or two (tapped) in series, although the latter might be a 'better' option...

Also, just to confirm, by connecting two 03.H inductors in series you can get a total inductance of 0.6H (?)
I read somewhere about the mutual inductance but I'm not sure in which cases this mutual inductance occur.

thanks again for your help
w.

 

[PRR]

> I'm not sure in which cases this mutual inductance occur.

For audio(*): only when they are on the same core.

High-inductance needs something better than air. Most of the flux stays in the core. Two cores mounted together may have 0.1% mutual coupling, which doesn't change your results. Two coils on the same core tend to 99% coupling, and then you must figure different.

The much lower inductances used in radio, with a small ferrite slug or just air, can have "significant" coupling even when "not very close". But such techniques don't give the high inductance values used in audio.

(*) The exception is the large air-core chokes used in loudspeaker crossovers. Stacking a couple bun-coils together will mess you up. You see them mounted far apart or at right angles to minimize coupling. Or these days, with some form of core because the added cost of core allows less copper, and I assume a lower total cost.

But a pound of heavy magnet wire is only a milliHenry, only a few ohms in the middle of the audio band. Your diagram shows values 100+ times higher, typical for interstage filtering. Doing this air-core (with usable DCR) would be too heavy to lift, so you are going to do it with a core.

You are not going to find what you want as a standard product. Not enough market for a high-H many-tap coil. Even if there were, every designer would want a different set of tap values.

Get one of those 70V loudspeaker transformers with lots of watt-taps and mess around.

http://www.coilcraft.com/hexa-path.cfm has a 6-winding core, but the largest value is 200 microHenries per winding, 3.6 milliHenries for all 6 together. You could ask for a quote for that part wound with far smaller wire, but they may not do the very-fine wire you need to get your values.

Look at:
http://makearadio.com/misc-stuff/t-725.php
http://www.1n34a.com/parts.htm#transformers

 

[warpie]

thanks PRR 

>Get one of those 70V loudspeaker transformers with lots of watt-taps and mess around.
>Look at:
http://makearadio.com/misc-stuff/t-725.php
http://www.1n34a.com/parts.htm#transformers

so, are you suggesting to get this transformer and unwind it until I get the desired inductance on each tap?
sorry about asking but I'm a bit confused  and I've never dealt with inductors before :

 

 

[PRR]

> so, are you suggesting to get this transformer

You asked for this:

600mH, 500mH... 100mH.

That Bogen has been measured as having 800mH, 400mH, 200mH, 100mH taps:

All values are relative to the black (blk) tap.
Color    Inductance
White     24     H 
Gray      12.04  H 
Violet     6.06  H   
Blue       3.04  H   
Green      1.565 H     
Yellow       787 mH
Orange       398 mH
Red          197 mH
Brown         98 mH

A private communication suggests you may really be looking for more H. Well, this Bogen seems to have it. 24H for a dozen dollars may be a Good Deal; if not, it is No Big Deal.

 

[warpie]

cool, I'll give it a try! thanks