Some questions about inductors and resistors

[Deang]

I probably should have just posted here to begin with. I posted these questions/comments at the DIY/Tweaks forum at the Asylum, and I did not get a sufficient amount of responses to get my mind settled. I was wondering what some of you might think.

The physical condition of several of my inductors is somewhat "rough". There are quite a few knicks/deep scratches, and some of the copper is darker than the copper around it. I did brush some clear fingernail polish (enamel), and brushed it across the damaged areas.

I would really like to change them out, and spent much of my spare time last week reading and learning about inductors. I learned that along with my inductance value, I needed to know the DCR. Searching various archives, I found out that all I needed to do was break out my multi-meter and run the leads to each end of the inductor to find the DCR.

This is what I found.

1.27mH -- DCR .8

.80mH -- DCR 2.0

.20mH -- DCR 1.0

These are fairly high values by today's standards, and I can't just use inductors with the same mH. The resistence values are part of the circuit. I did manage to find 1.25mH/DCR .7 coils at Madisound for the low frequencies, and I decided I could live with 1/10 of an ohm difference.

The other two inductors are irreplaceable. To get a DCR that high, I need at least 24 gauge wire with over 300 turns (I used several different inductor calculators). I finally got to the point where I decided I could probably wind my own, and no sooner had I convinced myself I could do it -- did I find that no one, NO ONE, makes the spool size I need.

One of the responses from the Asylum below. It doesn't sound "right" to me, but I don't have the background to figure out what it is.

"Just put a resistor in series with them. Mills makes some fairly low values, you should be fine if you're with in 10%. Parts Express sells some 18 gauge air cores, and Mills resistors. Welborne has even more low values. PE sells .20mH/.18DCR chokes, Welborne sells .8ohm Mills and there you go. You can also check out Handmade for chokes and Mills."

Moving on to resistors:

What is up with this "inductive", "non-inductive" resistor business. It seems these non-inductive types are all the rage right now. What I find so curious about this is that so many designs, especially vintage designs -- employ(ed) inductive types. I wonder first -- if non-inductive types are superior, than why are/were the inductive types used? Secondly -- and this especially applies to vintage designs, if the actual voicing of the speaker was done with inductive resistors in the circuit(s), then what purpose is served by using the non-inductive type? It stands to reason, (at least in my deranged mind), if inductive resistors are used, then whatever electrical properties they have (good or bad), was factored in and accounted for by the engineer while doing the design.

I called Regar, who told me the original resistors for the DQ-10 were non-inductive. After I finally learned there is an internal, physical difference between the types (the wire in a non-inductive type crosses back over itself to the other end where it started) -- I just went down to my basement, grabbed a hammer, and busted one of the old resistors open. The old resistors are most certainly "inductive".

So here is the problem. Everyone says I should use non-inductive resistors, but I can't help but think that the inductance value of the resistors was factored into the circuit(s). I think it's odd that people never talk about the fact that capacitors, inductors, and resistors -- all have capacitance, inductance, and resistance!

I originally ordered these really cool NTE 1% aluminum housed inductive resistors. I would love to use them. However, Joe from Sonic Craft told me not too, if the leads were ferrous metal. Sure enough, magnetic as hell.

So, opinions?

This message has been edited by deang on 08-31-2002 at 08:55 PM

[Al Klappenberger]

deang,

I think you have gotten some strange advice along the way!

Here is the straight dope, at least for Klipsch speakers:

* You want DCR as low as you can get it. Do NOT add resistors to inductors

if there were not there to begin with!

* For woofer inductors us soild wire of AWG #14.

* For the small inductors in the tweeter and squawker circuits use coils

wound of "Litz" wire. Again, use AWG #14 Litz. Litz wire has as much as 1/3

the loss of the same size soild wire in squawker / tweeter filters.

* There is no need for non-inductive reisitors. Wire-wound types, like

"cement" types, have inductance but it is extremely low and insegnificant.

* Replace capacitors with Polypropylene types.

All of these parts are available from Solen of Canada: http://www.solen.ca/

Al K.

This message has been edited by Al Klappenberger on 08-31-2002 at 08:46 PM

[Deang]

Al,

Thanks so much for the response.

This Klipschead is rebuilding a set of 25 year old Dahlquist DQ-10's. More for nostalgic reasons than anything else. After the Advent, it was speaker that got me started, and then ended with Klipsch.

But you did tell me the main two things I wanted to know.

What about the leads on the resistors made out of ferrous metal?

[djk]

There is no point in changing the inductors in DQ10 crossover unless they are damaged. If you change them you must get ones with the same DCR. If the correct DCR is not available, you must add a series resistor. Failure to do so will destroy the tonal balance and imaging of the DQ10. Resistors should have non-ferrous leads. Inductive vs non-inductive matters for amplifiers(and only if the design has feedback), not speakers. Resistor leads must be of welded construction, crimped is not acceptable(some cheap cement boat resistors are crimped).

People like a lower DCR inductor in Klipschorns because the midrange is too loud. With a 3R5 woofer a difference of 0R35 in the series inductor will raise the level of the woofer about 1dB. One of your inductors has a DCR of 2R0. Think about it.

If you have very early DQ10s with electrolytic caps, and you change to polypropylene, expect the midrange to be louder.

Mirror imaging is worthwhile(if yours are the older style).

[Deang]

Cool.

I have very low serial numbers, 2354 & 2355 - however, they have the yellow mylar caps. I figure sonically, the polypros are very similiar to the mylar - agree?

The 2RO inductor you mentioned is for the midrange.

How does one test an inductor?

This message has been edited by deang on 08-31-2002 at 10:16 PM

[djk]

"I have very low serial numbers, 2354 & 2355 - however, they have the yellow mylar caps. I figure sonically, the polypros are very similiar to the mylar - agree?"

Go ahead and change these out, a small but worthwhile improvement.

"I certainly understand about the DCR on the midrange inductor. How does one test inductance?"

I use a LCR bridge with DF and Q. If the bridge doesn't have DF and Q it cannot accrately measure components with high ESR. It is also accurate to 10 milli-Ohms DCR(0R01). The typical hand-held VOM is ~0R4 when you short the leads. This is too high to be of any use for speakers. The B&K 875B is $165. The MCM 72-960 is $189. The $50 meters don't cut it.

[Malcolm]

Time for a sanity check. The DCR you quoted for the 1.27 mH inductor would be about right for a 20 gauge air core inductor. The others would be right with even smaller wire. Is that what are? If not, did you zero your meter before measuring the resistance of the coils? Cheap meters don't even have a zero function. If you have one of these, you need to measure the resistance between the two probes and subtract it from what you get when you measure the coils. Even after that, the measurements won't perfect.

I have never seen an schematic for your crossovers, so I will generalize.

An ideal inductor has a DCR of 0. Practical inductors don't. Generally, the choice comes down to what works well enough, what you can afford and what you have space for. Hopefully the DCR of the inductors used has been taken into account by the designer. Changing the DCR of an inductor can change the parameters of the circuit and not necessarily for the better. If you decide to keep DCR the same, putting a resistor in front of a new coil wound from larger wire is a waste of money. Might was well use the same size as original. For that matter, IMHO replacing an inductor for cosmetic reasons is a waste of money. How often do you look at your crossover?

An ideal resistor has no inductance or capacitance. The inductance of the part chosen by the designer is either neglegible or has been taken into consideration. Again, the choice comes down to what works well enough and what you can afford. Used to be that the only place you saw non-inductive resistors was in critical RF circuits because a little inductance in the wrong place could detune a circuit.

[Al Klappenberger]

deang,

On my recomendations before.. Remember, I did say "for Klipsch speakers"! Dahlquist is a different story. I have no experience with those!

AL K.

[Deang]

Malcom,

I am using a digital multimeter. I've measured the resistence on these inductors a half dozen times over several days -- and always get the same reading.

The DCR's of these inductors were most certainly factored in as part of the circuits for the individual drivers. Of this there is no doubt. As the frequencies climb, the wire size decreases -- increasing the resistance value.

The crossovers for the DQ-10 are mounted on the backside of the cabinet, they are not in the cabinet. They are completely exposed. It is possible these speakers may have been sitting in front of windows, with the sunlight settling on the crossovers over many years. The mylar caps were actually peeling apart like an onion.

The normally light colored sand bar resistors were very dark. Turning several of them over revealed most of material gone from around the wound wire. I also discovered that one of the tweeters was buzzing. I am replacing the tweeters with some Scan-Speak D2010's, and I will be wiring a 12ohm/12 watt resistor in parallel to match the impedence of the old tweeters -- at least, that is what I was told the purpose of the resistor is.

These things appear to have been well driven for 25 years. I do know how to tell whether any of the inductors are "burnt" -- however, one of the .20mH/2DCR inductors looks like someone dropped a can of beer on it. However, I know all the inductors function, because I did fire up the speakers when I got them, and all the drivers worked.

I'm really going the extra mile with these. I have considerable time invested in refinishing every part -- right down to the screws.

I would certainly be willing to email a copy of the schematic to anyone who is interested in looking at it -- the crossover design is quite a piece of work.

As far as the inductors go -- I am definitely going to install the new Madisound inductors for the bass and midbass. They are 19 gauge (instead of 20), one winding short, and within a 10th of an ohm on the DCR.

I am going to wind my own pair of .80mh/2DCR inductors. I will be posting plenty of pictures of how I'm going to do this, along with the finished product.

Any volunteers to test them for me?

This message has been edited by deang on 09-01-2002 at 11:41 AM

[Malcolm]

Wow, sure does sound like you are going the extra mile. Now I understand why you want to replace the inductors and resistors.

If I were doing this, I would go with the same sand resistors to keep the original look. But if you want something cooler looking, aluminum resistors are fine. If you have any concerns about the ferrous leads you might want to rebuild one with a sand resistor and one with the aluminum resistor and see if you can hear a difference.

I doubt you need to measure the inductors you wind. If you follow the formula you should be very close.

Looking forward to pictures of your project.

[Deang]

This is bizarre.

Check out these two charts on wire gauge and ohms per 1000 feet.

http://www.tfcbooks.com/referenc/wiresize.htm

http://www.lalena.com/audio/calculator/inductor/help.shtml

They are not the same. What gives?

I knew I would need 26 gauge wire, so I simply divided 41.02 by 1000 (.0412), and then multiplied by the number of feet (an unknown, and I just started out with SWAGS) -- which ended up being 49, to give me 2.02 ohms. Then I stumbled across the 2nd chart, and the numbers are different.

[Malcolm]

Yep, and I have other charts with different numbers. And the wire you use may be a little different from any of the charts. The charts are all pretty close, so I wouldn't worry. The DCR you wind up with will be the DCR you wind up. The key point is to pick the right size wire so you will be in the ballpark.

[Deang]

"In the Ballpark"! I'm not sure if I like the sound of that.

So, is it better to err on the side of inductance, or resistence?

[Malcolm]

Don't let the term scare you. Everything in your crossover was selected to be "in the ballpark." Wind your inductor with the correct size wire and the DCR will be close enough you won't be able to hear a difference.

[Deang]

Thanks Malcolm. Guess I'll get the old drill warmed up