Resistance is Futile

[Deang]

Since a capacitor supposedly doesn't pass DC -- why does it matter what the DCR of an inductor is in an LCR circuit?

There is a lot of live and learn with DIY. It's like working on a huge jig-saw puzzle, and you spent most of your time crawling on all fours looking for the pieces.

I'm really hung up on this DCR thing, and I can't seem to let it go. I did finally change out my inductors for the bass, going back to the correct DCR to bring the driver/cabinet/port relationship back in line. After getting the Qts of the drivers for the RF-7's and using some online calculators -- I realized it was the smartest thing to do. This made sense though, as the woofers hang right off of the inductor. John Warren surmised that Klipsch used the DCR of the inductor to tweak the Qts of the woofers, and I think he was right. A "spanking" by DJK prompted me to revert back to the correct value.

What doesn't make sense is when looking at the tweeter circuit. While poking around on the board with my MM, I did confirm that I couldn't get DC to pass through any of the caps. So, if there is a cap on either side of an inductor, what the hell does it matter what the DCR is? It sure doesn't look like it's going anywhere.

Another thing I'm lost on is this idea of hanging a resistor off of an inductor to tweak the DCR. I know people do this, but I don't think it's "right". I spent some time measuring the inductance and capacitance of various resistors in my possession -- and the values that were coming up were surprisingly high. I suppose one could do this kind of thing as long as all three values (resistance, capacitance, and inductance) were considered, and factored in. However, my gut tells me there is still a missing piece to this.

[Klewless]

The fact that a capacitor does not pass DC has no bearing on the DCR of an inductor located in the high frequency part of a network. You may consider that an inductor is composed of two "perfect" components; a coil (with no resistance) and a resistor (with no inductance). You cannot have a "perfect" component.

However the DCR (the resistance of the wire in the coil) does affect the circuit wherein it lies. It is either in series with its driver (midrange/tweeter) or is parallel to it. In both cases it will have an affect on the performance of the drivers because the resistance does change the Q of the circuit.

Perhaps the thought of "DC" Resistance is misleading. Resistance is always measured as a DC component (it does not change with frequency). The frequency dependent conterpart to resistance is impedance (resistance which does change with frequency).

It is a common thing for people to use the internal resistance of components to advantage since it is cheaper than adding another part to reach their goal. In the case of a coil one can use smaller wire to take advantage of more resistance (assuming the circuit needs more resistance) without actually having to put a resistor in the circuit.

Hope this does not confuse the issue for you. The bottom line is that resistance is resistance and it does not matter whether it is surrounded by DC blocking components or not.

[gerbache]

In circuit analysis, the overall impedence, similar to resistance but for AC, is the resistance of the circuit, which you are calling the DC resistance, plus the reactance, which is the effect of the capacitors and inductors. The resistance of the circuit is always in place and does not change, but it appears to change as the reactance of the circuit varies with frequency.

When you changed the resistance of the inductor, you made the overall impedence change for the circuit, which may or may not cause a problem in the speakers, but definitely is no longer to factory specs. How much did you change the resistance, anyway? If it's only changed a small amount, you may actually be within the tolerance of the circuit. Still, Klipsch probably picked an inductor with that value for some sort of a reason, so I'd probably try to keep it at close to the factory resistance as possible. Hope this helps!

[JohnA]

Dean,

The impedance of any component is the DC resistance plus the AC reactance. So, the DC resistance of an inductor increases its impedance and causes a loss of voltage to the driver. That wastes power and reduces driver output, not always what you want. If there is an inductor in a circuit that sees low current flow a smaller wire can be used and higher resistance tolerated becauses the losses will be low. However, that resistance makes the circuit behave a little differently than the equations predict, so you need to keep it as low as practical.

[Deang]

I really miss Mark and Leo when they pull the vanishing act, but you guys (as well as DJK) really helped me out with the excellent posts.

I had some trouble reconciling what John was saying with the other provided information, but after juggling it all around in my head and sleeping with the schematic taped to my forehead -- I was able to get my RF-7's completely dialed in.

The first thing I did was revert back to the original DCR on the bass inductor. I've been wanting to try a copper foil inductor, but nothing was really available until now. Parts Express is carrying a new line of inductors by a company called Jantzen. The CFAC's and round copper coils by this Danish company really fill much needed gaps (values). I was able to use a CFAC that puts me at +.065mH over stock, with a DCR that was dead on. I'm estatic over the increase in performance in the bass and midbass. The "whack" is back in droves. I think I'm also hearing better articulation in the midrange too, but I won't know for sure until I go through several more CDs.

The original inductor in the high pass has a DCR of .7. I don't care for the original inductors for several reasons. The core is to small, and the variance between the two (left and right channels) was .9mH. I had Madisound custom wind replacements which came back with the correct mH, but were .45 DCR instead of .7. I used them anyways without ill effect. Leo had told me that since the cap sitting behind this inductor is already passing less energy, the effect of the lower DCR in this part of the ciruit would be marginal. The fact that I didn't notice any change in the output level of the horn confirmed this. The critical inductor is the one in the LCR parallel notch filter (damped bandpass) controlling the horn's peak. The replacements from Madisound were perfect, and so no concern here.

My original question related to two things. One was related to the bass inductor. Before I ran across the Jantzen CFACs, I was toying with the idea of tagging a resistor with the 10AWG Northcreek coils I was using to drive the resistance back up. The other related to the inductor in the LCR circuit. Here's why: The stock RF-7's can be a little hot on top, especially with the titanium tweeter. There can be quite a bit of sibilance. Leo's fix was to change the resistor in the LCR circuit to a value that attenuated the horn's peak. Contrary to what I thought, it was by actually lowering the value that Leo accomplished this. I followed suit, although I went down less than he did -- I think I like a little more heat on top than Leo. However, we are only separated by a 10th of an ohm.

As I was looking through the new Jantzen coils and I came across the idea of using a lower DCR inductor in the LCR circuit to attenuate the horn's peak instead of the changing the resistor value. So that is what I did. I went back to the stock value on the resistor, and used an inductor with a lower DCR. A little bit of math showed me an inductor with a DCR that would attenuate the horn's output the same amount as the resistor I had been using. At least, this was my thinking.

I replaced both of the Madisound scramble wound coils with new Jantzens. To get the mH match on the high pass inductor I had to unwind a few coils, but then designed my board to account for the extra lead length. The Jantzens coils are cool. To unwind the wire, you just peel it. The coils are "baked" after they are wound. This is ideal, and much better than dipping in varnish, which doesn't have the viscosity to penetrate through the whole coil.

I'm dead on with the inductor in the LCR, and still -.3 DCR in the high pass. So I'm done. CFAC's on the woofers, Film and tin foils in series with the tweeters, paralleled Auricaps in the LCRs, and superior inductors in the high frequency sections than either stock or the Madisounds.

The outcome was interesting. The horn's output level actually sounds closer to stock than it did. The lower DCR of the inductor doesn't seem to attenuate the horn's output as much as changing the resistor did. Still, no sibilance evident in anything so far, and I think I've taken my RF-7's as far as I can take them. If there is distortion, I am incapable of hearing it.

[djk]

"Since a capacitor supposedly doesn't pass DC -- why does it matter what the DCR of an inductor is in an LCR circuit?"

In a notch filter the DCR of the inductor and the ESR of the capacitor determine the 'Q', or shape of the filter, how wide or narrow a bite the filter takes out of the signal.

". So, if there is a cap on either side of an inductor, what the hell does it matter what the DCR is?"

When an inductor is used in the high-pass circuit the DCR only affects the ultimate roll-off of the crossover, usually a couple of octaves below the crossover point where you are 20~40dB down anyway.

An exception to this is the first order series crossover. The DCR of the inductor in parallel with the tweeter is critical and must be as low as possible. You cannot 'tweak' the Qes of the woofer with the wire size in a series crossover, it will let too much low frequency show up in the tweeter.

"Another thing I'm lost on is this idea of hanging a resistor off of an inductor to tweak the DCR. I know people do this, but I don't think it's "right"."

If I need to add resistance in series with an inductor I usually save the money and order an inductor with a smaller wire gauge.

There is a case where adding a resistor is 'right' as opposed to using a smaller wire size, and that would be adjusting the woofer Qes in a very high power system.

In professional situations I've had the woofer inductor get so hot that it burns all the insulation off the magnet wire and charred the 3/4" thick board the network was constructed on all the way through to the speaker cabinet.

This was a good time to go from bi-amping to tri-amping despite the cost and complexities involved.

[leok]

Dean,

I don't disappear. Others provide explainations, as is the case above, that are generally more understandable than mine. If I have a different opinion or think there is something that should be mentioned, I'll speak-up. Now that you've change the inductors back, how is the sound changed?

leok

[Deang]

My, you almost sound a little...testy

HF: I took L1 to .16 DCR, and put R2 back to 2 ohms. The reduction in resistance is basically the same as when I had R2 at 1.75 ohms. The outcome was a slightly more pronounced output than when I had the lower resistor value in place, though still not nearly as pronounced as stock. The sound is still exceptionally clean at all volume levels -- without assault.

LF: The Solen's DCR of .18 is for the most part acceptable as Al K and Jon Risch said. My Northcreek coils going down to .06 was an idiotic move. Big changes here reverting back to .3 using the CFAC's. Last night I had decided that I was noticing improvement in every area relating to the cones. I'm saving the embellishment and overstatement for the email I'm drafting for you. I will probably PM you, since I don't have your email address with me here at the other work-site. You may find some of the things I have to say interesting.

[leok]

Dean,

Thanks for the note. I did a quick read and will look more closely later.

Upper frequencies covered by the cones, and speed of the bass have been weakness of the RF-7s. (This is very relative, I think they are terrific speakers) So, I'm particularly interested in your comments there.

Leo

[Deang]

The CFAC on the woofers is undeniably the cure. Fasten your seat belt.