RS 2.5 owners, assistance requested

geoff727

geoff727

Well-Known Member
Hello all,
I am rebuilding and upgrading a pair of 2.5's for another forum member. As part of the process, I am considering the replacement and upgrade of each crossover component individually, and I've run into a curious thing with the inductor in the circuit for the Watkins woofer. I'm hoping there's a lurking circuit expert on the forum who can shed some light on this...

The circuit for the 2-ohm coil on the woofer is formed by a series resonant circuit consisting of a 700uf cap and a 20mH inductor. Together, these two make a resonant frequency of about 42Hz, which should also be the closed-box resonant frequency of the Watkins woofer if the right woofer surround is used to keep this circuit 'in tune'.

When I measured the 20mH inductor, it measured as....10mH! This was true for the inductors on both speakers, and also one that I de-soldered and measured on my own 2.5's as well. They are all labeled as 20mH, but all measure around 10mH.

I measured them with both an LCR meter, and the Dayton WT3 on a laptop, just to make sure one reading wasn't way off. The LCR meter measured slightly higher than the WT3, which could mean that my "Zero adjust" might be a hair off. Nevertheless, I did not see anything close to what I expected.

So, first question to the electrically knowledgeable....why would all the inductors measure out at only half the inductance that they are supposed to be?

Cont'd....

20mH inductor LCR.JPG 20mH inductor my speakers.JPG 20mH inductor WT3.JPG
 
Last edited:
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So, with a 10mH inductor and a 700uf cap in series, we should have a resonant circuit with a center frequency at about 62Hz. In theory, this should grossly mistune the interaction between the resonant circuit and the Watkins, yielding an impedance dip in the low-60hz region.

Plotting that on a simulator, it looks like this graph below. The high point of the frequency plot, and the zero-phase crossing angle, should be a point of zero impedance in the circuit (not counting the DCR of the inductor). Apologies for the size of this....the peak of the top graph is just past the 50Hz line, but far below the 100Hz line.

With 10mH inductor.jpg With 10mH inductor.jpg
 
Looking at a 20Mh inductor and a 700uf cap, we have a resonant frequency in the mid-40Hz region, what we'd expect. You can see the top of the frequency plot and the zero-phase crossing point has now moved a little below 50Hz.

With 20mH inductor.jpg
 
Here's the theoretical plot of the circuit with the components as measured: 10mH with .3ohms DCR, and 680uf (the cap's tolerance is a little over 2% off, not too bad).

Since the website is down-sizing the images, I've included labels to make it easier to read.

With 680, 10, and .3.jpg
 
A simpleton like me would automatically think, rip these inductors out of the circuit and replace them. But, here's where it gets REALLY interesting:

Soldering back up the inductor and the cap together in series to measure the resonant circuit, we get this....

The lowest impedance and zero-phase point is at 47Hz, almost right where it's supposed to be! (What the heck happened to 61Hz?!)

Anyone care to explain this?

RS2.5 owners, does anyone care to de-solder an inductor and measure it?

Infinity's can be crazy, but this is one of the most curious things I've yet found.
Geoff

Resonant circuit WT3.JPG WT3 plot of actual components.jpg
 
Don't have an answer for you. However, Bill Watkins is a member of the InifinityClassics Yahoo group and sometimes chimes in. You might consider reposting your question there.
 
So nobody knows the answer to this?

In addition to that weirdness, I have measured the 2.5/5.0 woofer inductor. One set of leads measures .25mH (a tenth of what it's supposed to), another set of leads measures about 1.8mH, and in series the leads measure 3.0mH. I am absolutely confounded by these things. None of it makes sense. If my measurement methods were inaccurate, I could accept that. But, the inductors in the midrange and tweeter circuits measure spot-on.
 
Very interesting problem. Hopefully someone can provide an answer.

I am not an expert at all, but I would suspect some issue with the (LCR and Dayton) measurements. In particular that the default frequency used in the measurement is not anywhere close to the actual operating range of the inductor. Can you feed the coil a baseline frequency in say 100 Hz range and measure the inductance under such conditions?

See at the bottom of the page here:
http://www.radio-electronics.com/info/t_and_m/lcr-meter/basics-tutorial.php
 
Seppo: Thanks for the link. I have never had any measurement anomalies from either of these two instruments, which I've been using for years. I've measured countless parts with them (lots of inductors), and have never seen anything close to this. The WT3 makes a sweeping measurement, from the lowest to the highest frequencies. I will have to read more on E-cores. Maybe there is something about these that is different that I have not seen before. Still, though, for the 4-ohm coil inductor to measure .25mH and 1.8mH, that's just crazy. If that's actually the case, there'd be hardly a low-pass function at all to the woofer crossover; the woofer's own properties would be rolling the frequencies off before the inductors ever had a chance to.

Tubed: Yes, it is the 700uf that's in series with the 20mH. Interestingly, on the actual crossover board, the position of the two components was reversed compared to what's on the schematic. Non-consequential, just interesting.

I am almost tempted to buy all-new inductors for this project, sticking with what I know is a "known quantity" in inductor design and value. My whole point for digging these inductors out and testing them was to see if replacing them would yield an increase in performance in the form of an increased damping factor from the amplifier. This is achieved through a lower DCR of the inductor. My thought here is to provide one more avenue in overcoming an issue that has plagued the 2.5 since it's inception: bass that doesn't match the midrange. One thing that truly surprised me, though, measuring the 20mH (which measured as 10mH), was the very-low-DCR that existed in that inductor. I expected a value of at least half an ohm, but it's actually about .3, lower than I expected. Now, individual drivers will vary in resistance by this much, easily. But that difference in resistance in series with the output stage of the amplifier makes a big change in damping factor. The amplifier that the new owner uses is going to have a very good damping factor already, and I want to take advantage of that. If I go with inductors which I am more confident in, but that have a slightly higher DCR, I reduce the damping factor. Not the result I want to achieve, especially since these woofers (if I remember right) already have a fairly high Q at resonance. So, I have to give very careful thought about what I'm doing here and approach each step with a lot of patience.

Anyway, these speakers are proving to be an interesting experiment, and we will have a full-build thread on them in the coming months after they've been completed and fully set up in the new owner's home.

G~
 
^^^^^^
It just seems curious that on other Watkins Infinitys, the largest inductor is always in series with the largest capacitor.
Perhaps I'm reading the 2.5 schematic wrong.
 
tubed said:
^^^^^^
It just seems curious that on other Watkins Infinitys, the largest inductor is always in series with the largest capacitor.
Perhaps I'm reading the 2.5 schematic wrong.
Click to expand...

The values of the cap and inductor depend on what resonant frequency is needed from the circuit, which is determined by the resonant frequency of the woofer inside the enclosure. So values will vary, depending on those needs.
 
New 20mH inductors are here for the project. They are advertised as 20mH, say 20mH on them, and measure......20mH!!! That's with the LCR meter, the WT3, and the DATS 2.

I have ideas about why all the others measured so far off, ideas that give me no doubt as to the validity of changing the inductors. Anyway, this project will be getting new inductors in key places. Namely, the 20mH and the 5mH. The midrange and tweeter circuit air core inductors will be staying. The 2.5mH inductor in the woofer circuit will be removed from the circuit entirely in favor of a more optimal bi-amp scenario.

G~
 
I just read this and have a question for you. At what frequency did you measure the coil. Believe it or not it makes difference. Try measuring it at 1000 HZ and then at 40 HZ . It may suprise you.
 
rs1b said:
I just read this and have a question for you. At what frequency did you measure the coil. Believe it or not it makes difference. Try measuring it at 1000 HZ and then at 40 HZ . It may suprise you.
Click to expand...

Well, I think the LCR meter measures at 1kHz. The DATS does a frequency sweep, but I don't know which frequency it uses to pin down a component's value.

Still, though, how can one explain the fact that the old 20mH inductors measured as 10 while the new 20's measure spot on at 20? Difference in inductor design? Difference in construction? The measurement equipment was the same.

And, if the old inductors really swayed that far in inductance depending on the measurement frequency, there's only one frequency where the inductance is correct...and that would have to be the designed crossover frequency. Otherwise, the crossover frequency would differ from what was intended. And, how does one explain the measured L-C resonant frequency (47Hz), when the 'theoretical' frequency (over 60Hz) would have been quite higher, using the same test equipment (i.e., same measurement frequency)? Any thoughts?

G~
 
G
Almost every inductor I have tested on the Watkins circuit has been far off the mark. Usually 12-18 mh . But so has the cap used. On my 2.5's the coil is 15 mh and the cap although rated at 700 mfd tests out at 900 mfd. Still ends up resonating at 43 hz. If you do the math it tests out. Also keep in mind that it is pretty forgiving. It could be 10 hz either way and not a huge deal. I know people prescribe magical power to the Watkins circuit but really it is at best a basic tuned circuit that is simple and it works. That is what is so great about it and why it is so easy to add it to any system with a sealed box. You would laugh at some of killer systems I have made using this circuit to add on subs to sealed boxes without extra amplification.
Another misconception is that as it approaches the resonate frequency all power goes to the 2 ohm circuit. Not so. It gets divided between the 2 and 4 ohm with more current flowing in the 2 ohm side for obvious reasons. Knowing this you can just parallel the wires on the speaker terminal and send it off to another box with a watkins woofer with a coil and a cap just as used in a watkins circuit ( wire the 4 and 2 ohm coils in parallel so that you get the best use of the voice coils. On the woofer in the 2.5 box wire those coils in series.. Now you can have your 2.5's handling the bass to around 45 HZ and the rest will go to the other woofer in the other box from around 65 HZ and down as there is some overlapping. No extra amps needed.
 
Okay, so the inductors are way out of spec. Every large value inductor I've measured in these things so far, not just the 2.5's, but every Infinity I have, has been way out of spec. However, the small value inductors (like air-core's in the tweeter circuits) have all been spot-on in their value. In your 2.5's, the cap happens to also be far enough away from it's rated value that the resonant frequency still holds good for the circuit (using the 1/2*pi blah blah blah equation for circuit resonance). But, the question remains, how can my circuit have a theoretical (i.e., equation-derived) resonance of over 60Hz, yet yield a resonance of 47Hz when measured using the same test equipment that tells me the 20mH inductor is only half it's rated value at the frequency where the measurement is taking place? It just confounds me, because it makes no sense at all.

I follow your plans for the outboard Watkins woofers. I like it. Nice and simple. I don't quite follow, however, why one would do this on a 2.5. You already have a dual-coil woofer operating. Why wire in a second one? Power handling? There would be a dB increase over the frequency range the second voice coil is playing, because now you have two woofers operating in phase with each other while outputting sound over twice the surface area. On the flip side, though, wiring both coils from both woofers in series would cancel that efficiency boost out. So I can see a benefit from the increase in power handling. I can really understand doing this on an RS-II or -IIa/b because you'd be adding an extra half or full octave to the low frequency extension. For me, though, to extend the low frequency response on a 2.5, it makes sense to add external servo-controlled subs (true servo's, not the Infinity SSW series:)). Then you could gain a response extension from 30-ish Hz down to the lower limit of human hearing.

Please keep going, I love this kind of discussion.
G~
 
It would be nice to extend the bass response of any speaker.
But if the speaker is designed for a set response(inherent in it's design), I be more inclined to add multiple subwoofers ala Geddes ideas.
I've done this and the results are remarkable.
The only drawback are the multiple cords.wires, amplifiers etc. inherent with utilizing more speakers.
https://drive.google.com/file/d/0BxgUOGOB5HbfR0JTRF9XZjkyUms/view?pref=2&pli=1
https://drive.google.com/file/d/0BxgUOGOB5HbfR0JTRF9XZjkyUms/view?pref=2&pli=1
 
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