Newbie Inductor Question

[hj808]

Hi-

In trying to troubleshoot, and possibly refurbish/upgrade the crossover in my Infinity Renaissance 90, I have run into some curious inductor readings.

First, the schematic:


This all stems from an intermittent EMIM midrange, and the driver itself tests a nominal 3.5 ohms, so I dug into the speaker to start looking at the crossover. Because I am inclined to replace the nearly 20 year old NPE's, I started looking at the woofer crossover first, using my newly purchased Extech LCR meter. This is what prompts my question.

The three inductors in this crossover unit all read substantially below their rated value:

18.3mh = 14.5mh
12.7mh = 10.1mh
6.1mh = 4.58mh

These readings are all at 120hz, with the inductors out of the circuit. At 1khz, the readings are even lower.

Am I failing to measure these correctly? The smaller inductors in the mid/high crossover all read pretty close, except for L6 which reads app. 2.9mh vs. the 3.5mh rating.

I bought the LCR meter so that I would be able to do some pretty exact cap matching for these crossovers if I did re-cap them, and I figured that it would also be accurate for checking the inductors. Is this meter just not appropriate for inductors in the higher ranges?

Also, any suggestions on NPE's for this unit? There seem to be very few to choose from, and definitely none that match that monster 882uf in the 2ohm voice coil circuit. And finally, any suggestions on upgrades to the Solen film caps? I understand that they would be considered only moderate quality, but size is also an issue here, considering the large 72uf C5.

Thanks in advance for any suggestions or opinions-

Herb

[sumaudioguy]

Well it could be a lot of different things. Try measuring some quality air core inductors out of circuit and see if the reading are the same at 120Hz and 1kHz. Those should be. Air core inductors tend to be the most linear with current and frequency especially at audio frequencies. On the other hand the field from the air core can get into things unexpectedly (even other coils) but that is another story.

If the coils you measure are very different at the two frequencies then it should likely be the coil is different at the two frequencies.

[gedlee]

Those seem like large difference, but I would not call them extreme. I have often measured large differences between published values and what I get.

Solen caps are fine. Don't waste your money on more expenesive ones, but I would avoide the NPEs due to aging problems and the like.

[hj808]

Originally Posted by gedlee

Those seem like large difference, but I would not call them extreme. I have often measured large differences between published values and what I get.

Solen caps are fine. Don't waste your money on more expenesive ones, but I would avoide the NPEs due to aging problems and the like.

I'm still very uneducated about crossover theory, but wouldn't a 20% error in an inductor value cause some aberrations to the intended design? I guess I'm particularly concerned since this is part of a dual voice coil Watkins woofer, which is apparently kind of a strange beast to begin with.

Yes, the NPE's do worry me, for those reasons. Can't really avoid them though; no way within reason to get those values in film caps. I guess my question about NPE's is whether there are more choices out there than the ERSE and Bennic (and a couple of generics) variety that seem to be available.

I'm sure the Solens are fine, I am just trying to consider the benefit of upgrading while I have these things apart. These speakers are a pain to dismantle, and also, they are keepers, so I am willing to sink some time and money into making everything as right as I can.

Thanks for the responses-

Herb

[Magz]

Hey Herb,

You could always do what I did with my Infinity RSIIb's, and put the crossover outside the box in a separate enclosure. Then you can try whatever you like and tweak to your heart's content. Check it out:

[sumaudioguy]

Might not have a high wife acceptance factor but other than that external crossovers really make a lot of sense for service and tweaking. Mine are just mounted to the outside back of the speaker...don't look...just listen.

[hj808]

Thanks for the responses. While an external crossover is probably not something I would pursue, I can definitely see the benefits to having the access. I'm not really inclined to experiment too much, just want to make sure that everything is up to the original specs with possibly higher quality components than originally used. As with all things electronic, the oem costs were a consideration for a production run, but on an individual basis the expense of higher grade components might be justified.

I did get a chance today to do some alternate testing on one of the inductors that was reading low compared to the marked value. Using the following instructions:

Measure an unknown inductor

I calculated the inductance measurement on the 18.3mh inductor and found that this method produced essentially the same value as I got using the LCR meter. The metered value was 15.1mh (my original measurement in my first post was mistakenly noted) and the result of the signal generator/oscilloscope and lots of math test was 15.07mh. So, the meter seems to be pretty accurate, and this inductor is almost 20% out of tolerance.

Should I attribute this difference in value to plain being out of tolerance, which would surprise me that they would have shipped these speakers like that, or is it possible that there is a shorted turn(s) that has altered the inductance?

At this point I'm leaning toward replacements.

Thanks- Herb

[kyheng]

Well, putting the crossover indeed is not a good way, but for me, if short term for tuning wise, it is good. Because I no need to take out speakers often. But it will be messy ometimes.