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Speaker crossover capacitor & inductors values: schematic & implementation?

I've been repairing/restoring/rehabilitating a great 30-year old pair of speakers and I have some questions stemming from my novice inexperience.

1) In the woofer part of the crossover, the schematic shows a "100uF 100V bi-polar" in parallel with the woofer.  My existing crossover, spare crossover that I purchases, and in every photo that I've seen of the crossover, they use two 47uF caps instead of one 100uF cap.   Why?

Why not two 50uF caps (which certainly appear to be available)?  I understand that two smaller value caps might perform better than one large value cap (correct me if that understanding is wrong), but why use 47uF caps?
I will certainly re-build this cross over with some type of Metalized Polypropylene cap.  But do I use two 47uF (maybe with an added higher-quality bypass cap) or two 50uF caps?

2)  In the midrange section of the crossover, it has two 10uF caps in parallel wire in series with the midrange drivers. See attached snippet from the schematic:

As with the woofer circuit, I will probably replace these with Metalized Polypropylene caps.  I assume I can use one 20uF poly cap rather than two 10uf poly caps without any significant difference in quality.  Or will two 10uF actually be much better?

3)  In the tweeter circuit there are 3 inductors. The first in series with the tweeters is a very small 0.011uH ferrite core inductor with a Rdc of 0.1 ohm (the other 2 are 0.5mH and 1.3mH inductors wired in parallel with the tweeters). See: 

None of the speaker parts sellers (Madisound, Parts-express, Parts-connexion) have any inductor that small.  Is that possibly a typo?  (I will have to see if I can measure the inductor)  0.011uH = 0.000011mH = 11nH.   Does a value that small make any sense to any of you?  I would like to replace it with an air core (for obvious reasons) and I can find air core inductors in roughly that size (0.010 uH) on digikey.com.  But the size does seem strange to me. Further, if it is correct, the Rdc of the inductor will almost certainly be different. Do I need to compensate for the different resistance in the circuit, and if so how or where would I do that?  Thanks!

lotusm50

Showing 3 responses by erik_squires

erik_squires
16,774 posts
 

BTW, if you are doing this, get something like the Dayton Audio DATS.  It's immensely useful for speaker diagnosis, part measurement and creating accurate schematics.   

erik_squires
16,774 posts
 

Let me add a little more... if I have a choice between a smaller coil with more DCR or a big coil and separate R I'll pick the smaller coil because coils handle high power better. 

erik_squires
16,774 posts
 

It has been suggested that when switching to low ESR film caps, to add some compensating resistance in series at the driver

Well, the importance of this really depends.  If this is in series with the driver, and the difference is < 0.3 Ohms it’s probably OK, but you might still hear a difference exactly because you’ve changed the level of that driver relative to everything else.   Modders do this and go "OMG!! I’m a genius this sounds so much better..."  Well, it’s different because you’ve now altered the crossover.  It’s not necessarily that your $150 cap is so much better.

When ESR or DCR (for coils) really matters is when they are parallel (go to ground) to the driver.  In some cases you may even see small value resistors in series.  Those very small values should be a clue that the total R is a critical value.   Very small total R values in this leg can really throw off the entire speaker’s impedance curve.   When I pick coils and caps to use in a speaker I take these DCR and ESR values into account.  They aren't an accident.  I did not randomly pick a small gauge coil because I could not afford bigger, so assume the original designers did the same. 

These situations are where doing a complete electrical analysis  and using a crossover simulation like XSim or VituixCAD can really help you evaluate what you are doing. 

This is a discussion best held in DIYaudio where a lot of builders and experts hang out. 

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