AR-4x: unfamiliar crossover

[RoyC]

Based on this new cap value info, I am including a possible cap combo (see pic 3) for comment, made up from caps on hand. For the tweets, I had planned on using these 20 mf Solen caps; and I had previously purchased a handful of these 10 mf surplus caps based on Kent's value recommendations. Trying to assemble 24 mf for the woofs, I am showing two 10 mf surplus caps plus either a 3.3 mf Carli or a 4.7 mf Carli. Close enough or not? [i had been planning to use these Carli caps to possibly build a 8mf value for testing my 4x to 4xa conversion project, but I may be willing to sacrifice them here.]

Thanks so much, guys. Further thoughts?

Your meter appears to be able to read up to 20uf. Just switch it down to the 20uf mark in the "F" section of the dial and connect the leads to a 10uf cap. If you get a reading of around 10uf, you've just figured out how to use your meter. Try it to get some practice. If you have the meter I think you have, you will not get a reading for any cap value over 20uf. Meters used for speaker work should be able to read caps to at least 400uf.

Those surplus 10uf cap are great, but they often measure just a bit on the low side (typically within 3%), so I would pair them with the 4.7uf caps to make your 24uf caps. The result will easily be close enough for the crossover. Even if the total were to be 25uf, it would be well within the +/- 10% specification for the 4x.

It seems you are all set to go!

Roy

[dynaco_dan]

Hi there

I went to the Amazon site regarding the meter.

It is s&h prepaid.

Scroll down their page for owner reviews, not too promising in my opinion, from reading their feedback.

The paint stripper I used for my Dynaco A-50's is Made in Canada and is called Green Solv.

I bought it at our local Lee Valley's store.

Go and read the information of this fine product for some advice.

Not shippable to the USA, sadly.

You can check out their USA web site for an equivalent at least.

[ra.ra]

I am still working on these 4x's, but before I have anything presentable to post, I wanted to follow-up and report that I think I have been able to figure out how to measure capacitance with this silly multi-meter. The normal alligator clip test bears no results, but when the capacitor leads are inserted into the slots on the face of the meter, supposed accurate results are reported on the digital screen. Pics show the 10 mf surplus cap from Madisound, but I also confirmed this with 2 mf Solen and 6 mf Erse caps.

This insertion detection is not foolproof - much like an A-E potentiometer, I have to find the 'sweet spot' in order to get a settled reading.

[ra.ra]

Weeks later, and again thanks to all who have helped me with this discussion. Just a brief progress update here, as well as another crucial question. Pics attached show: current state of cabinet restoration; original crossover; and proposed schematic.

Thanks to Roy C here and a distant post from Zilch (RIP) on AK, it was determined that this x-o is a somewhat rare transitional version for the AR-4x which used the earlier AR-4 woofer. My question is this: in my schematic sketch, am I showing the correct wiring for the 24 mf woofer capacitor? Also, more 'normal' 4x speakers seemed to have woof and tweet with opposite polarity, but this schematic suggests differently. Electronics wizards, please speak up! Is this schematic consistent with the original crossover photo?

About the cabinets - - - I was originally hesitant to begin the paint stripping process because the paint was so thick and tenacious, but ultimately I decided to persevere and soldier on with some elbow grease. Current state is pretty good, with no finish at all (yet), but it was not easy. Each face took at least three applications of heavy duty chemical stripper, and each step was unique. First coat wrinkled and peeled up easily (like a prom dress, John?), second coat required detailed muscle scraping, and third coat was steel wool cleaning up the leftover white which residue which had left a 'pickled' look.

Advice with crossover would be greatly appreciated.

[JKent]

VERY nice job on that cabinet! So many options now. How to decide?

No comment on the xo--I'll leave that to Roy.

Kent

[RoyC]

Advice with crossover would be greatly appreciated.

The crossover drawing looks OK from here, ra.ra.

It is very likely the original polarity was as you have drawn it, and AR reversed it upon the elimination of the parallel crossover cap in later versions. AR appears to have been playing around with polarity in a number of early iterations of various models. If you are curious, reversing tweeter polarity can be interesting to experiment with. It won't damage anything to do so.

Roy

[ra.ra]

Delayed response, but thanks to both of you for recent comments.

On the original crossover, I remain a little perplexed about the original wiring when two caps are situated within the same wax block, so I just want to be sure that my first sketch in post #29 seems correct - - - this revised sketch here was another thought, but on second glance, it doesn't seem to make any sense - - - it seems that in this configuration, both caps would have an affect on the tweeter.

On the cabinets, I have not done any further work yet. While I do like the somewhat unusual look of "blonde" wood on these speakers, I have never found this wide-grained pine plywood terribly attractive. At one point, I had the idea of re-veneering with vertical grain fir, which has a beautiful warm softwood appearance, but I've decided I'll probably not go there. Now that I've labored to strip these, I'm working to find the best way to get them to 'glow' with a minimal finish. I am inclined not to use any stain, which will typically emphasize the grain contrast, yet I wish to warm up the raw wood with a very simple finish. Oil?.... probably not, since it does not enhance pine the way it does with walnut. Wax?.... maybe, but will it warm the color? Shellac?.... perhaps, since it can be tinted and thinned easily, plus it can be easily removed with denatured alcohol.

One thought that interests me, if I ever do this type of restoration again, might be to create a pair of small AR's in "soft black", as described in this thread. Those LST's look great with the off-white linen.

http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=7855

[RoyC]

On the original crossover, I remain a little perplexed about the original wiring when two caps are situated within the same wax block, so I just want to be sure that my first sketch in post #29 seems correct - - - this revised sketch here was another thought, but on second glance, it doesn't seem to make any sense - - - it seems that in this configuration, both caps would have an affect on the tweeter.

Your revised drawing takes the 24uf parallel cap and places it in series with the 20uf cap and the tweeter. Not good! You completely eliminated the parallel woofer cap and reduced the total sereis capacitance with the tweeter by nearly 50%.

Your drawing in post #29 is correct, and very easy to implement. Regardless of how they are housed, you simply have two caps in this crossover. They are both connected to the same input signal on one end, but their output leads carry the filtered signals to two different places. You could literally glue a new 24uf cap to the back of the woofer and connect one of the cap's leads to the + terminal and the other to the - terminal of the woofer and achieve the proper result. Your new 20uf cap, of course, would then be connected in series with the tweeter as always. Having 2 caps in one casing was just a less costly way to go. AR probably got a good deal on 'em.

As for your cabinets...the unfinished pine version has become very collectible. Yours look great. You may want to consider leaving them as is.

Roy

[ra.ra]

Thanks, Roy, for confirming our earlier correspondence and dismissing this other ridiculous notion about the x-o - - - - when I see comments like "Not good!", I tend to listen closely.

I do know that these unfinished pine cabs have gathered a sort of rare cult status, and since I've already gone to the considerable trouble of stripping them bare, I am very dis-inclined to do anything that will compromise their value. Thanks, again, I'll post again once these speakers get re-assembled.