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unpredictableape 2nd April 2017 05:46 PM

Adapt Polk Audio R20 for Classix II build or build box?
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Hi guys, I'm new to speaker diy and want to get started with an easy-ish project to get done quickly. I'm not sure that I've chosen one though. In order to avoid having to build and finish a cabinet, I'm considering just swapping out the drivers and crossovers in a set of Polk Audio R20s I have sitting around with the drivers and electronics from a Classix II kit. My main concern in doing so is the difference in distance between woofer and tweeter -- will the results be so profoundly different or worse that I'm wasting my time and money? Or is that even that can be known without building it?

I'm comfortable wielding a soldering iron, and while I have a table saw and miter saw, my woodworking skills are not too impressive, and are very seldom-used. What started me looking into getting a set of decent speakers is that my old surround speakers -- the Polk Audio R20s -- really aren't sounding too good to me these days. They were great as surrounds years ago, but compared to a set of Polk Monitor 4.5s from the 80's they sound pretty lifeless. The cabinets look great though, and match my office furniture perfectly. The Miami Vice era Polks don't look so good in the office, and they're bigger than I'd like.

My budget is fairly flexible but I like small numbers better, especially starting out with this. If the Caritas kit were around I'd be considering it. I found the Classix II design and it looks like a no-brainer. Except the cabinet dimensions are slightly different. I'm concerned mainly about the acoustic offset and somewhat about the difference in tweeter diffraction and cabinet volume. These are all things of which I have only a basic understanding. (Most of the wave mechanics I've read about was in regard to electrons, not sound or liquid. The concepts overlap a lot, but the applications are very, very different.) The C-to-C offset from woofer to tweeter in Paul Carmody's design is 5-3/4" or 5-7/8", depending on whether you're following the plans for front-ported or rear-ported cabinets. The distance on my R20 cabinets is right at 6-1/4", though it looks like I'll either have to surface mount the tweeters or very carefully drill an expansion to the tweeter cutout in order to get the BC25TG15 tweeters to fit. This should give me the opportunity to move them slightly closer to the woofer, but not a whole lot because the front port on the R20 cabinet interferes with expanding it straight toward the speaker. I've attached a picture of the front of the R20 for reference.

So even if I expand the tweeter cutout straight down, the tweeter-to-woofer C-to-C distance will likely only decrease to just under 6-1/8", so I think it'll end up around there. Does anybody have an idea, or know how to calculate if that is going to degrade performance, introducing peaks or troughs around the crossover point? The main reason I want a pre-designed driver and crossover package is to get good sound without relying on my nonexistent design abilities, so if this deviation form the design negates that benefit, I should probably prioritize building the cabinet, or find out what I'd need to design a crossover for the same drivers in this R20 cabinet. But really I just don't know if it's likely that big a deal.

There also is the question of the somewhat smaller volume of the R20 cabinets. The Classix look like they're ~0.53 cubic feet, or 15 liters. The R20 is 11.78, or .42 liters. I can shorten the vent tube to compensate but It'll sound a little different no matter what. I don't think there's anything I can do about that, and I don't know for sure how big a deal it is. I think it's less of a problem than the woofer/tweeter distance change though.

So I guess I'm fishing for a go/no go consensus from people with more experience than I have. I'll likely buy the kit with a faceplate and get started on cabinets no matter what, but without the ability to do a side-by-side I won't ever know whether my matching R20 cabinet or the homebuilt and finished Classix cabinet is the best choice. I really only have the time and ability to finish in paint or maybe a vinyl wrap.

I'm also tinkering with the idea of finding anohter tweeter that is a better physical fit. The cutout in the R20 is around 3-3/8" so the BC25TG15, though it sounds like a great tweeter and a great tweeter for the money, isn't a great fit physically. Deviating from the Classix II design puts me in design-my-own speakers land though, which I doubt I'm ready for. It's still tempting to go that route though, as I have the better-performing MW6502 woofers in the Monitor 4.5s, which should fit fine in the R20 speakers. I'm not certain they would perform as well in that cabinet, or what, if any modifications ot he port would be needed. Then if I found a tweeter to fit the R20 that pairs well with the MW6502, I'd need to how to design a crossover to make the two work together, which sounds like a pretty sophisticated task. And I imagine that when desiging a crossover, one builds the board and components outside the speaker and tests them after doing the research. And I imagine a fair number of de-soldering and re-soldering events occur before the best sound is found, especially for beginners.

So if you have any recommendation, I would greatly appreciate it. At this point I think I'll order the Classix II kit, plug the components in the R20 cabinets and go from there by sound. Any comments or recommendations are welcome.


Lojzek 2nd April 2017 10:00 PM

2 Attachment(s)
You can freely put the Classix II speaker units into Polk Audio R20 cabinet for the differences are mostly in the bass region and some very small because of baffle shape. Black curve is Polk's cabinet, grey original 15 lit. Both of these versions are tuned approximately the same, Polk version having a bit longer vent than original Classix one.

It's a kind of a shame not tu modify the XO filter of Polk because I believe this is a similar league of speaker units. In order to do that you'd have to do some measurements which isn't that hard but takes time to learn how.

unpredictableape 3rd April 2017 12:54 AM

Oh wow, that looks like the cabinets are really very similar then. So as far as the woofer goes, I'm in good shape with the R20 cabinet.

I had not considered modifying the crossovers I already own. That drops the budgetary concern a bit, or frees some up for a tweeter that fits physically if I can find one that matches the woofer as well as the BC25TG15-04. I read so many things about the importance of a custom crossover I didn't think for a second about modifying the one I have instead of building from scratch. I have a multimeter and can read stripes, so I think I can get the resistance and the inductance but I don't think I have a way to test capacitance. Capacitors aren't an expensive part, and are probably the most likely to need modification, so I'm pretty sure I can figure this out.

Thanks for the advice! I don't suppose you'd know a 4 ohm tweeter that's also a good match for the DC160, has a good near-field sound dispersion and whose overall diameter is around or under 3.375"/8.57cm? :) Wait, there's not a speaker driver database floating around anywhere, is there? So far the BC25SC06-04 and the Dayton ND25FA-4 are the closest I've found, but I've only looked at sensiticity as a factor so far. I guess I could just buy a router bit and make the right tweeter fit.

Lojzek 3rd April 2017 07:25 AM

I wouldn't know of any driver database but it's more likely that you'll have to modify either baffle or the tweeter. IMO the right way to start any kind of DIY within loudspeakers is first to use measurements to find out what kind of performance your speakers offer now and then you will see clearly what to do.

Nowadays it's a common thing to use free software ( measures all kinds of things you can imagine) like excellent ARTA which does everything as the version you pay for except for not being capable of exporting plots, but you can always use copy/paste function. Got to and get the manuals and latest software version. A normal decent sound card is more than enough. Basically you'd need a microphone with phantom power supply that you can put together yourself or use soundcard phantom power if it's available. There is lots of threads on this topic so you should find out all the answers.

As long as I have been modifying loudspeakers out of curiosity, in almost all cases the factory crossover filters with cheap loudspeakers were very defficient in parts count and more important, the values were such that holes in frequency response were significant and it's far better to fill these holes with sound than without it.

Learn to use measurements and spend your money first on that, then the drivers.

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