Crossover rebuilds & ear fatigue update
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Coblio
Posts: 13
Howdy Y'all,
I posted here a week or so ago under the title "Taming TL Ear Fatigue" and wanted to pass on a few things I've learned along the way. I've overcome several stumbling blocks in my efforts to improve my various aging speakers and wanted to offer some halfassed wisdom, for whatever it's worth. It's all one man's opinions, feel free to disagree, most audio matters are subjective and arbritrary--I'm simply relating my own practical experiences and observations, and maybe saving someone else from repeating my own mistakes.
For those who responded, thanks for the expert input--I purchased the silk dome RD0198-1's to replace the screechy SL-3000's, removed the premium bypass caps I had installed because I bought into the audiophile hype regarding their benefits, and my RTA-11TL's are singing better than ever before, even though the new crossovers and drivers are far from being broken in.
When I called Polk to order the new tweeters and told the tech about my problems in trying to get the RTA's correctly dialed in, he was very helpful but didn't accept that the irritating sound resulting from my upgrade mods was the fault of the tweeter itself; he felt that the degree of harshness I was describing was due to an error I had made in constructing the crossovers. To re-describe what happened, several years ago I started out with Polk RTA-11t's and on the advice of a tech purchased SL-3000's to replace the original SL-2000 tweeters. I built new crossovers from an RTA-11TL schematic using premium inductors and caps. With great expectations I then fired up my stereo only to discover I had created a monster.
Some of the problem, I'm sure, was due to my new crossovers--though I used the same component values as the RTA-11TL, I replaced the original inductors (which I felt were substandard) with 14awg Goertz air core foils, used Solens to replace the electrolytic and Mylar caps, bypassed all the caps with Audiocap PPT Thetas, built a new wiring harness from Supra OFC low-inductance 12ga hookup wire, and replaced the input terminals with gold-plated copper Vampires.
The SL-3000's just didn't sound like they belonged in the system; the speakers sounded very wrong and not enjoyable at all.
On the advice of several folks here at Club Polk I ordered the RD0198 replacement tweeeters. Since the silk dome replacements are drop-ins for the SL-3000 and share the same electrical characteristics I didn't expect a truly major transformation, but my 1987-vintage speakers sound much better than they ever have before, and are replacing the $1700 Celestion SL-12's which have been my primary stereo setup for some time.
The original 20awg 0.3mH inductors had a DCR of 0.6 ohms while the Goertz foils are 0.2 ohms, so I thought I might need to add a resistor in series with the inductor or even replace it with an inexpensive 19awg wire coil to get back in the area of the original design specs. However I've been listening in joy for several hours and this will not be necessary. Yes, the foil inductor does produce a somewhat extended high end, but it is not harsh or irritating--the quality of the foil inductors and caps results in a smoother and more pleasing HF output, I feel.
Likewise with the large 14awg 1.5mH LF inductor I used in replacing Polk's original part, which was a scramble-wound coil of narrow-gauge magnet wire loosely wound around the outside of the terminal cup. The foil inductor produces bass response that's noticeably tighter and deeper. I believe the lower DCR in the woofer leg adds to the clarity and accuracy of the speaker. The slight changes in DCR and capacitance resulting from the modifications are not significant enough to alter the crossover point or frequency characteristics of the system, so far as I can tell, and if the parameters have changed it's for the better.
I am not a golden-ear audiophile who claims to hear audible differences between different capacitors and brands of hookup wire--for me the only test is listening pleasure.
For anyone interested, here are my personal conclusions after devoting considerable time and money trying to obtain the best sound my Polk RTA's are capable of delivering. The same drivers were used in many other Polk systems and most crossovers had similar or identical designs, so the same principles should apply to other Polk models.
1) For Polks using SL-2000, SL-2500 or SL-3000 tweeters, dump 'em and invest in the silk dome RD0198 ($100 a pair). Polk has a different silk dome replacement for the SL-2000, which was more efficient and crossed over at a slightly lower frequency, but I've heard both versions and rather than using this driver I'd obtain a schematic for the later model variations which replaced the SL-2000 with the SL-2500/3000. Polk continued using the same LF drivers such as the MW-6510 and MW-6502 as they upgraded their tweeters, so the low-pass crossover values stayed the same while only the high-pass sections were redesigned. So even though I started out with the RTA-11t and SL-2000 tweeter, I chose the SL-3000 driver replacement and used the newer RTA-11TL schematic in determining component values for rebuilding the crossovers.
2) Though some folks differ, I believe that high-quality pure-copper foil inductors with low DC resistance offer lower distortion and superior dynamic range compared to small-gauge wire coils. The cost is not that great, good 14awg or 16awg inductors in the values needed cost around $10-$15 each. While the lower DCR does alter the crossover characteristics somewhat, you just might find that you prefer it. If not, a small-value 5W resistor in series with the HF inductor will quiet the tweeter response.
3) Use good-quality non-inductive resistors such as Mills or Kiwame. They run about $2 each.
4) Solen metallized polypropylene films are excellent crossover caps, and even the large values such as 33uf are under $10. Electrolytic caps have no place in crossovers, and were only used in older systems due to size limitations and unavailability of large film caps. If size is still an issue, Black Gate bi-polar electrolytics are an exception to the rule, and are outstanding in crossovers.
5) A ton of audiophiliac hype has been written regarding bypassing large-value film crossover caps with smaller-value high-end metallized film or film/foil caps. Some speakers might benefit from this, possibly more inefficient designs, but after recent experience installing bypasses in both my Polks and a pair of Klipsch KG4's it didn't require much listening to tell me that while I did hear a bit more detail, I also picked up high-end harshness l didn't care to listen to. My first impression was positive, but after an hour or so my ears got tired and I had a listening headache. After de-soldering my $60 worth of AudioCaps I was much happier with both speakers. The one exception regarding bypassing is if you've got electrolytic caps still living in your crossovers; by nature, 'lytics (with the exception of Black Gates) have poor HF characteristics and contribute a rather muddy quality at audio frequencies. They audibly benefit from bypassing--the general rule of thumb is to use a film cap that's around 1% of the base cap value. The bypasses should also have a higher voltage rating than the base caps, preferably at least double.
6) The rear connector terminals in many speakers are pretty tacky quality--if yours are plastic knobs and white metal, spend ten or 12 bucks on some five-way Vampires, Daytons or something else decent. There's no need to pay $50, just use terminals that are mechanically solid gold-plated brass or copper.
7) As long as you're poking around inside the cabinets, if your internal wiring harness is cheapo lamp cord replace it with some decent wire, like 12awg Monster, Dayton or Supra. I prefer to get rid of crimp-on spade connectors and hard-solder things together, but this makes it a hassle if you decide to make changes later so at least use pure copper or gold-plated spades instead of tin ones. The steel connector blades on the individual drivers pick up oxidation over time so polish 'em up with fine sandpaper and squirt 'em with De-Oxit before you finish putting things back together.
8) For ease of construction, many speaker designers built the entire crossover networks onto the back of the terminal cups, and this in itself often required using smaller, lower-quality inductors and caps. Get the assemblies off the cups and build your crossovers on a larger, high-quality printed circuit board--when completed you can either glue it or screw it to the bottom or back wall of the cabinet interior. I use "Big Foot" PCB's, about $6 from Madisound. They measure about 4" x 7", have male spade connectors pre-mounted and the layout is very versatile, with multiple soldering points for whatever design you might concoct.
9) It probably needn't be said, but double-check to make sure you've observed proper polarity in reassembly. If your solder joints are solid but the speakers sound like crap, you've most likely reversed the phase somewhere in the wiring.
When all's said and done, you shouldn't be out of pocket any more than $200 max and might very well decide that you don't really want those Magnepans or B&W's you've been coveting after all...with a little finesse and some new-generation tweeters a 20-year-old pair of speakers can sound every bit as good as anything on the market these days.
Of course if that doesn't turn out to be the case, don't blame me.
I posted here a week or so ago under the title "Taming TL Ear Fatigue" and wanted to pass on a few things I've learned along the way. I've overcome several stumbling blocks in my efforts to improve my various aging speakers and wanted to offer some halfassed wisdom, for whatever it's worth. It's all one man's opinions, feel free to disagree, most audio matters are subjective and arbritrary--I'm simply relating my own practical experiences and observations, and maybe saving someone else from repeating my own mistakes.
For those who responded, thanks for the expert input--I purchased the silk dome RD0198-1's to replace the screechy SL-3000's, removed the premium bypass caps I had installed because I bought into the audiophile hype regarding their benefits, and my RTA-11TL's are singing better than ever before, even though the new crossovers and drivers are far from being broken in.
When I called Polk to order the new tweeters and told the tech about my problems in trying to get the RTA's correctly dialed in, he was very helpful but didn't accept that the irritating sound resulting from my upgrade mods was the fault of the tweeter itself; he felt that the degree of harshness I was describing was due to an error I had made in constructing the crossovers. To re-describe what happened, several years ago I started out with Polk RTA-11t's and on the advice of a tech purchased SL-3000's to replace the original SL-2000 tweeters. I built new crossovers from an RTA-11TL schematic using premium inductors and caps. With great expectations I then fired up my stereo only to discover I had created a monster.
Some of the problem, I'm sure, was due to my new crossovers--though I used the same component values as the RTA-11TL, I replaced the original inductors (which I felt were substandard) with 14awg Goertz air core foils, used Solens to replace the electrolytic and Mylar caps, bypassed all the caps with Audiocap PPT Thetas, built a new wiring harness from Supra OFC low-inductance 12ga hookup wire, and replaced the input terminals with gold-plated copper Vampires.
The SL-3000's just didn't sound like they belonged in the system; the speakers sounded very wrong and not enjoyable at all.
On the advice of several folks here at Club Polk I ordered the RD0198 replacement tweeeters. Since the silk dome replacements are drop-ins for the SL-3000 and share the same electrical characteristics I didn't expect a truly major transformation, but my 1987-vintage speakers sound much better than they ever have before, and are replacing the $1700 Celestion SL-12's which have been my primary stereo setup for some time.
The original 20awg 0.3mH inductors had a DCR of 0.6 ohms while the Goertz foils are 0.2 ohms, so I thought I might need to add a resistor in series with the inductor or even replace it with an inexpensive 19awg wire coil to get back in the area of the original design specs. However I've been listening in joy for several hours and this will not be necessary. Yes, the foil inductor does produce a somewhat extended high end, but it is not harsh or irritating--the quality of the foil inductors and caps results in a smoother and more pleasing HF output, I feel.
Likewise with the large 14awg 1.5mH LF inductor I used in replacing Polk's original part, which was a scramble-wound coil of narrow-gauge magnet wire loosely wound around the outside of the terminal cup. The foil inductor produces bass response that's noticeably tighter and deeper. I believe the lower DCR in the woofer leg adds to the clarity and accuracy of the speaker. The slight changes in DCR and capacitance resulting from the modifications are not significant enough to alter the crossover point or frequency characteristics of the system, so far as I can tell, and if the parameters have changed it's for the better.
I am not a golden-ear audiophile who claims to hear audible differences between different capacitors and brands of hookup wire--for me the only test is listening pleasure.
For anyone interested, here are my personal conclusions after devoting considerable time and money trying to obtain the best sound my Polk RTA's are capable of delivering. The same drivers were used in many other Polk systems and most crossovers had similar or identical designs, so the same principles should apply to other Polk models.
1) For Polks using SL-2000, SL-2500 or SL-3000 tweeters, dump 'em and invest in the silk dome RD0198 ($100 a pair). Polk has a different silk dome replacement for the SL-2000, which was more efficient and crossed over at a slightly lower frequency, but I've heard both versions and rather than using this driver I'd obtain a schematic for the later model variations which replaced the SL-2000 with the SL-2500/3000. Polk continued using the same LF drivers such as the MW-6510 and MW-6502 as they upgraded their tweeters, so the low-pass crossover values stayed the same while only the high-pass sections were redesigned. So even though I started out with the RTA-11t and SL-2000 tweeter, I chose the SL-3000 driver replacement and used the newer RTA-11TL schematic in determining component values for rebuilding the crossovers.
2) Though some folks differ, I believe that high-quality pure-copper foil inductors with low DC resistance offer lower distortion and superior dynamic range compared to small-gauge wire coils. The cost is not that great, good 14awg or 16awg inductors in the values needed cost around $10-$15 each. While the lower DCR does alter the crossover characteristics somewhat, you just might find that you prefer it. If not, a small-value 5W resistor in series with the HF inductor will quiet the tweeter response.
3) Use good-quality non-inductive resistors such as Mills or Kiwame. They run about $2 each.
4) Solen metallized polypropylene films are excellent crossover caps, and even the large values such as 33uf are under $10. Electrolytic caps have no place in crossovers, and were only used in older systems due to size limitations and unavailability of large film caps. If size is still an issue, Black Gate bi-polar electrolytics are an exception to the rule, and are outstanding in crossovers.
5) A ton of audiophiliac hype has been written regarding bypassing large-value film crossover caps with smaller-value high-end metallized film or film/foil caps. Some speakers might benefit from this, possibly more inefficient designs, but after recent experience installing bypasses in both my Polks and a pair of Klipsch KG4's it didn't require much listening to tell me that while I did hear a bit more detail, I also picked up high-end harshness l didn't care to listen to. My first impression was positive, but after an hour or so my ears got tired and I had a listening headache. After de-soldering my $60 worth of AudioCaps I was much happier with both speakers. The one exception regarding bypassing is if you've got electrolytic caps still living in your crossovers; by nature, 'lytics (with the exception of Black Gates) have poor HF characteristics and contribute a rather muddy quality at audio frequencies. They audibly benefit from bypassing--the general rule of thumb is to use a film cap that's around 1% of the base cap value. The bypasses should also have a higher voltage rating than the base caps, preferably at least double.
6) The rear connector terminals in many speakers are pretty tacky quality--if yours are plastic knobs and white metal, spend ten or 12 bucks on some five-way Vampires, Daytons or something else decent. There's no need to pay $50, just use terminals that are mechanically solid gold-plated brass or copper.
7) As long as you're poking around inside the cabinets, if your internal wiring harness is cheapo lamp cord replace it with some decent wire, like 12awg Monster, Dayton or Supra. I prefer to get rid of crimp-on spade connectors and hard-solder things together, but this makes it a hassle if you decide to make changes later so at least use pure copper or gold-plated spades instead of tin ones. The steel connector blades on the individual drivers pick up oxidation over time so polish 'em up with fine sandpaper and squirt 'em with De-Oxit before you finish putting things back together.
8) For ease of construction, many speaker designers built the entire crossover networks onto the back of the terminal cups, and this in itself often required using smaller, lower-quality inductors and caps. Get the assemblies off the cups and build your crossovers on a larger, high-quality printed circuit board--when completed you can either glue it or screw it to the bottom or back wall of the cabinet interior. I use "Big Foot" PCB's, about $6 from Madisound. They measure about 4" x 7", have male spade connectors pre-mounted and the layout is very versatile, with multiple soldering points for whatever design you might concoct.
9) It probably needn't be said, but double-check to make sure you've observed proper polarity in reassembly. If your solder joints are solid but the speakers sound like crap, you've most likely reversed the phase somewhere in the wiring.
When all's said and done, you shouldn't be out of pocket any more than $200 max and might very well decide that you don't really want those Magnepans or B&W's you've been coveting after all...with a little finesse and some new-generation tweeters a 20-year-old pair of speakers can sound every bit as good as anything on the market these days.
Of course if that doesn't turn out to be the case, don't blame me.
Post edited by Coblio on
Comments
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Excellent write up. I thought you'd like the new tweeters.Political Correctness'.........defined
"A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."
President of Club Polk -
Great write-up!!! Right now I have my 20 year old RTA-11t's and a new pair of Lsi9's. And while the 9's are stellar in HF response and male/female vocals and brass and string instruments but I'm having issues with what I feel is a large unnatural midbass peak (perhaps around 80Hz or so) that just isn't there with my RTA's. I keep going back and forth on wether upgrading x-over components and tweeters in my RTA's will get me closer to what I'm looking for then having to expend the money on the 9's which are stellar in every respect except the midbass peak I'm hearing.
Don't get me wrong the 9's do almost everything extremely well and I know the procedure you outline will improve the RTA's but they will never have the HF response or vocal reproduction like the 9's. However I feel the RTA's have a much better mid-bass reproduction and LF extension. On the 9's some music sounds really good in midbass and others just sound outright unbalanced. I find what were great recordings on my RTA's don't sound as good on my Lsi's and some recordings that sounded not so good on the RTA's sound great on the Lsi's............go figure
I suppose in the end I should just keep the 9's for awhile and if I find I just can't live with the exaggerated mid-bass I'll just move up to the 15's. The stock RTA's are almost unlistenable now compared to the Lsi's simply because of the tweeter issue. Long live the Vifa!
H9"Appreciation of audio is a completely subjective human experience. Measurements can provide a measure of insight, but are no substitute for human judgment. Why are we looking to reduce a subjective experience to objective criteria anyway? The subtleties of music and audio reproduction are for those who appreciate it. Differentiation by numbers is for those who do not".--Nelson Pass Pass Labs XA25 | EE Avant Pre | EE Mini Max Supreme DAC | MIT Shotgun S1 | Pangea AC14SE MKII | Legend L600 | BlueSound Node 3 - Tubes add soul! -
I agree quite strongly that choice of source material makes a big difference in how a particular speaker sounds--not only the type of music but the quality of the recording. For what it's worth, I'm told that the RTA-t's SL-2000 tweeter has an unpleasant spike at around 13kHz that was corrected with the SL-3000...as I mentioned in my writeup, however, I feel that the SL-3000 is unpleasantly harsh and the newer silk dome RD0198-1 offers the best of both worlds. Like you, I've always liked the midrange and upper bass smoothness the RTA's provide, and now I'm pleased with the high end as well, although it took a good deal of research and experimentation for me to get there.
My primary test CD while doing the modifications was Alison Krauss' "Live at Union Station," about as challenging a vocal recording as you can come across. Alison's soprano is the voice of an angel, but with the wrong speakers can be piercing. The recording also features a lot of violin, banjo, Dobro guitar and mandolin--all instruments extending well into the upper ranges, as well a male tenor and baritone voices. All I can say is, I feel that the modified Polks do the recording justice.
However, having other speakers for comparison I can't really say they're the greatest speakers for high range vocals...I have a pair of Klipsch KG4's, a mid-line 80's-vintage speaker that I've modified with new crossovers and new polymer voice coil/dome assemblies for the horn tweeters that replace Klipsch's original phenolic domes. Klipsch's trully do excel in the high end (as least I think so) and the revitalized horns are something special, the soprano vocal harmonies on the same Krauss recording cause me to stop and take notice even though I've now heard them 30 times over.
If there's one speaker out there that does it all it's certainly one I'll never be able to afford. I split my listening about evenly beween CD and vinyl and my listening tastes include bluegrass, jazz, classical and hard rock. I also have both tube and mosfet power amps. I have a third set of mains, Celestion SL-12's. It makes my living room pretty crowded, but there's no getting around the fact that different material and different amps shine best on different speakers.
If you want a significant improvement in your RTA-11t's, though, you might consider a simple path--buy a pair of Polk's silk dome direct replacements for the SL-2000 tweeter (I don't have the part # at hand), replace the caps on the existing crossover with Solens (or at least replace the bi-polar electrolytic cap with something better--poor high frequency characteristics aside, 'lytics have a limited shelf life and degrade over 20 years). You might not get quite the result you're looking for, but I think you'll find that the old RTA's have some life in 'em yet.
Best,
Dave -
Great writeup Dave! My experiences are similiar with the replacement of the SL2000 tweeter with the soft dome RD-0194. I personally have not had the problem with the bypass caps on the two sets of SDA crossovers that I have rebuilt. However, my next rebuild on my Polk CRS's I am not planning to use any by-pass caps. I prefer the Sonicaps to the Solens. The Solens have a great reputation on the Polk Forum but have mixed reviews elsewhere. The criticism being they can sound a little harsh.
I am with you about building a crossover board from scratch. Especially if you get some of the BIG film caps. Plus, the point to point wiring is always better than the cheap trace boards.
Thanks for the writeup! Maybe post some pic's of your work!Carl -
Thanks for the tip on the Sonicaps. I've used Solens out of habit for years because there were few other choices and they come in any value you can name, but since I still haven't permanently mounted the crossovers and my ears have become ultra-attuned to subtleties with all the test listening, I might just check out the Sonicaps.
Since the SDA's are larger and more complex than the RTA's it makes some sense that you've done better with bypassing than I have--my 11TL's share the one tweeter with a pair of 6" MW-6510's and dual 8" tuned passive radiators, plus my listening area is fairly small and needs acoustic dampening, so even minor HF changes are not subtle--with some added distance, thick carpeting and wall hangings I think the bypasses might well be a good thing--at first I was impressed by the added detail and when it became tiring my room acoustics may have been largely to blame.
I'm also trying out some higher-DCR HF inductors before I button things down, which should benefit more from bypassing than the big foil units. I've taken it this far and have different resistors, caps and inductors to spare so I still want to try some different things--even though I like the way I've got things dialed in now, it's an opportunity to learn hands-on about the finer points of crossover components so I might as well take advantage. Book learnin' only goes so far, I like to get my hands dirty and own my opinions instead of repeating what other folks have to say.
Thanks to everyone for the input and support--this is an excellent forum with real-world enthusiasts. Places like AudioAsylum are great places to learn, but the atmosphere and opinions can be a little out there, if you know what I mean (though I certainly can't contradict the indisputable audible benefits of cryogenically treated unidirectional-electron feng-shui corrected $12,000 1-meter interconnects or similar essential hi-fi components).
Cheers,
Dave
