Taming TL Ear Fatigue

Coblio

Some time back I reacquired a pair of Polk RTA-11t towers I had traded to a friend some years previously and undertook an upgrade project--even though I had thoroughly enjoyed the original speakers, I felt there was room for improvement so I bought a pair of the newer RTA-11TL (SL3000) tweeters from Polk and built new crossovers using premium components.

I did too good a job, because they're now obnoxiously strident--while the midrange detail and bass clarity are outstanding, better than the Klipsch KG4's and Celestion SL12's I've got for comparison, I develop a listening headache pretty quickly. I found out too late that the original RTA-11T (SL-2000?) drivers are in demand as replacements for the later model I installed, and I had sold my originals to a fella with a pair of Polk SDA's. They're unavailable at any price as far as I know--they're similar to the Peerless 810665, used in some yet-older Polk systems (the mounting dimensions of the SL-3000 are identical to the Peerless).

Polk was forthcoming in providing me with crossover schematics and selling me the SL-3000 tweeters, but the design specs of the drivers are proprietary engineering data. All I'm told is that "in general terms" the impedance should be regarded as 5 ohms and they should be crossed over somewhere between 2,000 and 3,000Hz. The original tweeters had phenolic domes while the new ones are titanium I believe, and the HF crossover values are somewhat different--a 0.3mH inductor replaces the original 0.4mH, the originals had a 2.7-ohm resistor in series with the inductor while the new version has no resistor in this position, and capacitance in the positive leg is reduced from 28uf to 24uf.

To the tame the problem I'm thinking of simply installing an adjustable L-pad to adjust the tweeter volume--I hate putting yards of cheap resistance wire in the signal path of my new high-end crossover parts, so I planned on finding the proper values through listening, then installing fixed resistors in place of the adjustable pad. However I'm no speaker designer and I'm not sure this is the right approach--the problem might be in the crossover frequency or even in the drivers themselves. In upgrading from a 16ga wire LF inductor and 18ga iron-core HF inductor to 14ga Goertz air-core foil inductors, plus replacing the electrolytic and mylar caps with Solens bypassed with Audiocap PPT Thetas, I've changed the electrical characteristics considerably.

The old RTA and SDA series' have a cult following and can be quite impressive...even now the detail and accuracy jump out and the first impression is very favorable; the new crossovers noticeably improve the midrange and the bass is tighter and deeper. However extended listening becomes uncomfortable due to the hyper-highs. My L-shaped living room and hardwood floors don't help--in a different environment they still might shine, but this is what I've got to work with for now.

So here I am, soliciting help from the experts. Any suggestions for knocking down my screechy top end a notch or two would be greatly appreciated. Is a proper L-pad likely to solve the problem, should I be looking at changing the crossover frequency, or should I be investigating different tweeters?

Thanks for any insights or suggestions you can offer.

RuSsMaN

First of all, what are you running on the back end, amp, pre, cables, etc?

Second, I agree, no more inductors, but what about experiementing with a simple resistor across the tweeter terminals? I don't have a schematic for that particular speaker, so I don't know if Polk was doing that already in the crossover.

Welcome to the boards, you certainly seem to have some audio in your bag.

Cheers,
Russ

Coblio

Howdy,
Thanks for the input. I'm currently running a B&K ST1400 Series II power amp, Parasound P/HP-850 pre, Adcom GCD-575 CD player, Linn Axis turntable, SME 3009 SII imp. tonearm, Goldring 1042 MM cartridge, Audioquest interconnects and Kimber cable. I've got a VTA-80 35wpc tube amp on my bench that'll soon be replacing the B&K (a Dynaco ST-70 mod, though there's nothing Dynaco left except the transformers), plus a Van Alstine Super PAS-3 pre (again, though the Dynaco design was the starting point it's an entirely new circuit using a regulated power supply with 12AX7 phono and line stages). I expect that tube amplification alone will tame some of the stridency from the speakers, though the B&K strikes me as warmer-sounding than most other mosfet amps.

I've got the RTA-11TL schematic and there's no resistor across the tweeter, though there was a 2.7-ohm 5W resistor in the RTA-11T which it superceded. It's just a matter of trial and error finding the right solution, I guess. A single resistor might well do the trick, I don't know if an L-pad solution is really necessary to maintain proper impedance or not.

Warm Regards,
Dave

RuSsMaN

Keep us posted, I'll be interested in the journey - the trials and errors, and where you finally end up.

Just looking at the list - the Parasound could very well be the source of top end grain / fatigue in that setup. The weak link, if you will.

Cheers,
Russ

F1nut

Welcome!

That's the first I've heard of SL2000 tweeters being in demand. They have a nasty 5dB spike at 13kHz that the SL3000 doesn't. Anyway, you don't want either of those tweeters, IMO. What you want is the new silk dome replacement tweeters, part number RD0198-1. Start there before trying other methods of taming the highs. Next, remove the bypass caps. I've found that at first they seem to make the highs sparkle with detail, but after awhile realized that they make the highs annoying and harsh. The speakers sound much better without them. Also, if at all possible put down a large rug on your floor.

After trying the above and it's still bright, I'd try the resistor idea.

Coblio

I personally feel that the Parasound is a real sleeper value for a solid state pre; until it was discontinued two years ago you could pick one up new for about $275. I've recommended it to two other people and they were also pleased. It's pretty robust under the hood--Black Gate caps, metal film resistors, ALPS volume and balance pots--though like everything there's room for improvement. I estimated what it would take to rebuild mine with premium film caps and an upgraded power supply and figured it wasn't worth the expense. On my project pile I've got an Adcom GFP-555 along with new replacement opamps that I'm looking forward to trying out. I also just came by a B&K Pro10 pre on Ebay that should be arriving tomorrow ($215 for that plus an ST-140 power amp. I've owned half a dozen ST-140's and come up with a $150 modification package that kicks butt, if I do say so myself; I triple the power supply capacitance, install Schottky hexfred bridge rectifiers, replace the output board electrolytics with Black Gates, use premium film/foil caps for coupling and bridging, rewire with OFC/teflon hookup wire and bias the mosfets at around 230mA). Much of my joy in audio comes from knowing which end of a soldering iron to hold, I like to tell folks how little I've had to spend for nice sound.

Coblio

F1 Nut, thanks for the tip--I didn't know about the new silk domes. The bypass caps are a mixed bag; in my 20-year-old Klipsch KG4's I recently installed newer horn drivers and bypasses, and the combination really cleaned up the muddiness I had set out to cure. However you're probably right on regarding the Polks, since more high-end detail is the last thing I want. I'll take those pricy caps and put them someplace they'll do more good, like an amplifier.

RuSsMaN

Opamps.....

Go discrete, period. I view opamp upgrades like audio bandaids, you still aren't REALLY solving the problem, so to speak.

Jesse makes a STELLAR point, silk dome replacements are the order of the day for good sound from vintage Polks. You won't need your iron, but they (should) floor you with the difference. When you said you ordered from Polk, I assumed they were RD's, not SL's......

Cheers,
Russ

ohskigod

might want to consider sound treatments to the room itself. I had a nasty glare problem that it took me a while to figure out was the room itself. some sound foam on the walls in the right places helped greatly. I cant stress the differences it made enough.

the more detailed the speaker, the worse the fatigue I would get was (and it would set in quicker)

I would get fatigue listening to Bozaks before my room treatment, which you might allready know, is near impossible since Bozaks are so laid back.

throw some thick blankets on the wall to see if it helps even a little, if it does, then it will be worth it to go the sound treatment route.

also, while I am a fan of the Parasound php850, it could be considerred on the forward side, which can enhance any brightness problem you have, a tube pre is allways a good thing to consider if your sensitive to listeners fatigue

danger boy

i say start with the silk dome replacements first... that's a no brainer place to start.. that may just tame the harshness you are having. it did for me.