Improvements to the SDA SRS

[DarqueKnight]

Introduction

There have been questions concerning what is required to “upgrade” the SDA SRS to the SDA SRS 1.2TL. The answer is: NOTHING! Although the original SRS and the SRS 1.2TL look nearly identical to the casual observer, they are entirely different speaker systems. The only things they have in common are the stereo drivers (MW 6503), passive radiator (SW150), and the physical arrangement of the drivers on the baffle board. The cabinets, crossover circuits, SDA interconnect cable, binding posts, SDA drivers, tweeters, and SOUND are different between the two speaker systems.

The set of SDA SRS loudspeakers discussed here was manufactured in September of 1987 and purchased be me in November of 2002. The original owner purchased them in July of 1988 and subsequently sold them to his brother. The second owner was looking to sell them prior to going off to graduate school and moving into a smaller space. I received the speakers in near-mint condition with original boxes, manual, packing material, and sales receipt. The only thing that kept them from being in absolutely mint condition was an easily repaired 8-1/2” long by 1/32” wide crack in the left speaker’s top oak cap.

The modifications consisted of the usual replacement of the grille cloth, binding posts, the resistors and capacitors in the crossover network (Figures 1 through 3), and the replacement of the SL2000 tweeters with the SL2000T tweeters (figures 5a and 5b).


Modification Costs

The total parts cost for the modification was $553.45 and was worth every cent. I paid $1400 for the speakers. This price included out-of-state delivery. Therefore, the modification brought my total investment up to a little under $2000.

Parts were sourced from the following vendors:

1. SL2000T Tweeters
Polk Audio Customer Service
800-377-7655
www.polkaudio.com

2. Mills MRA-12 Resistors
The Parts Connexion
866-681-9602
www.partsconnexion.com

3. Solen PB Series Film Capacitors
The Parts Connexion
866-681-9602
www.partsconnexion.com

4. Ponte Knit Cloth
Hancock Fabrics
Can be purchased locally or ordered online
SKU #708545
www.hancockfabrics.com

5. Cardas HCBP-S Binding Posts
DIY Cable
360-452-9373
www.diycable.com


Enlarge
Figure 1. From front to rear: Cardas HCBP-s binding posts, Mills MRA-12 wire wound resistors, Solen PB series polypropylene film capacitors, and Hancock Fabrics “Ponte” knit grille cloth.

For this modification, I chose to replace the electrolytic capacitors with Solen PB series polypropylene film capacitors. Although I prefer the sound of AudioCap film capacitors over Solen, AudioCap does not make film capacitors in the large values (92 uF and 130 uF) needed for the SRS crossover circuit. I could have connected smaller values of AudioCap capacitors in parallel to give me the larger values I needed, but arranging and securing such a conglomeration on the crossover circuit board would have been a topological nightmare. While the cost of such an arrangement would not have qualified for nightmare status, it would have been a very disturbing dream. I also could have used AudioCaps for the smaller capacitance values and used Solens for the larger values, but I did not want to mix capacitor brands on this particular project.

In comparison tests, the AudioCaps provided a little more detail for a lot more money. However, the Solens are no slackers in the performance area. For example, they were the used by Dunlavy Audio Labs in reference speaker systems costing up to $35,000. Unfortunately, like many well-regarded high-end speaker companies, Dunlavy Audio Labs has ceased operation. While the company was in business, Dunlavy speakers were marketed as “the world's most accurate loudspeakers for discriminating audiophiles and professional recording engineers”. That may have been a little bit of marketing hyperbole, but I will say that I was very impressed with what I heard when I visited the DAL factory in Colorado Springs, Colorado in December of 2000.


Enlarge
Figure 2. SRS crossover circuit board and replacement components.

Although the crossover circuit board of the SRS is luxuriously spacious compared to other SDA models (see Figure 4), the advantage afforded by that spaciousness is moderated by the three large capacitor values required (two each of 130 uF and one of 92 uF).


Enlarge
Figure 3. Comparison of stock electrolytic crossover capacitors with their polypropylene film replacements.


Enlarge
Figure 4. Modified SDA SRS 1.2TL crossover (top) and stock SDA SRS crossover (bottom).

Modifications to SDA loudspeaker crossovers is not an undertaking for the inexperienced, impatient, or fainthearted individual. It is a task that requires the dexterity of a contortionist and the insight and patience developed through solving many difficult jigsaw puzzles.



Enlarge
Figure 5a. SRS speaker with stock SL2000 tweeters. The original SL2000 had a 5 dB resonance at 13 kHz. This added a bit of brightness and "detail" that some listeners liked.


Enlarge
Figure 5b. SRS speaker with upgrade silk dome RD0194-1 tweeters.

Figures 6a and 6b show the stock binding posts and the replacement Cardas HCBP-S posts. With my ears and my equipment, I have noticed no sonic benefit in upgrading the binding posts of my speakers. However, the all-metal, higher quality posts do provide a more secure connection than the stock posts with their plastic retaining nut. Since I have no current interest in bi-wiring and bi-amping, I decided to wire the high frequency and low frequency sections of the crossover to a single set of binding posts. I did leave my options open by drilling larger holes in the binding post plate to accommodate a second set of HCBP-S posts in the future. The upper holes were sealed on the inside with thick plastic adhesive pads and finished on the outside with dark felt circular pads.


Enlarge
Figure 6a. SRS stock binding posts.


Enlarge
Figure 6b. Cardas HCBP-S posts.

The upgrade grille fabric provided both sonic and aesthetic benefits. I tested prospective replacement fabrics by holding the fabric up to the light and by draping a yard of the fabric over my head and having the sales lady stand ten feet away and talk to me. She seemed to really enjoy helping me with my research.

Modification Procedure

I installed the new tweeters and binding posts in the left speaker and did A/B comparisons between it and the stock right speaker with the SDA cable disconnected. Next, the left speaker crossover was modified and A/B'ed with the stock right speaker, again with the SDA cable disconnected.

The time for all the modifications took a total of 10 hours and 45 minutes. Four hours of this was the time it took to remove, take apart, modify, reassemble, and reinstall the first crossover board. The second crossover board took only 2 hours and 45 minutes since I did not have to figure out how I was going to arrange components on the board. The SDA SRS 1.2TL crossover uses snap-in wiring harness connectors to for the driver wires. The SDA SRS driver wires are soldered to its crossover board. Therefore, the mere removal of the SRS crossover board is a tedious process. All components were resoldered using Cardas high silver content Quad-Eutectic solder.


Enlarge
Figure 7a. Side view of the modified SRS crossover circuit. The resistors that were at the top edge of the board were mounted on the underside of the board in the area above the large inductor coil under the center of the board. Prior to reinstallation, the large capacitors were secured to the board with plastic cable ties and heavy duty packing tape.


Enlarge
Figure 7b. Top view of the modified SRS crossover circuit.

Listening Evaluations

The improvement in sound quality was astonishing! I will post a detailed review of the the modified SRS in two weeks after the new components have had an opportunity to properly burn in. The review will compare the modified SRS to the stock SRS and will compare the modified SRS to the modified SRS 1.2TL.

[Airplay355]

damn man what do u do for a living? i think i may have found my future career lol

[PolkWannabie]

Raife,

Talk about HOF ... Great post and great showcase ... Thanks for the education.

[DarqueKnight]

Thanks. Glad you enjoyed reading it.

[DarqueKnight]

Originally posted by Airplay355
damn man what do u do for a living? i think i may have found my future career lol

I am an electrical engineer. It's nice work if you can get it.:)

[bikezappa]

Report Card

Information A+

Details A+

Clarity A+

Thoroughness A+


When can you do my speakers?

E=IR

[dorokusai]

Excellent information and pictures. Thanks for posting this Raife.

[F1nut]

Beautiful!

[AsSiMiLaTeD]

Nice Set-Up you have...

The Terminatrix is hot...

[kelley]

Nice job. I'm after bikezappa for my crossover work.

[madmax]

Great!

Can you supply the actual values for the Caps and Resistors? I have a pair exactly like this and am going to do the same modification. I'm ready for a speaker upgrade now!

Thanks!
madmax

[DarqueKnight]

Capacitors (quantity per speaker): 4.4uF (1), 12uf (1), 20uF (2), 92uF (1), 130uF (2).

Resistors(quantity per speaker): 2.7 ohm (1), 7.5 ohm (1), 15 ohm (1), 22.5 ohm (2).

A 4.4 uF and 92uF capacitor was unavailable in the Solen PB series, so I used 4.3uF and 91uF respectively. The small differences between the stock and upgrade parts was within the 5% tolerance range of the original part

A 22.5 ohm part was unavailable, so I used 22 ohms. The 0.5 ohm difference was within the 10% tolerance range of the original part.

Have fun.:)

[madmax]

Wow!
Thanks Raife, I've been considering this for awhile but it always comes down to needing the values, which means taking the xover out which always causes me to put it off... Over and over again.

What voltage value for the caps and what wattage rating for the resistors. Sorry to be such a problem here but I'm betting others are interested as well. (I could probably figure it out but some may not be able to, maybe I couldn't either :))
Thanks again, I'm actually going to do this. I am going to hold off on the SL2000T's to start with but I'm betting that will happen soon.
Thanks again, (and I plan on adding this to my copy of your manual as well),
madmax

[HBombToo]

Originally posted by madmax

What voltage value for the caps and what wattage rating for the resistors. Sorry to be such a problem here but I'm betting others are interested as well.
madmax

Great question mm. I was thinking about staying with cap rating of 250V at the minimum. Better safe than sorry I'm thinking.

HBomb

[DarqueKnight]

madmax, Hbomb,

It's no problem. All the stock cerment type (they look like little bricks) resistors are 5 watt, 10% tolerance parts.

The upgrade Mills resistors are 12 watt, 1% tolerance parts.

The 4.4uF and 12uF stock mylar film capacitors are 100 volt, 5% tolerance parts.

The 20uF, 92uF, and 130uF electrolytic capacitors are 50 volt, 5% tolerance parts.

The upgrade Solen polypropylene film capacitors are 400 volt, 5% tolerance parts.

I highly recommend the SL2000T tweeter.

[HBombToo]

Originally posted by raife1
madmax, Hbomb,

The upgrade Solen polypropylene film capacitors are 400 volt, 5% tolerance parts.

I'm assuming the 400V cap is for the high pass???

HBomb

[DarqueKnight]

All of the upgrade caps I used are rated 400 volts. All of Solen caps in the PB series have a 400 volt rating regardless of capacitance value.

[HBombToo]

ohhh... I get it now. My bust, I was heading down the wrong path there.

Thanks
HBomb

[madmax]

Thanks Raife,
I will soon be upgrading. Did you upgrade the wiring or wire connections? I was thinking they should be soldered rather than clipped on. What is your opinion? I have soooooo many SL-2000's I hate to switch to the 2000T's but maybe I can sell off some of them. We will see.
Great thread, thanks for taking the time to make such a professional presentation!
madmax

[gidrah]

Great post! Hell, I don't even have any SDAs (yet) and I read it with great enthusiasm. I will have to go back and read it again many times over.


That has to be one; if not THE, best post ever. Some threads get close to as good over time, but BAM! You came out like a gangbuster. Great job!

[DarqueKnight]

I did not change the internal wiring.

Prior to modifying my SDA 1B speakers in 1990, I called Polk and asked about the benefits of particular parts upgrades. The engineer I spoke with, Chris, said that there would be no benefit, audible or otherwise, from changing the internal wiring because the distances were so short and the stock wiring was of the appropriate gauge.

I replaced all the wiring anyway just to hear for myself. I used 12 gauge monster cable and soldered the wires directly to the drivers. Just as Chris predicted, there was no audible improvement. I switched back to the stock cables with the clips.

Chris did recommend upgrades to the resistors and capacitors and adding foam tape to the driver baskets to dampen vibration. He did not recommend adding damping material to the inside of the cabinet or changing any of the drivers.

For the SDA 1B modification, I did apply foam tape to all the driver baskets, but I did not hear any audible improvement, therefore I did not do it on any of my subsequent modifications.

Chris further stated that upgraded binding posts would provide only mechanical rather than audible benefits.

I ordered a couple of the SL2000Ts to "try out" in the CRS+s in my home office rig. I liked them so much that I ended up replacing all of the SL2000s in all my SDAs. Thats 22 tweeters (ouch!).

I'll probably end up replacing the SL3000s in my 1.2TLs with the new SL3000T tweeters also (more ouch!).

[DarqueKnight]

I almost hate to bring this up because, depending on your equipment and listening habits, it could lead to trouble.

All SDAs use a semiconductor self-resetting tweeter protection switch in the crossover circuit. Even when the switch is functioning perfectly, it causes a very slight loss of high frequency detail. Some of these switches, as they have aged, have caused significant loss of high frequency detail. Some have outright failed.

Polk does have an upgrade tweeter protection switch available, but I have not evaluated it. I preferred to remove the switch from the crossover circuits of my SDAs by shorting it out with a wire jumper. The tweeter protection switch can be seen as the small blue object near the lower left edge of the circuit board in Figure 7b of my initial post.

If you are using lower powered amps or receivers and/or frequently listen at high volume levels (I consider high volume to be >85 dB, C-weighted) I definitely do not recommend removing the tweeter protection switch.

[madmax]

One item on my list was to dampen the baskets with pieces of dynamat. Another was a brace from the back to the front where the tweeters mount. (I notice a pretty good vibration going on there). The fuse is a touchy subject for sure. I have played with the polyfuses at work and am not convinced they are the greatest for audio. I was considering taking the speakers to a known level where the tweeters shut off, placing a fuse holder in line and try different fuse values until I get it to blow at about the same level. I have found the operation of the polyfuse handy. I'm sure it saved me a few times. I've also wondered if there would be a benifit if the xover were enclosed and/or isolated from vibration. Mostly because of the coils. Maybe just mount the coils on rubber. Any thoughts on that?
madmax

[DarqueKnight]

I doubt that you would get audible benefits from further isolation of the crossover. However, try it and see.

The large coils are not physically attached to the main crossover circuit board and are securely bolted to thick metal plates in the crossover frame. The three small coils located on the crossover circuit board are secured with plastic bands and have foam damping material between the coil and the board.

I thought about securing everything with hot glue, but that would cause problems if I decide to change a component or required repairs in the future.

[madmax]

Just bumping this back to the top because it is so damn cool!
madmax

[BobMcG]

Excellent article. Without a doubt one of the best SDA technical posts I've ever read. This will no doubt help and inspire a lot of people looking to improve the performance of these classic speakers.

[pjdami]

Very nice article raife1. I could swear that you work for Polk after reading that article.

Although I do not own any SDAs personally, those were my dream speakers when I was growing up. I just never could afford them back then when I was in high school / college. One of these days.....if any of them show up within a few hundred miles of south Louisiana, I may just jump on an opportunity for a local purchase.

Perhaps Justin should change your status from Polkazoid to SDA Virtuoso or something honorable like that.

[rskarvan]

I agree completely and totally with your endorsement of the SL2000T. I love this new tweeter in my CRS+'s.

[DarqueKnight]

SRS Modification Listening Evaluations

Differences between the SDA SRS and the SDA SRS 1.2TL

With the exceptions of the tweeters, binding post plates, and slight diffenences in wood end cap construction, the SRS and the SRS 1.2TL look identical. There is only a two pound difference in the cabinet weights of the two speakers (182 pounds for the SRS vs. 180 pounds for the SRS 1.2TL). The SRS uses extensive internal bracing to reduce cabinet resonances. The SRS 1.2TL uses a heavier, stiffer “monocoque” cabinet structure to reduce cabinet resonances. A monocoque structural shape is one that bears loads on the skin of the structure. Therefore, the SRS 1.2TL cabinet weighs about the same as the SRS cabinet, even though it contains much less internal bracing. The cabinet structure of the SRS 1.2TL provides a significance increase in bass definition and resolution.

Some listeners prefer the original SRS SL2000 tweeter to the sound of the SRS 1.2TL SL3000 tweeter. These listeners subjectively feel that the SRS 1.2TL sounds “dull” and “rolled off” by comparison. I think the SL3000 and SL2000T sound more accurate. Listeners who like a little treble emphasis in their music may not like the sound of the new SL2000T tweeter. The SL2000’s 5 dB tweeter resonance at 13 kHz added a bit of brightness to the overall sound. The SRS 1.2TL’s SL3000 tweeter has fairly flat response from 2 kHz to 25 kHz and sounds more “laid back” by comparison. I requested phase and frequency response charts for the new SL2000T tweeter, but my request was declined by Polk’s engineering department.

The SRS 1.2TL uses improved SDA drivers (MW6511) and a simplified crossover circuit. The nominal impedance of the SRS 1.2TL is 8 ohms compared to a nominal impedance of 4 ohms for the SRS. Differences between the SRS and SRS 1.2TL are summarized in the table at the end of this post.

Listening Evaluations

Listening evaluations were done in six phases:

Phase 1 - Evaluation of the stock SDA SRS making careful notes of clarity, placement of instruments and vocals within the soundstage, and soundstage dimensions.

Phase 2 - Install SL2000T tweeters in the left speaker and do an A/B comparison between it and the stock right speaker with the SDA cable disconnected.

Phase 3 - Install upgrade crossover components in the left speaker and do an A/B comparison between it and the stock right speaker with the SDA cable disconnected.

Phase 4 - Evaluation of the modified SRS immediately after modification with SDA cable connected.

Phase 5 - Evaluation of the modified SRS two weeks after modification to allow proper burn in of components.

Phase 6 - Comparison of the modified SRS to the modified SRS 1.2TL two weeks after modification.


The crossover components and new tweeters were burned in over a period of two weeks by playing the radio 24 hours a day at a low level (60 dB, C-weighted) and by 10 hours of movie viewing at moderate to high levels (85 dB to 95 dB, C-weighted).

Source material (All CDs):

"Time Out" by Dave Brubeck, tracks 3 and 4
"Kind of Blue" by Miles Davis (20 bit version), tracks 1 and 2
"Saxophone Colossus" by Sonny Rollins (20 bit version), track 2
"The Song Lives On" by Joe Sample, track 6

Equipment

Home Theater System Components:

Sony TA-E9000ES Preamp, version 2.1 firmware*
Sony DVP-S9000ES DVD/SACD/CD Player
Adcom GFA-5500 power amp
Monster Cable Z2 Reference Speaker Cables
Monster Cable Z100i Interconnect (pre/power amps)
Audio Research Coaxial Cable (DVD/preamp)

*Tone controls were set to “flat”.

Two Channel Audio System Components:

Adcom GFP-750 Preamp**
Adcom GCD-750 CD Player
Adcom GFA-5802 Power Amp
Signal Cable Premium XLR V2 Interconnect Cables
Monster Cable Z3 Reference Speaker Cables

**This preamp does not have tone controls.

In the home theater system, the SRSs are 8 feet-3 inches apart inner edge-to-edge, 19 inches from the rear wall, 4 feet-8 inches from the left wall and 5 feet 3 inches from the right wall. The listening position is 12 feet from the plane of the speakers. The room has a ceramic tile floor and a 12 foot ceiling.

Phase 1 Listening Notes (Stock SRS)

Instruments are well-defined and stable within the space between the speakers. There is no projection of instruments and vocals beyond the front plane of the speakers. On some material the soundstage extends two feet beyond the outside edges of the speakers. Image depth is 3 to 5 feet behind the front plane of the speakers.

Phase 2 listening notes (Install SL2000T tweeters in the left speaker)

The balance control was moved back and forth between the modified left speaker and the stock right speaker with the SDA cable disconnected. It was immediately obvious that I was on to a good thing. The left speaker sounded louder, though it did not measure louder on the analog sound level meter. The increase in high frequency smoothness and detail, without increased brightness, was significant. It sounded like I was comparing two completely different speakers.


Phase 3 listening notes (Install upgrade crossover components in the left speaker)

The balance control was moved back and forth between the modified left speaker and the stock right speaker with the SDA cable disconnected. I heard little details, like percussion instruments, that I did not hear in the stock speaker.

Phase 4 listening notes (Modified SRS immediately after modification)

The SDA cable was reattached and a completely new set of speakers presented themselves. In addition to the improvements in high frequency detail, extension, and smoothness provided by the SL2000T tweeters, the upgrade crossover components provided significant improvements in imaging and soundstaging.

For example, on track 3 of Dave Brubeck’s “Take Five” CD, the piano was projected two feet out from the front plane of the speakers. The bass had more articulation and definition. There was more “shimmer” and overtones on the drum kit’s cymbals. Paul Desmond’s alto saxophone, which sounded clear with the stock SRS, was crystal clear with the modified SRS

With the stock SRS, on track 1 of Miles Davis’ “Kind of Blue”, John Coltrane’s tenor saxophone solo was placed at the same location as the left speaker. In the modified SRS, Coltrane is standing a foot to the left and behind the left speaker. Bill Evan’s piano, which was projected two feet to the left of the stock SRS left speaker and confined to the front plane of the speakers, is still projected two feet to the left of the modified SRS left speaker, but is now projected four feet in front of the front plane of the speakers! Piano overtones and the subtle growl of the bass notes were more evident in the modified speaker. When Miles comes in with his first trumpet solo he is dead center and two feet in front of the speaker plane. Jimmy Cobb’s high-hat notes are coming from an airy cloud two feet from the outer edge of the right speaker.

With regard to home theater applications, the SL2000T tweeter is a closer timbre match to the LSiC's tweeters.

Phase 5 listening notes (Modified SRS two weeks after modification)

All the sonic attributes heard in the immediate post-modification listening session are still evident. The only differences now are that the images are more “solid” and the bass definition and articulation are better.

Phase 6 listening notes (Modified SRS compared to the modified SRS 1.2TL)

In the two channel audio system, the SRSs and the SRS 1.2TLs are 6 feet-8 inches apart inner edge-to-edge, 14 inches from the rear wall, 34 inches from the left wall and 15 feet-2 inches from the right wall. The listening position is 12 feet-6 from the plane of the speakers. The room has a hardwood floor and a 10 foot ceiling.

The modified SRS 1.2TL is still the champion, although the sound of the SRS comes commendably close to that of the SRS 1.2TL. I would subjectively quantify the modified SRS’s sonic attributes as 85% of those of the modified SRS 1.2TL. To be fair, it must be considered that the modified SRS 1.2TL has the advantages of (1) a simplified crossover (less components for the signal to pass through), (2) higher quality polypropylene capacitors (AudioCap PPMF series), (3) an improved cabinet structure, (4) improved SDA drivers, and (5) improved tweeters.

The midrange reproduction between the two speakers was close, with the SRS 1.2TL providing a little more detail, presence, and image solidity. Whereas the soundstage of the SRS is fully three-dimensional, the soundstage of the 1.2TL was more holographic. The SRS 1.2TL gives more of a sense of space between the individual instruments in the soundstage. The SRS 1.2TL was far better in reproducing upper treble and lower bass frequencies.

With the SRS 1.2TL, tenor and alto saxophone notes have a more “reedy” and “airy” quality. Rim shots on the drum kit have more impact and transient attack. The high-hat has more metallic shimmer and overtones. The bass notes are stronger, more articulate and more defined. In summary, the SRS 1.2TL presents a more “real” aural illusion.

Conclusion

This was the first SDA modification I have done that resulted in a "night and day" difference in the sound of the speakers. I feel that the sonic benefits I received from the modification far outweighed the costs in time and money.

[PolkWannabie]

Raife,

Aren't there now also SL3000T replacement tweeters available from Polk ?

Have you heard these ?

Wonder what noticeable differences there'd be with replacement of the original tweeters in the 1.2 TL's. Any thoughts ?