Following is a description of the modification procedure for my 1989 vintage SDA 1C’s. The modifications consist of the replacement of the grille cloth, binding posts, crossover resistors and capacitors, SL2000 tweeters, and SDA interconnect cable. Spiked feet were also added. These modifications were not all done at the same time. The crossover/binding post/grille cloth replacements were done first, then the silk dome tweeters, then the SDA custom interconnect, then the spikes.
Modification Costs and Parts
The total parts cost for the modification was $584.10 and was well worth every cent. Parts were sourced from the following vendors:
1. RD0194-1 Tweeters and Carpet Spikes (part #RF-1400-1)
Polk Audio Customer Service
2. Mills MRA-12 Resistors (Two 7.5 ohm and one 2.7 ohm resistor per speaker.)
3. Multicap PPFMX Series Polypropylene Film Capacitors (Two 6 uF (or one 12 uF), and one 20 uF per speaker)
The Parts Connexion
4. Sonicap Gen I Polypropylene Film Capacitors (One 4.3 uF per speaker)
5. RelCap PPMF Series Polypropylene Film Capacitors (One 40 uF per speaker)
6. Ponte Knit Cloth
Can be purchased locally or ordered online
7. Vampire BP-HEX Binding Posts
The Parts Connexion
8. Metal ¼” Insert Nuts For Spikes (House Mates Hardware part #54454)
Figure 1. Modified SDA 1C crossover.
Partsconnexion was out of the 12 uF value capacitor that the schematic called for. I used two 6 uF capacitors in parallel. I also substituted a 4.3 uF (5% tolerance) capacitor for the 4.4 uF specified in the schematic. The schematic specifies a 4.4 uF 5% tolerance Mylar capacitor or a 4.4 uF 10% tolerance electrolytic capacitor.
Figure 2. Placement of the crossover board inside the SDA 1C. The snap-on wiring harnesses used with 4th generation SDA’s really facilitate easy removal and reinstallation of the crossover board. The crossover is accessed by removing the passive radiator.
Unsnap the three wiring harnesses, which are designated P1, P2, and P3. Harnesses P2 and P3 both have four pins, and it is possible to plug the wrong harness into the wrong receptacle. The harness numbers are printed on the circuit board. Immediately after removal of the P2 and P3 harnesses, you should write the appropriate harness number on each four pin plug.
Figure 3. Underside of SDA 1C crossover assembly.
The crossover circuit board is attached to a large inductor coil with four plastic standoffs. The inductor coil is secured to the back of the speaker cabinet with a large metal bolt. Use a 5/32” hex wrench to remove the bolt while holding the crossover assembly with one hand. The bolt screws into a nut on the other side of the coil. The nut is held in place by a blob of hot glue. When I was replacing both crossovers after modification, the glue, which had become brittle with age, peeled off the surface of the coil frame and I had to remove and replace the old hot glue. If this happens to you, be careful to cover the top edges of the nut with hot glue while not getting any on the nut threads. I tacked the nut in place with a couple dabs of super glue so that it would not shift while the hot glue was being applied.
After the crossover assembly is removed, remove the two black wires that connect the large coil to the circuit board. Use needle-nose pliers to pinch the tips of the standoffs so they can be pushed out of the coil frame and the circuit board separated from the coil.
I removed the 750 pF silver dipped mica bypass capacitor because I did not feel it was required with a high quality film capacitor. I also removed the tweeter protection polyswitch and replaced it with a solid wire jumper.
How you orient your parts will depend on their size and shape and your personal preference. The 1C’s crossover circuit board is adequately spacious enough to accommodate larger film and wire wound resistors. I oriented the Mills resistors standing up vertically, just as the stock cermet resistors were placed. I used solid wire jumpers to extend the reach of the top Mills resistor leads.
Cardas Quad-Eutectic silver content solder was used to secure upgrade parts to the circuit board.
Figure 4. RD0194-1 silk dome replacement tweeters.
The RD0194-1 silk dome replacement tweeters are drop-in replacements for the stock SL2000 tweeters. They are a smoother, more neutral sounding tweeter which you may or may not like depending on your listening preferences.
Figure 5. Replacement of stock binding posts and SDA jack with Vampire BP-Hex posts.
The stock binding posts with their flimsy plastic retaining nuts were replaced with Vampire BP-Hex posts. A single post was installed above the SDA cable jack to accommodate the new custom heavy gauge SDA interconnect. There was no sonic improvement with the better binding posts, just a better mechanical connection for speaker wire terminations. Larger ¼” holes were drilled for the Vampire posts.
Figure 6. Custom SDA interconnect.
A custom SDA interconnect was made from a 15 foot length of Monster Cable Z2 Reference speaker cable. In the stock pin/blade cable, only the pin connection carries a signal. The two 12 gauge conductors of the Z2 cable were soldered together at each end and terminated with Monster Lock Pins, which allow different connectors (different size spades and bananas) to be screwed on. The larger gauge, lower resistance cable made noticeable improvements in soundstage depth and overall detail.
Figure 7. Spikes were installed, just in case.
I no longer like, or have, carpeted floors, but I installed spikes anyway.
Figure 8. Modified SDA 1C speakers with upgraded grille cloth.
After many, many extended listening sessions, I found that I required a degree of sonic transparency that was unattainable with the stock grille cloth. The thinner, darker, silkier fabric also provides a more appropriate aesthetic and is a better match for my contemporary décor. The stock grille cloths of my SDA 1C’s were attached to the grille frames with approximately 999 staples. It was fun removing all of those.
Figure 9. Modified SDA 1C speakers with grilles removed.
A substantial improvement in sound quality was evident immediately after the modifications were completed. Bass definition and impact and clarity throughout the entire frequency range continued to improve over a period of two weeks. The image stability of instruments within the SDA soundfield was also substantially improved with the use of higher quality crossover components and the larger SDA interconnect. Remember that, as with all things in audio, your sonic results may vary depending on your associated equipment, acoustic environment, and ears.
For a number of years I owned a pair of modified SDA 1B’s concurrently with a pair of modified SDA 1C’s. The question one might ask is “which was better?”. As with all things in audio, “better” depends on the source material you are listening to and your listening preferences. The modified 1B’s certainly beat down the stock 1C’s. When comparing the modified 1B’s to the modified 1C’s using source material without “heavy” bass content, I struggled to hear a difference between the two. On material with heavy bass content, the 1B apparently produces “more” bass, but the increased bass output is actually due to the higher amount of cabinet resonance. The 1C has an improved cabinet structure that minimizes cabinet resonances. It produces slightly “cleaner” bass, which may or may not appeal to some listeners, and a bit more midrange detail.
While my ears did not hear much of a difference between the modified SDA 1B and SDA 1C on a lot of source material, an amplifier will certainly “see” a big difference. The SDA 1B was a 4 ohm speaker with a moderately complex crossover. It was not a particularly easy load for an amplifier. The SDA 1C was a 6 ohm speaker with a relatively simple crossover and was easy load suitable for use with a wider range of amplifiers.