This report discusses the crossover modification procedure for installing a custom printed circuit board and custom 16 mH SDA inductor into my SDA SRS loudspeakers. A similar modification was previously performed on my SDA SRS 1.2TL loudspeakers (Link: Custom Printed Circuit Board for SDA SRS 1.2TL).
Before and After Pictures
Figure 1. Some SRS crossover wires were soldered directly to the board.
Figure 2. A big pile of SRS crossover goodness...but a mess.
Figure 3. New SDA SRS crossover assembly: main board, support board and crossover cover plate.
Figure 4. Left to right: 130uf capacitor board, main board, 16 mH Northcreek inductor, stock 2.75 mH inductors.
Figure 5. The soup can sized 130 uF Solen capacitors were mounted on a separate circuit board tucked in the corner.
Figure 6. Large inductor mountings. There is a cabinet brace just below the crossover opening that is 4" wide at the front and 1-3/4" wide
at the sides. It provided a good place to mount the 2.75 mH and 16 mH inductors. The inductors were mounted with aluminum nuts and
Figure 7. Back during the Jurassic, when my SDA SRS's were manufactured, Polk had not started to use the nice wiring harnesses that
were used with later SDA speakers. The original connections were of the soldered and metal tab/quick disconnect type. I replaced all of
those with plug-in wiring harnesses.
Burn In Procedure for 16 mH SDA Inductor
The upgrade SDA inductors for the SDA SRS 1.2TL's had 100 hours of music played through them prior to burn in on the Audiodharma Cable Cooker. With the 1.2TL inductors, overcooking (blurred transients, small loss of overall detail and clarity, diminished sound stage height, diminished bass slam, loss of micro growl details in bass) was audibly evident after 58 hours of cooking.
The SDA SRS's upgrade inductors had 10 hours of music played through them prior to burn in with the Cable Cooker. With the upgrade inductors for the SDA SRS, I stopped hearing and measuring improvements at the 150 hour mark, but did not hear evidence of overcooking. I processed the inductors for another 93 hours in increments of 24, 24, 24, 12 and 9 hours. Even after an additional 93 hours of Cooker processing, for a total of 243 hours, I did not hear evidence of overcooking. I wondered about this and sent an email inquiry to Northcreek Audio asking if the wire and insulation of these inductors was different from the first set. I have not yet received a response (although I did receive the inductors for my CRS+ upgrade today ).
Another curious thing was that this pair of inductor's post burn in oscilloscope curves never reached the degree of smoothness that the first set did.
Figure 8. Cable Cooker (left) and inductor outputs at 0 hours.
Figure 9. Cable Cooker (left) and inductor outputs at 150 hours.
Figure 10. Inductor output at 150 hours (left) and 243 hours.
Figure 11. Cable Cooker (left) and inductor output pulse rising edge at 150 hours.
Figure 12. Cable Cooker (left) and inductor output pulse falling edge at 150 hours.
Figure 13. Inductor output pulse rising edge at 0 (left) and 150 hours.
Figure 14. Inductor output pulse falling edge at 0 (left) and 150 hours.
Listening Evaluation and Discussion of Results
Figure 15. The inductor upgrade facilitated a sonically heavier and more detailed front stage in my home theater system.
My SDA SRS's are used primarily as the front stage speakers in my 5.1 home theater system and secondarily as two channel speakers for background music when entertaining.
I was curious about whether the new board made an audible difference. I thought about doing listening evaluations between the stock and custom circuit boards. However, Gimpod mentioned that he heard improvements with his new boards over those provided by the upgrade Sonicraft Sonicap capacitors and Mills MRA-12 resistors. I have no reason to doubt this as it is reasonable that a better quality board with heavier circuit traces would provide better noise performance.
The upgrade SDA inductors provided Such Good Sound improvement over the stock inductors that I was reluctant to take them out for further intensive burn in processing. Prior to burn in, the upgrade inductors provided more weight at the sides of the front stage for both music and movies. After burn in, I heard even more image weight and more sonic detail.
I was concerned about some loss of integration with the dual LSi 9 speakers used as center channel speakers. After the inductor upgrade, there was tighter integration between the LSi 9 centers and SDA SRS fronts and between the SDA SRS fronts and LSi 15 surround speakers. This most likely would not have been the case had I not performed crossover modifications on the LSi 9's (discussed here) and LSi 15's (discussed here).
I thought the inductor upgrade might make the SRS's too bass heavy, since bass heaviness due to cabinet resonance is a known issue with this speaker. Some people actually prefer the tipped up bass presentation of the SRS over the more accurate bass of the SRS 1.2 and SRS 1.2TL. The inductor upgrade resulted in bass refinement to the point that it sounded like I had less bass. I didn't actually have less bass, I just had a much cleaner bass signal which translated to less cabinet resonance (and apparently less bass). On the other hand, as with the 1.2TL's, I realized more bass tactile sensation after the inductor upgrade.
The direct current resistance (DCR) of the modified SRS's decreased from 4.7 ohms each to 4.4 ohms each with the new 16 mH SDA inductor.
The SRS crossover upgrade, driver and passive radiator rings, Mortite and Dynamat Xtreme all contributed to a cleaner, more detailed bass signal, with a commensurate reduction in cabinet resonance.
After the SRS inductor upgrade, a frequent visitor to my home remarked:
"I don't like this. Now they sound just like the speakers (SDA SRS1.2TL's) in your living room."
Of course, the inductor upgrade did not make the SRS's sound "just like" the 1.2TL's, but they sounded closer to them in the bass region.
There are much lower cost alternatives to the circuit boards and inductors that I used. You don't even need to use a printed wiring board. A thin wood board with point-to-point wiring works very well. Even if you do not do a full crossover modification, I highly recommend replacing the 16 mH SDA inductor if your amplifier is comfortable with the lower impedance. All circuit boards were manufactured by ExpressPCB.
1. Pair of main PCB's,............................................ ...$268.18
2. Pair of support PCB's,...........................................$ 132.37
3. Pair of 16 mH, 14 AWG North Creek inductors..........$241.95
4. Pair of PCB's for 130 uF capacitors..........................$112.35
4. Mounting hardware for PCB's and inductors..............$ 75.29
Total............................................. ........................$830.14 (A modest amount in the culture of modifications for big SDA's. )
Figure 16. All the parts are in for my second SDA CRS+ crossover modification.
Link: Improvements to the SDA SRS.
Link: Custom Printed Circuit Board for SDA SRS 1.2TL
Link: LSi 9 Crossover Modification Project
Link: LSi 15 Crossover Modification Project
Link: Audiodharma Cable Cooker Review