Improvements To Modified SDA SRS 1.2TL Crossover
A preliminary report was posted here.
After years of being on my very long audio "to do" list, I decided it was time to give my SDA SRS 1.2TL's a proper crossover modification with an appropriately sized printed circuit board (PCB). This was first suggested to me a few years ago by forum member Madmax. In 2008, I discussed with Ken Swauger, Polk's customer service manager at that time, my interest in having a custom crossover board fabricated. However, Ken could not locate any information on the stock board's specifications or vendor, and I let the idea slip back into obscurity. More recently, forum members Gimpod and Janne discussed their good results with custom crossover circuit boards and improved custom SDA inductors.
The stock capacitors and resistors of my 1.2TL's were replaced immediately after purchase in December of 2001 with AudioCap polypropylene film capacitors and Mills MRA-12 resistors. The AudioCap capacitors were replaced with Sonicap polypropylene film capacitors in June of 2008.
This modification replaced the stock printed circuit boards and 16 mH SDA inductors.
Figure 1. My big Sonicaps now have room to stretch out and breathe.
The new PCB (printed circuit board) measures 12" long by 4.2" wide (50.4 sq. in.) with the standard thickness of 0.062". The new board is over twice the area of the original board, which measures 5-3/4" long by 4-5/16" wide (24.8 sq. in.). The populated new board main board weighs 1.8 pounds. For additional support against flexing and vibration, the main board is attached to a second board with eleven nylon standoffs. The support board rests on small sheets of Dynamat Xtreme vibration damping material. The boards were manufactured by ExpressPCB (www.expresspcb.com) using their express 4-day service. The boards were ordered on a Monday and were received the following Friday. I did the board design using Express PCB's ExpressSCH and ExpressPCB free schematic and board layout software.
The new boards are fabricated from FR-4 fiberglass epoxy with 1-1/4 ounce copper plated traces, plated through holes, silk screened top sides and solder masked under sides.
Figure 2. The original board (left) was already a very crowded configuration. Replacing the stock capacitors and resistors with larger
upgrade parts was a topological challenge. The diameter of the new custom 16 mH SDA North Creek inductor was too large (5") to fit
inside the cabinet opening (4-3/4" high) and could not be mounted flat on the crossover cover plate like the stock inductor. I asked North
Creek if they could wind the inductors to be taller with a maximum diameter of 4.5". They said their winding machines could only wind to
a maximum height of 2".
Figure 3. Stock SDA SRS 1.2TL crossover (photo by forum member Vmaxer). I was surprised to find this tiny little board inside my
1.2TL's, since the original SRS uses a large board similar in size to the new custom boards.
Figure 4. Modified 1.2TL crossover with piled-on Sonicap capacitors. The Mills resistors had to be mounted on the underside of the board.
Figure 5. The original crossover had the large 16 mH SDA attached to the crossover cover plate, then the board attached to the inductor
spool with four nylon standoffs. The new inductor is mounted on a cabinet brace. The silver foil is Dynamat Xtreme vibration damping
Figure 6. The wires for the new SDA inductor were first soldered to the board, just like the wires on the stock PCB. For greater removal
and installation convenience, a 2-pin header and wiring harness were installed for the inductor.
I did not think of adding a pin header for the SDA inductor until after both boards had been populated. Coincidentally, the copper traces for the SDA inductor were conveniently located so that all I needed to do to accommodate the pin header was to enlarge one of the existing holes and drill another hole 0.25" away in an adjacent trace.
Figure 7. Silk screening on top side. This picture was taken prior to the SDA inductor wiring header being added at location L3 near the
Figure 8. Solder masked copper traces on board rear.
Figure 9. The red jumper is an 18 gauge solid copper wire that replaces the polyswitch.
I provided a space on the new board to replace the polyswitch with a resistor if I chose to do so. The RXE135 polyswitch has an untripped nominal DCR range of 0.12 to 0.19 ohm and a tripped DCR of 0.3 ohm. I measured the DCR's of four new RXE135 ployswitches (link). Two measured 0.16 ohm and two measured 0.17 ohm. Mills makes MRA-12 resistors in 0.1, 0.15 and 0.18 ohm. However, since removing the Polyswitches is a Polk recommended modification and since the RXE135 polyswitches have a next-to-nothing DC resistance of under 0.2 ohms, I am not in a big hurry to experiment in this area.
Mounting The SDA Inductor
The stock 16 mH SDA inductors are comprised of 18 gauge wire and have a DC resistance of 2.8 ohms. They were replaced with custom 16 mH inductors from North Creek Music Systems (14 gauge wire and 1.3 ohms DCR). Replacing the SDA inductors lowered the overall DCR of the loudspeakers from 3.8 and 3.9 ohms respectively to 3.4 ohms. My amplifiers did not seem to mind the slightly lower DCR and did not run any hotter than usual.
I experimented with three different locations for the SDA inductor: 1. A 2" wide cabinet brace running from front to back located just inside the cabinet's crossover opening. 2. The rear cabinet wall just behind, and near the top of, the passive radiator. 3. The rear cabinet wall to the side of the binding post plate.
I was surprised to find that this inductor was so sensitive to placement. Mounting the inductor behind the passive radiator (figure 12) resulted in muddy bass and diminished midrange detail. Mounting the inductor next to the binding post plate (figure 13) did not sound terrible like passive radiator location, but there was a significant loss of bass detail, articulation, and speed and some loss of midrange detail. Mounting the inductor on the narrow brace resulted in sound that was outstanding.
I sent these results to North Creek and this was the response I received:
"RE the 16mH coil, chances are it interacted with the bolt. Anything metallic anywhere near by will change its sound. It is not vibration dependent but we generally recommend mounting it to the bottom back corner so it can have adhesive on the bottom and two sides."
I used the same non-magnetic aluminum bolt, washers and nuts in each location. Perhaps the steel basket of the passive radiator had some effect. I don't know why the location near the binding post plate sounded bad. The vertical orientation might have also contributed to the sonic deterioration.
Figure 10. Toroidal inductors do not appreciate having magnetic metals inserted in their cores as this causes a rise in the amount of
inductance. The SDA inductors were attached to the cabinet brace with aluminum threaded studs, washers and hex nuts. Nylon nuts
and bolts were also tried, but were rejected in favor of sturdier metal parts.
Two inch wide aluminum fender washers for the tops of the inductors were not available locally. Steel washers were used as they only caused a little over a tenth of a mH increase in inductance, which was well within the 10% design tolerance. In contrast, a steel bolt through the center of the inductors caused a 2.5 mH increase. The 1/4" 1-1/2" steel bolt though the stock SDA inductor raised the inductance from 16.25 mH to 17 mH.
Figure 11. The inductor's mounting spool is a 3/4" plastic coupler. I found it in the plumbing department at Home Depot. It was exactly the
right length and diameter (1-9/16" long and 1-5/16" in diameter) to fit inside the inductor cores. The outside surface of the couplers had to
be ground flat in eight places to correspond to the inductor's eight binding straps.
Figure 12. SDA inductor installed.
Figure 13. Nice location. Horrible sound.
Figure 14. Nicer location, nicer sound, but nowhere near as good as the sound from the brace location.
Part 2: 1 mH And 2 mH Inductor Replacements and 16 mH Inductor Burn In
Replacement Of Other Low Frequency Inductors
The use of a high DCR SDA inductor was a design compromise to ensure compatibility with the amplifiers commonly available 20 years ago. Replacement of this inductor with one with a low DCR is a Polk recommended modification, provided your amplifier(s) can deal with the lowered impedance.
The replacement of the other inductors in the low frequency crossover section with lower DCR inductors IS NOT a Polk recommended modification as it will change the voicing of the speaker. Whether or not the change in voicing is sonically pleasing will depend on a variety of factors such as the the amount of change in DCR and listener preferences. Refer to the comments from Matthew Polk and Stu Lumsden on inductor replacement in the document attached below.
I decided to try lower DCR North Creek replacements for the 1 mH and 2 mH inductors in the low frequency section of the crossover. I knew better than to mess with the high frequency inductors unless I kept the DCR's the same or unless I committed to a circuit redesign.
Figure 15. The stock 1 mH and 2 mH take a break after 20+ years. After the first session with the replacement inductors, I couldn't wait to
get them back in.
Figure 16. North Creek Music Systems 1 mH and 2 mH inductors and mounting hardware.
Figure 17. North Creek 2 mH (front) and 16 mH inductors.
Figure 18. North Creek 1 mH inductor mounted.
The 1 mH and 2 mH replacement inductors provided sound that was harsh, disjointed (lack of coherence between low and mid/high frequencies) and the bass was muddy. I relayed these results to North Creek and I was told the inductors needed to burn in over a few days. The overall harshness went away after three days, but the other sonic aberrations remained. The 1 mH and 2 mH inductors were returned.
SDA Inductor Burn In
I was very reluctant to begin a burn in session with the Cooker because I was already enjoying Such Good Sound from the upgrade SDA inductors (and new PCB) that I didn't want to interupt my fun....but I did it anyway. It does not make sense to have wonderful conditioning devices at your disposal and not use them.
The Audiodharma Cable Cooker can be used to burn in other items besides cables. Electronic components such as resistors, inductors, transformers, capacitors and bulk wire can also be burned in on the Cooker. The Cooker's manufacturer recommended a burn in time of 4-5 days for these inductors if they were brand new, less if they had some hours on them. These inductors had 100 hours of use prior to putting them on the Cooker.
The inductors were first measured individually and then connected in series to one set of the Cooker's speaker cable outputs. The Cooker's conditioning signal was ran through the inductors in the same direction that power flows through them when they are connected to the crossover.
Oscilloscope displays were recorded at the beginning, 8th hour, 17th hour, 20th hour, 29th hour, 35th hour, 54th hour and 58th hour. Listening evaluations were done at the beginning, 17th hour and 58th hour.
As I noted in my preliminary report, I realized modest gains in bass clarity and definition, but huge gains in bass tactility. I was feeling a lot more bass rather than hearing a lot more bass. Curiously, Gimpod reported similar results with just a PCB upgrade and the stock inductor. Gimpod's results were not surprising when you consider that a circuit board is as much a circuit component as the resistors, capacitors, inductors, etc. For stereophonic audio applications, a board with larger surface area and with wider and thicker copper traces might exhibit better noise performance due to lower resistance and better power handling capability.
At the 17th hour listening session, which lasted two hours, I noted more presence and detail in high frequency percussion sounds, particularly hand claps and finger snaps and an apparently louder sound level, which was evidence of a lowered noise floor. There was a little more detail in the micro growls in the bass. The oscilloscope plots showed less ripple and ringing noise in the rising and falling edges of the conditioning signal pulses.
Since I planned to condition the inductors up to four days on the Cooker, I did not schedule another listening session until the halfway point, or 48 hours. However, some social obligations prevented a listening evaluation at the 48 hour mark. I did not have an opportunity to listen until the 58th hour. I was surprised, but pleased to find evidence of slight overcooking (blurred transients, small loss of overall detail and clarity, diminished sound stage height, diminished bass slam, loss of micro growl details in bass) at this point. I was surprised because I expected to have to cook these inductors three or more days. I was pleased because evidence of overcooking meant that I didn't have to do any more removal and reinstallation.:smile:
I did note some sonic improvements even with the overcooked inductors. The apparent sound level had increased to the point where I needed to turn the volume control down a couple of clicks. There was also an enhancement of depth at the sides of the sound stage wherein some sounds were brought almost to the sides of my seating position.
Recovery from overcooking took an unusually long time. After 5 hours of play time and 18 hours of rest, the bass was still not "correct" with regard to clarity and detail. After an additional five hours of play time, the bass clarity and detal had improved beyond the pre-cooking levels. There was also an increased sense of air and ambient information on some recordings. After five more hours of play time, I did not hear further improvement. I assumed everything was as good as it was going to get. However, the next day, after 18 hours of rest, there was more bass detail, texture, micro growls and articulation as I was hearing bass details in familiar recordings that I had not heard or documented before. The total cooking time was 58 hours followed by a recovery time of 51 hours.
Figures 20-27 show a reduction in signal deformation due to noise as the inductors were processed by the Cable Cooker. The close-up plots of the rising and falling edges of pulses are particularly interesting as they show a decrease in noise rippling with continuous processing by the Cooker.
Figure 19. Left to right: Audiodharma Cable Cooker, 16 mH North Creek inductors, laptop computer for data recording and comparison,
Tektronix TDS 2010 oscilloscope.
Figure 20. The Cooker's conditioning signal pulse train output from the inductors at the beginning (left) and at the 58th hour.
Figure 21. The Cooker's conditioning signal single pulse output from the inductors at the beginning (left) and at the 58th hour.
Figure 22. The Cooker's conditioning signal rising pulse output from the inductors at the 8th (left) and at the 58th hour.
Figure 23. The Cooker's conditioning signal falling pulse output from the inductors at the 8th (left) and at the 58th hour
Figure 24. Input (left) and output conditioning pulses to and from the inductors at the 58th hour.
Figure 25. Closeup of input (left) and output conditioning pulse to and from the inductors at the 58th hour.
Figure 26. Closeup of input (left) and output conditioning pulse rising edge to and from the inductors at the 58th hour.
Figure 27. Closeup of input (left) and output conditioning pulse falling edge to and from the inductors at the 58th hour.
Part 3: Cost Breakdown, Conclusion, References
Cost Break Down
This third crossover modification cost a bit more than previous mods, but I moved much further along in my quest for greater stereophonic realism and emotional thrills. Within the context of increasing and sustained aural satisfaction, it was money well spent.:smile:
1. Pair of main PCB's............................................. ....$268.18*
2. Pair of support PCB's............................................. $132.38*
3. Pair of 16 mH, 14 AWG North Creek inductors..........$243.47
4. Mounting hardware for PCB's and inductors..............$ 58.28
Total............................................. .................................................. $702.31 (A modest amount in the culture of modifications for big SDA's. :smile:)
*Boards are available from other vendors at much lower cost, but longer delivery times.
This crossover modification was more costly time wise than previous crossover mods due to the time required to learn the PCB design software and design the boards (29 hours total, 7-learning the software, 13-basic design, 9-fine tuning).
This was one of my more satisfying SDA modification experiences as didn't require any contortionist's skills, didn't produce any nasty surprises and it resolved a long-standing issue with board space and a serious crossover circuit design compromise. My next scheduled modification is a remodeling of the wood surfaces on my SDA SRS 1.2TL's. I have not decided if I am going to do similar modifications for the SRS's in my home theater system and my three pairs of CRS+'s.
As always, I will not be offering a board design or crossover modification service to the public. I'm saving such fun stuff until after I retire from real world employment.:wink:
Gimpod's SDA 1C Custom PCB Thread
Janne's 1.2TL Inductor Modification
Audiodharma Cable Cooker Review
Selected Previous SDA SRS 1.2TL Modifications
SDA SRS 1.2TL Sonicap Upgrade
Mortite Speaker Seals
SDA Steel Retaining Rings
Custom SDA Crossover Cover Plates
SDA Tweeter Brackets
Foam Rubber and Dynamat Xtreme SDA Driver Basket Insulation
AI-1 Dreadnought Isolation Transformer
Improved SDA Interconnect Cables