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.