Several years ago, after I had posted a thread on an SDA crossover modification, forum member Madmax mentioned the option of having custom printed circuit boards made for my SDA crossover modifications. This stayed in the back of my mind and was recently brought to the front by the recent custom printed circuit board work of forum member Gim54pod.
I have also appreciated the sharing of results by those who have replaced the inductors in their SDA's (Janne, Ben62670 and others). In addition to a custom crossover circuit board, I have replaced the large 16 mH SDA inductor in my SDA SRS 1.2TL's.
The custom board was designed (by me ) with ExpressPCB software (free from www.expresspcb.com). The high frequency components are on the board's left and the low frequency components on the right (figures 1, 2 and 5).
Figure 1. Stock 1.2TL crossover circuit board and 16 mH SDA inductor on left.
Figure 2. Custom 1.2TL crossover circuit board with AMP Mate-N-Lock wiring connectors installed.
The polarity markings for the tweeter wiring connector are wrong (upper left in figure 2). I did not remember that the 1.2TL uses the same board as the crossover board as the 1.2. I did not realize the error until after a discussion about the 1.2TL circuit board with Gim54pod. The polarity markings are correct for the 1.2 but not the 1.2TL.
I also could have added another inch to the right side of the board in order to provide more space for the standoff nut between L1, L2 and C2 (figures 1 and 5). The nut is accessible with needle nose pliers. However, after assembly, I found that I would have preferred more space around the nut. Aside from this and the tweeter polarity marking error, I am very pleased with the way the board turned out.
Figure 3. Bottom of custom circuit board showing circuit traces.
Figure 4. The stock modified crossover is at top, next is the custom crossover board, at the bottom is the support board for the
The stock board was already crammed to overflowing with "normal" sized parts. The stock modified crossover with larger, higher performance capacitors was a jumbled mess. The resistors even had to be mounted on the underside of the stock board. The new board provides an orderly platform and easily facilities the replacement of parts if required or desired.
The new board measures 12" wide by 4.2 inches high. The cabinet opening for the crossover is 14-3/16" wide by 4-3/4" high. The support board prevents flexing in the long main circuit board (see figure 7).
Figure 5. Custom circuit board populated.
The two wires leading from the bottom right in figure 5 connect to the large 16 mH inductor. That inductor has an diameter of 5 inches, which is too large to fit inside the cabinet opening if it were attached to the crossover cover plate.
Figure 6. The red jumper is an 18 AWG 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 RXE153 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.
In the future, when I am more dedicated to audio than I am now, I am going to investigate replacing the jumper with an appropriate resistor.
Figure 7. Populated custom board assembly.
Figure 8. Installed 16 mH inductor coil.
A cabinet brace was conveniently located just under the cabinet opening. The coil is sandwiched between two 1/8" neoprene rubber pads. A plastic spool was inserted in the coil core and the coil was secured to the brace with a 3/8" diameter-5" long threaded nylon stud and nylon nuts.
Using a steel bolt through the center raised the inductance value to 18.3 mH. The metal retaining washers on top raised the inductance of the coil from 15.8 mH to 15.93 mH. The steel center bolt of the stock 16 mH coil raised the inductance from 16.25 mH to 17 mH. Aluminum bolts, washers and nuts are another option.
The upgrade coil is attached to the wires with quick disconnects and the other end of the wire is soldered to the board. After installing the 16 mH inductor, it occurred to me that it would be better to solder the wire leads directly to the inductor and then attach the wires to the board with an AMP Mate-N-Lock connector.
I am going to investigate replacing the 1 mH and 2 mH inductors in the low frequency section with North Creek inductors. The new inductors, which measure 1.25" high and 3.25" and 3.75" in diameter, will be mounted on a cabinet brace just above the cabinet opening and will be connected to the board with Mate-N-Lock connectors. Fortunately, the circuit trace arrangement is such that the board can be easily modified to accept the new connectors.
I have some concern that the much lower DCR of these signal path inductors may objectionably affect the sound balance (0.7 ohm vs. 0.21 ohm for the 1 mH inductor and 1.3 ohm vs. 0.31 ohm for the 2 mH inductor). However, if I don't like the sound I'll just go back to the stock inductors.
I did not initially plan to replace the 1 mH and 2 mH inductors with low DCR coils, but after hearing the increased image weight and enhanced SDA effect after the 16 mH was replaced, I decided to look into it.
The three stock inductors in the high frequent section will be left in place. The parallel inductor (0.4 mH) and one of the series inductors (0.4 mH) have DCR's of 0.8 and 0.9 ohm respectively. The DCR of the series inductor (0.7 mH) for the 4th tweeter is 1.9 ohm, which is in series with a 7.5 ohm resistor. I expect that replacing that inductor with a lower DCR inductor might require a redesign of the tweeter network in order to bring it back into sonic balance.
Gim54pod's SDA 1C Custom PCB Thread
Janne's 1.2TL Inductor Modification