Hi folks, I take the opportunity of reporting my own modest experience with clocks and SB. Actually, it's a SB2 I played with, so components references and pinout in the following may differ for a SB3.
I elected a D-Clock from NewClassD, the new home for Lars Clausen, formerly at LCaudio. Advertised intrinsic jitter is a mere 1.5 ps. Small enough, I guess. So I ordered a 12.2896 MHz clock, keeping the 12.28 MHz part of the circuit untouched. The output of this clock is floating, using a pulse transformer. At first, I ried to connect the cable directly to pin#42 of the Xilinx, after cutting the trace between this pin and the pin#2 of the HCU04. My concern was not to use the crystal pad, as this one is fed through two successives gates before it gets to the Xilinx FPGA. And these gates are not free of any power supply noise, which is a well known cause of jitter. A convenient ground was found by inserting a short piece of thin component lead (from a signal diode, I think) in a via close to the Xilinx. Just peel of the varnish on the bottom side of the PCB, then gently scub the ground plane in order to make a good solder. After that, you get some kind of a robust pin to hook the cable shield to. Adequate power supply was obtained from the pins of the conspicuous big 3300µF cap in the vicinity. Alas, it didn't work: as reported by another member in this thread, no progression on the timing bar. I drop a line to Lars Clausen, and he was kind enough to answer back promptly. I followed his advice to go first through a gate of the HCU04, assuming that this device would be more compliant to the symetric, low voltage, clock output (actually, it's a +/-1.6V instead of a 3.3V). He added that the transitions of his clock are very sharp, and shouldn't be affected jitterwise by the HCU04. However, I decided to keep the gate count as low as possible: no more than one! So I connected the clock cable to pin#1 of the HCU04, actually using one of the pads of the previously removed 12.2896 MHz crystal. Of course, I repaired the previously cut trace between the output of this gate (pin #2) and Xilinx pin#42. This setup works perfectly now. In order to get clean of unwanted signals, I removed also the caps C6 and C15 (the small companions of the crystal) and, as a final touch, I lift up the pins #3 and #4 of the HCU04, effectively removing of the circuit the Pierce oscillator gate. Doing this, I'm pretty sure there is no other component involved in the path of the clock output. Oh, yes, one end of the resistor R1 (the one paralelling the crystal) is still connected to it... but the other end is now floating around, so it shouldn't hurt. This is for my story: I hope it will be helpful for somebody. Well, now it works, and well, but does it worth the expense? If using the analog out of the SB, maybe yes, assuming you also do your homework on power supply and the like. But if you go for a first class external DAC, and in my humble opinion: no. Last month, I purchased a Lavry DA10, and, as I also own an untouched SB3, I set up some comparison, one SB using the Toslink, and the other the coax: the Lavry allows for a convenient switching between these inputs. I felt unable to tell a difference. I exchanged the connection, too, in order to eliminate possible difference in electrical vs. optical interface. I made some spectrum analysis, using the sound pattern from Julian Dunn, and I must say there is some improvement in the picture for the reclocked SB2 over the stock SB3. All well below the -130 dB theshold. Hardly audible. As a matter of fact, I'm wondering if the guys at Slimdevices didn't do some fine tuning in the SB3 over the SB2, with improved PCB layout, or different components, I don't know, but my feeling is that the SB3 is slightly better than the SB2, regarding jitter (I didn't pay much attention to analog output, however). So this is my (not so humble) advice: don't bother modding the SB3. Just buy a decent DAC. The Lavry DA10 is a dream, BTW. JLM -- jlmatrat ------------------------------------------------------------------------ jlmatrat's Profile: http://forums.slimdevices.com/member.php?userid=10656 View this thread: http://forums.slimdevices.com/showthread.php?t=32761
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