As many of you know by now the SB Touch includes inexpensive electrolytic caps on its outputs. These caps are polarized, which means they are supposed to be run with a bias voltage across the cap, this bias actually makes the cap work properly. But in the case of the touch there is NO bias voltage, I've been trying to find out what this lack of bias voltage does to the sound.
I finally tracked down some articles on the web where people have been looking into this. It turns out that without bias a polarized capacitor still behaves as a capacitor for small AC voltages, BUT the distortion through the capacitor goes up exponentially as the AC voltage increases. The upshot is that at the full output of the Touch (2V RMS) the unbiased cap has 22 times as much distortion as a film capacitor does. At lower output voltages this distortion goes down significantly, but even at its best it's still about 5-6 times as much as a film cap. This has some interesting ramifications for connections to a system. The conventional wisdom says that you shoulkd run the Touch at 100% volume all the time and use an analog domain volume control, thus maximizing the bits used and the signal to noise ratio. But that doesn't take into account this exponential distortion. A number of people have reported that they get much better sound running the Touch directly into a power amp and using the digital volume to bring the level down. This is usually attributed to preamps not being very "transparent", but in this case it might be the significantly lower distortion when you run the Touch at a lower output level. Of course all the above is JUST for the analog outs, it has nothing to do with running the digital signal into an external DAC. So what to do about it? You can use an external DAC, that has been talked about a LOT. You can open up the Touch and bypass the output caps. Opening the Touch and performing surgery and getiing it back together is not an easy task, so don't attempt this unless you know what you are doing and are willing to risk destroying it. There are threads on this over in the DIY forum. You can also build an external bias circuit that biases the capacitors without having to open up the Touch. I'll try and get a thread on this started over in the DIY forum in a day or two. I'm kind of surprised that none of the companies that supply "tweaks" haven't marketed one of these yet. It very simple, a 6V source (AC supply or battery) and 2 resistors. You can put it in a box with connectors, but then you need an extra set of interconnects. You can have pictails sticking out of a box, you can take an interconnect and cut it in half and insert this etc. There are several ways you can do it. Its still not as good as getting rid of the capacitors all together, but its much lower distortion than what you get out of the box, and you don't have to do anything to the Touch itself. John S. -- JohnSwenson ------------------------------------------------------------------------ JohnSwenson's Profile: http://forums.slimdevices.com/member.php?userid=5974 View this thread: http://forums.slimdevices.com/showthread.php?t=93875 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/audiophiles
