Mnyb;693176 Wrote: > 22 times ! as measured separately or -in this actual application- ?
I think he's referring to a Cyril Bateman article in which Bateman measured the THD of an electrolytic cap at about 22x that of a low-distortion film cap. I have the Bateman articles, which he used to provide for free at his web site, but that web site is gone now - it was on geocities. I've provided an alternate link 'here' (www.diy-audio-engineering.org/web_dl/papers/cap_dist.zip). The thing is, this distortion was measured with 1VRMS across the cap. But this is a coupling application, and if the cap is chosen to be large enough, the AC voltage across it is negligible. That's why the Douglas Self excerpt I posted shows the distortion approaching zero as the cap value is increased. Bateman did find that, for polar electrolytic caps, a reduction of distortion could be had by DC biasing. But that technique is wasteful. The absolute level of the second harmonic distortion component is proportional to the square of the amplitude of the AC voltage across the cap, while the absolute value of the third harmonic is proportional to the amplitude cubed of the AC voltage component across the cap. So decreasing that amplitude gives great benefits. I've attached a plot of that relationship from Bateman's articles. That's the relationship that Self is taking advantage of by limiting the AC voltage component across the cap to tens of mV. Bateman also found that non-polar electrolytics had lower distortion than polar ones, so they are a good choice. +-------------------------------------------------------------------+ |Filename: polar_cap.jpg | |Download: http://forums.slimdevices.com/attachment.php?attachmentid=13128| +-------------------------------------------------------------------+ -- andy_c ------------------------------------------------------------------------ andy_c's Profile: http://forums.slimdevices.com/member.php?userid=3128 View this thread: http://forums.slimdevices.com/showthread.php?t=93875 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/audiophiles
