[R-390] Orange Drop vs ceramic disc

[2002tii]

Bob wrote:

>Granted that at super low distortion levels  the effects of  ceramic
>types can be measured, but  in a typical communications receiver  audio
>amplifier section where THD  products of  5 percent  or 10 percent 
>are the norm,
>I doubt if distortion products down  60 db or more are going to be
>measurable let alone heard by mere mortals.
>
>And the tests were all done on the smaller SMT caps  and I'd like to
>know  if physical size makes any difference,  especially on the DC voltage
>applied parameter.

There have been many tests, back to at least the '40s (long before 
small surface-mount ceramics were a gleam in anyone's eye), 
documenting this behavior of ceramic caps.  Bob Pease published a 
chart in EDN in the early '80s comparing the dielectric 
absorption-related distortion of various types of capacitors that 
showed measured distortion of 1% or more at low audio frequencies for 
ceramics (though NP0 caps were much better).  (Note that this is a 
different mechanism than the voltage coefficient of capacitance, and 
that both mechanisms cause distortion independently.)

We comprehensively tested all kinds of capacitors in audio coupling 
circuits in the '80s, and I assure you that the distortion from 
ceramic caps is clearly audible.  Yes, we used a high-resolution 
audio system, not a communications radio -- but different types of 
distortion are heard more or less independently (that is, one type -- 
for example, even-order harmonic distortion, which dominates tube 
communications radio distortion -- does not mask another type -- for 
example, high-order intermodulation distortion or digital 
quantization errors, even when it is present at much higher levels), 
so the distortion due to ceramic coupling caps may very well be 
audible even in a circuit with 10% even-order harmonic 
distortion.  We also used signal cancellation techniques to listen to 
the distortion products alone, and the distortion products of ceramic 
caps are extremely ugly-sounding (high-order, non 
harmonically-related -- easy to spot at very low levels).

Since it is no effort whatsoever to avoid ceramic caps in coupling 
applications, there is simply no reason to use ceramics in those 
applications (indeed, it is simply good engineering practice to avoid 
possible ill effects when there is little or no cost to do so).

Best regards,

Don


Copyright 2011.  Not for redistribution