Re: Recommended capacitors for AF circuits

Winfield Hill wrote:

Indeed. While I don't go around promoting myself as an expert,
I do usually stick to talking about things I know about, and
perhaps that gives me a certain credibility. With respect to
capacitors and distortion, I've enjoyed Walter Jung's writings
on the subject, and I especially enjoyed Cyril Bateman's series
on capacitor distortion in the 2002 issues of Electronics World.
But I give them credence not only because of what they write,
but because of my own measurements, which I'll get to below.

As I've said many times, it's important to distinguish between
the contribution a capacitor makes to an circuit when it has
no or little AC voltage across it to one when it has a large
voltage. For example, consider a simple R-C lowpass filter.
At low frequencies where the filter has no effect, you won't
be able to measure distortion no matter what type of capacitor
you try. This is the common case for a coupling capacitor.**

Likewise at high frequencies, where the capacitor is shorting
out the signal, it won't matter what type of capacitor you use.
This is the common case for a bypass capacitor.

Sadly, some of the capacitor-distortion measurements I've seen
were taken under one of these conditions, and therefore they
can't teach us much about the issue.

The critical region for measuring capacitor distortion is where
the AC voltage drop across the capacitor is large. For example,
in an R-C rolloff filter is voltage drop should be similar to
the drop across the resistor, i.e. near the 3dB -45-degree phase-
shift point. One does not often encounter such a condition in
audio circuitry, but three places one does are tone controls,
speaker crossover, and RIAA phono compensation. Folks who have
not taken actual AC measurements on capacitors under the -3dB
condition may be expected to have all kinds of opinions about
it, but surely actual measurements trump their ideas.

Here're some actual R-C filter measurements, taken at 1kHz with
a Krohn-Hite 6200B distortion analyzer, on some random parts.

. R C dielectric dist comments
. --- ----- ---------- ----- ---------------
. 22k 8.2nF polypro 0.002% Panasonic 2% ECQ-P1H822GZ
. 22k 8.2nF film 0.003% Phipps & Bird subst. box
. 22k 8.2nF ceramic 0.071% CK05 100V MIL jellybean
. 160 1uF tantalum 0.046% 50V gum-drop ECS-F1HE105K
. 160 0.1uF tantalum 0.040% 50V radial molded Kemet
. 160 1uF electrolytic 0.017% 1uF 25V radial
. 160 1uF electrolytic 0.053% at 3 kHz, -15dB

As I cautioned Thomas, the common popular ceramic capacitors,
which are fine for bypass, etc, are bad news in audio filtering
circuits. This shows with 0.07% distortion values. Moreover,
the distortion output of the 6200B that shows what's left after
subtracting the fundamental, reveals the distortion products
contain a number of evil-looking strange harmonics, which no
doubt are as evil-sounding as they look.

Although the 0.07% ceramic capacitor distortion is only a few
times higher than a LM1036 makes at low signal levels, it's
present at all signal levels, and I'll wager it sounds worse.

Although the electrolytic I tried showed plenty of distortion,
it had much less than I expected to see.

The film types had distortion values near the 6200B's limit,
and the monitor output showed more analyzer noise than signal.
So they're pretty good in my book. We'd have to turn to Cyril
Bateman's equipment designs to discover their true distortion.

** This means I do not generally subscribe to the notion that
selecting the right kind of coupling capacitor will improve an
audio amplifier. An exception may be the use of electrolytics
for low-frequency signals, where the coupling cutoff is not far
below the signal frequency. Is this why one sees 5Hz rolloffs?

--
Thanks,
- Win


I'll take your lab data over a troll's blathering any day. Keep up
your usual good work, Win! :-)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
.

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