Rectification in aluminium electrolytic capacitors

[Eeyore]

I've just been having a discussion elsewhere about the performance of aluminium
electrolytic caps for audio coupling in a zero bias situation. This is a method
very widely used nowadays.

I've observed, indeed I first heard of it decades back, that provided the signal
*across* the capacitor doesn't exceed about 100mV ac there is no measureable
non-linearity. I've confirmed this using Audio Preciosn test gear which measures
down to 0.0008% THD (-102dB). Hence if the value of the cap is such that 100mV
is never exceeded, there will be no distortion contribution from the cap. I've
also confirmed that if the 100mV is exceeded, you do indeed get distortion.

The mechanism seems to be electrolytic rectification AIUI. I presume that the
absence of any effect below ~ 100mV is due to the 'forward' voltage of this
rectifier, and current only flows when it's exceeded.

Any more thought on this ?

I did find this page about electrolytic rectification btw.
http://home.earthlink.net/~lenyr/borax.htm

The oscillograms are rather interesting.

Graham

 

[Damon Hill]

Eeyore <rabbitsfriendsandrelations@hotmail.com> wrote in
news:46369117.4405B657@hotmail.com:

I've observed, indeed I first heard of it decades back, that provided
the signal *across* the capacitor doesn't exceed about 100mV ac there
is no measureable non-linearity. I've confirmed this using Audio
Preciosn test gear which measures down to 0.0008% THD (-102dB). Hence
if the value of the cap is such that 100mV is never exceeded, there
will be no distortion contribution from the cap. I've also confirmed
that if the 100mV is exceeded, you do indeed get distortion.

Was this tested with non-polarized electrolytic capacitors as well?
Any of the more esoteric capacitors like Black Gate (graphite mixed
with the electrolyte) or polymer-based caps?

It would seem to confirm that electrolytic caps should ideally be
biased.

--Damon

 

[Eeyore]

Damon Hill wrote:

Eeyore wrote

I've observed, indeed I first heard of it decades back, that provided
the signal *across* the capacitor doesn't exceed about 100mV ac there
is no measureable non-linearity. I've confirmed this using Audio
Preciosn test gear which measures down to 0.0008% THD (-102dB). Hence
if the value of the cap is such that 100mV is never exceeded, there
will be no distortion contribution from the cap. I've also confirmed
that if the 100mV is exceeded, you do indeed get distortion.

Was this tested with non-polarized electrolytic capacitors as well?
Any of the more esoteric capacitors like Black Gate (graphite mixed
with the electrolyte) or polymer-based caps?

I did test one example of a non-polarised type. Interestingly, once that 100mV
threshold had been exceeded it produced twice as much distortion (at low signal
levels) as a polarised type. It would seem that's simply because it is
internally 2 such caps back to back. I've therefore concluded that for 'low
level' signal coupling the NP types offer no advantage and in fact degrade the
signal more.

No, I haven't tested any more esoteric types. The last I heard, Sanyo ? were
stopping manufacture of the Black Gates.

It would seem to confirm that electrolytic caps should ideally be biased.

Ideally. I have used them 'back to back' with the junction polarised to rail via
a very high value R. It's cumbersome though.

The zero bias method works well provided you don't exceed ~ 100mV across the
cap. A value of 100uF deals with this in most real world audio circuits.

Graham