[sdiy] emulating bi-polar capacitors

[David G Dixon]

For my application, I needed to go down to 0.1Hz.  For this, I found 22uF to
be adequate, so two 47uF caps.

  _____  

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Tom
Wiltshire
Sent: Wednesday, February 16, 2022 1:31 AM
To: Brian Willoughby
Cc: Synth DIY
Subject: Re: [sdiy] emulating bi-polar capacitors


[CAUTION: Non-UBC Email]	
Why do we need such large capacitance values anyway for DC blocking caps? Is
everyone feeding low impedance loads and trying to reproduce sub-bass
signals? 

Putting 100K/100n into 1/2PiRC gives 15Hz, and you can easily go two octaves
lower (220n, 470n) with standard film caps, or reduce impedance accordingly.



On 16 Feb 2022, at 06:30, Brian Willoughby <brianw at audiobanshee.com> wrote:

While writing firmware for a digital mixer, the folks over in the
digital-controlled-analog-preamp team discovered distortion that was traced
to the polarized capacitors. I did not look at the schematic, so I don't
know what the exact problem was. They changed the capacitors for the next
prototype and got rid of the distortion. I don't remember whether they
switched to bi-polar caps or just found polar caps with a higher voltage
rating. I seem to recall it was the latter, but it stuck in my mind that the
electrolytics could cause distortion due to their polarized nature - at
least for large input signals and/or high gain settings.

Sorry for the non-answer. Seems like there's definitely the potential for
problems that can't be ignored, but the solution is not necessarily
bi-polar.

Brian


On Feb 15, 2022, at 12:30, Mattias Rickardsson wrote:


Hej Danjel and others,

I don't recall seeing any hard facts about non-polarized electrolytics
performing better than ordinary polarized in audio circuits, but still they
occasionally turn up in designs. Would be interesting to hear why they
sometimes are preferred by audio designers, though! :-)

I searched for "non-polarized" in Douglas Self's reference book "Small
Signal Audio Design" and found a couple of applications where they actually
do make sense - but it's a practical reason rather than an audio performance
reason:
DC blocking in inputs & outputs, where it's possible that the connected gear
pulls the voltage way off ground level, and you never know in what
direction.
Douglas Self writes:

"C2 is a DC-blocking capacitor to prevent voltages from ill-conceived source
equipment getting into the circuitry. It is a non-polarized type as voltages
from the outside world are of unpredictable polarity, and it is rated at not
less than 35 V so that even if it gets connected to defective direct-coupled
equipment with an op-amp output jammed hard against one of the supply rails,
no harm will result."

Any other good reasons for using them?
And sorry, no - I don't have any better advice than what has already been
said. :-)

/mr



On Feb 15, 2022, at 11:32 AM, Mike Bryant wrote:


I've never bothered, I've never noticed a difference in just using a 47μ
instead of playing about with back-to-back capacitors even after the thick
end of 40 years the earliest stuff I built that way still has capacitors
that capacitate just fine.


--


Gordonjcp



Agreed.  Unless you actually have a reverse DC bias voltage (in which case
rotate the capacitor) I've never understood any need for the non-polarised
capacitors.  Most mixing consoles are full of thousands of them either
feeding the input or fed from the output of an opamp via a resistor to
ground.




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