[sdiy] emulating bi-polar capacitors
steve at bendentech.co.uk
Wed Feb 16 11:00:41 CET 2022
just because we can "get away" with using standard polar electrolytics
(and we often can) doesn't mean that it is good engineering or that
there will be no negative effects under any circumstances.
Polar electrolytics will withstand something like half a volt of reverse
polarity with no ill effect. Above that, they start to leak current, and
if that current is not controlled then they will suffer damage up to and
including exploding. We've all seen what happens to PSU caps
accidentally fitted backwards.
In the case of audio coupling caps, the cap forms a single-pole highpass
filter with the input resistance of the stage it's feeding, i.e. a
reactive/resistive potential divider. At frequencies well above the
corner frequency of the filter, virtually all of the voltage appears
across the resistance. Great, no voltage across the cap (in either
direction!). But what happens as the frequency drops towards the corner
frequency? More and more of the voltage appears across the capacitor
until eventually the negative signal peaks cause the cap to leak
current. At that point you will start to get signal distortion. The cap
will rarely be damaged because there is usually enough resistance in the
circuit to limit the current to a safe level, but fidelity might be.
That is why, as previously mentioned, you see bipolar electrolytics in
speaker crossovers. The corner frequencies of the filters in a
crossover, by definition, are located in the middle of the audio band,
so there will be substantial signal voltage across the capacitors at all
times. In addition, a speaker is usually a low-impedance system, so
there is nothing to limit any reverse leakage currents to a safe level.
Polar electrolytics would be a disaster there.
So in a nutshell, polar electrolytics for coupling are fine as long as
you can be sure that there will never be any substantial signal content
below the corner frequency of the filter that the cap forms with the
input impedance of the stage it's feeding.
Of course, this raises the question of what the cap is doing there at
all. If the above condition is met, the cap isn't doing any highpass
filtering. And it can't block unpredictable DC offsets (i.e. ones that
could be of either polarity) greater than half a volt. The answer is
that it is generally blocking small opamp offset voltages (of the order
of tens of millivolts) and preventing their being amplified by
subsequent gain stages.
Benden Sound Technology
On 16/02/2022 06:30, Brian Willoughby 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.
> 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. :-)
>>> 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.
>>> 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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