[sdiy] emulating bi-polar capacitors
mbryant at futurehorizons.com
Thu Feb 17 11:38:53 CET 2022
> 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 don't remember whether they switched to bi-polar caps or just found polar caps with a higher voltage rating
Oddly enough this is what I've been researching on my latest project and electrolytics on the input, polarised or otherwise, appear to give slightly less distortion than polyesters, but more than polystyrene. You have to use 63V electrolytics anyway because of the phantom voltage so I suspect they didn't go any higher in voltage, but Douglas does show in graphs how going up to 1000uF does kill off the distortion.
Of course with the polys the available values are smaller so you have to raise the input impedance to recover low frequencies so the noise floor rises slightly unless swamped by a really low mic impedance, but the polystyrenes do seem a potentially good choice. Been looking through schematics to see if anybody does actually use them though. Electrolytics seem pretty universal even in the highest end gear .... well unless your surname begins with an N and you use a transformer :-)
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Brian Willoughby
Sent: 16 February 2022 06:31
To: Synth DIY
Subject: Re: [sdiy] emulating bi-polar capacitors
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.
> > --
> >> 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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