[sdiy] emulating bi-polar capacitors
steve at bendentech.co.uk
Wed Feb 16 12:57:41 CET 2022
That's less an endorsement of playing fast and loose with electrolytics
than a case of marvelling at how they got away with it at all!
C29 in conjunction with R116 (10K) is acting as a lowpass filter,
decoupling the 5V bias voltage being fed to the VCF.
If R116 was much lower in value - or not there at all - how long do we
think C29 would last?
The unpredictable reverse leakage current through C29 probably means
that the bias voltage varies quite a bit from unit to unit and perhaps
with other things like temperature too. By all means claim
retrospectively that this contibutes to the magic of the TB303 but I
would be amazed if it were deliberate and it isn't good engineering.
Ah, you might say - good engineering doesn't matter if it sounds right.
But what we don't know is how many units got binned because the leakage
was so bad that the VCF didn't work at all...
Benden Sound Technology
On 16/02/2022 10:42, rburnett at richieburnett.co.uk wrote:
> There is at least one polar electrolytic capacitor in the Roland
> TB-303 that is fitted backwards and has significant average voltage
> across it (the wrong way!)...
> The polarity of the capacitor is correct on the schematic in the
> service manual, but the PCB ident is the wrong way round. The actual
> component is fitted as per the PCB ident, not the schematic!
> On 2022-02-16 10:00, Steve Lenham wrote:
>> 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.
>> Steve L.
>> 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.
>> Synth-diy mailing list
>> Synth-diy at synth-diy.org
>> Selling or trading? Use marketplace at synth-diy.org
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> Selling or trading? Use marketplace at synth-diy.org
More information about the Synth-diy