Aleph 5 DC feedback capacitor (C5) reverse polarized

jwb · 1st October 2003, 03:32 AM

The capacitor C5 is used in the Aleph 5 to provide 100% feedback at DC. The capacitor can have forward or reverse voltage applied, however in the drawings this is a polarized capacitor. I'm not too worried about this, because most electrolytic caps can withstand 1-1.5V applied in reverse polarity. But when attaching or detaching cables, a high-voltage spike may be encountered (hence the zener protection for the MOSFET gate), and I'm wondering if this has any potential to damage the capacitor.

Is it possible for an AC signal to damage an electrolytic capacitor, or is that a problem only with extended DC reverse charge? Should I use a solid electrolyte capacitor with higher reverse tolerance?

The failure mode is not spectacular: it will simply lead to less feedback at DC. However this might change the DC output offset of the amp and damage the speaker.

Nelson Pass · 1st October 2003, 05:42 PM

I have not found the polarity to make a difference. Any spikes
that would go through a zener have been of such short duration
that they don't get noticed by the capacitor.

jwb · 1st October 2003, 06:14 PM

Thanks Mr. Pass. As an engineer-by-training, I'm naturally curious about modes and rates of failure. It's always good to hear the voice of experience on such matters.

tiroth · 1st October 2003, 06:20 PM

I believe electrolytic capacitor failure due to reverse-biasing is related to power dissipated; within reason, reverse voltage without significant current won't damage them. An AC signal with sufficient current can most certainly blow these caps, but one that supplies only a few mA, probably will cause no problem or a slow degrading of value.

1st October 2003, 03:32 AM	#1
jwb diyAudio Member Join Date: Mar 2002 Location: San Francisco, USA	Aleph 5 DC feedback capacitor (C5) reverse polarized The capacitor C5 is used in the Aleph 5 to provide 100% feedback at DC. The capacitor can have forward or reverse voltage applied, however in the drawings this is a polarized capacitor. I'm not too worried about this, because most electrolytic caps can withstand 1-1.5V applied in reverse polarity. But when attaching or detaching cables, a high-voltage spike may be encountered (hence the zener protection for the MOSFET gate), and I'm wondering if this has any potential to damage the capacitor. Is it possible for an AC signal to damage an electrolytic capacitor, or is that a problem only with extended DC reverse charge? Should I use a solid electrolyte capacitor with higher reverse tolerance? The failure mode is not spectacular: it will simply lead to less feedback at DC. However this might change the DC output offset of the amp and damage the speaker.

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1st October 2003, 05:42 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	I have not found the polarity to make a difference. Any spikes that would go through a zener have been of such short duration that they don't get noticed by the capacitor.

1st October 2003, 06:14 PM	#3
jwb diyAudio Member Join Date: Mar 2002 Location: San Francisco, USA	Thanks Mr. Pass. As an engineer-by-training, I'm naturally curious about modes and rates of failure. It's always good to hear the voice of experience on such matters.

1st October 2003, 06:20 PM	#4
tiroth diyAudio Member Join Date: Dec 2001 Location: Pittsburgh, PA, USA	I believe electrolytic capacitor failure due to reverse-biasing is related to power dissipated; within reason, reverse voltage without significant current won't damage them. An AC signal with sufficient current can most certainly blow these caps, but one that supplies only a few mA, probably will cause no problem or a slow degrading of value.

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