Diode across signal carrying capacitor - hmm....

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cuelist

Well-known member
Joined
Aug 21, 2004
Messages
248
Location
Sweden
From time to time I see circuits where a diode is put in parallel with a polarized capacitor (electrolytic). I am talking about signal carrying circuits, not power supplies.

I understand that the diode is there to clamp any reverse biasing of the capacitor but would it not have a detrimental effect on the signal?

Anyone here who has looked into this?
 
Here are a couple of examples:

Preamp from Crest XVCA consoles (see diode over capacitor in series with gain setting pot.

preamp1.jpg


Output stage from old Midas console (see diode across output capacitor).

driver1.jpg
 
[quote author="tv"]looks like inverse polarity protection[/quote]

Yes, that is crystal clear to me BUT does it not have any detrimental effect on the audio signal?
 
likely not since the diode is reverse biased and thus blocking the current flow. even if the signal were to go over 1.5v it would only see itself.
 
imo the diode is invisible to the signal if everything is o.k.

if there's a circuit failure diode protects the component
 
Hi
Yes I read Doug's books and it refers to diodes across capacitors. As a summary, yes the distortion does increase as the fequency is reduced because when the reactance of the cap is enough to get to probably about 0V3 (my guess on warm diodes) the diode will start to conduct. In one of Doug's designs he uses 2 diodes to keep clear of the problem rather than making the cap even bigger.
I hope this is useful.
Matt S
 

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