Topic: DIY/electronic guru questions: Why use tantalum coupling caps?

[poorlyconditioned]

I just looked at my DMIC20 and noticed tantalum caps coupling the preamp output to the ADC.

I've also seen tantalum caps in various audio-technica phantom power modules, couping the (DC) transistor amp to the (AC) output transformer.

So, why not use an electrolytic, or better yet, an electrolytic bypassed with a film or polyester cap?

Any electronics gurus out there know the answer?  If not, I'm going to rip the sucker out!!!

  Richard

[Chanher]

try the oade boards too, it always seems like Doug is willing to share his knowledge.

http://www.oade.com/Tapers_Section/Forum/dcboard.php

[SparkE!]

I just looked at my DMIC20 and noticed tantalum caps coupling the preamp output to the ADC.

I've also seen tantalum caps in various audio-technica phantom power modules, couping the (DC) transistor amp to the (AC) output transformer.

So, why not use an electrolytic, or better yet, an electrolytic bypassed with a film or polyester cap?

Any electronics gurus out there know the answer?  If not, I'm going to rip the sucker out!!!

  Richard

Tantalums generally make lousy audio coupling caps unless you are working in low S/N systems where cost and space are at a premium.  It surprises me that any high end audio gear would use tantalums as coupling caps.  If they do, at least they should put two of them in series with each other with opposing polarities.  The problem with tantalums is that thier capacitance depends on the instantaneous voltage across the cap, so your signal can modulate the capacitance.   Also, they are prone to the effects of dielectric absorption.  (Electrolytics suffer from dielectric absorption too, but not quite to the same degree.)  In order to understand what dielectric absorption is, it's easiest to describe the behavior associated with it.  Suppose you put a DC voltage across the cap and let it sit there for a little while and let the charge get all distributed.  Then remove the capacitor from the charging voltage.  Naturally, it maintains essentially the same voltage across it as the charging voltage.  Now, if you temporarily short the cap to force 0 voltage across it, you'd expect it to remain discharged once you remove the short, but that does not happen.  You'll actually see the voltage come back up. It won't go back to the original voltage, but it doesn't stay at 0 either.  So there is this latent charge storage mechanism, kind of like a battery.  When you combine the signal dependent capacitance and the dielectric absorption, you can get some charge pumping going on that will end up as a DC offset in your recordings and some increase in even harmonic distortion.

So, if you can get the same electrical size of caps in some sort of plastic film caps (styrene, mylar, polyester, etc) or ceramic caps, I'd say go for it and make the change.  It's unlikely to do anything but help.