Electrolytic vs Wima Film cap in high filter circuit issue

EastPoint

Recycler
Subscriber
Hi folks. I was recapping a Marantz 1550 receiver and I had a strange issue. I rebuilt the board that has the low filter and high filter switches on it, and afterwards I noticed that when I pressed the high filter button there would be a huge pop and the receiver would go into protection briefly. There was a .47 uf 50v electrolytic capacitor (one of those bright orange aluminum caps) behind the switch, and I had replaced it with a Wima film cap. When I put the original .47 electrolytic cap back in, there were no issues. I swapped in a couple other film caps and it did the same thing with all of them. Only the electrolytic was ok. Any idea why this happened?
 
This could be where low ESR is not such a wonderful thing. The density of an electrolytic is much higher, and the charge build "spike" can be buffered more by its resistance. The film cap is more likely to charge quickly and pass the remainder of the signal through, and if the "signal" happens to be a transient spike, guess what gets through.
 
^^^ Yup--the "charging" cycle of the electrolytic acts as a "buffer"--the WIMA film caps will pass signal virtually "immediately"--basically as if there were no cap there at all.
 
One of the often-touted benefits of the film cap is that it is very good at limiting transients in power supply and switching applications. It must be understood, however, that in these applications the capacitor is most frequently connected from a switched source or power bus to ground (or the opposite pole) and that the capacitor is not intended to pass signal, but filter. Applying this "attribute" to a cap that is intended to pass signal is usually a good thing for signal integrity, but you've discovered one of the weaknesses.
 
Interesting - I ran into a minor 'noise' issue with the low / mute switching circuit on a re-capped NAD 3140, namely, an annoying 'click' every time the low / mute switch was operated. I don't remember if it was in both switching directions, but I had fitted Wima film caps at the main amp inputs, just after the low / mute switching... one to re-visit I think, maybe a couple of KL's will be a better choice.
 
thanks for a great explanation- I suspect I will revisit a couple of wima placements in light of this
 
Thank you for this thread! I put Wimas at the 4.7uf/35V locations on a Marantz 2325 PU01 board (really fun board to access :rolleyes:) and am getting a nasty pop when pushing the Low Filter button. I'll bet this is why electrolytics were recommended for these locations. Lesson learned!
 
this is nice to see because i know films should not replace every 'lytic even tho many people preach that as the gospel.

i read it somewhere but i have no idea where.

i've love a thread stating every place they should NOT be.

i stopped automatically replacing 'lytics with films a while back.

i stopped using films in phonostages because i had noticed more hum and edginess in the sound. that is on place i would not hesitate to use UFG unless they need to be UKL.

carry on. and thank you.
 
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