Changing coupling cap from tantalum to film???? PM1000

So I replaced the input coupling caps on my PM1000 from a polarized tantalum cap, to a non-polarized film, and I think I'm having some problems due to this. Is it possible that because it isn't polarized that I'm sending voltage BACK to whatever is plugged into it?

If I use mics with the two modules that I made this change on, it works fine, and sound really nice, registering no level of any frequency on my VU's or in Pro Tools. However, if take the mix buss outs of my TAC Scorpion and plug it into the inputs (mic and line go in the same input and there is no mic line switch, only a stepped gain switch) then the Scorpion freaks out a little bit. The meters on the master buss go nuts and slam. There's no audible sound of any noise happening to warrant the meters slamming up like this, and when I play music through the Scorpion it still sounds the same, and when I listen off the headphone amp of the PM1000, it also sounds fine. Could this be an oscillation? It only does it when the OUTPUT of the tac scorpion isplugged into these modules with the changed film caps. I actually replaced ALL of the tantalums (3 of them, I didn't recap the echo sends) with film caps.

Seeing as I recapped two modules and they are both behaving the same way, and the ONLY difference between these two modules and the rest that I recapped is the film caps and direct outs (which is a transformer wired off of C37) which was not plugged into anything during the test, and the other modules, it has to be these film caps.

Any thoughts? Here's the schematic:

http://home.new.rr.com/lordjimbo/PM1000E_2.pdf

Thank you!

Jay Lison

C1, 0.47/35V? How enormous is the new cap? You can't put fat caps on front of a high-gain high-impedance amp like that without risk of MHz oscillation. And while it may not sound awful, it IS doing bad things to the audio.

Yep, that's the one. I put a 3uf film in there. The schematic has it as .47, but they actually put 1uf caps in most of the modules, though there were a few that had .47 tantalums in there.

Thanks for the info PRR, I'll try reducing it. I'll take my scope with me on my next trip to the studio and see what I can find out....

Thanks,

Jay

You can get pretty small Wima MKS2 high-value polyester caps..

Jakob E.

Actually, I'm not sure if that's the problem. I replaced that same cap on about 6 other modules with a 4.7uf tantalum, and this problem isn't going on with those modules. I can plug the output of my Scorpion console into those modules, and the scorpion is happy as a clam. Or is the impedance issue related to being a film cap?

Or is the impedance issue related to being a film cap?

Click to expand...

Yes, I think it´s only related to big film caps, as tantalum are electrolytic caps. Try 1uF/63v film. That should be small enought, or maybe you can buy a lower voltage on, like 50v or 35v, but that´s not very easy to find.

I see! I will do that! Thanks for the help guys!!!

> I replaced that same cap on about 6 other modules with a 4.7uf tantalum

Let me clarify my question.

A one-uFd Tantalum is about this big. A one-uFd Film is about this big.

Just as the big type catches your eye, the physically big cap catches stray field from the output stage and couples it back to the input.

0.2 to 1.0uFd is a good size electrically. It doesn't hurt the frequency response to be this small; it has some effect on subsonic noise and mostly a 1.0uFd Tantalum is the same price as a 0.47uFd Tant so they just use the same cap everywhere.

Film caps are much bigger physically. Even a 0.47uFd Film may be acting as a bigger antenna than the little Tant. Also if the board is laid out for a snot-size Tant, where did you hang a several-uFd Film? You can't "stretch the wires" and remote-mount an input cap in a high-gain input like this one. You have to keep it all tight and small.

Try the 0.5uFd 50V Panasonic B-series green-film; it is about as physically small as a film-cap that uFd can be.

Tubejay - did you get to the bottom of this problem?

I still have not dealt with those two modules. My modules with tantalums in that spot are fine though, so it has to be that the size was too large. I'll post back here when I fix them, though that probably won't happen for a few months. My console is stuck at a studio right now, so that's why my work has been delayed on it.