OSCON & Equivalent Placement

[SyncTronX]

Question:
I've had OSCON or AlumPoly type capacitors recommended to me
in a solid state amp, in place of Non-Polar/Bi Polar caps that were
originally used in the device.


It's a medium power standard amp made by Carver.

The location is along the center rail where it branches to each
channels first feed to the differential pairs output base then through
a resistors follow by by CapResistor filter then down to another
differential pair regulating the protection relay.

When I started looking into the leakage of these AlumPoly caps,
they are leaky! I mean this really isn't something I want is it?
Let me scan a page from the Schematic that shows where it is.

I typically would use a Nichicon Bi-Polar cap for these, instead of
an AlumPoly NPCAP PSG Series.
Yes, they have great endurance 15K-20K hours AND 500uA leakage.

Here is a link to the schematic LINK
see page six.


or better yet, here is a cut down version of the schematic.



What do you think?


Cheers,

 

[jean-paul]

Can’t see the schematic but in general it is wise to replace electrolytic bipolar capacitors for fresh new electrolytic bipolar caps or, preferably, film caps when they fit physically and have the same or higher voltage ratings.

However, the SAL series solid aluminium caps Philips produced could often be used in situations where originally bipolar caps were used. Too bad they are out of production. In case you might want to try these, I have many values still in stock.

Another possibility is to use 2 good quality polar caps in series but you already know this I think.

 

[SyncTronX]

Here is the schematic,
the cut down version.
The amp is (cliometrics, semitropical, simetrical, simetricol)
symmetrical and I shoved the inputs over to see them.

The values are 220UF 16V with a 3.5mm lead spacing.
I can order some of the 220uf caps that will fit.

The Oscon type were recommended to me,
when I looked at the specs...I saw the high leakage,
even more then BP/NP Caps.
This was moved to update the pic as the original

had and error in it.


Cheers,

 

[sgrossklass]

Yeah, those need low ESR about as much as a hole in the head. Bipolar is not strictly necessary in normal operation (I think there'll be a slight positive DC offset) but may help avoid destruction during a fault condition.

High leakage is directly associated with high 1/f noise. (See kgrlee MC preamp thread in Analog Source.) More of a problem if there's actually any DC on it that would allow leakage current to flow, but still, not exactly the thing you want looking into a critical high-impedance node, including your inverting input. A thing to consider when relying on RC filtering e.g. for PSRR.

With C701 the benefits of low ESR are likely to outweigh these considerations (plus it's in parallel with a zener anyway), and the LTP would have enough supply rejection for any potential noise of C702 not to matter.

BTW, 0.01 CV (a typical leakage spec on a good standard cap) would be 55 µA on a 220µ/25V if I'm not mistaken. So unless you were looking at a substantially bigger part, 500 µA wouldn't be great.

 

[SyncTronX]

sgrossklass,


Are you saying that C701 would be a better place of the
AlumPoly CAP? As opposed to the Highlighted locations?


So If I understand your comment, I might as well stick to the NP
caps to where they were originally placed.


Or If I understand what you are saying High Leakage correlates to high 1/f noise,
would it be any benefit to use a low leakage cap in these three locations hightlighted?
If I recall Nichicon UKLs state they have 0.002 CV, which is the best I've seen.
Also, 220uF is too big for a film cap...there is no room.


So maybe you are correct in stating the BPs might be beneficial during a fault
condition.



Thanks for the insight.


cheers,

 

[johnego]

When I started looking into the leakage of these AlumPoly caps, they are leaky! I mean this really isn't something I want is it?

I have been using OSCON for decades as feedback capacitors without thinking because when I compared long ago it was good (but very bad as input coupling) and more importantly, I have made many great sounding amps with this cap in the feedback which means it shouldn't be a problem! Many times I was confused because looking or hearing about the specs, I don't know why I have to keep using it.
But today, especially when sgrossklass mentioned that high leakage is directly associated with high 1/f noise, I questioned again my decision to use this OSCON because if 1/f is so high impossible it can sound good enough for me!
And surprise! There are two types of electrolytes used by OSCON. The one with plastic sleeve uses 'organic semiconductive' and leakage is rated below 100uA. Even the one I have used (type SP) is said to be suitable for audio!

 

[jean-paul]

The bipolar 220 µF 25V in the feedback loop really is better replaced by a bipolar cap. In fact using a polar electrolytic cap in the feedback loop as done in many amplifiers is a design error driven by cost cutting. I would also not experiment by changing bipolar caps to polar caps in an existing design as the designer probably had good reasons to choose exactly that type of cap.

FWIW my experiences with various types of OSCON is that they are excellent in digital circuits but I would never use them in analog applications except maybe for decoupling supply lines. There are many types that perform better in such applications. Same counts for polymer caps. These are both mainly designed for low ESR operation in computer/SMPS applications where other types failed in the past.

If you are the "anything goes" type then don't hesitate and throw in something, you can open a new thread about the new phenomena that may occur If you want the device to operate like it used to then simply buy Nichicon MUSE bipolar caps preferably with a slightly higher voltage rating (as long as they fit physically).

 

[SyncTronX]

@johnego,


That is interesting I'll have to check further on that.



@Danke Jean-Paul,

That was my thinking also, sometimes it is just good to ask
and get confirmation that I'm not missing the boat.

While things are opened up, I try to install a higher voltage
that still has same lead spacing.

Cheers und Vielen Dank,