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Old 24th January 2013, 08:09 PM   #1
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Default big film caps for decoupling?

I got a company who can manufacture film caps of 100uf + so I thought to give a try for decoupling but wanted to know which are best suitable for decoupling..

metalized polyprop or plain polyprop?
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Old 25th January 2013, 03:15 AM   #2
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Default Another Rainbow

Have a look at the size, price and availability of a 100uf MKT (metallized polyester) film cap. (the smallest, cheapest film type).
A 100uF polyester film cap is $32-1,000 from Mouser and non-stock. Here is a Kemet data sheet for cheap MKT types but to save you the read, the size for 100uF 63V is 42x40x20mm - big! http://au.mouser.com/pdfdocs/KEMETR60Datasheet.pdf.

100uF Polypropylene film types, generally best for audio, are not available from Mouser, at least. The sizes and prices however, would necessarily be staggering.

Are you trying to build a museum piece of high-end extremism, a practical amplifier or just tell us about your dreams of perfect, monster and unaffordium parts?
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Last edited by Ian Finch; 25th January 2013 at 03:20 AM.
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Old 25th January 2013, 04:08 AM   #3
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Such an approach is impractical and of little benefit in my opinion.

What are you decoupling, anyway? Line level or power amp?

You can get the same result that you are attempting to achieve (for much less money and much less board space) by paralleling appropriate capacitors. I have recently done some informal tests with this and have achieved subtle subjective differences in sound. My tests are incomplete and inconclusive, but promising.

For line level, you can bypass with 22 or 47 uF electrolytics ( which have to be close to the opamp but not right on top of it) in parallel with maybe a 1 uF poly (closer) and a 0.1 uf ceramic (soldered directly under the opamp right on the supply pins). You can try smaller electrolytics and bigger polys and see how it works.
Your results may vary.

For headphone amps and power amps, the caps will obviously have to be scaled. You're on your own here but I intend to do some research myself on this topic. I did experiment a little with capacitor banks consisting of multiple electrolytics in parallel for headphone amps, and the results were very promising. Since smaller electrolytics have better high frequency response than larger ones, you might obtain excellent results with this approach without using polys (but still use the ceramic bypass!) Do some experimenting on your own. Parallel capacitor banks have lower esr than equivalent large capacitors, so you get an added advantage.

For the record, I used rat shack metallized polyprop caps of 1 and 2.2 uF that I have had sitting around new in the package for years. I will be ordering a lot of new parts in the future, but right now I'm experiment with parts I have laying around.

Don't be sucked in by the woo of boutique capacitors. Excellent results can be obtained with ordinary parts if you employ appropriate design. Some of the ideas I have suggested are not "industry standard" by any means but all have been discussed here in some capacity.
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Old 25th January 2013, 04:32 AM   #4
gootee is offline gootee  United States
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While the large-value PP caps are quite large, physically, and I haven't seen radial versions, they are not all that expensive, at the places that sell speaker crossover supplies, such as madisound and parts-express. Their lead length would be way too long for high-frequency decoupling, unfortunately (which is also needed, for power amps).

The answer is, I think, arrays of electrolytics, on unetched 2-sided PCBs.

A 10x10 array of 1000 uF electrolytics should give less than 1 nH of inductance!

Terry Given made one, and measured it with a network analyzer.

Here are most of the relevant links (near the bottom of the post at the following link):

LM3886 component selection

-----

I developed some equations for calculating required capacitances and maximum tolerable inductances, for decoupling and reservoir caps, at:

Power Supply Resevoir Size

and

creating 1 watt rms power from 5 watt chip amp?

and

paralleling film caps with electrolytic caps

and the links at the bottom of the post at

Power Supply Resevoir Size

-----

The decoupling and reservoir caps are the main signal path! THE SOUND that comes from the speakers is directly from CURRENT that comes from the decoupling and reservoir caps, and occasionally from the rectifiers. See the image at:

Power Supply Resevoir Size

Cheers,

Tom

Last edited by gootee; 25th January 2013 at 04:48 AM.
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Old 25th January 2013, 04:41 AM   #5
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Quote:
Originally Posted by gootee View Post
While the large-value PP caps are quite large, physically, and I haven't seen radial versions, they are not all that expensive, at the places that sell speaker crossover supplies, such as madisound and parts-express. Their lead length would be way too long for high-frequency decoupling, unfortunately.

The answer is, I think, arrays of electrolytics, on unetched 2-sided PCBs.

A 10x10 array of 1000 uF electrolytics should give less than 1 nH of inductance!

Terry Given made one, and measured it with a network analyzer.

Here are most of the relevant links (near the bottom of the post at the following link):

LM3886 component selection

Cheers,

Tom
This is pretty much what I'm working on! You made my work too easy. I feel like I'm cheating now.
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Old 25th January 2013, 04:53 AM   #6
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100 uF through hole polypropylene is probably no better than than 100 uF surface mount polymer electrolytic or polymer tantalum given the mounted inductance.
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Old 25th January 2013, 07:15 AM   #7
JMFahey is online now JMFahey  Argentina
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Quote:
wanted to know which are best suitable for decoupling..

metalized polyprop or plain polyprop?
Which one is the most expensive?
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Old 25th January 2013, 10:36 AM   #8
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plain polyprop is expensive
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Old 26th January 2013, 04:40 AM   #9
gootee is offline gootee  United States
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Quote:
Originally Posted by Fast Eddie D View Post
This is pretty much what I'm working on! You made my work too easy. I feel like I'm cheating now.
Well, heheh, sorry, but at least it looks like you were/are definitely on the right track!

One thing that I've been trying to follow-on with, for that, is how to best mount and connect everything. It would be a shame to have 0.5 nH inductance and then ruin it by using a stupid connection scheme for the load.

For chipamps, I'm pretty-much down to thinking that one rectangular pcb will hold two cap arrays (one for each polarity), and the chipamp will be between the two arrays.

But not wanting the external components and traces for the chipamp circuit to break up the planes, I'm thinking the chipamp circuitry, minus capacitors, will have to be on a daughterboard, on very short standoffs, straddling the dividing line between the arrays' power planes, probably over the non-component side of the arrays' board. Then the power and ground pins and connections can all go basically straight down through the daughterboard and into the array board, to either side (power or ground), with minimum connection lengths.

Alternatively, the array pcbs could be mounted parallel to each other, with their non-componnt sides close together, with standoffs between them, and the daughterboard could be, maybe, perpendicular to them, and mounted very close to one edge, so that the chipamp pins could directly reach the array boards.

The heatsink mounting is somewhat awkward, either way. But I can't see any way to not have anything at all behind the rear plane of the chipamp. Both of the schemes above would occupy the space "below" the chipamp pin level, behind the rear plane of the chip. So the heatsink can be connected but it can't go "downward", at all, behind the chipamp.

Wait, maybe the chipamp package could end up parallel to the non-component side of the array board, with the front of the chip facing the board, and the heatsink plane would then also be parallel to the board. That is one way that the entire half-space behind the chip could be available, enabling use of a large heatsink mounting face that extended in all directions in the plane of the chip's rear surface.
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Old 26th January 2013, 11:57 AM   #10
Telstar is offline Telstar  Italy
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The biggest film cap I have seen is 470uF, and I'm gonna try it
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