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Old 19th March 2012, 12:32 AM   #1
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Default Tantalum caps in the filament supply?

Hi all,


I'm repairing a cary ph301 phonostage. One of the things wrong is a burnt out tantalum cap on the heater supply. This is a dc 12v supply, and each of the valves has a 10uf tant on the sockets. Seems like a good idea to replace those with something else - but film caps ain't gonna fit. Seems like the only choice here would be a small electrolytic or another tant.

Any idea which is better/ok to use?


Fran
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Old 19th March 2012, 12:55 AM   #2
tomchr is offline tomchr  United States
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Tantalum caps tend to have fairly high ESR (100-ish mOhm). You can do better with modern electrolytics, such as the ones made by Nichicon and others. I would not use ceramic caps due to their voltage coefficients -- or at least, if you do use ceramics take the voltage coefficient into account.

~Tom
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Old 19th March 2012, 01:16 AM   #3
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Thanks tom,

So tants have no special properties obvious in this application other than their small size?

Fran
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Old 19th March 2012, 03:15 AM   #4
tomchr is offline tomchr  United States
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Tantalum offers high capacitance in a small package. Aside from that, they have no magic properties.

Ceramics - specifically the newer X5R or X7R types - offer quite a bit of capacitance in a small package as well. But not the cap/size ratio of a tantalum cap. Modern MLCC's, however, offer ESR's in the single digit mOhm range. They do suffer from a fairly harsh voltage coefficient, though. At 50 % of the rated DC bias, you have about 50 % of the rated cap, and it drops dramatically from there. More importantly, however, the surface mount MLCC caps offer very low ESL (equivalent series resistance). This can be of importance in RF circuits (including switching power supplies).

Film - specifically polypropylene - offers very low loss tangent and dielectric absorption. Hence, they're used a lot for coupling caps as they tend to be transparant (i.e. not cause harmonic distortion). The better ones reach single digit mOhm ESR as well. They're fairly bulky, though. But can be made to withstand high voltages. They can also handle *A LOT* of transient current.

Electrolytic caps do suffer some from dielectric absorption, hence, aren't typically used in the signal path of audio circuits. They offer a good cap/size ratio and the modern polymer types intended for use in switchmode power supplies offer very low (10's of mOhm) ESR. One of Nichicon's capacitors reaches 7 mOhm... That's pretty solid for an electrolytic cap.

That's the executive summary...

~Tom
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Old 19th March 2012, 08:52 AM   #5
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Thanks again tom, appreciate your help and advice on this one!


Fran
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Old 19th March 2012, 09:57 AM   #6
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Quote:
Originally Posted by woodturner-fran View Post


but film caps ain't gonna fit.


I wouldn't worry much about the value, even 1uF polyprop will sound much better than the tantalums in this position. Or experiment between a film and a nice sounding larger value electro. A 47uF BG-N is what i often use on heaters with voltage regs. Definitely prefer current regs though.
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