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croccodillo 28th November 2005 02:23 PM

Motor caps
 
I've read in some threads here that somebody uses polypropilene motor caps in their projects: are those capacitor good enough?
I'm trying to build a para-feed SE amplifier, so I could use such a cap for output stage (I have lot of them).

Ciao,
Giovanni

Bas Horneman 28th November 2005 02:28 PM

Quote:

polypropilene motor caps in their projects: are those capacitor good enough?
They are excellent! Some of the best you can get IMO. The only things possibly better are some of the paper in oil motor caps.

tubemaster 28th November 2005 03:12 PM

Quote:

Originally posted by Bas Horneman
The only things possibly better are some of the paper in oil motor caps.
Hello,

btw. where can you get paper-in-oil motor caps for power supply use?

Thanks
Michael

croccodillo 28th November 2005 03:23 PM

Just an idea: I work for a company that produces microwave ovens for medical applications; into every microwave oven (yes, also the one you have on your kitchen desk) there is an high voltage (4500V), paper in oil capacitor, usually with a capacitance of about 1-2 microfarad.

Ciao,
Giovanni

Bas Horneman 28th November 2005 04:03 PM

Quote:

btw. where can you get paper-in-oil motor caps for power supply use?
Don't have an address for them really. Have some russian ones I got from e-bay. And there we have the big drawback of paper in oils. If you get NOS there is apparently a big difference in quality...not all paper in oils are created equal. (same for polyprop but I would guess less variation)



Giovanni's suggestion is a good one. Unfortunately most are indeed 1uF -2uF.

Sherman 28th November 2005 06:29 PM

Quote:

Originally posted by tubemaster


Hello,

btw. where can you get paper-in-oil motor caps for power supply use?

Thanks
Michael

Try eBay. I got some very nice caps of 40uF, 55uf and 100uF for a very good price. Be sure to just search 'All Categories' as listers put them all over the place.

One important thing to note- get motor run caps and not motor start caps. Motor runs are designed for continuous duty. Also, motor caps will be rated for AC voltage. I've been told you can double that for DC use but I use the AC * 1.4 rule.

Shoog 28th November 2005 10:35 PM

I am using salvaged Microwave oven caps in my Parafeed amp. I am using them for power supply filtering. They are rated at about 1500V and look very robust. If you use a CCS source in your design, it is surprising how little capacitance you can get away with in your power supply. The CCS, if good, will supply clean DC at a constant current, so all that heavy duty filtering you usually need for SE designs becomes largely redundant. I do however use two large filtering chokes.

Shoog

speaker 29th November 2005 01:27 AM

What is the prevailing wisdom as to why motor run caps are good? I've also seen them used as tweeter highpass filters. Do they have an especially low ESR & DF or other factor? I understand the reliability aspect but what else attributes to their suitability?

Bas Horneman 29th November 2005 10:27 AM

Quote:

What is the prevailing wisdom as to why motor run caps are good?
Excellent question.

My answer is : I don't know.

But like you said reliability is good.
And there is anecdotal evidence that the oil "dampens" leading to good sonic qualities. Why else would a lot of "boutique" caps use oil? Maybe someone in the know can shed some light on this.

The paper in oil thing ...once again...just speculating here..is a tone thing...paper also has better di-electric qualities compared to teflon. But this can only be said of paper with a certain type of "dryness" I suppose.

Last but not least..if using older caps with oil in putting them in your amp will save them from being dumped in some landfill where they will pollute groundwater with heavy metals.:angel:

jsn 30th November 2005 08:26 PM

This is the closest thing I have seen to quantitative proof that paper in oil is a good thing:

http://members.aol.com/sbench102/caps.html

I really like polypropylene in oil caps becuase they are inexpensive, easy to get as motor run caps, and relatively easy to work with.

Here is an amp that uses a bunch of inexpensive 10uF poly-in-oil caps:

http://boozhoundlabs.com/monkey/

I also like these for speaker crossovers. You can sometimes get good deals on low voltage (200V or so) Russian surplus PIO caps on eBay.

To me the sound of oil caps in general is clearer, more relaxed, and perhaps tonally a bit darker. Of course meaningfully describing sound is a hard thing to do, so ymmv.

High voltage, low value oil caps like those mentioned in microwave ovens would make great coupling caps, and really great parallel feed caps.

jsn


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