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Old 24th January 2007, 05:49 PM   #1
nhuwar is offline nhuwar  United States
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Default B+ filter caps

Is there a good brand to use for the b+ filter caps? Ones that most of you all find superior to others?

Thanks

Nick
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Old 24th January 2007, 05:53 PM   #2
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Depends a bit on the type of caps, but when it comes to electrolytics I alwasys prefer Evox Rifa.
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Old 24th January 2007, 06:08 PM   #3
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I like ASC motor run caps a lot. You can get them at Allied Electronics, or at Percy or Parts Connexion.
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Old 24th January 2007, 06:28 PM   #4
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Motor runs from ebay.
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Old 24th January 2007, 06:33 PM   #5
nhuwar is offline nhuwar  United States
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Is it ok to use electrolytics in series
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Old 24th January 2007, 06:50 PM   #6
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Yes but you should have a resistor parallell with each cap.
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Old 24th January 2007, 07:54 PM   #7
nhuwar is offline nhuwar  United States
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Ah That part I do know. I actually bought the resistors for the caps the other day. The reason for this post is I want to use a pi filter network and I wanted to know a good brand to buy.

Does anyone remimber off hand the formula for figuring minimum required capatitance for stated ripple I cant find it in my arrl book?

Thanks

Nick
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Old 24th January 2007, 09:10 PM   #8
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I used Motor-run caps in my Aikido PSU. I managed to find GE caps at a local surplus/reclamation facility and had hundreds to choose from. The cost per cap is pretty low this way.

Good Luck with the B+

Charlie
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Old 25th January 2007, 12:32 AM   #9
Sherman is offline Sherman  United States
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Motor runs here also. I've put them in the last four amps I've built and like them a lot. The main drawback is size. I've drilled holes in the top plate and have them sticking through, it is harder to hide them inside the amp.
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Old 25th January 2007, 01:34 AM   #10
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I Use a good quality Panasonic electrolytic from Digikey (two or more in series if needed) for the first cap in a CLC supply. Then I use a 100uF 370VAC ASC motor RUN cap ($12 on Ebay) for the second cap. These are good to about 500 VDC so a series combination may be needed for HV applications. On some amps I use both Panasonic electrolytic and a motor RUN cap in parallel for the second cap. Sometimes it it is advantageous to put a small resistor between the electrolytic and the motor RUN cap. This damps the ringing that can occur due to interaction between the inductance inherent in ALL electrolytics and the capacitance (very low ESR) in the motor run cap. This technically makes the supply CLCRC, with R being in the 5 to 50 ohm range.

It is important to know the difference between a motor RUN cap and a motor START cap.

Motor START caps are used to create a phase shift when starting an AC motor. The cap is only used for a second or two at startup and then mechanically switched out of the circuit. It is made to handle a high current for a short time and can not handle continuous operation. These are usually non polarized electrolytics. Their ESR is not characterized and they are not useful for audio amp use.

Motor RUN caps are used to create a phase shift (against the motors inductance) for a multi pole motor on a single phase power source. This type of capacitor is always in the circuit, and a large AC current is flowing through the capacitor. Any resistance in the capacitor (ESR) is pure loss in the motor circuit, so these caps are optimized for low ESR. These caps are rated for a maximum AC voltage with AC current flowing through the cap. I have seen many recommendations for operating these caps on DC, some as high as the AC rating times 2. I use 1.41 for a safe bet. These caps are usually metalized polypropylene. Since low ESR, high voltage and current handling capabilities are all qualities we want in power supply caps, these work good in tube amp power supplies, often very good. The ESR is usually much lower than any electrolytic, and maintained ofer the entire audio range. This becomes real obvious when you spark a charged one with a screwdriver (don't). The main drawback is, as mentioned before, size. The 100uF 370VAC caps that I get from Ebay are about the same size as a beer can.
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