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Old 12th July 2005, 03:08 PM   #1
Wynand is offline Wynand  South Africa
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Default Is feedback cap polarized or Not

I see that National's LM3875 PDF shows that Ci, the Feedback Capacitor, a Non- polarized type cap is. The fact that the value is 10uF and that I've seen other schematics with a electrolytic cap there confuses me.

Could I take two 4.7uF and connect the negative poles together to form 9.4uF non-polarized.

PS amp I'll build has a dual supply PSU.
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Old 12th July 2005, 03:14 PM   #2
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No, but you could take two 22uF caps together like that and make a bipolar 11uF. Caps in parallel adds values.
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Old 12th July 2005, 03:17 PM   #3
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It can be a regular polarized elytic, atleast thats what i used. It would be best with a non electrolytic tho.
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Old 12th July 2005, 03:21 PM   #4
Wynand is offline Wynand  South Africa
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Shot, thanks
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Old 12th July 2005, 03:28 PM   #5
jcx is online now jcx  United States
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in a "signal path" application like this if you cannot use a plastic film the next best best choice is a non-polar Al electrolytic - much lower measurable distortion than any combination of polar electros

the older recomenation of series polar caps w or w/o bias is now known to be inferior to using non-polar Al electro which has full thickness Al Oxide layer on both electrode foils - higher V rating which gives thicker Oxide and lower V stress also lowers distortion
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Old 12th July 2005, 04:19 PM   #6
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Or, as a much better sounding solution replace the cap with a short. Offset is generally not a problem and input DC can be better handled with an input cap or transformer.
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Old 13th July 2005, 06:20 AM   #7
Wynand is offline Wynand  South Africa
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analog_sa: Where do you get your LM chips from. I live in Cape Town and the only LM chip I can find is the LM3876 and I want the LM3875.

P.S. I think I'll play around with the "short" but leave space for a cap. Thanks.
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Old 13th July 2005, 11:11 AM   #8
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Default LM's in SA

Sorry I am in Pretoria, and here we get them at Communica or RS Electronics apparently... R55 ex vat at Communica, dunno if they have branches in Capetown...

These guys are in CT I think and they have apex chips, maybe they have LM's along with the aerospace/surveillance stuff...

http://www.eagle.co.za/
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