Is feedback cap polarized or Not

Wynand · 12th July 2005, 03:08 PM

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.

richie00boy · 12th July 2005, 03:14 PM

No, but you could take two 22uF caps together like that and make a bipolar 11uF. Caps in parallel adds values.

Tekko · 12th July 2005, 03:17 PM

It can be a regular polarized elytic, atleast thats what i used. It would be best with a non electrolytic tho.

Wynand · 12th July 2005, 03:21 PM

Shot, thanks

jcx · 12th July 2005, 03:28 PM

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

analog_sa · 12th July 2005, 04:19 PM

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.

Wynand · 13th July 2005, 06:20 AM

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.

agrobler · 13th July 2005, 11:11 AM

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/

12th July 2005, 03:08 PM	#1
Wynand diyAudio Member Join Date: Mar 2005 Location: Cape Town	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.

12th July 2005, 03:14 PM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	No, but you could take two 22uF caps together like that and make a bipolar 11uF. Caps in parallel adds values. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

13th July 2005, 11:11 AM	#8
agrobler diyAudio Member Join Date: Jul 2004 Location: Pretoria	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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12th July 2005, 03:17 PM	#3
Tekko diyAudio Member Join Date: Jan 2005	It can be a regular polarized elytic, atleast thats what i used. It would be best with a non electrolytic tho.

12th July 2005, 03:21 PM	#4
Wynand diyAudio Member Join Date: Mar 2005 Location: Cape Town	Shot, thanks

12th July 2005, 03:28 PM	#5
jcx diyAudio Member Join Date: Feb 2003 Location: ..	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

12th July 2005, 04:19 PM	#6
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	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.

13th July 2005, 06:20 AM	#7
Wynand diyAudio Member Join Date: Mar 2005 Location: Cape Town	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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