disc ceramic HF feedback cap

Circlotron · 18th October 2010, 12:04 PM

Just today I started building the Silicon Chip Ultra L-D amplifier (the 100W version) from a kit and one of the feedback caps is a disc ceramic. Now I know for a fact that non-NPO ceramic caps generally vary their value all over the place with changes in voltage so I would have thought that using a ceramic here would have been a really bad idea. What should I replace it with, if in fact I should replace it? It is the one from Q8 collector to Q7 base.

http://www.siliconchip.com.au/cms/ga...l?a=102474&i=4

Samuel Groner · 18th October 2010, 01:36 PM

How do you know its not a C0G/NP0? Small values (below 1 nF) use hardly any other dielectric.

Samuel

DF96 · 18th October 2010, 06:55 PM

The switchover from NP0 to non-linear ceramics takes place at different values for different capacitor ranges. There is a colour code - I think a black band indicates NP0?

You could try silvered-mica or polystyrene - these are the other common low capacitor value dielectrics, but I don't know what effect they may have on sound.

Ian Finch · 18th October 2010, 10:42 PM

When you have recovered from the cost and lead time to buy 100pF caps in better than non -NPO/COG ceramic in Oz, you will find it hard to get assurances or indications like the black top on NPOs from retailers or distributors any more. That is my recent experience anyway.
Rockby Electronics might be your best bet in Melbourne, though as surplus resellers, they may have little stock of such popular values.

DCPreamp · 19th October 2010, 03:53 AM

I'd just use the cap provided. Considering the lengths the circuit designer went to to keep distortion low and quality high, they must have also considered that cap's value and type. Plus, that cap's value only comes into play way into the KHz range, so you'll never hear it or even a better part anyway.

Paul

djoffe · 19th October 2010, 04:20 AM

Try to verify that the cap is NPO or COG. You don't want a bad dielectric here...they've cascoded the driver to avoid nonlinear Ccb...why replace it with another nonlinearity in this cap?

Circlotron · 20th October 2010, 12:45 PM

Quote:

Originally Posted by DF96

There is a colour code - I think a black band indicates NP0?

The cap does indeed have a black band - looks like a very short haircut

so maybe it is an NPO then. I didn't know they came in disc version.

embrown057 · 22nd October 2010, 04:22 AM

I like to use the Kemet Multilayer to replace npo's. The CGO class 1 by Kemet are very stable and have down to 5% tol. Mouser has stock for them depending on the values.

http://www.kemet.com/kemet/web/homep...01_GoldMax.pdf

pradigmshift · 22nd October 2010, 09:53 PM

Ceramic can have poor capacitance stability with respect to voltage. Some ideas: I've seen mention of silvered mica said to sound sweeter/sharper and polystyrene should be the most neutral but it can't resist high temperatures, so carefull with soldering. Polypropylene is a good alternative.
However, I've yet to see a measurement of distortion spectrum vs type of feedback cap... some measurements would be interesting.

You can always try to swap parts to see if you can hear a difference.

18th October 2010, 12:04 PM	#1
Circlotron diyAudio Member Join Date: Jun 2002 Location: Melbourne, Australia	disc ceramic HF feedback cap Just today I started building the Silicon Chip Ultra L-D amplifier (the 100W version) from a kit and one of the feedback caps is a disc ceramic. Now I know for a fact that non-NPO ceramic caps generally vary their value all over the place with changes in voltage so I would have thought that using a ceramic here would have been a really bad idea. What should I replace it with, if in fact I should replace it? It is the one from Q8 collector to Q7 base. http://www.siliconchip.com.au/cms/ga...l?a=102474&i=4 __________________ Best-ever T/S parameter spreadsheet. http://www.diyaudio.com/forums/multi...tml#post353269

18th October 2010, 10:42 PM	#4
Ian Finch diyAudio Member Join Date: Apr 2010 Location: Coffs Harbour, on the east coast	When you have recovered from the cost and lead time to buy 100pF caps in better than non -NPO/COG ceramic in Oz, you will find it hard to get assurances or indications like the black top on NPOs from retailers or distributors any more. That is my recent experience anyway. Rockby Electronics might be your best bet in Melbourne, though as surplus resellers, they may have little stock of such popular values. __________________ regards

19th October 2010, 03:53 AM	#5
DCPreamp diyAudio Member Join Date: Jun 2005 Location: Sacramento, CA	I'd just use the cap provided. Considering the lengths the circuit designer went to to keep distortion low and quality high, they must have also considered that cap's value and type. Plus, that cap's value only comes into play way into the KHz range, so you'll never hear it or even a better part anyway. Paul __________________ "Believers cling to the myth despite the evidence, reinterpret the myth to suit the evidence, or lie about the evidence to support the myth." "To err is human; to blame errors on external factors is even more so."

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Help with Ceramic Disc Capacitor Numbers	uh1hhuey	Everything Else	1	30th June 2010 07:00 AM
Flat round and old ceramic disc capacitors	AndrewGM	Parts	1	6th October 2008 10:19 PM
Is a OPA445 AC stable with 100% Cap feedback at HF?	Fanuc	Solid State	0	30th July 2007 09:07 PM
Replacing ceramic disc caps with silver mica caps?	G	Solid State	5	18th November 2002 01:02 PM
100n ceramic disc capacitor	markkanof	Solid State	7	20th June 2002 09:46 PM


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

18th October 2010, 01:36 PM	#2
Samuel Groner diyAudio Member Join Date: Jun 2005 Location: Zürich	How do you know its not a C0G/NP0? Small values (below 1 nF) use hardly any other dielectric. Samuel

18th October 2010, 06:55 PM	#3
DF96 diyAudio Member Join Date: May 2007	The switchover from NP0 to non-linear ceramics takes place at different values for different capacitor ranges. There is a colour code - I think a black band indicates NP0? You could try silvered-mica or polystyrene - these are the other common low capacitor value dielectrics, but I don't know what effect they may have on sound.

19th October 2010, 04:20 AM	#6
djoffe diyAudio Member Join Date: Oct 2008	Try to verify that the cap is NPO or COG. You don't want a bad dielectric here...they've cascoded the driver to avoid nonlinear Ccb...why replace it with another nonlinearity in this cap?

22nd October 2010, 04:22 AM	#8
embrown057 diyAudio Member Join Date: May 2010 Location: Florida	I like to use the Kemet Multilayer to replace npo's. The CGO class 1 by Kemet are very stable and have down to 5% tol. Mouser has stock for them depending on the values. http://www.kemet.com/kemet/web/homep...01_GoldMax.pdf

22nd October 2010, 09:53 PM	#9
pradigmshift diyAudio Member Join Date: Jun 2010	Ceramic can have poor capacitance stability with respect to voltage. Some ideas: I've seen mention of silvered mica said to sound sweeter/sharper and polystyrene should be the most neutral but it can't resist high temperatures, so carefull with soldering. Polypropylene is a good alternative. However, I've yet to see a measurement of distortion spectrum vs type of feedback cap... some measurements would be interesting. You can always try to swap parts to see if you can hear a difference.

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read