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Old 18th October 2010, 12:04 PM   #1
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Default 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
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Old 18th October 2010, 01:36 PM   #2
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How do you know its not a C0G/NP0? Small values (below 1 nF) use hardly any other dielectric.

Samuel
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Old 18th October 2010, 06:55 PM   #3
DF96 is offline DF96  England
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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.
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Old 18th October 2010, 10:42 PM   #4
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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.
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Old 19th October 2010, 03:53 AM   #5
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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
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Old 19th October 2010, 04:20 AM   #6
djoffe is offline djoffe  United States
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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?
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Old 20th October 2010, 12:45 PM   #7
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Quote:
Originally Posted by DF96 View Post
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.
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Old 22nd October 2010, 04:22 AM   #8
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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
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Old 22nd October 2010, 09:53 PM   #9
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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.
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