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Old 5th August 2013, 10:49 PM   #1
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Default Capacitor types - what to use and where?

Hi All....

I am obsessing about the "best" types of capacitors to use for my next project. Its pretty obvious where electrolytic caps are required but I'm struggling with the other values.

For the 100nF decoupling caps circled in BLUE.... what's the best choice here? Any old ceramic? Or perhaps a COG ceramic or a film cap of some description?

It was suggested to me that the 1uF input capacitor should be a silver mica. Problem is I can't find any mica that big. What would be the next best thing here? How about the 220pF cap next to it? Does it need to be anything special or would any old ceramic suffice?

Thanks.
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Old 6th August 2013, 09:17 AM   #2
dfy is offline dfy
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Pretty sure there are no 1uF silver mica caps. The 1uF is for AC coupling - the standard choice for that are polypropylene film caps, but you can go as expensive and exotic as you want, paper in oil, various kinds of film coated with gold or silver, etc etc. The actual results of exotic caps are hotly debated.

Most of the 100nFs are for power supply decoupling. They are also in parallel with larger electrolytics. I personally see no reason not to use standard X7R ceramics there, but I'm sure you will find different opinions.
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Old 6th August 2013, 09:27 AM   #3
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Thanks for your comments dfy - much appreciated.
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Old 6th August 2013, 09:48 AM   #4
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X7R is excellent choice for 100nF decoupling caps, but the problem is voltage rating. With the amp on the schematic, with more output transistor pairs, 50V may not be enough. For that reason alone film (MKT, polyester) caps could be required with 63V or 100V rating. There is no supply rail voltage on the schematic, but judging by the number of output pairs it is high!

1uF input cap should be high quality film, but type of this cap will depend on the space available on pcb and lead spacing (1uF MKT is much smaller than 1uF polypropilene).

220pF input cap should be polystyrene, polypropilene or multilayer ceramic
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Old 6th August 2013, 10:25 AM   #5
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This amp will be built with 70V rails.
No problems finding 100V rated X7R caps:

http://au.mouser.com/ProductDetail/K...c8%252bjtHA%3d
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Old 6th August 2013, 10:34 AM   #6
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Any recommendations for C5 and C17?
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Old 6th August 2013, 11:25 AM   #7
AndrewT is offline AndrewT  Scotland
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All the blue decoupling should be ceramic on very short leads and traces.
The green input cap probably polypropylene film or foil, but you can get away with MKT and MKS.
C05 & C07 should probably be deleted.
RF attenuating C04 probably polypropylene or polystyrene or C0G
Zobel cap C17, MKT or MKS
C10 silver mica or C0G

Delete D5
Reduce R28 and L1

This looks DX like. Many values and many component types seem badly chosen.
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Old 6th August 2013, 11:40 AM   #8
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Quote:
Originally Posted by Ranchu32 View Post
Any recommendations for C5 and C17?
C5, multilayer ceramic NP0, cheap, excellent, easy to find
C17 polyester MKT (or if footprint allows MKP, but I doubt if footprint will be that big)
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Old 6th August 2013, 11:52 AM   #9
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When I recapped my amp I used these --> KEMET SMR High Temperature Film Capacitors for the decoupling caps. It was a long time ago, and they didn't mention anything about audio back then. I chose them mainly because of good specs and excellent temperature stability. I got them from RS Australia, but I don't believe that they stock them any more.

C17 you want something beefy, something like this should fit the bill B32022A3473M - EPCOS - CAPACITOR PP FILM, 0.047UF, 300V | element14 Australia Others may have different opinions of course!

for the 220pf I'd use these --> http://au.element14.com/wima/fkp2d00...xed/dp/1005983

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Last edited by wintermute; 6th August 2013 at 11:56 AM. Reason: add 220pf
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Old 7th August 2013, 12:54 AM   #10
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ivanlukeit, AndrewT, wintermute, appreciate your comments. I've now got a good idea of what needs to go where...

AndrewT, you are correct; its a DX MkIII amplifier. Some of the values are incorrect; notably, the miller compensation cap, which should be 82pF and not 180pF. D5 will be deleted and R23A&B reduced from 68R to 22R.

I will have to think through the implications before removing C05 and C07 as you suggest.

I also wish to increase the gain by reducing R09 to 820R. In another thread you made the comment:
"But the ratio of C03:C06 has not been maintained when R09 was changed. Either C03 must be made smaller, or C06 must be made bigger, or a combination of both. This should be determined by the required input filter the user needs for his bass preference and then the NFB cap adjusted to a significantly WIDER passband than the input filter."

Are you able to give me any general advice or specific recommendations on that matter? I don't know how to determine what is an appropriate input filter. Presumably it is a value somewhere between DC and 20hz?
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