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Old 11th November 2011, 07:12 PM   #1
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Default Ceramic Cap in Line Level Input RFI Filter?

I am designing a opamp input network and I'd like to include a RFI/EMI Filter, but unfortunately the only caps I have down in the pF range are ceramic discs.

I know Jung and others warned against using ceramics in audio but would it be okay if I were to use them as RC Lowpass shunt at about 4x the highest frequency of interest?

I have calculated a worst case source impedance of about 4k, resulting in Fc=85khz with a 470pF cap, allowing the commonly used rotation on the input pot resulting Fc in the range between 125khz - 300khz.

The filter seems a good margin above and audio band, and I am not expecting too much garbage up there so voltage induced distortion in ceramic shouldn't be that bad, or is ceramic in the signal path more like asking for trouble?

I would really prefer not having to go hunt down tiny film caps or resort to cascading them.
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Old 11th November 2011, 08:15 PM   #2
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The curve in the ceramic cap capacitance vs voltage has a slope related to the voltage of the cap. So I think if you use 200v or 1000v ceramic caps, which are pretty common, you won't see much capacitance variation over the 1.6v signal you might get on an audio input signal.
I've violated this Iron clad rule and am not going back. I used 50V 5uf ceramic Z5U caps as input caps to the first transistor in my power amp. I like the resulting sound a lot better than 5 uf tantalum caps I took out, which were replacement tantalums I bought at a distributors "front table" decades before E-bay. I think stuff on the front table was probably rejects or stale stuff- these had a frying pan noise I always blamed on the transistors not being original "selected" BC108's. The ceramics got rid of that. The amp sounds now just like a CS800s amp which has no capacitors in or out on my speakers at 1V out, . I think <1.6 signal on a 50V cap may have covered up my "sin".
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Last edited by indianajo; 11th November 2011 at 08:21 PM.
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Old 12th November 2011, 10:44 AM   #3
marce is offline marce  United Kingdom
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COG MLCC are the best for RF decoupling, small size with minimal parasitic inductance is what you want.
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Old 12th November 2011, 11:21 AM   #4
AndrewT is offline AndrewT  Scotland
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4k worst case source impedance from the attenuator does not sound good.
Have you considered a buffer after the attenuator?

Variable RF filtering due to volume knob rotation changes does not sound good if it extends down into the audio spectrum. At the other extreme, very little attenuation of RF when source impedance drops to near zero.

These are far worse concerns than what type of capacitor to use.

Add a C0G/NP0 22pF to 68pF at the input socket to convert the interconnect L into an LC RF filter.
Add a plastic film 220pF to 1000pF at the PCB input to create an RC RF filter.
MKS/T or MKP, or Teflon, or Polystyrene. Cheapest is MKT/S
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Old 12th November 2011, 04:33 PM   #5
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Quote:
Originally Posted by AndrewT View Post
4k worst case source impedance from the attenuator does not sound good.
Have you considered a buffer after the attenuator?
4k is being quite pessimistic, that's worst case 600 Ohm source impedance + 2.5k worst case from a 10k pot + series 1k resistor. I might actually scale 1k down to about 100 Ohms.

Most modern day sources have an output impedance of around 100 ohms I believe, that puts the filter Fc at about 125khz.

Perhaps I was being too aggressive designing the filter?
In the mean time I may look into the buffer, I am thinking maybe a simple opamp stage or simple 2n3904 emitter follower.
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Old 12th November 2011, 09:36 PM   #6
AndrewT is offline AndrewT  Scotland
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Old 13th November 2011, 09:29 AM   #7
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COG ceramics for values up to 330pF are the best part for RF filtering and are well behaved with very low temperature and voltage coefficients, no problem at all for audio.

If you use a buffer, be sure to use a series output resistor of 50~100R. Emitter followers and many opamps turn into RF oscillators if asked to drive a capacitive load without this.
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