C0G/NP0 Ceramics in Active Filter Circuits

[speakerguy79]

After reading Cyril Bateman's published articles from Electronics World, I am wondering if C0G type ceramics would be superior to film types in active crossovers and filter circuits. While they did have a hint of 3rd order distortion just barely measurable at ~-130dB in his test jig, they performed better than some Mylar types and as Cyril noted had much better reliability than any of the film types, as he found in the several thousand caps he tested that many axial and/or radial leaded parts would occasionally have problems with non-ohmic connections to the lead wires (obviously not a problem with surface mount caps).

After reading his work, I'm at a loss for any reason not to use them. We all know X7R and other type II dielectrics are not at all acceptable for active filter circuits, but is NP0/C0G a viable alternative?

[john curl]

Mylar is not good for audio filters.

[speakerguy79]

Cyril's testing did show much higher distortion for PET (harmonic and intermodulation) vs. PP, Film/Foil, and NP0/COG ceramics, so yes I agree they are not good. But he noted the very occasional problem of non-ohmic connections on leaded parts on all type of film capacitors, which lead to primarily odd-order distortion products. This is why I am wondering if, in a production environment or for the hobbyist who cannot test these things, if C0G is the better choice since it only measured minorly worse than the best film/foil he tested (and produced mainly even order distortion). Cyril even went so far as to say that for any value 10nF or less that chip ceramic NP0/C0G was his preferred choice. I should note his tests of X7R type ceramics show them to be among the worst next to tantalum, so I am in no way discussing anything other than type I dielectric ceramics.

[john curl]

My tests published 30 years ago showed that X7R ceramics were awful, as well as tantalum. Unfortunately, Cyril did not footnote either the 'IEEE' or 'Audio' paper on this. Nonlinear distortion is just one factor, DA is just as important.

[SY]

Did you check any NP0? I've always wondered about that myself.

In my own active filters, I just use PP/foil and Teflon. And why not? They're reasonably cheap and reliably good.

[john curl]

NPO is well known, but not practical. Just look at the size and price. Polypropylene is better, and polystyrene and teflon, even better.

[anatech]

Completely agree with that. I guess that if you have them, then try them out. It's not like you can break anything by trying.

I have been considering a test for these NPO/COG types. I see a few of the better old amplifiers that did use these types and sounded pretty good.

As far as reliability is concerned, a few ceramic types have shorted or become noisy due to metalization going around the ceramic disc a bit. Imagine my surprise after tracing the faults down to a part I didn't think was prone to failure.

-Chris

[jcx]

when building in surface mount technology np0/c0g can be acceptable, but I have measured more 2nd order IMD than Bateman's results suggest - it could be manufacturer and test condition related

I don't know of PPS film cap measurements to this level - the other smt film technology (besides "mylar" relative PEN)

Originally posted by jcx
lynx soundcards can do this directly, add some cleverness from jerald graeme's "optimizing op amp performance" book and with a dedicated low noise +30 dB amp in front of the lynx (and a lot of averageing to get below the noise floor) i can see to -160 dB - as evidence that i have really measured distortion, i found that 2 series 200V np0 caps gave ~ -110 dB distortion with 8Vrms while relpacing them with polystyrene dropped the harmonic to the measurement noise floor (caps in Av +4 sallen-key 30KHz low pass, 1 Kohm Rs in a compound/buffered op amp filter circuit) - the fundmental is also visible at the correct level calculated from loop gain

[lineup]

NP0 ceramic is sometimes a good choice.
Especially when good film caps with wanted values are hard to find.

Besides the electrical qualities,
we often want 1% or max 2% tolerance caps for active filters.

Good axial polyproylene can be found with 1% from 47pF to 47nF.
For example these Vishay BC Components type 460 to 464