DC voltage across a cap vs. Signal transfer - Page 2 - diyAudio
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Old 27th October 2012, 11:00 AM   #11
DF96 is offline DF96  England
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Frequency response won't help, as any deviation will be very small. Two ways to do it: Fourier analysis to look for harmonics or IM, or a bridge balance and look at the residual.
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Old 27th October 2012, 12:05 PM   #12
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Quote:
Originally Posted by yagoolar View Post
I was asked why I used cathode bias vs. Fixed bias. Well, started thinking. In cathode biased tube DC voltage across a coupling cap between a driver an a power tube is lower than in fixed bias (grid biased with negative DC voltage related to cathode). I wonder if there are any measurements of caps "behaviour" which give data to compare two topologies?
you can use a good amp meater inline wth caps to see how the cap is holing up and the amps being drawn through the circuit
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Old 27th October 2012, 12:48 PM   #13
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I have worked in RF for 33 years and never heard claims that dc across the sort of capacitor used in RF has any effect. You would not be using polarised types and HiZ ceramics as coupling in RF circuits
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Old 8th December 2012, 10:26 PM   #14
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http://www.niccomp.com/help/VoltageC...-032012-R1.pdf
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Old 8th December 2012, 10:35 PM   #15
dmills is offline dmills  United Kingdom
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Some of the class 1 dielectrics do however exhibit measurable intermod at surprisingly low imposed voltages, and that is before you even think about something awful like Y5V (But why anyone whould use that for anything that mattered is an interesting question).

I have even measured IMD in thick film resistors used in an RF attenuator (THAT caused much headscratching, especially as said attenuator was being used for IMD tests)!

For me the key thing with capacitors of the coupling sort is to make them large enough that they do not have any meaingful signal voltage across them, even electrolytics are fine for audio with a little standing bias and only a few mV of signal voltage across them, filters of course ask rather more of caps.

Regards, Dan.
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Old 9th December 2012, 07:54 AM   #16
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RF IMD is measuring levels that are way beyond anything a real speaker or cartridge can achieve.
Only filter circuits have large audio voltages appearing across the capacitor and nobody would use X7R or worse ceramics for that. Sometimes electrolytics end up being used in filters as plastic types are not sensibly available in the value. Amplifier coupling does not fall into this category as the value should be large enough to have negligible ac voltage in the audio range
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Old 9th December 2012, 09:40 AM   #17
dmills is offline dmills  United Kingdom
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Yep, the RF guys deal with dynamic ranges and levels of selectivity that make us hyper sensitive to things like IMD, because when designing a high performance rx it **really** matters (IMD3DR is possibly **THE** figure of merit).

For audio linearity is almost always dominated by the transducers in play (condenser microphones, even high end ones, are interesting to measure IMD on, particularly at high frequency, the reputation the things have for a richer sounding mid range suddenly makes a lot of sense!).

I am in agreement that for any coupling cap the trick is simply to make it so large that no meaningful signal voltage that matters appears across it (And if electrolytic to ensure a little bias voltage is present).

There are now C0G ceramic caps in values interesting for audio filter networks operating at sane impedances which is nice as they are available is reasonable SMT packages.

Regards, Dan.
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Old 9th December 2012, 02:10 PM   #18
DF96 is offline DF96  England
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I'm not sure what was the point of the link in post 14. It is well known that high-k ceramics are very non-linear dielectrics, which is why they are only used where this does not matter. The writer of that piece from the cap manufacturer seemed to be labouring something obvious; perhaps he was a junior engineer who had only just realised it himself? High-k ceramics are never used for coupling in serious audio systems.
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Old 9th December 2012, 02:36 PM   #19
jcx is offline jcx  United States
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even dissing "Ceramic" is too broad - NP0/C0G are as linear, low loss, low DA as the "better" film for audio
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Old 9th December 2012, 02:45 PM   #20
DF96 is offline DF96  England
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I was only dissing high-k ceramics, and even they have legitimate uses such as RF decoupling. Low-k ceramics can be very linear, as you say, but are sometimes ignored due to 'guilt by association'.
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