John Curl's Blowtorch preamplifier part II

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#92,241
Not to confuse the issue, but Demian, do you really think of a COG cap as a 'high q ceramic cap' ? I don't, and that is why I qualified my serious criticism of ceramics. Oh, of course, you have plenty of 2.2uf COG or NPO ceramics in miniature size. '-)
By the way, how did those COG EQ caps work out for RIAA? I think that I will stick to polystyrene, thank you.
 
#92,247
john curl said:
Not to confuse the issue, but Demian, do you really think of a COG cap as a 'high q ceramic cap' ? I don't, and that is why I qualified my serious criticism of ceramics. Oh, of course, you have plenty of 2.2uf COG or NPO ceramics in miniature size. '-)
By the way, how did those COG EQ caps work out for RIAA? I think that I will stick to polystyrene, thank you.
Click to expand...

C0G and NP0, not COG and NP0.
 
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#92,249
We are identifying...Cee-Zero-Gee and En-Pee-Oh, no it's En-Pee-Zero. 🙂

Okay, would one of you gentlemen be kind enough to teach me the
proper method for hooking up a band pass filter from a bread board.

No problems with the high pass itself (that I am aware of.
No problems with the low pass itself that I am aware of.

But, when I hook them in up series to make a bandpass,
the dad gum thing has no bode curve, its just flat all the way through.

I'm so embarrassed to admit it's got me stumped.

Chit,
 
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#92,252
When it comes to ceramic caps, most of you tend to use high Q (multiplying factor) caps that have a lot of capacitance in a relatively small package. That is WHY you would pick a ceramic cap over a Mylar, etc most of the time. NPO caps can be pretty good, but values over 0.1uf are not generally available, so they make lousy coupling caps, no matter what. Hi Q caps are made of materials that are generally non-linear so you could get more distortion from the cap than the rest of the circuit, if you get unlucky. That is why we usually avoid high Q ceramic caps from serious audio circuits. It is not just non-linear distortion, but also dielectric absorption and vibration sensitivity as well that high Q ceramics have problems with, that compounds the problems with these caps.
 
#92,254
First question is active or passive filters? If passive the input and output impedances matter. Generally a zero ohm source and an infinite ohm load. So two passive filters in series won't work.

Now most active filters also expect a zero ohm source.

That usually means an opamp buffer to driver the filter and for a passive filter one between stages and one after.
 
#92,255
john curl said:
When it comes to ceramic caps, most of you tend to use high Q (multiplying factor) caps that have a lot of capacitance in a relatively small package. That is WHY you would pick a ceramic cap over a Mylar, etc most of the time. NPO caps can be pretty good, but values over 0.1uf are not generally available, so they make lousy coupling caps, no matter what. Hi Q caps are made of materials that are generally non-linear so you could get more distortion from the cap than the rest of the circuit, if you get unlucky. That is why we usually avoid high Q ceramic caps from serious audio circuits. It is not just non-linear distortion, but also dielectric absorption and vibration sensitivity as well that high Q ceramics have problems with, that compounds the problems with these caps.
Click to expand...

John, how good are NP0 caps for use in amp compensation and feedback? Do you have any experience with piezoelectric behavior of the NP0(C0G) caps?
Damir
 
#92,256
John,

As usual everyone else knows way more than you! 😉

C0G capacitors are made from a composite dielectric that combines both positive and negative temperature coefficient ceramic oxides to get a low temperature coefficient dielectric.

Static temperature (meaning slow changes compared to one over audio frequencies.) variations nicely cancel. However that does not guarantee that the thermal tracking is tight enough for in particular low audio frequencies. (High frequencies do not have as much effect due to thermal mass.)

The correct test is to match 10 units for value and use the 4-1-1-4 bridge technique. This has shown many "distortonless" capacitors do distort with an interesting frequency response curve.

But I have no issue with letting others use ceramic capacitors particularly as a feedback resistor bypass (compensation ) where there can be not only large voltage swings but also typically higher temperatures.

But then I can be a nasty S.O.B. As the comic books used to call it. (Confused me as a kid since I thought that sob was just a word not an abbreviation.)
 
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