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John Curl's Blowtorch preamplifier part III
John Curl's Blowtorch preamplifier part III
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Old 11th March 2018, 08:59 PM   #1021
DPH is online now DPH  United States
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Well, oxides have good electrical impedance over wide bandwidth so it makes sense. Oxides also cover a huge huge huge swath of materials, so one needs to drill down a little more.

Many high-K mlcc caps are made from barium titanate derivatives, i.e. Barium titanate - Wikipedia

While you can get BaTiO3 out into the k >= 5000, many of our more linear oxide insulators, e.g. TiO2, HfO2, ZrO2, top out with k < 100. Al2O3 is around 10 if I remember correctly. SiO2 is a mere ~4. Hard to make a big capacitor with SiO2 cheaply.
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Old 11th March 2018, 09:00 PM   #1022
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Thanks PMA and George for the cap references.
I might point out that PMA's data sheet does not DIRECTLY infer non-linear distortion or DA (dielectric absorption) or linear distortion. X7R ceramic caps have both, big time. This is where many engineers go wrong. They try to get away with a ceramic cap that is the right size and price, that blocks any DC, BUT has distortion, both linear and non-linear when used as a coupling cap. Just look at Sony products! This is WHY many serious audio designers (not mid-fi) have designed out the vast majority of the coupling caps. It is just a more linear approach, and it is not that hard to do.
For example, the JC-3 Jr, which has up to 80 dB low frequency gain, gets away with a single simple servo per channel. It's cheaper than adding a quality cap to do it this way. Wake up guys, I am trying to give you some free advice, not sell you something.
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Last edited by john curl; 11th March 2018 at 09:16 PM.
 
Old 11th March 2018, 09:00 PM   #1023
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I have seen many shielded cables where one end was not connected to the ground (shell in the case of RS422/485/232/TIWAY). In some cases, it makes a difference which end is grounded.

This was done to prevent ground current in the shield.

I have also seen double Faraday shielding with two shields, one connected at each end. This can also make a difference depending on which way the cable is connected.

With audio, unless there is a ground fault with major potential differences in the ground points, I see no need for this and there should be no effect on switching the direction of the cable.
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Old 11th March 2018, 09:03 PM   #1024
scottjoplin is offline scottjoplin  Wales
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There's some interesting discussion/links here Cable shield as a Faraday cage
 
Old 11th March 2018, 09:09 PM   #1025
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John, the enormous nonlinearities in capacitance vs. (temp, voltage, frequency) etc do directly infer massive non-linear distortion.

Look at AVX's charts on C0G dielectrics
http://datasheets.avx.com/C0GNP0-Dielectric.pdf

Or this:
https://ec.kemet.com/mlcc-dielectric-differences

It's staring you right in the face. It's pretty patent that the use of a x5r/x7r as a bypass cap is an exercise in cost cutting or a failure to read.
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Old 11th March 2018, 09:24 PM   #1026
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NPO or COG caps are perhaps OK for SMALL VALUES, personally I prefer polystyrene or mica, but maybe that is because I am used to them. They just are NOT PRACTICAL in values above 0.1uf, AND they get expensive as well. That is why the parts distributors will steer you to X7R or equivalent caps. They are cheaper, smaller, and available in large values, BUT they distort BADLY! But you will not necessarily easily catch their problem with a simple sine wave measurement, and trust me, you can destroy the sound quality of a remarkable design with just using X7R caps in a serious audio stage.
I got fired once by the Grateful Dead, because I (along with my colleagues) made such an error, back in 1971. I didn't know why my design failed until 3 years afterward, when Tektronics showed me what X7R ceramic capacitors behave like. You don't want to make the same mistake as I did, it is just not worth wasting your time.
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Old 11th March 2018, 09:24 PM   #1027
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John Curl's Blowtorch preamplifier part III
Typically you want a shield to conduct capacitvely coupled noise currents to the lowest noise reference point. if the whole system is inside a continuous shield one connection at the transducer would be ideal. That does not happen in audio. Usually you would connect the shield at the preamp end so the noise currents are not shared with the return line from the transducer (Phono cartridge?). However every system may be different.
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Old 11th March 2018, 09:26 PM   #1028
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Quote:
Originally Posted by john curl View Post
Thanks PMA and George for the cap references.
I might point out that PMA's data sheet does not DIRECTLY infer non-linear distortion or DA (dielectric absorption) or linear distortion.
It does clearly show nonlinear distortion as capacitance change vs. AC voltage. It also shows capacitance change as a function of DC voltage, which again means nonlinearity. My point is that some engineers should learn to read the datasheets. Linear distortion is shown as capacitance change vs. frequency. So all your objections are wrong, IMO, except that of DA. DA was not shown in those examples.

Last edited by PMA; 11th March 2018 at 09:31 PM.
 
Old 11th March 2018, 09:56 PM   #1029
gpapag is offline gpapag  Greece
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John Curl's Blowtorch preamplifier part III
Quote:
Originally Posted by DPH View Post
While you can get BaTiO3 out into the k >= 5000
We should avoid it.
Affordable ultrasonic piezoelectric probes are made of BaTiO3

George
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Old 11th March 2018, 10:35 PM   #1030
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OK everybody, here is the Sony FM radio analog output using 2.2uF and 1uF caps. I would like to use COG (NPO) ceramics for this. Where do I find these caps? Who makes them? Come on PMA or DPH, help with the answer to my need!
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