John Curl's Blowtorch preamplifier part II

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You may underestimate the sensitivity of the human condition .
Actually, over the years, I've been accused of overestimating the sensitivity of the human condition.

When an effect is orders of magnitude below "the human condition", it is of no concern. When it is at or near the level of discernment, it cannot be discounted.

Banging a cap with a ball peen is inconsistent with actual use.

The vibrations-capacitors subject is to be looked from two points -
1) external force induced vibrations of capacitor, i.e. EMF caused by external vibrations
2) electro-mechanical effects in capacitor's interior, i.e. distortions induced by internal vibrations of capacitor components due to signal.
3. modulation caused by the movement of charged plates relative to each other.

jn
 
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Of course, I was not thinking of a 0.1uF cap as a COUPLING CAP, unless you can SCALE and in your mind, move the drive signal to 10Hz and the load to 100KHz, and realize that the distortion should be similar. However for a FILTER like RIAA, or a bass rolloff filter, a cap like what I measured, could easily be used, inadvertenly, because their 'form factor' is so ideal. This was just a gentle reminder.
PMA thanks for the 0.1 Mylar measurement. We already use them for less critical applications.
 
I was waiting for the SCALE suggestion, but then I use 1uF with 100k or 2.2uF with 47k ;)

For RIAA, we may use something like WIMA polypropylene, with no added distortion.

However, I have a question prepared - why the cap distortion is that much considered, though tube distortion, similar or MUCH higher than average cap distortion, is considered OK and harmless? :)
 
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Same measurement as (2), for a 'stone age' (1975 plus minus) ceramic capacitor. We can see huge distortion, into 1k.

In this measurement, (marce), the ceramic cap dielectric was made from 'supermit' compound. This was probably the worst thing use for ceramic caps ever. I am sorry, it has no code like X7R etc, and the properties were that bad that the manufacturer did not declare them in the datasheets.
 
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Banging a cap with a ball peen is inconsistent with actual use.

jn

Why not drop a phono stylus down on the cap to measure vibration? just making/breaking mechanical contact should separate electrical from mechanical pickup.

Remember the force on the plates of an unbiased capacitor is always attractive so the vibration is all 2nds. Several easy to perform experiments here.
 
I was waiting for the SCALE suggestion, but then I use 1uF with 100k or 2.2uF with 47k ;)

For RIAA, we may use something like WIMA polypropylene, with no added distortion.

However, I have a question prepared - why the cap distortion is that much considered, though tube distortion, similar or MUCH higher than average cap distortion, is considered OK and harmless? :)

My question is: Can either of you perform the test with the capacitor biased?

Perhaps two in series, with a 1 or 10 meg resistor used to charge the pair's center tap?

jn
 
Why not drop a phono stylus down on the cap to measure vibration? just making/breaking mechanical contact should separate electrical from mechanical pickup.

Remember the force on the plates of an unbiased capacitor is always attractive so the vibration is all 2nds. Several easy to perform experiments here.

I'd also worry about magfield pickup, so it needs to be inside a mu metal box.

jn
 
Personally, I solved the problem of cap distortion, decades ago. I just want to remind others of the differences in cap in this regard. OF COURSE, I am showing 'reasonable worst case' as that is important to design to.
I am not the 'circuit police' but if I was, I bet I could cite a number of amateurs, (and professionals) for using a high distortion cap in a critical position in an audio product. It is sometimes very difficult to judge a cap's distortion characteristics from the case quality. Personally, I had no idea what I would measure with this ceramic 0.1uF cap. I just opened a package of 100 'something' that I got surplus, and this is what I measured. I have thousands of caps like these, now what do I do with them?
 
My question John is why make the jump from a polystyrene cap to a Teflon cap and not use a polypropylene cap if it has virtually the same distortion graph between the two caps? It seems that it has been shown many times that the Teflon is actually the inferior cap due to its tendency to mechanical vibration distortion, so why use those caps except for the obvious marketing factor of the word Teflon?
 
My question is: Can either of you perform the test with the capacitor biased?

Perhaps two in series, with a 1 or 10 meg resistor used to charge the pair's center tap?

jn

Just tried mylar, two in series, center tap, biased with 10M resistor, 15Vdc. Biased/unbiased - no difference in distortion. I would not bother with ceramics, though it might show something.
 
My question John is why make the jump from a polystyrene cap to a Teflon cap and not use a polypropylene cap if it has virtually the same distortion graph between the two caps? It seems that it has been shown many times that the Teflon is actually the inferior cap due to its tendency to mechanical vibration distortion, so why use those caps except for the obvious marketing factor of the word Teflon?

It's playing the numbers game. Teflon has a lower dielectric constant than polypropylene.

se
 
Teflon CAN be a better cap than Polystyrene, I would presume. However, I am going to stick to Polystyrene in most of my designs. About 15 years ago, we had Rel make us a run of Teflon caps, and that is what is used in the RIAA of my own personal preamp. We also offered a modification to Teflon for some Vendetta customers, until the caps ran out. I have not reordered, however.
However, while I cannot generally fault Polypropylene, I just don't need its 'advantages' because I prefer film-foil, rather than metalized film caps. I have a 'winner' cap in any case, why change, unless I want something much cheaper and smaller? That was my ORIGINAL question here: What is a pretty good Polyropylene cap that could be used instead of the Rel RT's that take so much room?
 
Pavel,
I have looked into the electrical data sheets on the different polymer capacitors in the past and have seen that the polystyrene seems to be one of the best materials for capacitor applications. At the same time it appears that for some manufacturing reasons, perhaps just cost that there is a limit to the values that these styrene caps are made, the larger caps that I needed for passive xo's were typically not available and therefor I used polypro caps in those instances. Does anyone have an answer why we don't see large values of polystyrene caps, is it only a cost/value situation that leaves polypro as the alternative?
 
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