| Conrad Hoffman |
As Rod Serling used to say on The Twilight Zone, "Offered for your consideration"... are two capacitors. One, a 3 uF polypropylene, with 0.02% dielectric absorption, and a dissipation factor of about 0.0013. The other, a 3 uF polyester (Mylar), with 0.17% dielectric absorption, eight times worse, and a dissipation factor of 0.019, more than ten times worse.
To make the values exactly equal, and this is very important, a small polyester was paralleled with the large polyester. The above measurements included this.
One end of each cap was connected to a GR 1390B noise source, and the other end of each cap terminated to ground with a 1% metal film 20K resistor. The common lead of the caps was driven with 2 VRMS of random noise covering the entire audio band.
A high gain differential amplifier (Tek 1A7A) compared the signals across each 20K resistor. If there was any difference at all between the signals passed by these two caps, it would show up. The results? The residual common mode of the amplifier with both probes connected to the same cap was about 100uVRMS, plus a trace of 60hz pickup.
Measuring between the caps gave the same 100uVRMS signal, plus the same trace of 60hz pickup.
That means that any difference between the two caps due to DA and DF was less than 0.005%. Considering the CMRR of the amplifier, a lot less. It also means that any difference from unknown mechanisms is also less than that, because the test doesn't care where the difference comes from. IMO, this test is pretty representative of how coupling caps are used.
Now, maybe there's some condition of non-linear source or load impedance where a larger difference would show up (I gotta give people something to hold on to), but logic says if the signals are the same, the sound has to be the same. IMO, if less than 0.005% doesn't qualify as "the same", and you can hear it, caps are the least of your worries.
If you want to see larger differences, just mismatch the cap values a bit. The low frequency difference will skyrocket simply because the RC constant is slightly different. Very small mismatches shouldn't affect the sound, but it's a variable that has to be considered when caps appear to sound different. Another way to see larger differences is to test against an electrolytic or a ceramic!
Ok, did I listen to them? Not yet- too much junk on the test bench from running the test. But I will. The difference is that now I don't expect to hear any difference, unlike the frame of mind I'd be in if I had great expectations due to the DA and DF differences.
No doubt some will consider this test bogus and useless- my flameproof underwear is on, so give me your logic as to why. :devilr: |
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| panomaniac |
Cool test. I like the idea of the differential signal measurement. I proposed doing a similar test on speaker cabales in another thread.
If there are differences, your test should show them. And a random noise would seem a good source. How about trying it with music? That's what it would be used for anyhow, right? A musical signal would show differences too, if there are any.
The only "flaw" in the test I can think of is that since the caps were tied together on one end, any distortion would be transmitted from one cap to the other and also reflected back to the source. Possible? I don't know - maybe not, but it's worth thinking about. |
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| Hartono |
| try testing into very low impedance load, near the voltage rating of the capacitor at the same time. |
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| Hornlover |
| This illustrates my experiences over the years. I have been involved in a lot of blind listening tests involving caps, and what we found in every case was that polyester caps perform well for most audio applications. I know that there will be many that will flame me because of this, but this is what we have found. We assembled a group of experienced listeners, using a variety of systems (at different times, usually), and the results have always been the same. Most listeners can hear the difference between an electrolytic and a film cap, but once film caps are compared, no reliable identification could be made. We compared generic polyester and ploypropylene caps to the very expensive 'audiophile' caps, and no consistant identification could be made. For this reason, I feel that if you are using (even generic) film caps in your system, dont waste money on expensive varieties unless the rest of your system is already optimised. Put your money where it will really pay off, such as bi-amping. |
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| Hartono |
Hi Hornlover,
I wouldn't recall my experience to be the same (was selling "audiophile" caps for 3 years), caps do differ in their construction, and while I agree that trying to do A/B test to identify which one is which maybe a little hard,especially at different listening sessions on differing system.
But there is identifiable audible difference, switching capacitor into circuit shows this more clearly.
Many generic cap do perform well, but the more sophisticated construction/material caps also have their use in more critical application. (more expensive is not necessarily better constructed/material ) some generic one might do well, but the manufacturer wouldn't guarantee that, they can change the dielectric to same material but lower quality ones without warning. We can not expect the metalization for 5 USD caps and 50 Cents one to be of same quality.
The problem with the so-called "audiophile" caps is that the manufacturer who really make the good ones do not want to reveal what they did to improve the caps, and do not break in ( burn in is better term here ) the caps before shipping them, even worse is that they do not tell people how to use them and in what circuit.
So the user have no idea which one did which things better, many only to be confused after their expensive purchase of "some" famous brand that in fact no better than generic ones.
Yes, it would be good to spend money on Bi-amping , but for people who already have Bi-amp-ed system, or want to improve other part of their system would benefit from better quality caps.
Speaking of Bi-amping for example, using metalized polyester inside active crossover is a very bad idea. generic one tend to develop short/noisy that can't "self-heal" in low power/voltage circuit. film and foil type is best in this case.
Now I invite people to see the differences in caps, instead of listening :D , try changing your DVD player video coupling caps. Make sure the caps have been properly broken in, or it might lead to sub-optimal comparison. Yes the size of the caps in this application is big, yes it is expensive, yes the result is different.
If anybody have physically tiny metalized polyester/polypropylene in their amp zobel network, do yourself a favor and change them to more appropriate ones, this one is critical, and also that tiny resistor.
I'm not defending "audiophile" caps, I am defending "good quality caps", I only say that cap have differences, some is better quality and constructed than others, and must be used on the correct application. Vishay make caps and that's not "audiophile" brand, unfortunately that doesn't make their caps cheap :rolleyes:
Now I wouldn't comment on resistor differences :headshot: better ask knowledgeable RF and digital people about this :cheers: |
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| john curl |
| Conrad, in all fairness, you don't know how to measure caps. We have already done, and you can too, IF you follow directions. |
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| FastEddy |
john curl: " ... We have already done, and you can too, IF you follow directions. ..."
... got links to your comparisons? Appreciate it ... :smash: |
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| Conrad Hoffman |
| Yeah John, how 'bout expanding on that remarkably enlightening post. You might be surprised what I know about caps, or I might be surprised at how little, but unless you offer some direction, we'll never find out :cool: |
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| john curl |
| Go to Walt Jung's website. Find CLASSIC articles, Find article about caps written by Walt and me (John Curl) in 1985 or so. Read article. |
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| Conrad Hoffman |
Well John, I'm a bit baffled. I read that article when it was first published, and have read it again. It's basically the test I'm doing. The only significant difference is that I haven't varied the impedance, though I did use various sources. There's a rule of testing that says "test it the way you use it", which is why I chose the values I did. I can certainly force a difference to be seen, but I could just as well do that by using a standard (traditional) 4-arm bridge, and looking at the residual on a scope instead of a tuned null detector once the bridge is supposedly "balanced". They never are, you know. I don't think there's any reasonable disagreement as to what's going on, just with my opinion that the differences are below any possible audible threshold if they're buried in the noise floor. Now, if the caps were used at a low impedance level, say a crossover network, the results might be different. I didn't test that, and have no opinion on the matter (though I avoid electrolytics in crossovers if at all feasible).
Regards,
Conrad |
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| john curl |
Conrad, your resolution is not good enough. You MUST be sure of your common mode rejection with frequency, AND you must VERY CLOSELY match the RC time constants. This is why we use an instrumentation IC op amp with 120dB common mode rejection.
If you want to see nothing at all, why bother to measure in the first place?
For the record, mylar vs polypropylene is easy. polystyrene vs polypropylene is tough. |
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| Conrad Hoffman |
John, I can force conditions where I can see a difference, but IMO, it's a red herring. (The 1A7A is a matched diff amp of excellent specs BTW, especially if you take care to adjust it properly) Both our methods of testing are fundamentally flawed anyway. D=omegaRsCs, or Rs/Xs if you prefer. We know from measurements that capacitors tend towards a semi-constant D over a wide frequency range. Not exact, but it doesn't vary all that much. Thus, Rs *must* vary with frequency. That means that these circuits can only be balanced for one frequency, and will always show a difference for caps of differing D and a non-sinusoidal waveform.
Anything will look "bad" compared to Teflon, polystyrene, or polypropylene, but no 'X-factor" has been revealed, only the difference caused by conventional capacitor parameters. And, sure, with enough sensitivity you can even see the difference between those three. Non-linearities in things like electrolytics will show up, but they also do when you look at the residual in any 4-arm bridge method. Not sure about DA, but I suspect it's not much of a factor with reasonable frequency AC signals- little soakage occurs per cycle. Thus, if I want to get down to insanely low values of D, it's easier to just use a bridge designed for the purpose, remembering that the reference cap in most bridges will be a very low DF mica, with high DA, yet it has no real influence on the results.
Still, an interesting test would be to find two capacitors of exactly the same value, and exactly the same DF, but of different dielectric types. Measure those against each other in your circuit, and there should be little if any difference. IMO again, whatever difference does show up will be below audibility for any decent film caps, and I'm not aware of any properly conducted listening tests that suggest otherwise (exact matched levels, blind, etc blah blah). I'm not saying there is zero measurable difference on lab equipment, just zero difference in practical application.
Going a bit OT, I once breadboarded a circuit for a research application where capacitor defects were an issue. It involved using two identical caps, one in the circuit of interest, and one in a bridge configuration that matched the operating conditions of the circuit. The bridge was nulled, and the residual amplified. The amplified residual was then injected into the circuit of interest, cancelling out the flaws of the capacitor. It was too sensitive to drift in the balance to be practical as anything other than a lab curiosity, but the concept is interesting. |
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| serengetiplains |
Conrad, find Cyril Bateman's series on capacitors. That will give you an additional (and alternative) set of measurements that, should you be so ambitious, could give you a reliable rule of thumb regarding capacitor sonics.
Read: mylar caps, among others, suck. |
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| Hartono |
jcx :
"Bob Pease reported some sucess compensating DA in S/H caps"
yes, unfortunately we can not use that method for signal coupling caps.
:dead:
Cheers
Hartono. |
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| serengetiplains |
| second that |
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| rdf |
Hi Conrad. A question about process, was the spectrum of the differential examined? Your note on the 60 Hz component implies 'yes' but it wasn't explicitly stated. If so what was revealed? With the help of solid state CCS loads tube amplification stages with a 2nd harmonic 65 dB down, 3rd well below that and nothing above are possible. If the spectral ‘difference’ between caps, assuming the worst, is a tight band of harmonics at -85 dB off the right hand side of the frequency scale (as I've measured of one poor sounding amp) it’s still arguable the coupling cap defines the sound of the circuit. I’m willing to bet a cluster of -85 dB harmonics from 5th to 15th far outweighs the sonic thumbprint of a -65 dB 2nd. The Tek 1A7A might not be sensitive enough to reveal all meaningful differences.
A suggestion for expansion, a 2 volt excitation signal doesn't come close to mimicking the full range a coupling capacitor sees in normal tube circuit duty. Another forum member describes driving capacitors in series with an eight ohm power resistor directly from the output of a power amp and typically hearing the bodies ‘sing’. This might be one non-linear effect a 2 volt noise test doesn’t capture. DC bias might reveal (or hide) another.
A final suggestion, consider high order low-pass filtering the excitation signal and examining the residual differences in the stop band. This might provide greater insight into what's happening. BTW, it's good to finally see a test in which one end of the cap isn't tied to ground, shunting the signal the test is intended to capture. |
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| Conrad Hoffman |
| Rdf- good stuff. I didn't do SA, and that would almost have to reveal more. I'm building up my test arsenal with another computer, so I should be able to do decent SA via an external sound card soon. The diff amp has an output, so I should be able to start with that. My assumption, not actually tested, is that once anything is down in the noise floor of my (listening) system, it just doesn't matter. OTOH, I'm also working at reducing both the distortion and noise floor of my system, so the bar does get raised. You mentioned our thermionic friends, which have their own requirements, but I'm pretty much concerned with solid state inputs in the 10-20kohm region. I probably won't mess with tubes again unless I fall into some deal on a prebuilt amp. I used to have modified Dyna Mark IIIs, but sold them many years ago for what seems like a song today. :( |
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| FastEddy |
| Looks like a job for a wiki author to tackle ... entries to the various links and articles that rate / evaluate / quantifing caps by material v. reactance, reluctance, inductance, capacitance, etc ... No? :apathic: |
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| jcx |
| quote: | Originally posted by Hartono
jcx :
"Bob Pease reported some sucess compensating DA in S/H caps"
yes, unfortunately we can not use that method for signal coupling caps.
:dead:
Cheers
Hartono. |
What exactly do you think needs Correction in Linear audio coupling applications?
Bob Pease’ DA correction is based on his linear approximation of DA as applied to highly nonlinear S/H circuits where the effect is especially clear with multiplexed input signals
Using Bob's same 6 RC branch linear approximation model of the DA of a mylar cap it is easy to show that the frequency response variation from that of an ideal cap ranges from >1 mdB at 20 Hz to <1 udB at 3 KHz with Bob's 1 uF Mylar cap model and 100 KOhms of input impedance that would be common in a audio coupling applications, with ss amplifiers at lower impedance the deviation from the ideal C response is still 10X less than the DBT threshold of 0.1 dB frequency response variation at the -3dB RC response point, much less in the pass band
unless you have some new data showing ppM frequency response variations are audible, I have to conclude that the linear model of DA does not explain why different film dielectric capacitors could be audibly different in active filters or coupling applications at common impedances
| quote: | Originally posted by rdf
... BTW, it's good to finally see a test in which one end of the cap isn't tied to ground, shunting the signal the test is intended to capture. |
I'm also curious about why people who do have decent equipment want to handicap it while trying to make sensitive measurements; "ground" is a human applied label, not a fundamental E/M concept, the label can be “moved” without affecting circuit operation
Moving the "ground" from one side of the signal generator to the other in the RC coupling circuit test usefully removes the voltage source from the measurement, reducing the CMRR requirement and allowing a much more sensitive measurement within the limitations of common test equipment - when we are interested in differences it makes sense to arrange the measurement to measure the difference - not the common part |
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| Hartono |
Hi Jcx,
"What exactly do you think needs Correction in Linear audio coupling applications?"
Maybe you misunderstood my sentence, my point is: it would be nice to be able to remove capacitor(any component/circuit in fact) distortion/nonlinearity , perhaps compensating it somehow, but too bad that for caps in coupling application, there's not much we can do about it. and it's too bad that while Bob can reduce capacitor error in his application, we can not do much other than changing capacitor type.
"unless you have some new data showing ppM frequency response variations are audible,"
I never mentioned that miniscule frequency response is audible in this thread. Anything can sound different and not because of frequency response ;)
Best regards,
Hartono |
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| FastEddy |
... we should start with the glossary: http://www.diyaudio.com/wiki/index....=Audio+Glossary ... no entries for capacitors at all or the associated terminology ...
A simple "door opening" entry would work ... or a reference to the hydraulic model related to a capacitor ... :confused: |
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| rdf |
I don't think anyone suggested potential sonic differences between caps are due to frequency response aberrations. Too easy to measure. :)
Re: the term 'ground' was just useful shorthand to describe the common practice of putting a very low AC impedance - the capacitor - across the error voltage being measured. Every test I recall seeing placed a resistor in series with the cap, tied the other cap end to ground and drove the resistor end with the generator, measuring mid-point. Potential spuria generated by the cap are shorted by the cap. A good representative test for some applications like PS filtering but of limited value for coupling. |
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| FastEddy |
" ... Every test I recall seeing placed a resistor in series with the cap, tied the other cap end to ground and drove the resistor end with the generator, measuring mid-point. Potential spuria generated by the cap are shorted by the cap. ..."
... because that's where the electrons are? ... at the ground connection (usually at a more negative voltage).
I would bet there are differences in reactance of a capacitor when the resistor is in series with the cap through the ground connection, thus offering a "longer", more resistive pathway for electron flow ... No? :confused: |
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| rdf |
| The tests I referred to were similar to your setup #2 and measured across the cap. I was thinking more in term of the below, measured across the resistor. Any capacitor aberrations which manifest themselves as a current will be much easier to measure across a load resistor than across the cap due to the latter's low impedance. Conrad's test in effect was measuring voltage differences to ground caused by cap current flowing through the load resistors. |
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| Conrad Hoffman |
| And of course it's a very interesting question what happens when you reduce those resistors and increase the current level. I still question the value of the test, because of what I mentioned above. With increasing current, D becomes a factor, and the circuit is no longer balanced. I can add a resistor to balance it, but only at one frequency. Yet, I was under the impression that D had been sort of ruled out as the "x-factor" for why caps supposedly sound different. Still, it means ESR varies with frequency, and that doesn't seem completely benign. Some say it's dielectric absorption, but others say not. That's actually easier to test (I think) by building up a physical version of the DA model, and making DA really horrible, then seeing if it sounds different. Not enough hours in a day- right now I'm rebuilding an amp and working on my oscillation overthruster, er, sniffer. |
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| FastEddy |
rdf: " ... I was thinking more in terms of the [attached diagram above], measured across the resistor. Any capacitor aberrations which manifest themselves as a current will be much easier to measure across a load resistor than across the cap due to the latter's low impedance. ..."
mmm .. looks suspiciously like many amplifier inputs = the resistor representing the "load" or input impedence of the amp and the cap blocking any DC offset.
Typical resistor/load values might be from 10K to 250K Ohms. (Wirewound = no? Metal film = ? Other type = ? ...)
The capacitor of course must be good enough to pass through any and all audio information in with relatively flat response ( ~20 to ~20K Htz ? ... 5 to 50K Htz? ... 5 to 200K Htz.?). Hopefully the cap will be good enough to not interfer, distort or otherwise change the audio by its presence. (It is axiomatic for "golden ear" designs to eliminate these caps entirely from the signal path, where possible.)
Thus the whole point of this discussion and thread is to discover the optimum capacitor type for audio application. ... And possibly to include the latest "wise old men" theories and experience regarding selection of capacitors and associated components into a Wiki think piece ... this being, in my opinion, a critical piece of amplifier design and construction knowledge.
Consider, as many of you already know, the use of decent passive components at the imput can make a world of difference at the output ... an extra US$0.50 spent at the input can easily be worth more than US$5.00 spent on corrections at the output. ... :smash:
a chart:
Capacitor Type ... Material type and quality ... approximate costs ... Best usage ... ???
============================================================
Ceramic 200 volt ... inexpensive, radial leads ... $0.05 ... high frequency bypass (non audio?) ... etc. ...
Comment? Contributions? ... |
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| Hartono |
"optimum capacitor type"
I thought we already agree on this ;) |
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| cliff |
Fast Eddy posted:
... because that's where the electrons are? ... at the ground connection (usually at a more negative voltage).
I would bet there are differences in reactance of a capacitor when the resistor is in series with the cap through the ground connection, thus offering a "longer", more resistive pathway for electron flow ... No?
I am not sure whether this is in jest or not :xeye:
If not, you have a rather bizarre understanding of electrical current, IMO :eek: |
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| Hartono |
"thus offering a "longer", more resistive pathway for electron flow ... No? "
at least the resistive part is correct...... it's a resistor hey !!! :D [Joke] |
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| FastEddy |
cliff: " ... I would bet there are differences in reactance of a capacitor when the resistor is in series with the cap through the ground connection, thus offering a "longer", more resistive pathway for electron flow ... No? ... in jest? "
No, not in jest at all. It is not an opinion but a fact that the length of the electron path will certainly make a difference in the results. Distance and timing (speed of electron flow and signal), conductor capacitance and resistance variations ... all resulting in an added contribution to resistance, inductance and capacitance variations ... the RLC time constant being related to phase shift and frequency rolloff, effecting the overall response curve, etc. :dodgy:
(This is but another reason for making certain capacitor types "close coupled" to an op-amp power pins to improve op-amp performance.) |
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| cliff |
| quote: | Originally posted by FastEddy
cliff: " ... I would bet there are differences in reactance of a capacitor when the resistor is in series with the cap through the ground connection, thus offering a "longer", more resistive pathway for electron flow ... No? ... in jest? "
No, not in jest at all. It is not an opinion but a fact that the length of the electron path will certainly make a difference in the results. Distance and timing (speed of electron flow and signal), conductor capacitance and resistance variations ... all resulting in an added contribution to resistance, inductance and capacitance variations ... the RLC time constant being related to phase shift and frequency rolloff, effecting the overall response curve, etc. :dodgy:
(This is but another reason for making certain capacitor types "close coupled" to an op-amp power pins to improve op-amp performance.) |
That is so off the wall I don't know where to start! |
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| FastEddy |
cliff:
I don't believe that this is off the wall at all. The speed of an electronic signal down a copper conductor is approximately 60% of the speed of light on a vacuum ... If one conductor is a few feet long and another is a few inches, simple calculations will show that the two approximately identical signals would arrive at a common destination at differing times, thus producing a phase shift and associated harmonics and cancelations. (This is one reason of many why ground loops happen.) ... :eek:
And, Surly, one should also allow for the contribution of the stray capacitance [differential] of a short wire verses a longer one ... which might make a significant contribution to the overall circuit response ... :cannotbe:
These kinds of signal pathway analysis considerations often differentiate a good amplifier design compared to a really, really great design. ... By studying the evolution of op-amp designs over the decades, these manacinations have become critical to the success or not of precision lab equipment, and of course higher quality audio devices ... and is absolutely critical in the design of audio DACs, etc.
One example > check it out: " ... Note that the Timer Tick occurs at 40Khz, the BPF event occurs at 10Khz, and the Display Event occurs at 800Hz. Since the display has eight columns, the refresh rate is 100Hz. ..." ... from: http://www.web-ee.com/Schematics/Au...dioAnalyzer.htm ... but not quite, not exactly correct according to figure 8 ... and this device is constructed with careful attention to signal pathway lengths and RLC considerations ... ;) |
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| Conrad Hoffman |
Fortunately, IMO, all that stuff happens at frequencies where, hopefully, I've squashed everything to near zero.
The circuit looks like an amp input- specifically my amp input, which is 20K. I did do a wideband impedance measurement of the input under power, and it is almost entirely resistive- to my great surprise actually. That's why I've had less interest in lower impedances, and why I don't hear any real differences. With lower impedance, who knows, maybe something audible would show up. I need to get the current projects off my bench before I can do more with this.
Charts are readily available showing the properties of various dielectrics, and capacitors follow those charts pretty accurately- it's hard to screw up Teflon or Polypropylene during cap manufacture, though I do see some variation in D between caps supposedly made of the same materials. BTW, people rave about how good Teflon is, yet I don't see caps offered in any useful value I could use in an amp. What do people do with 0.1uF Teflon caps anyway? |
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| rdf |
| Russian military, tanks I believe. A good relatively cheap alternative for experimentation. Built like tankware too, unfortunately EMP proofed in solid steel with steel pin leads. Those inclined can easily cut and gut them with a standard plumbers pipe cutter. Also unfortunately, that steel case hides the evil of mediocre construction quality in many of the samples I've opened. |
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| FastEddy |
Conrad: " ... Charts are readily available showing the properties of various dielectrics, and capacitors follow those charts pretty accurately ..."
Got links? I've seen a couple of these before, but a decent link list might be in order "for the rest of us ..."
" ... it's hard to screw up Teflon or Polypropylene during cap manufacture, though I do see some variation in D between caps supposedly made of the same materials. BTW, people rave about how good Teflon is, yet I don't see caps offered in any useful value I could use in an amp. What do people do with 0.1uF Teflon caps anyway? ..."
Likewise, the only use I've had so far for 0.1 uF Teflon or smaller is as snubbing caps (I continue to rant), close coupled to the power pins on op-amps in parallel with electrlytics on the power rails. But the readily available plastic caps (poly-razzmatazz or whatever) seem to be a) as good or better, b) smaller physically, c) cheaper sometimes by a considerable margin. ... :smash:
" ... That's why I've had less interest in lower impedances ..." ... meaning like the input front end of a tube amp? |
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| Conrad Hoffman |
| I searched around for some links, and there are quite a few, but it's disturbing the amount of disagreement between various sources. The only ones I trust are from the sites of the big name capacitor companies like AVX and such. Older engineering reference books (remember those) also have some good info, but not for the most modern materials and construction methods. I have a somewhat incomplete table I'm in the process of fleshing out, and will put up a link to that soon. There's less info out there on dielectric absorption than I'd like, and for all the history of the things, very little real data on paper/oil caps like Vitamin Q and Black Beauties (BBs were considered garbage back then, and sell for $50 each now- go figure). |
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| Conrad Hoffman |
Well, here's a start. If you've got more into to fill in, or see some obvious errrors, let me know.
some dielectric properties |
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| Hartono |
| For starter, the background of your link make the text almost impossible to read on LCD at 1440x900. |
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| serengetiplains |
| Conrad, your dissipation factor values look close to what I've read for those materials ... as maximum values. Actual values, at least from my measurements, vary considerably and in typically much lower ranges. For instance, I've measured DF of, say, some of those Russian teflons as low as 0.000001, with typical measures in the range of 0.000005 to 0.000025. These measurements are respectively 1 and 2 orders of magnitude lower than the spec you quote. |
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| serengetiplains |
| Conrad, one further point for your research, you might want to glance at this website. I found it to be an excellent resource. |
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| Conrad Hoffman |
| Tom- good site! I compiled my values from a bunch of data books, web sites, and a few measurements. I'm sure manufacturers tend to specify maximums- my measurements are invariably lower. Mica is a unique case, since the raw material varies so much. I suspect the films vary in composition, especially the ubiquitous "plastic film" sold by Panasonic. No Teflon caps here to test, as I don't know where I'd use the available values to any great advantage. They'd seem to be ideal for an RIAA phono preamp, but I don't know if the necessary values could be easily had. Just curious how you measure those low DFs. My 1615 goes down to 0.000001, but I'd only attempt that with the cap installed in a completely shielded enclosure, and at a pretty high voltage. Even then I wouldn't trust the measurement to better than plus or minus a few counts. I also have a phase comparator that could do it in theory, but I've never tried it. Not sure what's happening with the background- here the text shows up with very high contrast on medium gray with a 17" LCD, and is plenty readable, but monitors and peoples adjustments do vary. |
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| Hartono |
"Not sure what's happening with the background- here the text shows up with very high contrast on medium gray with a 17" LCD, and is plenty readable, but monitors and peoples adjustments do vary."
Maybe my eyes is a bit tired ;) |
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| serengetiplains |
Conrad, I have a Quadtech 1920 LCR meter. For best results, I measure caps using a 2V bias voltage in a copper can I've grounded to the 1920. Even then, DF measures bounce around as much as a magnitude between successive measurements.
I've posted some of Bateman's findings here. I've also posted DA measurements here using an admittedly compromised test rig (a DMM), though the results still provide useful information. |
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| FastEddy |
Conrad Hoffman: (you da man!!) " ... I have a somewhat incomplete table I'm in the process of fleshing out, and will put up a link to that soon. ... ... If you've got more into to fill in, or see some obvious errrors, let me know. ..." Link: http://members.rpa.net/~choffman/cap_losses.htm
serengetiplains: (... what i'm talkin 'bout!!) " ... some of Bateman's findings here. [Link: http://www.diyhifi.org/forums/viewtopic.php?f=13&t=388 ] ... I've also posted DA measurements here. [Link: http://www.diyhifi.org/forums/viewtopic.php?f=15&t=550 ] ... "
(Appologies to all for the redundancies, but some folks out in the hinterlands live behind firewalls that may block hidden web link references or and some others who may wish to run the links directly through their favorite language translators.)
Now ... suggestions for the wording of the DIYAudio Wiki definations: should we just edit, amend or append the Wikipedia entry or make our own from scratch ?? [Link: http://en.wikipedia.org/wiki/Capacitor ] ... :confused: |
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| Conrad Hoffman |
FastEddy- I like Wikipedia. I use it quite often and am impressed with the quality of most of the content. Therefore I want it written by people way better than me! Maybe we can come up with something as a group effort, but I foresee a big problem the minute we try to say anything definitive about the way anything sounds. Far out at one end (close to where I stand) is the view that the cap distortions (other than electrolytics) are inaudible. At the other end are the people who claim to hear the difference between Teflon and polystyrene. My guess is the circuit the cap lives in has a huge amount to do with what you hear.
OTOH, in another few minutes I could change my whole science based view of things. I just rebuilt an amp with much higher quality parts, improved the stability and lowered the distortion. I'm about 80% sure it now sounds worse than it did before. Lifeless is about the best description I can offer. I'll spend tonight at the bench trying to figure out why. My initial guess is that I added a miller cap to better stabilize the thing, and that may have been a mistake. Fortunately the only other cap of real significance is the 220uF in the feedback divider (no couplers), and that's too large to do much about. Maybe I'll make it larger still. Just think, if it were Teflon, it would be bigger than the whole rest of the amp! :devilr: |
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| FastEddy |
Conrad: " ... Therefore I want it written by people way better than me! Maybe we can come up with something as a group effort ..."
Likewise ...
" ... I foresee a big problem the minute we try to say anything definitive about the way anything sounds. ..."
Is thus and ever shall be ... Opinions can be put in quotes or labled as a "subjective opinion". I have no problem statements regarding quality. Example of a subjective opinion:"Because of the higher frequency response and recovery time of polystyrene snubbing capacitors (in parallel with larger electrolytic capacitors), the results are generally better than when left out of the power supply circuit." - FastEddy.
" ... I just rebuilt an amp with much higher quality parts, improved the stability and lowered the distortion. I'm about 80% sure it now sounds worse than it did before. Lifeless is about the best description I can offer. ..."
I have a tube pre-amp between my DVD-A player and my MOSFET power amp for this and other reasons ... Years ago methods to "color" solid state equipment included the introduction wire wound carbon resistors in the audio signal path to make them sound more like the "mellow" sound of a tube amp. Sony did it. ... ;) |
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| Conrad Hoffman |
| Whew! Almost two in the morning because I couldn't stop listening. Fixed the amp. As you know, I'm into differential comparisons. The amp was simply producing more difference between input and output than it should have. Distortion was reasonable, though not terrible low. Response was ok. Not sure what name to put on the error yet. The culprit was the miller cap that I added because I couldn't get the thing completely stable. Finally I changed the whole ground strategy that the amp has had for the last 20 years; that made it stable and I removed the miller cap. In/out difference went down and the life came back. Couldn't hear a significant difference when I shorted the output inductors. Doug Self has a good point about the electrolytic in the typical feedback network- select it for THD, not for the low frequency rolloff. I need to make mine bigger, if only to reduce the in/out difference due to phase shift. IMO, he doesn't have a good point about miller caps- they aren't an elegant solution to several problems, but a problem waiting to happen. Maybe that's why people have been less than enthusiastic about blameless amps? Not that this amp is anywhere near blameless :whazzat: |
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| FastEddy |
| Conrad: Got a diagram? :confused: |
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| Conrad Hoffman |
| FastEddy, I assume you mean of the grounding? No diagram, but it's simple. The original scheme had the common heavy copper wire between the filter caps. Everything was single pointed to the center of that. I ran a stub off the center point to get away from the cross currents, but that didn't help a bit. The real problem was that the caps are pretty tall, and the PCBs are quite a ways away. Bringing back grounds for the PCBs (which have split grounds), the chassis, the bypass caps, all separately was no good. I'm sure the wire inductance was brutal, and the system essentially had no high frequency ground. What I did was establish a new star point right between the PCBs, near the amp inputs. Now the local bypassing works properly and the thing is stable as a stone. I made one other important change based on early and later schematics by SWTP (this is a copy of their old "Tiger" amp). The early amps had a 220pF cap from the base of the diff amp input to ground. The later amps were identical, but inserted a 1K resistor in the base lead. This seems to help stability quite a bit. I've seen the same thing with op-amps, where a cap to ground on the input will give rise to intractable oscillation. |
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| FastEddy |
Conrad: " ... The real problem was that the caps are pretty tall, and the PCBs are quite a ways away ... [I established] a new star point right between the PCBs, near the amp inputs. Now the local bypassing works properly and the thing is stable as a stone. ..."
Effectively balancing distance and ground path impedence between the PCBs ... You seem to be well in advance of my previous rant(s): " ... Distance and timing (speed of electron flow and signal), conductor capacitance and resistance variations ... all resulting in an added contribution to resistance, inductance and capacitance variations ...", your clear thinking, bench work and intuition making up for and being better than "book learnin'".
" ... a 1K resistor in the [cap] base lead ... the same thing with op-amps, where a cap to ground on the input will give rise to intractable oscillation. ..." I believe that I have also seen this in some of the older op-amp cookbooks ... the theory being that there was "reflection" or "a bounce back" from certain types / sizes of caps, re-enforcing feedback ... or some such (I might be wrong here).
(Speaking of books, you might take a peek at "The Black Swan: The Impact of the Highly Improbable" by Nassim Nicholas Taleb, new at Amazon, et al ... definately good reading for DIY types.) |
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| Conrad Hoffman |
| Yup, seen it before, but the big question for me is, "why?" Why is what makes the world go 'round. It must reduce some kind of positive feedback that causes the problem, but I don't really understand the path. |
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| FastEddy |
| Well, near as I can tell, there is an "RC time constant" relationship between resistor and cap at the (+) input ... the resistor / cap acting as a band pass or low pass filter to the positive feedback loop = allowing only the frequency band from ~0 Htz up to the intended upper limit of the amp or op-amp before out of control oscillations can set in ... and the resistor / cap does take care of that pesky "bounce" or reflection off of the cap, "squaring up" the leading edge of the signal ... If you have a 'scope you might be able to see this comparing a square wave signal trace with & without the resistor ... :cannotbe: |
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| cliff |
???
That CR is a classic method of keeping the DC loop gain at unity, while maintaining an AC gain of 20 (Rf/Ri) above the Fc of the cap.
Nothing to do with oscillations or anything wierd.
Or are you looking at a different schematic? |
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| FastEddy |
cliff: " ... That CR is a classic method of keeping the DC loop gain at unity ... Nothing to do with oscillations ..."
mmm ... Yes, approximately correct. (I'm wrong again :xeye: ) My hydraulic model suggests that the resistor (1K Ri) "pinches off" the flow of electrons into and out of the cap (22 uF Ci) = acting as a high pass filter at the (-) negative feedback input (rather than a low pass type at the (+) input as stated). That would, I believe, do approximately the same (required) thing = allow the higher frequencies into the (-) negative input (pin 9) and thus, via the (-) negative feedback, keep the amp from oscillating ...
I guess I should breadboard these thing up myself before shooting from the hip ... or at least pay closer attention ... other than that, I had a great weekend ... You? |
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| cliff |
"My hydraulic model suggests that the resistor (1K Ri) "pinches off" the flow of electrons into and out of the cap (22 uF Ci) = acting as a high pass filter at the (-) negative feedback input (rather than a low pass type at the (+) input as stated). That would, I believe, do approximately the same (required) thing = allow the higher frequencies into the (-) negative input (pin 9) and thus, via the (-) negative feedback, keep the amp from oscillating ..."
You are on a hiding to nowhere trying to analyse an audio amp feedback circuit using a "hydraulic" model of current flow.
There is no direct current in Ri because of the series cap.
Only using the frequency domain model makes sense:
1) Dc gain is 1
2) At high frequencies XCi is very low so gain is 20
3) As XCi gets to 1K at low frequencies the gain will be 3dB down on 20. This will occur at 7Hz
Whether the amp oscillates or not depends only on its internal frequency response and phase margin. It is impossible to tell from this block diagram what those values are. However, since this is a commercial audio amp IC, that will be detailed elsewhere in the datasheet. |
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| Conrad Hoffman |
| I'm not sure we're talking about the same resistor here. My interest is in Rb, the series resistor on the input. It prevents grounding the input when the pot is turned all the way down. If there were a RFI cap to ground (the situation in my amp), the resistor would limit the impedance to ground at high frequencies. No doubt it's something to do with the small but real input current, but I don't see how the signal gets reinforced to cause oscillation if the input is grounded directly. |
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