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Old 5th February 2013, 01:32 PM   #1711
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Quote:
Originally Posted by RNMarsh View Post
Notice what happens with DC added to the ac thd..... increases of 20-30dB are common.... worse in electrolytics. Note too that asymetrical waveforms have a DC component.
I have measured thd with asymetrical waveforms and find with FFT that it was primarily 2H. Thx-RNMarsh
Microphones and phono catridges do not pass DC and an AC coupled signal in cluding music and these "asymetrical" waveforms do NOT contain DC. No tests that I have seen have any indication that the capacitor non-linearities can not be modeled in a simple way.

Besides most of even Bateman's results were very low order (in many cases seconds only) and still below -90dB. The audibility of this is debateable, the usual anecdotal evidence aside.
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Old 5th February 2013, 04:44 PM   #1712
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by scott wurcer View Post
Microphones and phono catridges do not pass DC and an AC coupled signal in cluding music and these "asymetrical" waveforms do NOT contain DC. No tests that I have seen have any indication that the capacitor non-linearities can not be modeled in a simple way.

Besides most of even Bateman's results were very low order (in many cases seconds only) and still below -90dB. The audibility of this is debateable, the usual anecdotal evidence aside.
Semantics, i suppose. DC as in Direct Current -No. Average level that is DC (non-zero), yes. Not concerned any more about coupling cap's distortions and causes. We got rid got rid of the electrolytic coupling cap used in high-end audio (except Asian sourced recievers still use a lot of them). Then we got rid of the coupling cap altogether (in ss designs); DC servo took care of the major remaining issue of dc offset and drift issues... if they exist any more in better designs. But caps used for active or passive cross overs, filters, EQ and situations where a significant voltage is developed across them is of more concern now. [for the recording and playback chain]

Caps now need to be thoroughly tested/documented as they are still used... with voltage drops across them and with DC on them etc. Some tests show significant thd under these conditions.

Caps can still be sorted by audiophiles by their D.A for general selection and tests for THD generally hold up for such selection. The reasons for thd are many, but include the materials and construction methods and QA/QC etc. D.A. is just one issue of several for the designer to be aware of. All should be by now. All cap properties and parasitics are modeled as linear properties but the differences in thd are not modeled. So testing is best way for specifics.


Thx-RNMarsh

Last edited by RNMarsh; 5th February 2013 at 05:02 PM.
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Old 6th February 2013, 06:35 AM   #1713
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Semantics, I suppose. DC as in Direct Current -No. Average level that is DC (non-zero), yes.
That's not semantics. Taking the average of a signal is a (crude) form of low-pass filtering. If the (long-term) average of a waveform is non-zero, it has DC content. Asymmetric music waveforms are not a result of DC/non-zero average, but of specific phase relations of the harmonics.

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Old 6th February 2013, 06:49 AM   #1714
RNMarsh is offline RNMarsh  United States
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Geeez. Look here... Put up a repetitive pulse... zero to + some level for some period. It has a dc level or average. no filtering involved. Put up a pulse with + 1 and neg -.5 for same periods... Average is + or non-zero. Lets not make this more compex than it needs to be. Asymetric test waveforms with a dc/average level that is not zero. Thx -RNM

Last edited by RNMarsh; 6th February 2013 at 06:53 AM.
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Old 6th February 2013, 02:32 PM   #1715
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Yep, geeez here too. Sorry, I don't get your point at all. Why do you want to test with a signal that has nonzero DC content if audio does not..? Perhaps Scott can help out...

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Old 6th February 2013, 02:48 PM   #1716
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Quote:
Originally Posted by RNMarsh View Post
Geeez. Look here... Put up a repetitive pulse... zero to + some level for some period. It has a dc level or average. no filtering involved. Put up a pulse with + 1 and neg -.5 for same periods... Average is + or non-zero. Lets not make this more compex than it needs to be. Asymetric test waveforms with a dc/average level that is not zero. Thx -RNM
Don't associate these test exercises to music. I disagreed with the examples Walt provided 30 years ago and I still disagree. A trumpet blast looks asymetrical on a scope, it DOES NOT contain a DC component, the time spent above 0 will always time average to 0 with the time spent below 0. You certainly can not add sine waves in any phase relationship to create a DC voltage. Neither of your test waveforms have physical meaning in real world sound propagation, both postulate a permanent offset in local barometric pressure.

No Samuel I can't help I'm afraid we must just move on this falls into the too much to lose category.
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Last edited by scott wurcer; 6th February 2013 at 02:55 PM.
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Old 6th February 2013, 04:40 PM   #1717
RNMarsh is offline RNMarsh  United States
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Waveform is chosen to bring out issues or imperfections from ideal. Which it does. You can debate it amoungst yourselves as to whether it is relavent.

Many people try to describe what second or third or even vs odd harmonics sounds like. But its best if you hear them for yourself. Same with capacitors 'sound'. Other's described the sound of electrolytics vs films and then film against film. Those descriptions did not seem to be caused by esr, esl or Fr nor THD to me. The test for DA and the way it was done matches exactly the descriptions for the sound and in rank order. But you can best understand this if you heard those caps for yourself. If you dont hear anything with any cap type then there is no issue to explore. My role was to find out what the listeners heard and try to correlate it with a characteristic and then a test.

But-- I'll let you have the last word on this old and tired subject. Good night.

Thx-RNMarsh

Last edited by RNMarsh; 6th February 2013 at 04:58 PM. Reason: Cap test waveform choice -
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Old 7th February 2013, 04:12 PM   #1718
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At this point in cap measurement history, it is the voltage drop across the cap that I would now focus on. As I said before, there are many applications which develop a signal voltage across the cap -- such as, EQ, filters and oscillators. Using the lower DA films and better construction caps give lowest measured distortion in such use.

Thx-RNMarsh

Last edited by RNMarsh; 7th February 2013 at 04:19 PM. Reason: Caps with ac voltage drop across them -
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Old 7th February 2013, 05:54 PM   #1719
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It is true that acoustically it is impossible for sound waves to produce a DC average.
The pressure wave will always return to zero.
But it is not true of electrical signals.
A series of positive going pulses will produce an average DC signal.

It is just the difference between electronics and acoustics.
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Old 7th February 2013, 10:44 PM   #1720
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Could this be why tube amps sound better, DC can't get through the output transformer? [/snark]
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