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Old 29th July 2005, 09:43 PM   #21
sam9 is offline sam9  United States
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Quote:
quote:
Originally posted by scott wurcer

BTW that zetatalk site is run by one of the most notorious internet kooks of all time.
Yeah. I kept looking for a link to Scully & Mulder, Or it may be an elaborate spoof. McAfee, I read years back, started out running a really tacky on-line dating service long before they were deemed respectable. Now he's a "Master of the Universe".
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Old 29th July 2005, 09:44 PM   #22
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-120dB distortion? What's the point? 120dB is difference between threshold of hearing and threshold of pain, so for me it is pointless for audio, -80dB would be more than enough I think. Or arts for arts sake?
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Old 30th July 2005, 02:10 AM   #23
AndrewT is offline AndrewT  Scotland
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Hi,
low order even harmonics at -110db to -120db would be innocuous, but high order harmonics sound about as bad as multiplying them by their order number to comparee with say 2nd harmonic i.e. 9th order harmonic sounds as bad as 4.5 times greater 2nd harmonic to our ears.
Crossover distortion is VERY peaky with again high order distortion but over a very short part of the overall waveform. A straight % measurement under values the the sound effect by about the ratio of whole wavelength of the fundamental to the % time that the crossover peak shows on the residual, about 10 to 30 times as bad.
Combining these two effects together, short time scale and high order harmonics, means that the low level measurements of crossover distortion should be increased by between 20db and 40 db and this correction factor means apparently inaudible distortion can have a profound effect on sound quality far worse than other low order distortions at -70 to -80 db.
In my view we should strive to identify some types of distortion at very low levels and then decide if they need to be eliminated or reduced. We cannot make that type of decision unless we armed with the evidence.
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Old 30th July 2005, 03:13 AM   #24
Mr Evil is offline Mr Evil  United Kingdom
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Default Re: As an experiment .....

Quote:
Originally posted by mwmkravchenko
Mr. Evil

Can the refresh rate be altered? Gremlins in the mix seldom make any sense. Sometimes the only way to find them is to start a process of elimination.

Mark
I'll just turn them off when recording a sample - eliminate them completely.



Quote:
Originally posted by sam9
...In it he mentions that aluminum enclosures offer little shielding for low frequency -- for that you need a steel enclosure...
I hadn't considered magnetic shielding. I suppose more tests are in order.



Quote:
Originally posted by mzzj
...I built wien-bridge oscillator...
I did do a few quick experiments with oscillators, but came to the conclusion that the the effort needed to make one with low enough distortion was higher than cleaning up the output from from a soundcard.




Quote:
Originally posted by Lars Clausen
My own measurements suggest that even OPA134 and OPA2134 have quite high THD, especially at supply voltages below +/-8V. The 0.0008% claimed in the datasheet will only stand at the most optimal conditions. At normal line level the real value is more like 0.01% from 20 - 20.000 Hz.

I recommend OPA627 which has ultra low THD under almost any circumstances, making it the best choice for building THD measurement filters.
I know datasheets are always... economical with the truth, which is why I actually tested a bunch of op-amps. The best one was the OPA227, which is what I used.

Yes, OPA627 is an exceptional op-amp, but it's too expensive for me to justify using it for anything but extremely special projects. However, while it does have very low THD, it's nothing special when it comes to noise.



Quote:
Originally posted by darkfenriz
-120dB distortion? What's the point? 120dB is difference between threshold of hearing and threshold of pain, so for me it is pointless for audio, -80dB would be more than enough I think. Or arts for arts sake?
A number of reasons:
  • While -120dB may be inaudible, it's not quite proven definitively that it is never audible, so it seems only sensible to minimize distortion just to be sure.
  • So I can show off
  • To challenge myself and learn more.
  • Who said I would only ever be using this to develop audio equipment?
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Old 30th July 2005, 01:01 PM   #25
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Well, good reasons, especially 2nd and 3rd.
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Old 30th July 2005, 01:46 PM   #26
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Quote:
Originally posted by darkfenriz
-120dB distortion? What's the point?

It is "art for art's sake" -- it's the same reason some guys spend all weekend tweaking their BMW's -- but this tweaking advances the state of the art.
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Old 30th July 2005, 01:55 PM   #27
PMA is offline PMA  Europe
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It makes sense to measure distortions with -120dB level, especially for high-order harmonics.

Several measurements can be seen here. The text is in Czech, but every engineer would understand ...

http://web.telecom.cz/macura/spectra/xa2es.html
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Old 30th July 2005, 01:56 PM   #28
mzzj is offline mzzj  Finland
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Quote:
Originally posted by AudioWizard



As for me, I'm absolutely not convinced there is any real difference in terms of shielding between aluminium and steel, and especially in the low-frequency area. A good, grounded aluminium shielding should be more than adequate - if it's not, there may be some other problem with the design/layout. Almost all "EMI shielding boxes" used in electronics are made of aluminium. I may be wrong, but that's not something I would worry about...

Another thing: in this article, the guy seems to think the thicker the shielding, the better: I don't think that Faraday would necessary agree. I myself would tend to think that thicker is better against very high frequency EM radiation, but not for low-frequency.

Anyway, interesting topic nevertheless.
For electromagnetic fields steel is worse than aluminium. What matters here is conductivity. Thickness is not so important, quite thin layer of aluminium and you already have -200dB attenuation.

Magnetic fields are completely different beast, they are usually low frequency and need ferromagnetic material for shielding. Faraday gage has little or no help at all against these.

And then there is conducted emissions left to deal, sometimes rather nasty. Battery powered equipment helps a lot.
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Old 30th July 2005, 02:07 PM   #29
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Quote:
Originally posted by mzzj

Magnetic fields are completely different beast, they are usually low frequency and need ferromagnetic material for shielding. Faraday gage has little or no help at all against these.

And then there is conducted emissions left to deal, sometimes rather nasty. Battery powered equipment helps a lot.
One of the other problems with "mu-metal" was that bending it distorted the electro-magnetic characteristics.

FWIW, I have found copper tape (from 3M) to be effective in some of my instrumentation projects. I had a 12 bit ADC which was giving me "bigger than LSB" errors until I placed a ribbon of copper tape across the top of the device and tacked it to the analog ground plane.
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Old 30th July 2005, 02:46 PM   #30
DUG is offline DUG  Canada
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Electric fields (E-fields) are shielded at the surface of the conducting material so aluminum foil works as well as copper or tin plated steel...copper and tin plate are easier to solder to.

For magnetic fields (H-fields) ferrous material work the best vs price.

But copper, because of it's high conductivity actually works better than steel only below about 60 Hz

I have no data on aluminum but it may show the same results as copper (but is difficult to solder.)

Brass (copper alloy) may be easier to find and is stiffer for forming shapes.
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