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Old 30th July 2005, 03:49 PM   #31
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Quote:
Originally posted by DUG

I have no data on aluminum but it may show the same results as copper (but is difficult to solder.)
Actually, I have some soldering alloy that allows to solder aluminium at a pretty low temperature (around 200-230°C), so usable with a soldering iron. This alloy won a price in the "Concours Lépine" a few years ago, and I got it from a consumers show. Not sure how you can find it now via retail...
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Old 30th July 2005, 04:12 PM   #32
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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?
That's not really the point, actually.
120 dB of resolution is pretty nice to have if you're serious about high-end audio electronics.

As for distortion, don't forget that even if you think one peak here and there at -100 dB are not a big deal, all cumulated, that can add up to unacceptable distortion over the whole spectrum. And anyway, this kind of resolution can allow you to make relevant THD measurements.

That being said, I really doubt you'll get anything useful below -90 dB when using a sound card in a PC. Even the Yamaha one (I myself own a DS2416). So I wouldn't really trust what I see below this threshold... Just a thought.
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Old 30th July 2005, 04:30 PM   #33
mzzj is offline mzzj  Finland
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Quote:
Originally posted by AudioWizard



That being said, I really doubt you'll get anything useful below -90 dB when using a sound card in a PC. Even the Yamaha one (I myself own a DS2416). So I wouldn't really trust what I see below this threshold... Just a thought.
Thats whats for notch filter was used by
Mr evil.



---
Aluminium solders are nothing new, multicore for example has Alu-sol. Quite easy to solder with this and ordinary soldering iron. Elfa and Farnell used to have this stuff at stock when i ordered last time, rather expensive but real multi-use solder, solders everything that is solderable by any means.

Problem with aluminium soldering is long-term stability, depending on solder and aluminium alloy how stable and corrosion resistant it is going to be. ALUSOL is supposed to be better than many others in this area.
http://www.farnell.com/datasheets/12269.pdf
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Old 4th August 2005, 10:49 AM   #34
mzzj is offline mzzj  Finland
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Analog appnote AN347 has nice graphs for magnetic shielding, showing that at 1khz steel is superior compared to copper but at higher freq not so...
http://www.analog.com/UploadedFiles/...6254AN-347.pdf
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Old 4th August 2005, 10:59 AM   #35
tekman is offline tekman  Germany
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Quote:
Analog appnote AN347 has nice graphs for magnetic shielding, showing that at 1khz steel is superior compared to copper but at higher freq not so...

One reason for this is the skin effect, which you have noticeably at higher freuqencies.

This can be partly compensated by using copper plated steel, even better silver plated steel. Well, which you have to cover with lacquer or your shielding will suffer due to silver --> silver_oxide.


hth,
Andreas
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Old 29th August 2005, 08:26 PM   #36
mzzj is offline mzzj  Finland
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Mr Evil, did you play around with different notch filters or BP filters ?
twin-T is pretty classical but pain in *** to tune. Bainter notch and gyrator based designs should archieve deep notch with much less sensitivity to component tolerances, but i dont know their suitability for this task?


Just did matching for 1khz twin-t caps, 0.2% tolerance in caps should get rid of most of the tuning, too bad that these are mylars, not exactly famous for temp stability.
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Old 30th August 2005, 01:21 PM   #37
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by mzzj
Mr Evil, did you play around with different notch filters or BP filters ?
twin-T is pretty classical but pain in *** to tune...
I did try a few other types, but the twin-t gave the best overall combination of distortion and Q. I tuned it by hand-picking wide-tolerance parts, e.g. all the 4.7nF capacitors are actually 4.8nF. It's not that much trouble to do by watching the results in real time - input white noise and do an FFT on the output and you can see the notch change as components are changed.
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Old 30th August 2005, 02:12 PM   #38
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Quote:
Originally posted by Mr Evil

I did try a few other types, but the twin-t gave the best overall combination of distortion and Q. I tuned it by hand-picking wide-tolerance parts, e.g. all the 4.7nF capacitors are actually 4.8nF. It's not that much trouble to do by watching the results in real time - input white noise and do an FFT on the output and you can see the notch change as components are changed.
an impedance bridge is a wonderful thing to have -- unfortunately you don't see them as often on EBay as two or three years ago --
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Old 5th September 2005, 05:57 PM   #39
mzzj is offline mzzj  Finland
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Default twin-t attenuator sensitivity to opamp distortion

twin-t attenuator sensitivity to opamp distortion?

I simulated twin-t attenuator with feedback(to raise its Q) and found rather unhappy results, ie sensitivity to feedback opamp generated distortion. Click the image to open in full size.
IE distortion generated by X1B in this pic (C) Mr Evil

I inserted ac source between X1B output and "centerpoint" of twin-t and it shows 3db-9db gain for 2x twin-t tune frequency, ie multiplying 2nd harmonic generated by X1B (and X1A) by a factor of 9db

(actual value depending on feedback factor, being 3db for 0.7 and 9db for 0.9x feedback. 0.7 feedback is about minimum required, notch response is already -3db down at 2khz.)

Maybe it would be better to leave twin-t to be a passive, but then it is going to attenuate 2nd harmonic 9db. Good side is that it is easyer to archieve deeper notch as it is not so damn narrow.

If decided to stick with feedback I think it would be usefull to take feedback from unity gain buffer instead of 40x amplified and >40attenuated signal as in Mr evils schematic
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Old 5th September 2005, 06:47 PM   #40
mzzj is offline mzzj  Finland
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Forget my mumblings, thinking it twice it doesnt seem an issue, my assumptions were just little faulty. As if second harmonic (2khz) of measurement signal would be -100db and X1B would add 2nd harmonic at -100db level to this 2khz, it would appear at 4khz -200db below fundamental. Clearly not an issue. Or case of fundamental distortion happening in X1B, assuming 40db notch and if X1B adds -100db 2nd harmonic back to feedback point. Now my 3-9 db distortion increase comes in handy, as 2nd harmonic generated by X1B is multiplied thanks to feedback. This would result -133db...-137db lower measurement limit for 2nd harmonic.
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