Output Transformer Waveform- Is it good or not? - diyAudio
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Old 22nd May 2012, 01:18 PM   #1
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Default Output Transformer Waveform- Is it good or not?

This is the 1kHz square wave from a Lundahl LL1690, primaries connected to a ES9008 DAC with 195Ω source Z, secondaries into 8.9kΩ (pot before buffer).
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Connection schema:
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Is that slight overshoot (or is it ringing?) something to care about?
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Old 22nd May 2012, 01:27 PM   #2
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Based on purely unscientific information, when i tested a friends buffalo DAC, iirc, the square wave showed some of the same ringing... personally I would do something to damp it myself...

Check the xfmr with an analog square wave - maybe the calibrator signal from the scope?

Also check ur scope + probe using the calibrator and look to see if it has overshoot or not, then adjust the comp trimmer in the probe or scope for best squarewave.

_-_-bear
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Old 22nd May 2012, 01:29 PM   #3
SY is offline SY  United States
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Assuming that the ringing isn't from your scope probe (bear's advice is spot on), a series RC across the secondary will clean it up.
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Old 22nd May 2012, 01:32 PM   #4
rdf is offline rdf  Canada
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Isn't that pre-ringing at the end of every top? If so I don't think it can be the transformer.
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Old 22nd May 2012, 01:34 PM   #5
SY is offline SY  United States
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Quote:
Originally Posted by rdf View Post
Isn't that pre-ringing at the end of every top? If so I don't think it can be the transformer.
Better eyes than mine! You are of course correct, it's not the transformer, it's the digital source.
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Old 22nd May 2012, 01:36 PM   #6
DF96 is offline DF96  England
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If its coming from a DAC it could be the digital reconstruction filter. Quite normal for a brick-wall filter.
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Old 22nd May 2012, 02:57 PM   #7
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Quote:
Isn't that pre-ringing at the end of every top? If so I don't think it can be the transformer.
Quote:
If its coming from a DAC it could be the digital reconstruction filter. Quite normal for a brick-wall filter.
Please elaborate!

Meanwhile I tried some combinations of a rheostat and some caps. Only noticable tendency is getting the signal flanks less steep and the overshoot remains ...

Probe is well calibrated, of course.

Last edited by Groundloops; 22nd May 2012 at 03:01 PM.
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Old 22nd May 2012, 03:09 PM   #8
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Lightbulb Lesson learnt!

Google is my friend ...

Quote:
The truncation of the impulse response is equivalent to multiplying it by a rectan-
gular “window” function. This leads to an overshoot and ripple before and after
the discontinuity in the frequency response – a phenomenom known as Gibb’s
phenomenom
http://www.robots.ox.ac.uk/~sjrob/Teaching/SP/l6.pdf

Thank you very much, guys!

Last edited by Groundloops; 22nd May 2012 at 03:24 PM.
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Old 22nd May 2012, 04:57 PM   #9
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SY and RDF, et al, quite so, the chipzet is doing the ringing, but if the probe is off, the ringing may seem greater... as I said at the top, I have seen it before with this chipset.

Groundloops, you need the right rather small value of cap... use ur scope's split timebase function to measure the period of the ringing wave, invert to get freq, and then pick a cap that is suitable given the impedances... I might put it on the primary side and see if that works better or not.

Alternately a small series inductance might also prove useful, perhaps with a resistor in parallel...

And, actually, please do report back what freq you measure the ringing to be, as the xfmr seems to be doing an excellent job!

_-_-bear

PS. if you have the circuit, maybe a better place to address the ringing can be discovered. Post if possible.
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Old 22nd May 2012, 05:12 PM   #10
DF96 is offline DF96  England
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There is no ringing to address. This is Gibb's phenomenon, caused by a brickwall filter. The 'ringing' is the inverse of the sum of all the square wave Fourier components which have been removed by the filter. Of course, you can add some extra filtering to smooth out the edges but then you are modifying the sound too so you no longer hear what came out of the anti-aliasing filter before the ADC.
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