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Old 1st November 2010, 10:53 PM   #121
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Take a look at the phase shift at 20K or however high a frequency you can hear. I have been able to detect 0.8 degree of phase shift with certain circuits and electrostatic headphones and would aim for over 200KHz as the corner of a single pole filter. This may indeed not provide sufficient filtering and additional steps may be called for. I offer this only as a potential explanation of an audible effect of a filter that on first inspection would seem that it should be inaudible.
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Old 1st November 2010, 11:07 PM   #122
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Default I hear

ya on the phase shift as far as possible audible effects go, But the plots he posted show a low pass filter with a corner frequency well above 500 kHz reducing distortion components in the audio band.
I would like to hear possible explanations why such a high frequency filter (>500 kHz), which should not effect frequencies anywhere near the audible bandwidth appears to reduce distortion components from 10 kHz to 25 kHz? I must be missing something, because this makes zero sense to me.
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Old 1st November 2010, 11:09 PM   #123
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Looking at the FFTs there are actually less artifacts with the 1.8nF than in the 15nF plot. Could these two be mislabeled?
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Old 2nd November 2010, 02:20 AM   #124
leo is offline leo  United Kingdom
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Could it be to do with the type of cap used ? With the 15nf Wima's cymbals sound more like a soft hiss, even trying a lower value polypropylenes gave similar effect with cymbals . Remove the caps and things are noticeably clearer
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Old 2nd November 2010, 04:38 AM   #125
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Interestingly modelling the Legato 2 circuit in SPICE with and without the 15nF caps also showed an effect on reduction of harmonics at much lower frequencies than the filter knee. Without the caps there was a much increased presence of a first order ringing at twice the frequency being pumped in. Off the top of my head the reduction was in the order of 60dB. I do prefer the Legato without the caps though, even if I too detect a very slight increase in 'glassiness'.
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Old 2nd November 2010, 07:36 AM   #126
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Had a thought about my measurements in the traffic on my way to work - I'm measuring on single ended output whilst the filtering is before the balanced to SE opamp. Could some high freq rubbish that slips through when not filtering upset the opamp and cause oscillation/distortion?

Anyway, I'll do some more measurements using my 60MHz DSO and FFT and correlate my results.

Cornelius
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Old 2nd November 2010, 08:48 AM   #127
needsp is offline needsp  United Kingdom
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Hi

The 1n caps are also act as low pass filters. Something else to vary in value and type, and to listen to, measure and simulate!

Paul
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Old 2nd November 2010, 10:21 AM   #128
Spev is offline Spev  United Kingdom
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Yup, I have some 1n polystyrenes here to try in both positions. Looking out for some 100nf ones to try around the opamp as well...
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Old 3rd November 2010, 01:11 PM   #129
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Quote:
Originally Posted by barrows View Post
Does not seem right at all? Can anyone explain what we are looking at here? Are we looking at the output of the Legato alone, or the output of another component being fed the Legato's signal?
Why would a filter effective about 500 kHz have any effect here?
I am considering going 10nF REL polystyrene here, with R1-4 at 180R.
What you are seeing is the limitation of the test setup.

What I am extremely pleased with is the extremely low measured harmonics! All of the stuff I see above what would be harmonics looks like noise.

It also could be a the FFT is not measuring the balanced signal? You would get much better noise immunity then.
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Last edited by Russ White; 3rd November 2010 at 01:42 PM.
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Old 3rd November 2010, 01:26 PM   #130
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Yes those caps are important to getting the best THD.

Notice that both the 2 and 3rd harmonics are basically nil even when using a low value cap. Even 1nf will work fine.

If you look at the noise bands(they are not related to the fundamental at all) they are consistent in all three shots starting especially at about 6.8Khz and there are several harmonics of that. This is pretty normal looking noise in sound card measurements. You can usually tell the FFT to ignore them. As you said this could also be re-sampling hash. Likely a combination.

As for the Bal/SE opamp if you must run without the filter caps you will definitely want to add something like 20pf compensation on both nodes. This will make the opamp a bit happier

But really to get the best picture of the I/V stage itself you should measure the balanced output. If your test setup can sum two signals it should be pretty easy.

I am very please looking at your FFT in regards to actual distortion on the fundamental. That is pretty amazingly low.

Cheers!
Russ
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Last edited by Russ White; 3rd November 2010 at 01:33 PM.
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