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Old 18th April 2019, 02:34 AM   #21
abraxalito is offline abraxalito  United Kingdom
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Yeah the THD numbers aren't much, if at all affected I would agree. Which just indicates that traditional measurements aren't that useful in designing I/V converters intended to be listened to.
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Old 18th April 2019, 02:39 AM   #22
chris719 is offline chris719  United States
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Originally Posted by abraxalito View Post
Yeah the THD numbers aren't much, if at all affected I would agree. Which just indicates that traditional measurements aren't that useful in designing I/V converters intended to be listened to.
Perhaps .

Have you found any measurable metric that you can correlate with?

I agree that in principle it would ideally be better to not feed HF directly into the op-amp, but IMO it works better than it should. I don't normally see the things you see if you purposefully put a powerful GSM signal into some op-amps, like DC offset and spurious tones.

Last edited by chris719; 18th April 2019 at 02:41 AM.
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Old 18th April 2019, 02:40 AM   #23
abraxalito is offline abraxalito  United Kingdom
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Not yet no, but its been a concern of mine over many years....


Anecdote related to this - one prototype multibit DAC just a couple of days ago sounded like a dead-ringer for an S-D DAC (think ES9023) and I took it apart to investigate why. Turned out the AD815 filter stage was oscillating wildly in the 10MHz+ region. Tamed that and it went back to its usual dynamic sound.
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Last edited by abraxalito; 18th April 2019 at 03:08 AM.
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Old 18th April 2019, 08:29 AM   #24
dreamth is offline dreamth  Romania
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Originally Posted by abraxalito View Post
Besides that, testing with a sinewave input simply isn't at all realistic when the source in reality is a DAC with very fast edges between sample points. When the opamp model is a macromodel (as most are) then THD is moot to begin with.


Having just scrolled up a bit, if you're putting in a filter inspired by me do your best to have decent damping - that ~1MHz high Q resonance looks potentially problematic. Oh and also I put my filters prior to any active circuitry (including discrete transistors).

There's a first version described and built for a Philips cd610 mentioned in the first post which sounds fine , it has no filters built in, i just used some mpx filters from radios and rc first order passive filtering after that.For that i mentioned the advantage of larger drive capability for passive filters with using op-amps in parallel .

For this version, I have posted complex input with sine modulating two different square wave signals , but the distortions at 1khz aren't affected by the existence of those filters.Those filters of mine aren't working on output current of the dac, they are buffered and i saw that i don't need too large values to filter those high frequencies square wave signals i've put in the sim.

I used Chris suggestion of generating the dac output impedance too...It doesn't really matter if i use his signal model or my model, the difference is barely 10db, while the transistor bias gives up to 50...80 db difference.

It just takes 10x times more time to sim with a complex input and i can only assess the effectiveness of the filter in wiping out those fast edges.In most cases the sim doesn't show any ringing at all below 1meg, i wasn't curious to investigate the filter as i was more curious of how thd relates to the transistor bias and the base noise level is completely unrealistic, it's like the only player would be the Johnson noise in resistors...
That is why i emphasized at some point somewhere that it's weird that the only transistors that works at modelling with are bc337/bc327 and i tried pretty much all the low noise transistors, even way superior transistors than the bc337. With any other transistor the sim is a complete failure.I actually told Chris that there must be something wrong about the bc337 noise modeling in its own topics.The results are too low.

The only good thing i know is that the circuit works well in its unfiltered version, has a lot of potential and drive and the new added filter inside the feedback loop can be further tinkered in the real world.Every single player or dac would have its own needs for filtering , no two dacs would give the same high frequency content.
I could copy the Abraxalito filters and try with that, but i don't even know what is really there to filter...I just made gross assumptions.
Now the real purpose is to use very low slew rate op-amps but also very low noise , very high CMRR and PSRR and see if it works, that is why i'm building something based on two opa4227, just as a proof of concept.
Attached Images
File Type: png balancedivfilter.png (68.5 KB, 166 views)
File Type: png unbalancediv.png (83.5 KB, 167 views)

Last edited by dreamth; 18th April 2019 at 08:44 AM.
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Old 19th April 2019, 11:22 PM   #25
dreamth is offline dreamth  Romania
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just investigating a bit further...
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Old 19th April 2019, 11:25 PM   #26
dreamth is offline dreamth  Romania
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Also some investigations with lower input(some photos in the previous post too), higher freq(10khz) and a bit more agressive filter.I wonder how capable are my ears to hear a less perfect filter.
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File Type: png ivact15.png (77.9 KB, 14 views)

Last edited by dreamth; 19th April 2019 at 11:27 PM.
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Old 21st April 2019, 01:26 PM   #27
dreamth is offline dreamth  Romania
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Quote:
Originally Posted by abraxalito View Post
Having just scrolled up a bit, if you're putting in a filter inspired by me do your best to have decent damping - that ~1MHz high Q resonance looks potentially problematic. Oh and also I put my filters prior to any active circuitry (including discrete transistors).
I tried some of your filters inside my circuit and for some reason LtSpice is refusing to show me a clear waveform, but LtSpice is doing the same with all sorts of filters...On Bode plot i can see just that they are very aggressive filters , cutting anything over 20 khz, and the phase response has also a very sharp evolution.

I have a very hard time trying to make Lt Spice even to work on some filters...and anyway there are lots of things working very bad while Spice would say it's ok and viceversa, things that LtSpice is refusing completely to sim that wors very well...I'm just following my instincts, but indeed, in pretty much all my filters Spice shows all sorts of resonances btw 100k...1 megahertz.the only worrying thing to me is that the noise looks to be too low in most instances .

Last edited by dreamth; 21st April 2019 at 01:28 PM.
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Old 22nd April 2019, 12:02 AM   #28
dreamth is offline dreamth  Romania
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Well....today i went into fet transistors and fet op-amps and i had a huge surprise, i mean, i didn't need complex filters to get everything right:
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Old 22nd April 2019, 12:04 AM   #29
dreamth is offline dreamth  Romania
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to be continued:
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File Type: png ivfet20.png (90.7 KB, 3 views)
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Old 22nd April 2019, 12:06 AM   #30
dreamth is offline dreamth  Romania
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and again...
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File Type: png ivfet21.png (69.0 KB, 10 views)
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File Type: png ivfet30.png (88.7 KB, 10 views)
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