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Any chance for setup pics?
Also have you tested the IVY-III from a signal generator?
Nothing on the IVY-III changes simply because you changed volume on the DAC...
One thing to keep in mind when measuring IVY is that it's output are very low impedance and has a very high slew rate. You must measure it with care to add some series resistance to your probes.
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I have more simple test which triggers these patterns: - 1kHz sinus -1dB via I2S from BeagleBone -- I believe this can reproduced using SPDIF too - unbalanced outputs from IVY connected to the unbalanced inputs of ESI Juli@ - default ES9018 registers settings except volume registers 0-7 - value in volume registers set to 30 before the actual measurements - screen shot - change value in vol.reg. 0 (or 1 for other channel) to 31 and then back to 30 - repeat Here is the animation of 10 measurement screenshots: http://i.imgur.com/42PDIqU.gifThis is reproducible with other channel too. (Russ, I'm going to post pics and try other things you've requested later)
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The only thing that comes to mind is that the IVY is a device that uses lots of negative feedback, the Legato uses none.
It is also hard to rule out interaction between your 40db amp and the IVY.
Not sure that there is really anything to be done - and the signal is still very clean. :)
You could try some comp caps on the final OPA1632 - but I seriously doubt that would help, and could actually hurt.
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There was no +40dB amp used in these measurements ... just direct connection between unbalanced DAC outputs and the unbalanced soundcard. The noise floor of the used soundcard is -101dBV, but the noise floor in the narrow band of FFT bin is under -135dBV. IVY: powered by +/-15V DC voltage on unbalanced IVY outputs: 325 mV rms AC (1kHz) The issue is still there with SPDIF and original microcontroller. Shots of my setup: https://drive.google.com...sbk5OZGc&usp=sharing
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Some random ideas, but non-trivial to realize:
- replace OPA1632 with another fully-differential amplifier - there are couple of other choices
- or maybe try different PCB layout for this OPA1632?
Using IVY III the distortion is pretty stable with near maximum levels, but with lower levels it varies a lot...
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Miero - you have not done nearly enough to convince me of the need to change anything. :)
In any case you can't lay the blame at the opa1632 when it requires some external change to see what you see.
If there where a problem with the opamp it would show up even at 0dbfs
What you are seeing is likely due to the high bandwidth of the IVY and some HF aliasing with your ADC or possibly some IMD. You could experiment with sharpening the LPF etc - I will leave that to you. :)
I am using the device exactly as it is intended.
You could experiment with some circuit changes yourself if you like, but I am not really convinced there is any real issue here.
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Adding some sound recording of the (d)effect: The 1kHz -45dB signal from DAC was recorded via +40dB amplifier, then 1kHz fundamental was removed by notch filter and the result was then amplified by 20dB in SW. During recording I've been turning SW volume knob in the range eta. +/-3dB. https://soundcloud.com/m...tion-patterns-of-ivy-iii
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Miero - it seems most of your questions really should be directed to ESS - or perhaps Audio Precision. :) That fact that nothing ill occurs until after you start to adjust volume gives you all you need to know to ask ESS what could be going on.
In any case given your test rig - you can't expect it's ADC to cope correctly with the HF aliases which are very likely to be present with the IVY (as it would be with an opamp I/V stage that does not employ a brick wall filter) Brick wall filters can produce their own ills.
Legato on the other hand is not an opamp type I/V. It can't really run out of NFB - so it has an inherent brick wall - which is much more friendly to a measurement rig like yours.
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Not quite right, it differs also while I2S signal is coming to Buffalo III which is (re-)started by disconnecting its power. The harmonics are different (and stable) on each DAC (re-)start: http://i.imgur.com/CAzdfTl.gifI'll try to test it with amplitude modulated signal later.
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Here is fancy thing I've tried: B3 playing -30dB sinus at 1kHz -> Legato balanced Vout+ and Vout- (with capacitors to remove DC offset) -> V/I via 2x 680Ohm to IVY inputs -> IVY unbalanced -> +40dB AMP -> ESI Juli@ soundcard rms noise: -112dBV harmonics 100dB bellow signal => no distortion patterns as if IVY is connected directly to B3. Edited by user Thursday, November 6, 2014 10:53:47 PM(UTC)
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There is a very big clue there. :)
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A brickwall filter created by Legato?
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It looks like Legato in this case arrangement is simply filtering out the HF which was likely causing some IMD and aliases in the ADC. Legato has a natural brick wall.
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It looks like our replies crossed. :) Yes I am very certain that is what it is.
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BTW you could use much smaller resistors as input to IVY - 221R would actually be closer to the B3 output impedance.
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Yes, it is possible. The brickwall inside ADC does not work well.
Tomorrow I'll try to switch IVY and Legato...
BTW: the pre- and power amplifier could be also HF sensitive... so there might be good reason to use Legato instead of IVY.
But balanced headphones should be fine with such HF noise :-)
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I wish there would be I/V with brickwall and rms noise level better than -120dBV... :-)
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You have to think about how that signal is handled downstream - and it's amplitude. Which is tiny, but it is not hard to upset an ADC. This about the size of the harmonics (aliases) you saw... that means that the overtone which created it was also quite small. Not generally something to worry over. Also you could very easily filter it out! You have the IVY-III schematic - you should be able to see how it would be quite simple. :) Be warned though - doing so has nasty effects in other ways - like slew rate and phase shifts... ick...
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Do you mean adding another GHz opamp just to remove HF? :-)
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