Measuring Distortion on the Cheap

[tomchr]

My point is opposite. There is only one plot in dBFS and it in fact shows DAC performance, because ADC is much better. It is the limit. The rest is a mere scalling die to input dividers of the ADC.

True that.

Which Cosmos ADC did you get? The "0 grade" with the lowest THD or one of the other ones? Just curious. It doesn't matter greatly, I don't think, but it would be nice to know for completeness.

I am very well aware of units and scalling.

That comment was directed at anyone reading these pages and not directly at you. Sorry I stepped on your toes.

Tom

 

[PMA]

The ADC+DAC combo will have one sweet spot, i.e., lowest spot in the THD vs level graph. Dedicated analyzers like the QA403 and APx500 will have multiple sweet spots due to their input and output attenuators

Your assumption is incorrect. The DAC used has lowest distortion at 0dBFS output level, and the same applies to ADC input. There is no knee and turning point with distortion rising, up to the clipping. We have made direct comparison with QA403 and QA403 noise and distortion results are inferior. Yes, it has big advantage of autoranging and may be operated by (almost) lay person.
Cosmos ADC noise limit is about -124dBFS (flat 22kHz) with highest sensitivity of 1.7V/0dBFS. This clearly defines the intrinsic noise of the ADC part.

 

[tomchr]

Your assumption is incorrect. The DAC used has lowest distortion at 0dBFS output level, and the same applies to ADC input. There is no knee and turning point with distortion rising, up to the clipping.

Sounds like the system is limited by thermal noise then. That's not a bad thing, but it does indicate that there could be room to push further.

We have made direct comparison with QA403 and QA403 noise and distortion results are inferior. Yes, it has big advantage of autoranging and may be operated by lay person.

Who's "we"?

I can compare numbers too, ya know... 🙂 It is clear that the Topping+Cosmos combo performs better at its sweet spot than the QA403. It is further possible that the sweet spot of the Topping+Cosmos combo is wider than that of the QA403.
All I'm saying - to anyone and everyone who reads these pages now or years from now - is: Be careful when comparing the graphs as 0 dBV may or may not be equal to 0 dBFS. This makes it impossible to tell exactly how much better the Topping+Cosmos combo is. That is all.

The QA403 doesn't do auto-ranging very well. Unlike the AP, you can't just plug any signal in and have it go. You have to fiddle with the attenuator settings manually. It does offer a form of "poor man's auto-ranging" in the automated test mode, but that only applies to sweeps.

Have you tried Cosmos' own DAC? They have one based on the same ES9039Q2M DAC chip as the Topping D10s. $139 from Cosmos as I recall. So a few bucks more than the Topping.

Tom

 

[tomchr]

My point is opposite. There is only one plot in dBFS and it in fact shows DAC performance, because ADC is much better. It is the limit.

I haven't said anything to the contrary so I don't understand why you're arguing.

My point is that one should be careful comparing two graphs where one is in units of dBV and the other in units of dBFS.

Tom

 

[Gruesome]

All very interesting and helpful. Regarding comparisons: What irks me is when dBFS is used without specifying what 'Full Scale' is in physical units. That, and leaving out the denominator in spectral density units, like writing 'dB' when dB/sqrt(Hz) is meant.