https://www.ti.com/lit/gpn/pcm4222Do you have some measurements/specs of this one?
https://www.ti.com/lit/gpn/pcm4222Do you have some measurements/specs of this one?
Does Matt recommend terminating the unused outputs?
Well, they're not exactly shorted. He has 75 terminators attached.I second that question. Terminating inputs make sense, but I doubt the unit appreciates having its output shorted.
No, that is what it says on top of the box. And in the specs:56Vp-p looks too low even to test 50W@8ohm amps. Is there any mistake about this number, maybe 56VRMS yet?
That's my bad. Four of the terminators had come with the unit and I must have blindly put them on when it first arrived (months ago). Fortunately it makes no difference as I just tested.Well, they're not exactly shorted. He has 75 terminators attached.
I'm not exactly sure of the topology of the internal output interface, but in some cases a dual op-amp is used to generate both polarities and placing a big (dummy) load on one side could effect the operation of the other side, negatively.
I don't know that's the case here, but it did catch my eye in the photograph.
edit: I see these are dual OPA1612 op-amp sections with 47 ohm build-out resistors.
Dave.
Hmmm. I can't figure out how! The horizontal axis in FFT is fixed at 20 kHz for 48 kHz sampling.Can you test it for aliasing? That would be nice to know as well in a bit of test gear.
Feed it a sweep that goes up to 80 khz. You'll see what shows up below 24 khz which is the aliasing you are interested in. That is what I did here.Hmmm. I can't figure out how! The horizontal axis in FFT is fixed at 20 kHz for 48 kHz sampling.
When I was testing it, the clipping indicator came on when I was messing with it which led me to believe without the attenuator it couldn't handle more than 0.7 volt. New value for 2 volt input below.@amirm My recollection is that FS input was 2 V rms. Why test at 0.7 v rms?
Here is 2 volt RMS:Can you test the noise floor modulation with 0dBFS (2 V rms) and - 60dBFS?
There is no way to get a cumulative graph like that from QA401 software. It is just a free-running FFT for each FFT window worth of audio samples. So such a sweep just shows what is there at that moment and then erases quickly.Feed it a sweep that goes up to 80 khz. You'll see what shows up below 24 khz which is the aliasing you are interested in. That is what I did here.
View attachment 31973
You also could feed the ADC fixed tones above 25 khz at a few frequencies and see what the ADC picks up. That would be easier to see in an FFT using the FFT part of the QA401 software.
There is no way to get a cumulative graph like that from QA401 software.
The fixed ultrasonic tones should still work. Play a fixed tone at 30 khz and see how much is in the FFT at 18 khz. So on and so forth.There is no way to get a cumulative graph like that from QA401 software. It is just a free-running FFT for each FFT window worth of audio samples. So such a sweep just shows what is there at that moment and then erases quickly.
So that would let you use it with REW, Virtins MI or various other software. Sounds like a good addition.If you find yourself limited by the QA401 Analyser software, you can always fall back to ASIO401 with your analyser of choice. (Full disclosure: I wrote that driver.)
I run my QA401 with ARTA and ASIO401 and it works just fine. (I like to use the FR2 mode of ARTA.)If you find yourself limited by the QA401 Analyser software, you can always fall back to ASIO401 with your analyser of choice. (Full disclosure: I wrote that driver.)
When I was testing it, the clipping indicator came on when I was messing with it which led me to believe without the attenuator it couldn't handle more than 0.7 volt. New value for 2 volt input below.
Here is 2 volt RMS:
View attachment 31974
So performance drops about 3 dB or so from what I reported.
Here it is at 60 dB lower:
View attachment 31975
So there is about 20 dB of noise floor modulation based on me eyeballing it at 1 kHz.
Also note that there is no compensation for FFT window for the voltmeter. It shows 1.89 volts with Hann window even though the input is 2 volts. Flat top window fixes that but of course screws up the FFT. AP software compensates for this.
I waited a long time for this test. Fortunately, my $ 50 ASUS sound card is a little better than the QA. The best and cheapest method is still a pure oscillator and a notch filter.