I acquired this interface cheaply and have performed some measurements to evaluate its suitability for measuring amplifier etc. https://www.presonus.com/products/firebox This is an example of a several generations old budget audio interface.
Measurement setup is with a 3 conductor jack lead input 3 and output 1L. It's worth noting that the outputs of the firebox are not diffirential and are instead 'impedance balanced' which means that the -leg is connected to ground via a resistor instead of driven. I normally use the external AC adapter to power the interface as otherwise its flaky to initialize on boot-up without it but in loop-back I can see significant AC harmonics:
Remaining tests are conducted powered from Firewire only. The optimal level for this interface in loop-back (don't know if ADC or DAC limited) is around 5.7V . Beyond this voltage the performance utterly collapses with harmonic and non harmonic tones across the spectrum. I have conducted further testing at 4V as this is close to optimal and also used in ASR testing.
Next lets compare 96kHz sample rate:
and 48 kHz:
So noise is elevated by using 96kHz sample rate even if low and high pass filtering to 20-20kHZ is used. Further tests done at 48kHz sample rate:
Low frequency test:
High Frequency test:
Results:
22.5Hz: THD = 0.0026% THD+N = 0.0054%, SINAD = 85.4 dB (noise limited)
1kHz: THD = 0.0020% THD+N = 0.0048%, SINAD = 86.4 dB (noise limited)
7.75kHz: THD = 0.0028% THD+N = 0.0052%, SINAD = 85.7dB (noise limited)
Equivalent noise input voltage at 1kHz: 172uV
Looking at other loop-back measurements it looks to be possible to get around 94 dB SINAD from modern interfaces in the same price range so this seems OK for something so cheap and old.
Measurement setup is with a 3 conductor jack lead input 3 and output 1L. It's worth noting that the outputs of the firebox are not diffirential and are instead 'impedance balanced' which means that the -leg is connected to ground via a resistor instead of driven. I normally use the external AC adapter to power the interface as otherwise its flaky to initialize on boot-up without it but in loop-back I can see significant AC harmonics:
Remaining tests are conducted powered from Firewire only. The optimal level for this interface in loop-back (don't know if ADC or DAC limited) is around 5.7V . Beyond this voltage the performance utterly collapses with harmonic and non harmonic tones across the spectrum. I have conducted further testing at 4V as this is close to optimal and also used in ASR testing.
Next lets compare 96kHz sample rate:
and 48 kHz:
So noise is elevated by using 96kHz sample rate even if low and high pass filtering to 20-20kHZ is used. Further tests done at 48kHz sample rate:
Low frequency test:
High Frequency test:
Results:
22.5Hz: THD = 0.0026% THD+N = 0.0054%, SINAD = 85.4 dB (noise limited)
1kHz: THD = 0.0020% THD+N = 0.0048%, SINAD = 86.4 dB (noise limited)
7.75kHz: THD = 0.0028% THD+N = 0.0052%, SINAD = 85.7dB (noise limited)
Equivalent noise input voltage at 1kHz: 172uV
Looking at other loop-back measurements it looks to be possible to get around 94 dB SINAD from modern interfaces in the same price range so this seems OK for something so cheap and old.