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Here is a full set of measurements of the miniDsp 2x4 HD that I purchased a few years ago. The version I have is the old revision.
I tested the device as ADC, DAC (both in unbalanced and in balanced operation) and as standalone unit with dsp on and dsp off. Digital connection was over USB.
Unbalanced DAC:
THD+N: -96dB
Output 1 looks a bit worse than output 2. When only one output was fed, THD+N improved to about -99dB. When all 4 were active, THD+N worsened to -92dB.
THD vs level:
Dynamic Range: ~100dB
Frequency response:
THD vs frequency:
IMD:
Unbalanced ADC:
THD+N: -84dB
THD vs level:
Dynamic Range: ~87dB
Frequency Response:
THD vs Frequency:
IMD:
You can combine two unbalanced outputs into a single differentially balanced one by inverting the phase on the 2nd output. With the correct adapter cables, the miniDsp 2x4 HD turns into a fully balanced 1 IN, 2 OUT device.
This should improve CMRR, but also reduce common mode noise coming from the device itself. Let's see if this is in fact the case...
Balanced DAC:
THD+N: -100dB
Wow, did this ever clean up the signal. THD+N improved by 4-5dB but the actual measurement looks a lot cleaner than before. Most of the random noise spikes are now gone!
THD vs level:
Dynamic Range: 105dB
Frequency Response: (same as unbalanced)
THD vs Frequency:
IMD:
Balanced ADC:
THD+N: -96dB
Wow! THD+N improved by 12dB compared to the unbalanced input.
THD vs level:
Dynamic Range: 105dB
Frequency Response: (same as unbalanced)
THD vs Frequency:
IMD:
Unbalanced Round trip ADA: 76dB THD+N
Some users have commented that the performance of the unit degrades when you activate filers, so I went ahead and dialed in an input EQ as well as an output EQ.
I made sure that the overall gain at 1KHz stayed at unity and measured the channel with EQ active against the 2nd channel without EQ:
We can see that performance itself doesn't change. Most of the noise comes from the ADC, so you will increase that if you dial in positive EQ and decrease that by dialing in negative EQ.
This tells us that correct gain staging is particularly important for the input side of this unit, but the use of DSP itself does not affect the performance of the unit.
Conclusion
The dsp capabilities of the unit are nice and the new software looks a lot sleeker than the old one. When used in balanced mode, performance is quite good, but you would need a second unit if you're feeding it via RCA and you want to run it in stereo. EQ reduces headroom, which technically brings down the performance of the DAC in a theoretical sense, but since the ADC is the limiting factor here, that effect should be negligible.
If you want to use the device on your mains and you feed it via the RCA inputs, it's important to have gain staging on point, otherwise it is possible that the 2x4 "HD" audibly degrades performance. In balanced mode you get some extra leeway and I wouldn't worry much about it then. Still, I would've wished to see better noise performance overall. The unbalanced graphs look really dirty with noise spikes all over the place.
I tested the device as ADC, DAC (both in unbalanced and in balanced operation) and as standalone unit with dsp on and dsp off. Digital connection was over USB.
Unbalanced DAC:
THD+N: -96dB
Output 1 looks a bit worse than output 2. When only one output was fed, THD+N improved to about -99dB. When all 4 were active, THD+N worsened to -92dB.
THD vs level:
Dynamic Range: ~100dB
Frequency response:
THD vs frequency:
IMD:
Unbalanced ADC:
THD+N: -84dB
THD vs level:
Dynamic Range: ~87dB
Frequency Response:
THD vs Frequency:
IMD:
You can combine two unbalanced outputs into a single differentially balanced one by inverting the phase on the 2nd output. With the correct adapter cables, the miniDsp 2x4 HD turns into a fully balanced 1 IN, 2 OUT device.
This should improve CMRR, but also reduce common mode noise coming from the device itself. Let's see if this is in fact the case...
Balanced DAC:
THD+N: -100dB
Wow, did this ever clean up the signal. THD+N improved by 4-5dB but the actual measurement looks a lot cleaner than before. Most of the random noise spikes are now gone!
THD vs level:
Dynamic Range: 105dB
Frequency Response: (same as unbalanced)
THD vs Frequency:
IMD:
Balanced ADC:
THD+N: -96dB
Wow! THD+N improved by 12dB compared to the unbalanced input.
THD vs level:
Dynamic Range: 105dB
Frequency Response: (same as unbalanced)
THD vs Frequency:
IMD:
Unbalanced Round trip ADA: 76dB THD+N
Some users have commented that the performance of the unit degrades when you activate filers, so I went ahead and dialed in an input EQ as well as an output EQ.
I made sure that the overall gain at 1KHz stayed at unity and measured the channel with EQ active against the 2nd channel without EQ:
We can see that performance itself doesn't change. Most of the noise comes from the ADC, so you will increase that if you dial in positive EQ and decrease that by dialing in negative EQ.
This tells us that correct gain staging is particularly important for the input side of this unit, but the use of DSP itself does not affect the performance of the unit.
Conclusion
The dsp capabilities of the unit are nice and the new software looks a lot sleeker than the old one. When used in balanced mode, performance is quite good, but you would need a second unit if you're feeding it via RCA and you want to run it in stereo. EQ reduces headroom, which technically brings down the performance of the DAC in a theoretical sense, but since the ADC is the limiting factor here, that effect should be negligible.
If you want to use the device on your mains and you feed it via the RCA inputs, it's important to have gain staging on point, otherwise it is possible that the 2x4 "HD" audibly degrades performance. In balanced mode you get some extra leeway and I wouldn't worry much about it then. Still, I would've wished to see better noise performance overall. The unbalanced graphs look really dirty with noise spikes all over the place.
