This unit was loaned to me by a friend to test
The Denafrips Pontus 2 is an R2R DAC sitting in the middle of Denafrips’ product lineup. Coming in at just over $1850 USD at the time of writing, it’s a revision to the existing Pontus 2 that offers some upgrades primarily to the power supply and filtering areas, but also to the digital signal processing itself as we will see shortly.
One of the changes is that the 12th Anniversary editions of the Pontus 2 and other products now offer a ‘True NOS’ feature.
Previously I’d demonstrated that the Denafrips DACs were in fact not ‘NOS’ (Non-Oversampling) capable even though it was advertised as such, I additionally showed that the NOS mode on the Denafrips DACs did not provide a resulting output similar to an actual NOS DAC.
If you’re interested you can have a look at my measurements of the Terminator Plus, toward the end of the post:
Are the new 12th Anniversary Editions actually NOS? Well….they’re closer to it, but technically still not NOS….more on this later…..
The Pontus 2 has a more than substantial linear power supply, accepting all regional input voltages, and keeping the PSU encapsulated within a shielded compartment within the DAC.
You can see in the above image the massive bank of capacitors, providing an extremely low impedance, low noise supply to the DAC.
In terms of why there are so many capacitors, it is done this way due to the fact that many smaller caps in parallel provides lower equivalent series resistance than a larger cap(s) in series.
Upon looking at the main DAC itself, we can see even MORE capacitors in a large bank, with voltage regulators following.
The DSP board itself which handles processing and digital inputs, is galvanically isolated to prevent noise from the digital source having any impact on resulting performance or sound quality.
This then feeds two FPGAs, one for each channel, which in turn control the actual R2R ladder converters. Each channel is fully balanced.
It is worth noting though that the Denafrips DACs do not have an output buffer stage, and instead the ladders drive the outputs directly. This could be argued to be more ‘transparent’ as there is less circuitry in the signal path, but does result in an extremely high output impedance of 1250Ω on XLR and 625Ω on RCA, as opposed to a more typical 50Ω or so as you’d see from most DACs.
This is unlikely to be an issue, as most preamps and amps will have an input impedance in the realm of 50,000Ω or more, but there are some designs where this is not the case.
The Topping A90 for example is only 2000Ω input impedance, so do check your amplifier to make sure you won’t have any issues here.
– Audio Precision APx555 B-Series Analyzer with 200kOhm input impedance set unless otherwise specified
– USB Source: Intel PC via intona 7055-C isolator
– All measurements shown are with the DAC connected via SPDIF unless otherwise specified
– Measurement setup and device under test are running on regulated 230V power from a Furman SPR-16-Ei
– Audioquest Mackenzie XLR and RCA interconnects
– Intona Reference Impedance Characterized USB Cable
– Audioquest Carbon SPDIF and AES cables (1.5m)
– Exact analyzer/filter configurations for each measurement are detailed in the full reports
– Measurements shown are with the DAC in ‘OS’ mode unless otherwise stated
Full Measurement Reports:
Reports available here:
Dynamic Range (AES17): 118.1dB
IMD SMPTE: -79.6dB
Noise Level RMS: 2.7uVrms (20hz-20khz) / 5.1uVrms (20hz-90khz) – 12.1uVrms (20hz-1Mhz)
DC Offset: 3.1mV active, 3.5mV idle
Delay: 12.36ms (Nice! Latency used to be an issue on Denafrips DACs but it seems the issue is resolved)
1khz Sine, 0dBfs, XLR Out (OS):
1khz Sine, -3dBfs, XLR Out (OS):
Performance does increase a bit when not at max output (this is quite common and is why I do -3dBfs tests).
1khz Sine, 0dBfs, XLR Out (NOS):
Performance in NOS in terms of THD+N is seemingly identical to OS.
1khz Sine, 0dBfs, RCA Out (OS):
We do lose a notable bit of performance when using RCA, but given as the increase is predominantly in the 2nd order harmonics, this might actually have a subjectively pleasant effect and you may find the RCA outs to sound slightly ‘warmer’. Try both and see which you prefer.
Low level signal output:
Filter Ultrasonic Attenuation:
Note how there is a dip at exactly 705.6khz in NOS…..
This shouldn’t be there
Idle Noise FFT:
THD+N vs Frequency:
The sudden jump past 15khz or so is a bit unusual here. Usually we see either a relatively flat response, or a slow rise into the higher frequencies.
It seems Denafrips has fixed the transfer function issue that existed previously, which caused DACs to sometimes output incorrect levels in test conditions.
Intersample Overs Test:
When DACs oversample, they can sometimes encounter a situation where the reconstructed/interpolated waveform goes above 0dBfs (the maximum possible digital value).
This is particularly common with poorly mastered music that has been ‘brickwalled’ (To clarify: I mean brick-walled as in the loudness-wars term, not brick-wall nyquist reconstruction filters https://en.wikipedia.org/wiki/Loudness_war ).
DACs ideally should have a few dB of digital headroom to accommodate this and reconstruct properly, and many such as Chord, RME, Benchmark etc do, and will output signals with intersample overs without any issue/distortion.
But many do not do this, either out of lack of awareness of the issue or because doing so usually means sacrificing a few dB of dynamic range and/or THD+N performance. And as a result will not be able to properly reconstruct these signals.
We can test this by creating a signal that induces intersample overs.
See in the image below. All the 44.1khz samples (squares) are below the 0dBfs limit, but the actual waveform itself, is above this limit.
The Pontus 2 is susceptible to intersample overs, and unfortunately, in a particularly bad way.
Usually in the presence of intersample overs, a DAC will simply clip, as seen below:
However the Pontus 2 does not clip, but instead when a sample value reaches above the maximum, it ‘wraps around’ to the minimum negative value, causing a huge sudden transient which will be very audible and may appear as crackling/popping.
Unfortunately even at just +1dB the issue persists, and you can see where the peaks of the sine wave suddenly jumps/distorts to the bottom, and the bottoms suddenly jump/distort to the top.
If you hear crackling/popping from your Pontus 2 now and then, this is likely why. Hopefully Denafrips can resolve this in a firmware update, but failing that, add -3dB of headroom/volume attenuation in your media player to resolve the issue.
Is it NOS?:
The first thing to do to see if the DAC is NOS is to check the impulse response. A NOS DAC should output a rectangular/square impulse response, sometimes with a rounded profile if there is an analog lowpass filter present.
That looks pretty rectangular, BUT, there’s a few things to note.
Firstly, the left and right channel are not aligned. This is because the Pontus 2 has a small inter-channel phase difference, which at 20khz is just under 8 degrees.
Nothing major, and not actually related to the topic of NOS, but still worth mentioning (again, hopefully something Denafrips can resolve in a firmware update).
Now, whilst the impulse response is rectangular as you’d expect, there are some odd aspects including:
- No overshoot at all (uncommon unless there is an analog lowpass filter present or the device itself has a fairly limited bandwidth)
- The curve at the beginning of rising/falling edge is quite rounded and nearly symmetrical to the curve at the end. With either a low pass filter or limited slew rate, the actual effect would not be this way, and would look closer to the yellow waveform below (though less drastic), with the start being quite sudden/straight and the end being more curved.
This had me scratching my chin, and I suspected that actually what might be occurring is that the DAC is indeed oversampling, but using a ‘zero order hold’ filter at likely 16x or 32x the incoming sample rate. This would in many ways LOOK like NOS, and to be honest, probably there wouldn’t be an audible difference to genuine NOS assuming no other DSP is being done. But still, not technically NOS, and leaves open the question of what other DSP is being done.
To verify, we can just put a signal through the DAC and inspect the output at higher frequencies.
Here’s what it should look like on a genuine NOS DAC:
Note how the components of the square wave descend slowly as you go higher in frequency.
But on the Pontus 2:
Notice that big dip toward the right? That’s unexpected, and upon closer inspection it turns out the dip is at exactly….you guessed it…705.6khz, EXACTLY 16x the input 44.1khz sample rate.
And when changing the DAC to 48khz, it moves to exactly 768khz.
There’s also a little bit of a spike there even when the DAC is idle in NOS mode too:
This shows that the DAC is unfortunately NOT actually NOS, but instead oversampling in a way that mimics a NOS output.
Similar things are done on the RME ADI-2’s ‘NOS’ filter for example (though they’re quite transparent in the manual about what that filter is.)
I now have a higher bandwidth oscilloscope and if I get a 12th Anniversary DAC back in I’ll do some further testing.
Does this matter?
From an audible standpoint? Likely not. I can’t imagine the difference between genuine NOS and ZOH Oversampling at 768khz or higher is going to be audible so long as other forms of DSP and dithering are absent.
But no matter how inconsequential or unimportant something might be, don’t lie to your customers!
If a car doesn’t have a supercharger, don’t call it ‘supercharged’ even if it’s really fast.
If a DAC can’t do NOS, don’t say it can do NOS even if the oversampling mimics it very closely.
To be honest, I’m more concerned about the phase difference between the channels, and the intersample overs being handled in the way they are. This simply should not happen, and due to the massive, full scale transient that is created, if played through a chain without a low-pass/bandpass filtered amp, it could potentially kill transducers.
But hopefully can be fixed via a firmware update.