I have updated my REFERENCE•M2 Systems. I want to emphasize the following updates:
USB and DX (DBNC) Version. The DX (DBNC) version is now complemented with a USB version (here) which leverages USB-C Filter, RF•STOP SE Signal Filter x2 (below) and the amazing benefits of PURE•DC power on the Mojo2 USB signal path. The USB version has more discernable digital hash than DBNC despite the increase in resolution to 32-bits (USB) from 24-bits (SPDIF). After many listening tests, I am comfortable to say that the more expensive DX version is still my favorite.
PGGB•IT! EQ. My own setup is now exclusively headphones (HE1000se) to ensure I have the most reliable objective comparison during testing. Version 7 of PGGB includes updated equalization so I evaluated it. Wow. I was not prepared for the improvement in tonality and bass. I used the Harmon2018 convolution available in the online AutoEQ app. The processing overhead of EQ is minimal so I redid my reference library of music. I strongly recommend users EQ'ing their PGGB tracks!
PURE•DC Mode. In 5V Pure mode, the IanCanada design fluctuates between 5.25V and 4.65V (recharge limit) as it powers Mojo2. 5V USB power has an industry specification voltage range of 5.25V to 4.75V. The low voltage discrepancy of 0.1V is sufficient to occasionally trigger Mojo2 to power-down (undervoltage). I have requested a design change from IanCanada to resolve this. In the meanwhile, I recommend disabling Pure mode and this keeps the Mojo2 happy at a constant ~5.25V.
RF•STOP SE Signal (EXTREME) x2. While testing the M2 USB version, the lower noise floor of PGGB 32-bit output revealed Mojo2 sensitivity to RF noise from my downstream components. Hence, I now recommend adding another RF•STOP SE Signal Filter in series.
Solid-State vs Tube Head-amp. At the Toronto show I used my Benchmark HPA4 and Accustic Arts Pre-II as headphone amplifiers. Show visitors commented that HPA4 (solid-state) was faster and more transparent than the Pre-II (tube). Since the show, I have been using the two amps independently and in series (leveraging the clean high power of the HPA4 as input to the Pre-II). I will make the following comments:
- Tubes introduce euphonic distortion and make for a sound that is compelling, although less resolving and sluggish. The primary advantage of tubes is the air-gap in the signal path (inside the tubes) that aids in filtering downstream RF noise and this improves overall transparency.
- Solid-state amps tend to sound a tad clinical and their galvanic path from input-to-output allows ingress and passage of downstream RF noise. However, when presented with a high resolution PGGB signal and extremely well filtered chain using RF•STOP SE Signal Filter x2, the sound from my HPA4 is magical.
Hence, the Benchmark HPA4 really is an endgame choice for anyone considering my REFERENCE•M2 system with headphones.