But back to the REF10 NANO. As far as termination is concerned, we are on the safe side. It has four outputs on the rear: two 50 Ohm and two 75 Ohm. Correct termination also requires the right cable and the right connectors. With a clock rate of 10 MHz, clean termination across the entire signal path is extremely important - I can't emphasize this enough! Mismatches here can ruin the signal quality to such an extent you might as well do without a master clock altogether. I tried switching between 50 and 75 ohms and the difference is clearly audible. You lose sharpness and precision. At the outputs of the REF10 NANO, a square-wave signal with extremely high edge steepness is output in contrast to the usual sine wave signals of other clock generators available on the market. The steep edge of the square-wave clock signal should lead to significantly faster and more stable synchronization of the connected devices, which in turn should result in less jitter induction in the receiver circuit. According to MUTEC, this is one of the main reasons for the sound improvement achieved by the REF10 NANO. The heart of every master clock is, of course, the oscillator used. Here we are primarily interested in the frequency stability of the generated signal. A distinction is made between long-term and short-term stability. Long-term stability refers to the amount by which the absolute clock frequency drifts over a longer period of time, such as days, weeks or months. At MUTEC, they believe that short-term stability, i.e. the accuracy in the extremely short time interval between samples, is the decisive factor for the sound quality of digital audio material. Changes in the short-term stability are described metrologically with phase noise and jitter. The short-term stability of an oscillator depends significantly on the ambient temperature. MUTEC therefore uses OCXO oscillators in all REF10 variants, which are manufactured in Germany. OCXO stands for "Oven Controlled Crystal Oscillator". The quartz crystal and all other components of the oscillator are located in a heated chamber, which is always kept at a constant temperature, even if the temperature outside changes. The temperature in the chamber is individually set to the turn-over temperature of the quartz material used, i.e. to the temperature at which the quartz exhibits the least frequency changes and therefore provides the highest frequency stability.

The REF10 NANO uses the same oscillator module as its larger siblings. The difference lies in the degree of selection. With the help of highly specialized and cost-intensive measurement technology, individual oscillators are selected from larger production quantities for the highest quality level, the REF10 SE120, which have extremely low phase noise values of at least -120 dBc measured at a 1-Hertz distance from the carrier frequency of 10 MHz. The REF10 achieves values of at least -116dBc in this central metrological aspect, while the REF10 NANO has to make do with values better than -112dBc. Sounds a lot worse at first. But as Mr. Peters, CEO of MUTEC GmbH, told me, in practice the measured values of many REF10 NANO are closer to the REF10 than to the guaranteed, lower minimum value. Extremely low-noise clock distribution and amplification circuits are intended to ensure that the reference signal of the OCXO is really available at the four outputs of the REF10 NANO with virtually no loss. All output stages of the REF10 NANO are galvanically isolated to prevent hum interference from ground loops. The REF10 NANO uses virtually the same sophisticated circuitry as its larger siblings. Just as important as the quality of the oscillator is the quality of the power supply for digital audio signal processing. Here, the REF10 NANO has to make do without the toroidal transformer and the generous filter capacities of its big siblings and instead uses an off-the-shelf switching power supply module in the first stage. But what comes next is quite something and hardly differs from the REF10. We find extremely low-noise, multi-cascaded voltage regulators of the latest generation, which are said to be optimized down to the sub-Hertz range. The highly sensitive OCXO oscillator receives its supply voltage isolated from the rest of the circuit to prevent mutual interference, and each of the four outputs also has its own voltage regulation. All the electronics are neatly arranged on a large circuit board and housed in a simple steel casing. At this point, the steel housing has the advantage over the often used aluminum housings that the shielding against external, high-frequency interference is considerably better.

The REF 10 NANO is easy to operate. The device is switched on and off using the power switch at the rear. After switching on, the red LED on the outer left lights up. In addition, a blue LED flashes until the heater has warmed the oscillator to its operating temperature; it then lights up continuously. Even if the heating process is completed after about one minute, MUTEC recommends giving the REF10 NANO about 20 to 30 minutes before listening to music so the entire oscillator is completely warmed up and the highest frequency stability is achieved. The outputs are selected using the rotary switch on the front panel and switched on or off by pressing it. Four white LEDs indicate which outputs are active. On the rear, in addition to the four outputs, there is also an input labeled "DC" for connecting an external power supply unit with 15 volts/1 ampere. If an external power supply unit is connected, the REF10 NANO must be disconnected from the mains using the mains switch or, even better, the mains cable must be unplugged immediately. Two LEDs on the front panel visualize which type of power supply is currently active. Unfortunately, MUTEC has installed a socket at this point, I'm guessing Lumberg, which is not compatible with the usual hollow plugs. MUTEC does include a suitable plug, but it might not be everyone's cup of tea to solder a suitable cable. I quickly came up with an adapter. For the listening test, my two cascaded MUTEC MC3+ USB are the "natural" playing partners for the REF10 NANO. From my PS Audio DirectStream DAC it goes via S/P-DIF to the MUTECs and from there via USB to the Diretta Lucia Piccolo Bridge, which receives the music data via Ethernet from my Windows server with JPLAY. The two unused 50 Ohm clock outputs - the MUTECs work with 75 Ohm termination - are switched off with the rotary switch on the front panel to reduce unnecessary interference emissions as far as possible. My two MUTEC MC3+ USB are set to "externally referenced re-clocking mode". The two green LEDs "EXTERN" and "RE-CLK" therefore light up below "Mode" and the two upper blue LEDs "MAIN REF" and "RE-CLK REF" light up below "STATUS". The REF10 NANO leaves a clear, easily audible "footprint" in my chain in the categories of spatiality, musical flow and transparency. The first thing I hear is how the spatial impression improves with the REF10 NANO. In the "Intermezzo from Goyescas" by Enrique Granados with the New Philharmonia Orchestra conducted by Rafael Frübeck des Burgos (Decca Legacy Volume One - FIM UHD), the wind instruments no longer sound so diffuse and oversized, overwhelming everything from the depths of the room, but are better defined in terms of their size and placement in the orchestra, which lends clarity and conciseness to the spatiality of the recording. In the "Simple Symphony, op 4 - Boisterous Bourree" by Benjamin Britten with the TrondheimSolistene (The Nordic Sound - 2L audiophile reference recordings - 24/352), the minimal reverberation of the recording location, a church, is now even better emphasized, giving the spatial expression unmistakable depth. In addition, there is significantly more airiness, from which the fine details of this recording, such as the filigree pizzicato of the individual string groups, benefit in particular and which simply allows us to participate even more intensively in the special atmosphere of this recording.