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To provide a full perspective of what the NAT Signature Phono is, I need to add a few essential elements. First and foremost the Signature Phono is battery powered, making it the only one in this series (but not that uncommon among this breed where utter silence of operation is critical to preserve the minute signals originating from the cartridge). It includes five independent batteries with a smart and worry-free charge management circuit. Battery life expectancy according to the maker is between three and five year).

When plugged in, a small blue LED indicates that the logic charge circuit is activated. Once turned on and after a 45 second warm-up cycle, the ‘operate’ LED confirms that the Signature Phono is ready for action. The ‘charge’ LED will only turn on when the batteries are charging. This never actually happened during my listening sessions and only after I turned off the unit. NAT recommends to keep the Signature Phono plugged in at all times and to let the logic circuit manage the batteries for optimized sound quality and life extension. Play time from a fully charged set of batteries should average fifteen hours, more than enough in most cases to never require recharging the batteries during listening. Based on my experience and the completely fool-proof operation of the NAT over the past few months, it seems like a very smart thing to do. Just plug it in and let the machine worry about itself.

The NAT circuit uses six 6N23P-EV tubes (three per side) in a zero feedback design (even local feedback is avoided) and the Signature Phono comes standard with high-quality NOS tubes which are estimated for 5,000 to 10,000 hours each. The designer indicates that any 6DJ8 or E88CC may be freely substituted if desired. These triodes and the complete absence of any feedback were selected to provide ultimate transparency and tonal truthfulness. Listeners will have to be the judge of that claim but just as with the Symmetrical preamplifier, NAT took particular care with their power supply implementation as the foundation for great performance and followed it with a simple yet perfectly executed circuit.

Another particularity of the NAT versus the other phono stages I reviewed in this series thus far is the use of a premium Jensen step-up transformer for the low-output MC stage. This transformer boosts gain from 40dB for MM and high-output MC cartridges to 60dB for the low-output MC input. The latter is what I listened to most of the time using a Denon DL103 but I did give the MM input a reasonable listen using a Grado Sonata I. I suspect the presence of the Jensen transformer to not be completely innocent with regard to the overall performance of the NAT (in a positive way) but also be responsible for the higher than usual sensitivity to proper loading that I'll cover later.

Finishing up introductions, the Signature Phono uses completely passive RIAA equalization with no feedback and is claimed to achieve an impressive >90dB signal-to-noise ratio thanks in part to the battery power supply. On paper this figure may not seem that stout compared to the claimed 140dB of the Esoteric E03 but in practice the Serbian proved far quieter than the Japanese even with the ground lifted on the latter so be wary of specs. They don't necessarily tell the whole story. In this particular case, the tubed unit proved significantly quieter than the solid-state E03 and even more silent than the Flight Phono which was my reference so far.

One of the upsetting factors I noted in my review of the NAT Symmetrical preamp was a very loud pop that could be heard at turn-off and had me afraid I had blown a speaker the first time I experienced it. The Signature Phono is almost fully free from this issue. I say almost because on my high-sensitivity Zu Essence speaker, I could hear a faint pop and therefore still recommend muting the Signature Phono at turn-off for good measure. If you forget, you will not go into cardiac arrest as with the Symmetrical.