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From a 21st-century convenience perspective, the only note which Nagra's Jazz might at least for some play slightly out of tune is the lack of a numerical volume readout. The legacy laboratory aesthetics and modulometer conspire against it. But today's shopper could expect and demand the convenience of volume settings which are precisely repeatable from the seat. That requires a numbered display as offered by competitors from ARC, BAT, conrad-johnson et al. However offerings from Lamm, ModWright, Octave and Rogue side patently with Nagra. This would make desirability or need of this feature a split vote. All other basics the Jazz neatly ticks off.

On tech specs, essentials for a modern tube preamp are wide bandwidth, low noise and low output impedance. Here the Jazz might strike some as a mixed bill on the first count. Response ±0.5dB is just 10Hz-50kHz where something like Octave's entry-level €3.850 HP300MkII hits a stout 1.5MHz at -3dB or 200KHz at -0.1dB. Channel crosstalk for the Jazz is >78dB. Where paper begins to positively shout is on dynamic range. Think >112dB in the high-gain setting. In fact the unit-specific 4-page bill of health accompanying my loaner showed a colossal measured 117dB for each channel referenced against a full 4V output. Hola! Such numbers promise impressively low-noise performance. It's partially tweaked with Nagra's very stringent in-house tube selection/testing. It makes for a very high 60% rejection rate. Inveterate rollers might remember that. Output impedance is a very jazzy 50Ω on both XLR and RCA. Maximum input level to hit 0dB in either gain setting is 0.25Vrms. Without the external PSU, all of this tech occupies a compact footprint of 310 x 254mm (12.2 x 10") with a low height of 76mm or 3 inches. Weight is 3.3kg or 7.3lbs.

For circuit and control basics, 1 shows the l/r channel first-stage 12AX7/ECC83 double triodes configured in differential topology and polarized by transistors. 2 is the second-stage 12AT7/ECC81 responsible for most the voltage gain which is then trimmed via a series of resistors in the feedback loop to vary the selectable amplification factor from 0 to 12dB. 3 is the motorized power switch and input selector. 4 is the motorized Alps Blue Velvet balance pot without center detent. 5 is the motorized Alps Blue Velvet master volume pot. 6 is the high-voltage 200V supply which powers the valves' anodes via a turn-on time delay of multiple minutes. 7 are the balancing transformers for the XLR outputs (one per channel). 8 is space for the optional XLR symmetrical input circuit with Nagra transformers which insures that truly balanced sources like those from dCS don't encounter a shorted XLR pin which could cause distortion.

The supplied tubes are gold-pin ElectroHarmonix from Russia.

The basic Jazz board architecture consists of three or four main PCBs as shown in this breakout drawing. 1 is the motherboard, 2 is the 4-piece board for the user-controlled circuits behind the front panel, 3 is the 2-piece rear-panel board for the i/o socketry circuits and 4 is the optional symmetrical input circuit board with Nagra transformers.

All inputs use a floating system to allow for pseudo-symmetrical signal management prior to the gain circuit. Only the ground of whatever input is active gets connected. All others are switched out of the circuit altogether. This effectively kills potential for static ground loops. The XLR-only signal-path bypass loop for home theater applications (input to output) is active even when the Jazz is powered off. This avoids redundant tube wear.

The optional VFS platform shown on the previous page combines two layers of clear-anodized 7mm aluminium with embedded Alpha-gel pellet decouplers whilst the Jazz itself gets upgraded to pointy footers machined from amagnetic copper-nickel-zinc non-ferrous alloy with embedded Delrin ball ends which sit in precisely machined dimples of the upper VFS plate.
On the milled front panel 1 is the infrared eye with a yellow control confirmation LED below it. 2 is the signature motorized power and input selector. 3 is the output selector for XLR or RCA. 4 is the 0/12dB gain level selector.

5 is the motorized balance pot. 6 is the motorized master pot. 7 is the mute selector with yellow LED where flashing indicates that the Jazz is inside its soft-start phase. 8 is the mono/stereo selector

9 is the modulometer showing output level and 10 is the dimmer to lower the modulometer's backlighting in 7 discrete steps.

Having divested myself of all remaining valve amp inventory over the preceding six months—Octave MRE-130 monos for high-power push/pull, Yamamoto A-09S and Woo Audio Model 5 for low-power single-ended—my personal focus of interest for 2013 were valve preamps. After years of amusement mating power tubes to speakers via output transformers, I'd found more satisfaction with single-ended transistor amps from Nelson Pass. Still I admire particular tube contributions. Enter low-level circuits. My 6SN7-based ModWright LS-100 with select Psvane CV-181 and TruLife Audio Athena with 6H30 do exactly that downstairs and upstairs. A short-term reader loan of a €24.000 Concert Fidelity CF080-XLS with Psvane 12AU7 had introduced me to even more exciting performance—from a tube type no less which earlier hadn't impressed—albeit at a criminally excessive price and for that seriously underfeatured.

With Nagra's friendlier Jazz, a simultaneously arrived Octave HP300SE from Germany and a Thrax Dionysos from Bulgaria, I had my first broad opportunity to explore specific flavors of various small-signal tube types and associated circuit choices within my favored valve-pre/transistor-power-amp scheme.