Nagra PL-P preamplifier Page 2
Before use, the PL-P's internal jumpers need to be set for the specific system. A good time to do this set would be after installation of the battery packs. Try to avoid drooling on the thick, gold-burnished circuit boards as we take a little tour through the PL-P's alluring innards.
First, choose an RIAA EQ option in the phono stage, located on the left of the main circuit board. Remove the jumpers and the EQ curve will conform to a 1978 addendum specifying a -3dB rumble filter at 30Hz. Rumble is not a concern with the air-bearing Forsell Air Force One, so I installed the jumpers, nulling the rolloff.
The left- and right-channel main jumper blocks are located just below. I chose moving-coil operation rather than moving-magnet, and set the loading at 100 ohms. Additional loading options are 330 ohms and 1k ohm (47k ohms is the default, realized with no jumpers installed). MM cartridges can be optimized with jumpers for capacitive loadings of 100, 220, and 470pF, 47pF being the no-jumpers default.
Finally, another pair of jumper blocks selects or bypasses a pair of cute li'l MC input transformers. (I tried several cartridges with outputs in the 350-650µV range; all required the transformers for sufficient gain.) Interestingly, in MC mode with the transformers engaged, the phono inputs float relative to ground. Bypassing the initial step-up requires one or both grounds of the phono connector RCAs to be tied to chassis ground to avoid hum. This is accomplished with yet another pair of jumpers. With them, one or both of the phono inputs can be connected to chassis ground. This truly obsessive and extraordinarily welcome flexibility allowed me to adjust for best results using TARA Labs The One interconnect's interesting grounding options.
Next, the outboard PLP-CCC3 AC power supply is plugged into a locking Fischer-connectored port on the PL-P's rear. To accommodate the rear-mounted battery compartment, input RCAs are arrayed on the left side panel, outputs and the tape loop on the right. Once these were secured, I ran a Synergistic Research Reference AC Master Coupler to the CCC3's fused IEC input socket, turned the supply on, and checked the status LEDs on its backside.
When the power supply is operating correctly and the PL-P's internal batteries are being "properly managed," the leftmost LED lights up, its intensity indicating rate of charge. At maximum—400mA with the preamp powered down—flat batteries can be fully charged in 12 hours. In fact, you'll need a touch of maturity and patience—a flash of gray at the temples—to help you through the setup. "Rechargeable battery batons supplied from the factory must be fully charged in the PL-P before use." Anticipation, as they say...
If neither of the power-supply LEDs illuminates, or only the rightmost one, something's amiss with battery installation or the fuse. The comprehensive manual details troubleshooting procedures. It also contains a section on grounding and hum that's so complete it could serve as an industry primer on this vexing subject.
Thermionics & bicycle bells
Each of the PL-P's eight tubes gets a 12-hour burn-in at the factory and is subjected to 400 measurements before final selection. Peering into the preamp's attractive boudoir—it's red, gold, and blue in there!—I noted a single 12AX7 phono input followed by one 12AT7 output. The line-stage at right features a pair of 12AX7 inputs with a 12AT7 triangulated between. The same applies to the output stage. Once again, no circuit details are provided, but note the PL-P's low output impedance—it's likely the 12AT7s function as cathode followers. The "French Blue" capacitors are all sourced from Philips. The two headphone transformers sit to the right of the line-stage, looking like a pair of old-fashioned bicycle bells. (I don't think they ring quite the same way.)
Another element of this unique implementation is a pair of tube timers—one for the phono stage and one for the line stage—located at the rear of the circuit board. They look like little thermometers: when the tiny red strakes hit the 5000-hour mark, it's time to change the tubes. Very Nagra.