LDR3000T.V25 Tube Preamp

Whether the source is RCA, XLR, or a mix of both, the LDR3000T handles it — there are 3 sets of RCA inputs and 3 sets of XLR inputs usable in any combination. The RCA and XLR outputs carry the same output signal regardless of source type, although the balanced output has only one phase active (pin #2) when the source is single-ended.

The single greatest challenge for any preamp is remaining true to the source material while delivering the selected, attenuated audio signal to the amplifier honestly and without editorializing. The LDR3000T does this by combining Tortuga Audio’s LDR volume/preamp controller with a hybrid zero/low gain tube input/gain stage, followed by a simple yet highly effective solid-state JFET current amplifier (output buffer).

Most tube preamps and amplifiers run their tubes at high plate voltages and current to achieve high linear gain, but today’s sources, amps, and speakers need little to no preamp gain at all. Because of this, the LDR3000T runs its tube at a low "slack" voltage of only 45 volts instead of 250 or higher. At very low currents, many tubes will operate linearly even at such low voltage — with a compatible tube, the resulting zero/low gain sound quality is glorious.

This elegantly simple design keeps the signal path short and simple without compromising performance.

The LDR3000T is built around the 4th generation remote controlled V25 preamp controller, using digitally controlled analog LDRs to provide smooth 100-step volume control over a 60 dB range. Features include ramped muting, channel balance adjustment, input switching, both stereo and mono modes, and adjustable input impedance. Built-in calibration of the replaceable socketed LDRs ensures accurate attenuation, channel balance, and performance over the life of the preamp.

The LDR3000T is operated via an intuitive interactive menu driven control interface built around the 7-button Apple remote and a bright, easy-to-see 256x64 pixel white-on-black graphic OLED display.

The attenuated audio signal connects directly to the triode’s grid with no input coupling capacitor. The stock tube is the 9-pin 6H30Pi Gold triode from Electro-Harmonix, chosen for its reputation for sonic excellence, robust long life, and the fact that it’s a contemporary tube unlikely to go out of production for a long time.

While the 6H30 is a delightfully clean, neutral, yet smoothly muscular sounding tube with just the right amount of tube euphonics, the Genalex Gold Lion E88CC/6922 triode is offered as an alternative for a more lush sound. In fact, the LDR3000T is great for tube rolling most any tube in the 6922 family.

Designing around the 6H30 tube means the LDR3000T has a very robust power supply, since the 6H30 demands high current. This opens up the possibility of trying a wide variety of alternative high-transconductance pin-compatible 9-pin triode tubes, including the 6DJ8, ECC88, E88CC, 6922, 6N23P, and 6N11, in both current and NOS versions.

The only reason for a preamp to have gain is if the amp can’t put out enough power to drive the speakers loud enough — rarely the case in the real world. Hence the LDR3000T’s default gain is set to unity (0 dB), but it can be adjusted up to +6 dB gain for systems that need more volume headroom. Adjusting the gain requires disassembly of the preamp and flipping around 4 plug-in gain resistor modules on the tube board.

Each stereo channel has its own dedicated triode tube for the best possible channel separation, sound stage, and stereo imaging. With single-ended audio, only half of each triode is used; with balanced audio, each triode handles both phases of its own stereo channel.

The output stage is a discrete solid-state current amplifier built around a contemporary version of the Toshiba 2SK170 JFET, operating in Class A mode with a constant current source. This discrete buffer delivers powerful, robust dynamics into practically any amplifier at a very low output impedance.

The LDR3000T power supply starts with a robust linear supply built around a large 50 VA linear shielded toroidal transformer, an oversized bridge rectifier, and almost 38,000 uF of filter capacitance. This feeds 2 independent DC-DC switching regulators, one dedicated to the tube heaters. The second provides 48 volts into a second-stage linear regulator that supplies 45 volts to both the tube plate and the solid-state JFET output buffer — handling the current requirements of almost any 9-pin tube available.

The base price includes a modest yet high quality metalized polypropylene film output coupling capacitor. For the highest audiophile level, an upgrade was offered to either the VCAP ODAM (Oil Damped Advanced Metalized) or, for ultimate performance, the reference grade VCAP CuTF (Copper Foil Fluoropolymer Film) output coupling capacitor.

These specifications reasonably reflect the final production model.