Pro-Ject Tube Box DS3 B black

Pro-Ject Tube Box DS3 B: balanced tube phono preamp

The Tube Box DS3 B is the first phono preamp in the Pro-Ject Tube Box series to offer balanced XLR inputs and outputs alongside the usual RCA sockets. It amplifies the very weak signal from a turntable cartridge, whether moving magnet (MM) or moving coil (MC), to bring it up to the level of an amplifier’s line input. Its circuit, designed by Dr. Sykora, uses two E88CC tubes in a dual-mono configuration, with each channel featuring its own electronics. The unit is made in Europe.

Adjustable impedance while listening

Load impedance is the resistance the preamp presents to the cartridge. Chosen correctly, it gives an MC cartridge an accurate tonal balance; chosen poorly, it makes the treble dull or, conversely, overly bright. On most phono preamps, this setting involves small switches hidden underneath the unit, which have to be adjusted between listening sessions. The Tube Box DS3 B assigns it to a front-panel rotary knob, which continuously varies the impedance from 10 to 1000 ohms while the record is playing. You start with the value specified by the cartridge manufacturer, then fine-tune by ear, immediately hearing the effect of each position. For MM and high-output MC cartridges, a fixed 47-kilohm setting can be selected with a simple press of the Loading Ohms button.

Two turntables and three types of connectors

Two turntables can remain connected at the same time, one to the balanced input and the other to the RCA input, with a front-panel button switching between them. Gain, impedance, capacitance and subsonic filter settings are stored separately for each input, so each turntable returns to its own settings whenever you switch. On the balanced side, the preamp accepts a 5-pin mini-XLR connector, as used on Pro-Ject turntables equipped with the True Balanced output, or a pair of standard XLR cables. A balanced connection carries the signal over two conductors in opposite phase: at the receiving end, the preamp compares the two and eliminates noise picked up by the cable along the way, protecting a very delicate phono signal from interference in a modern home. The RCA input is accompanied by a screw terminal for the turntable’s ground wire.

On the output side, the XLR and RCA sockets work simultaneously, allowing the preamp to be connected to two amplifiers. The XLR output provides an additional 6 dB of gain.

E88CC tubes at the heart of the circuit

The two E88CC tubes, also known as 6922, stand out for their low background noise and low microphonics, meaning low sensitivity to mechanical vibrations, two valuable qualities in a phono preamp where the original signal measures only a few millivolts. They form the unit’s tube output stage, paired with audiophile-grade polypropylene capacitors.

Comprehensive front-panel settings

Gain ranges from 40 to 65 dB in 5 dB steps. Lower values suit MM and high-output MC cartridges, while higher values suit low-output MC cartridges, whose output voltage is much lower. This choice also allows the turntable’s volume to be matched to that of the system’s other sources, such as a CD player or network player. Input capacitance can be set to 50, 150, 300 or 400 pF; it affects the treble of MM and high-output MC cartridges and has no effect on low-output MC cartridges. For example, an Ortofon 2M Red (MM) is set to 47 kilohms, 150 pF and 40 dB, while an Ortofon Quintet Red (low-output MC) is set to 50 ohms and 60 dB.

The subsonic filter cuts frequencies below 20 Hz with a slope of 18 dB per octave. These inaudible sounds, produced by a warped record or tonearm movements, put unnecessary strain on the amplifier and speaker cones.

Chassis, power supply and startup

The aluminum enclosure protects the circuit from electromagnetic interference. Power is supplied by an external 18 V adapter, placed outside the enclosure to keep the electromagnetic fields it generates away from the circuit. The optional Power Box DS2 Sources linear power supply provides cleaner power and replaces the supplied adapter.

A front-panel button turns the unit on or puts it into standby, and a blue LED confirms that power is on. 12 V trigger inputs and outputs (2.5 mm jack) allow the Tube Box DS3 B to be switched on and off at the same time as other devices in Pro-Ject’s Box Design range. The enclosure is available in black or silver, and magnetically attached wooden side panels are available separately.

Technical specifications

Tubes and audio circuitry

• Tubes: 2 × E88CC / 6922
• E88CC tubes with low noise and low microphonics
• Tube output stage
• Audiophile-grade polypropylene capacitors
• Dual-mono circuitry (separate electronics for each channel)
• Circuit designed by Dr. Sykora

Compatibility and features

• Compatible with moving magnet (MM) and moving coil (MC) cartridges
• Simultaneous connection of two turntables (one to the balanced input, one to the RCA input)
• Front-panel input selection
• Separate settings memory for each input
• XLR and RCA outputs active simultaneously (connection to two amplifiers possible)

Cartridge settings

• Gain: 40, 45, 50, 55, 60, 65 dB (+ 6 dB on the XLR output)
• Input impedance:
  • continuously adjustable from 10 to 1000 ohms via rotary knob, including during playback
  • fixed 47 kilohm setting (MM and high-output MC cartridges)
• Input capacitance: 50, 150, 300, 400 pF (no effect on low-output MC cartridges)
• Switchable subsonic filter: 20 Hz, 18 dB per octave slope
• All settings accessible on the front panel

Measured audio performance

• Output voltage: 500 mV at 1 kHz for a 5 mV input (40 dB gain)
• MM signal-to-noise ratio (40 dB gain): > 92 dBV, 102 dBV A-weighted
• MC signal-to-noise ratio (60 dB gain): > 78 dBV, 87 dBV A-weighted
• Harmonic distortion at 1 kHz: < 0.003% in MM, < 0.02% in MC
• Harmonic distortion from 20 Hz to 20 kHz: < 0.005% in MM, < 0.03% in MC
• RIAA curve accuracy: ± 0.5 dB from 20 Hz to 20 kHz

Inputs and outputs

• Inputs:
  • 1 × 5-pin mini-XLR (balanced, compatible with Pro-Ject True Balanced)
  • 1 pair of XLR (balanced)
  • 1 pair of RCA phono with ground terminal
• Outputs:
  • 1 pair of XLR (balanced)
  • 1 pair of RCA
• XLR wiring: pin 1 ground, pin 2 hot, pin 3 cold
• 12 V trigger: input and output (2.5 mm jack)

Power supply and consumption

• External adapter: 18 V / 1000 mA
• Power consumption: 750 mA maximum, < 0.5 W in standby
• Optional Power Box DS2 Sources linear power supply

Construction and finishes

• Aluminium chassis shielding the circuitry from electromagnetic interference
• Made in Europe
• Finishes: black or silver
• Optional magnetically attached wooden side panels

Dimensions and weight

• Dimensions (W × H × D): 206 × 70 × 194 mm (223 mm deep including sockets)
• Weight: 1.56 kg without power supply