Phono Stages

Pass Labs XP-25 Phono Preamplifier

ARTICLE INDEX

The Design

The XP-25 is a dual-chassis design, with the power supply in a different chassis than the gain stages. The two chassis are connected together with a DB-25 cable on the rear panels. The power supply has two DB-25 ports, so you can power another Pass Labs product that accepts this type of power connection. Also on the rear panel are two pairs of RCA single-ended (unbalanced) inputs, along with a pair of RCA single-ended and XLR balanced outputs. Although the power receptable is grounded, there is a separate grounding terminal that can be connected to a ground screw on your preamplifier, or to the metal base of the turntable. This can make a big difference in the amount of hum in the signal, since one is dealing with output voltages as low as 0.3mV from the phono cartridge.

The front panel has rotary controls to select the Resistive Loading (100 Ohms - 47 kOhms), Capacitive Loading (100 pF - 750 pF), and the Gain (53 dB, 66 dB, 76 dB). On the right are buttons to select Input 1 or 2 (you would need these if you are one of those vinly aficionados who has a turntable with two tonearms), a High-Pass filter (removes frequencies below 20 Hz), and Mute.

The Resistive Loading and Capacitive Loading controls are used to provide the best possible impedance interface between the cartridge and the input stage of the XP-25. Again, this is very important because of the low voltage output from the cartridge. For MM (Moving Magnet) and Moving Iron cartridges, you adjust both the Resistive and Capacitive Loading. For MC (Moving Coil) cartridges, it is mainly the Resistive Loading that is used. However, it's important to experiment with different loading combinations to see (hear) what sounds best. The instruction manual suggests starting with a 100 Ohm resistive load. I use a Sumiko Blackbird MC cartridge, so I set the capacitive load to 100 pF (the lowest setting).

Depending on the output of the cartridge, you select the gain setting. If you have a low output MC cartridge, you would probably want to have the 76 dB gain. The Sumiko Blackbird outputs 2.5 mV (which is high for an MC cartridge), so I used the 66 dB setting and the RCA single-ended output (which actually results in a 60 dB gain, since the front panel markings are for the XLR outputs). I tried different settings on both the Resistive Loading and Capacitive Loading, as well as the Gain, and these were the settings that I ended up with: (100 Ohms, 100 pF, 60 Gain). It allowed me to get the kind of loudness I want from my speakers with the preamplifier near the middle of its volume control range.

There are two gain stages in the XP-25. Compared to earlier models, the first gain stage has lower noise transistors, controlled open-loop gain, and a small amount of local negative feedback. Low noise thin-film resistors are used throughout. Resistor leads and end-capacitors have been eliminated. At the point the signal reaches the input stage, it has been regulated and filtered seven times, which allows a specification of being able to handle a signal as low as 0.1 mV (there are several cartridges out there that have 0.2-0.3 mV output). The input stage is basically the same for MM or MC cartridges. You control how the signal is handled by the myriad choices of resistive and capacitive loading on the front panel.