- Written by John E. Johnson, Jr.
- Published on 06 May 2013
The XPR-1 is a fully-balanced (from input to output) design, with a power supply that has a 2.5 kVA toroidal power supply transformer and 240,000 μF of capacitance.
The front panel is very attractive, with two vertical slots that emit a blue light. From the left, are a standby button (main on/off toggle is on the rear panel), then Peak, Dim, Meter, and Orientation buttons. These all have to do with the viewing of the power meter which is a series of LEDs that can be set (oriented) to move from bottom to top, left to right, or right to left, indicating the amount of power that is being delivered. You can also dim the LEDs and turn off the panel lights altogether. Many consumers don't like their equipment rack to have visible lighting, but I like them a lot. For my use, I set the LEDs to move from bottom to top. The bottom of the LED column is located where the white dot is in the photograph.
The rear panel is at the absolute other end of the scale in terms of connectors, compared to a receiver or processor. The RCA unbalanced and XLR balanced input is selected (one or the other but not both at the same time) with a toggle switch. Underneath the inputs are a massive set of brass binding posts that will take just about any cable termination you can think of. Then, at the bottom, is the main power on/off rocker switch, and a 20 Amp AC receptacle. You need to prepare for this, because the other end of the included cable will only fit a 20 Amp AC wall socket, where one of the prongs is sideways oriented rather than vertical. If you purchase the amplifiers, but don't have that type of socket, they are readily available at Home Depot or other electrical supply stores. There is also a pair of trigger jacks, for automatically turning on the power amplifier(s) when you turn on your preamp or processor (which would need to have the appropriate output trigger jack).
If you look inside the chassis, shown below, you can see 24 power supply capacitors and the 2.5 kVA toroidal transformer. This power supply is massive! And that is the basis of any good power amplifier. Each rail has 18 output devices (bipolar transistors), for a total of 36 (having a large number of output devices is also necessary for a good, high powered amplifier).
The output stage is configured as triple Darlingtons. Here is a schematic of a triple Darlington output stage (shown below). For a balanced configuration, the triple Darlington (the direction of the arrows indicates these are NPN transistors) is referenced to another triple Darlington set on the opposite hot leg, but the arrows would be pointing in the other direction, i.e., PNP. So, there are six triple Darlingtons running in parallel on the + and six on the - legs. The advantages of the triple Darlington configuration include high current gain and very high impedance for the overall circuit, while its disadvantages include a more limited bandwidth and increased phase shift at high frequencies, so it can become unstable with negative feedback.
As further illustration, here is a diagram of the two basic types of bipolar transistors, PNP and NPN.
Other features of the XPR-1 include soft start (to limit in-rush of current to the power supply) and protection circuits in the case of a fault (e.g., shorting the outputs).
The amplifier runs in Class H, meaning that the voltage rails are modulated so that they stay a few volts above the output signal demand.