PASS LABORATORIES XONO
Owner's Manual

individual efforts and reflect a coherent underlying philosophy. They make a subjective and an objective statement of quality which is meant to be appreciated. It is essential that the circuitry of an audio component reflects a philosophy which addresses the subjective nature of its performance first and foremost. Lacking an ability to completely characterize performance in an objective manner, we should take a step back from the resulting waveform and take into account the process by which it has been achieved. The history of what has been done to the music is important and must be considered a part of the result. Everything that has been done to the signal is embedded in it, however subtly. Experience correlating what sounds good to knowledge of component design yields some general guidelines as to what will sound good and what will not: 1) Simplicity and a minimum number of components is a key element, and is well reflected in the quality of tube designs. The fewer pieces in series with the signal path, the better. This is often true even if adding just one more gain stage will improve the measured specs. 2) The characteristic of gain devices and their specific use is important. Individual variations in performance between like devices is important, as are differences in topological usage. All signal bearing devices contribute to the degradation, but there are some different characteristics that are worth attention. Low order nonlinearities are largely additive in quality, bringing false warmth and coloration, while abrupt high order nonlinearities add harshness. 3) Maximum intrinsic linearity is desired. This is the performance of the gain stages before feedback is applied. Experience suggests that feedback is a subtractive process; it removes information from the signal. In many older designs, poor intrinsic linearity has been corrected out by large application of feedback, resulting in loss of warmth, space, and detail. A very important consideration in attempting to create an amplifier with a natural characteristic is the selection of the gain devices. A single ended Class A topology is appropriate, and we want a characteristic where the positive amplitude is very, very slightly greater than the negative. For a current gain device, that would mean gain that smoothly increases with current, and for a tube or field effect device a transconductance that smoothly increases with current. Triodes, JFETs, and Mosfets share a useful characteristic: their transconductance tends to increase with current. Bipolar power devices have a slight gain increase until they hit about

an amp or so, and then they decline at higher currents. In general the use of bipolar in a single ended Class A circuit is a poor fit. Another performance advantage shared by Tubes and Fets is the high performance they deliver in simple Class A circuits. Bipolar designs on the market have between four and seven gain stages associated with the signal path, but with tubes and Mosfets good objective specifications are achievable with as few as one gain device in the signal path. Regardless of the type of gain device, in systems where the utmost in natural reproduction is the goal, simple single ended Class A circuits are the topologies of choice. The gain devices in the Xono were all selected for top performance in each part of the circuits. A quad of ultra low noise matched JFETS form the input stage, followed by power Mosfets for output devices. Resistors are all precision RN55D or better. Film capacitors are used for all values below 10 uF, and bypassed low impedance/high bandwidth electrolytics are used for greater values. The external power supply for the Xono consists of an oversized, shielded, toroidal power transformer delivering an unregulated 85 volts peak to peak DC through separate rectifiers and capacitors for each channel which is then passively RC filtered before being sent to the main Xono chassis. A custom manufactured shielded cable carries the DC power from the supply to the main Xono chassis. The main chassis of the Xono has separate stages of active regulation for each channel followed by passive filtering and then feeding the constant current sources which bias the various gain stages. There is a total of 120,000 mfd of filter capacitance and five stages of filtering and regulation. The power supply noise reaching the circuit is on the order of a microvolt, and is differentially rejected at the output in a balanced system. There are a total of 22 high performance power electrolytics with an aggregate capacitance in excess of 120,000 mfd used to accomplished energy storage and passive filtering of the power supply. The chassis of the Xono is made entirely of machined aluminum. The top cover is made from aluminum. The face plate too is milled from thick aluminum stock. The Xono was designed by Wayne Colburn.