Pass Labs First Watt B4 Active Crossover
The B4 is a stereo active crossover filter system designed for high performance and flexibility and is unique in several ways.
First, it uses only direct-coupled matched discrete complementary Jfets (the now unavailable Toshiba 2SK170 and 2SJ74) in unity-gain buffer circuits without feedback. These buffers allow accurate interface between a very wide range of impedances in the filter components. They are adjusted for zero DC offset and do not require coupling capacitors. The filter resistors are high quality 1% Vishay commercial metal film types, and the filter capacitors are Wima polypropylene film types.
Second, the B4 has a fantastic range and variety of filter values and characteristics. Each of the four filters offers 1, 2, 3 or 4 “poles” for each filter, corresponding to 6, 12, 18 or 24 dB/octave slopes. Each of these filter poles is independently adjustable within each filter, allowing wide customization of the character of the filter. Each pole is adjustable from 25 to 6275 Hz in 25 Hz increments, which is 255 settings per Pole, and the possible number of settings for the four poles of each filter is something like 400 million.
You might wonder why this variety would be necessary, and the answer is simple – the ordinary “text book” set of filters cannot make allowance for the enormous range of loudspeaker drivers, not to mention enclosures, environments and individual taste. Occasionally a standard filter will do a pretty good job, but you won't really know until you have tried some of the alternatives. Usually there is a “best” setting, but it is not often among the settings found in “canned” active crossovers.
The B4 is intended to give the audiophile access to these possibilities with a minimum of trouble. Inside are sets of clearly marked frequency switches for each of the four poles of each filter section weighted in binary increments - the frequency of that pole is simply the sum of the values of the switched which are turned ON.
If you want Sallen-Key values, they are clearly marked out. If you want filters whose high pass and low pass outputs still sum flat at 18 and 24 dB/octave, they are also either clearly marked or prescribed by simple formula.
The B4 has an input impedance of 10 Kohms and output impedance of 125 ohms. The bandwidth of the low pass is DC to the value of the filter frequency, and the bandwidth of the high pass is about -1 dB at 100 Khz.
The distortion of the all the circuitry in series is about .005% at 1 volt, and the maximum output is about 10V peak. The circuit will drive 600 ohm loads at slightly lower output and higher distortion, but still does a good job. The noise in the unweighted audio band is a few microvolts, or about -110 dBV, which is about -130 dB below peak output. The external regulated switching supply is passively RC filtered to give very low noise.