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The First Watt B4 is an outstanding active crossover designed by Nelson Pass. This unit has incredible flexibility in both the crossover point and the slope of the crossover.

The B4 is a stereo active crossover filter system designed for high performance and high flexibility. It is intended for those who feel the need for crossover network characteristics which are far more variable than the “textbook” filters offered by other products. Because the characteristics of every loudspeaker driver varies dramatically and uniquely from an ideal device, it is routinely the case that a standard filter is not optimal, and this is where the B4 is particularly useful.

The B4 offers 1, 2, 3, or 4 Poles for each filter, corresponding to 6, 12, 18 or 24 dB per octave slopes. Most importantly, each of these Poles is independently adjustable within each filter, allowing wide customization for the character of each filter. In the X1 range, each filter Pole can be varied from 20 Hz to 1,260 Hz in 20 Hz steps, and in the X10 range, from 200 to 12,600 in 200 Hz steps. These filter settings are adjusted by internal switches inside the B4.

A quick calculation shows that this gives approximately 270 million different permutations per filter. Most of these will not be of interest, but you never quite know what will do the job.

The Low Pass output levels of the B4 are variably and independently adjusted from the front panel, and the High Pass output levels are attenuated to calibrated levels by internal switches.

The active circuits are DC coupled JFET complementary followers operated without feedback.

The only capacitors in the signal path are the polypropylene filter elements. Filter portions which are unused can be bypassed, minimizing the active components in the signal path.

The B4 has a high input impedance of 100 Kohms and low output impedance of 150 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 0.5 dB down at 100 Khz.

Although DC coupled, each circuit has been trimmed for the very minimum of DC offset, and so you should expect to see about 1 mV of DC through the system. very low noise and distortion and wide bandwidth. It is important to note that any DC offset present at the input from source components will be passed through the B4 Low Pass circuit.

The distortion of the all the circuitry in series is about .003% at 1 volt, and the maximum output is about 10V peak. The noise in the unweighted audio band is a few microvolts, or about -110 dBV.

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First Watt B4 Active Crossover

Made in California, USA

$4,695

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The First Watt B4 is an outstanding active crossover designed by Nelson Pass. This unit has incredible flexibility in both the crossover point and the slope of the crossover.

The B4 is a stereo active crossover filter system designed for high performance and high flexibility. It is intended for those who feel the need for crossover network characteristics which are far more variable than the “textbook” filters offered by other products. Because the characteristics of every loudspeaker driver varies dramatically and uniquely from an ideal device, it is routinely the case that a standard filter is not optimal, and this is where the B4 is particularly useful.

The B4 offers 1, 2, 3, or 4 Poles for each filter, corresponding to 6, 12, 18 or 24 dB per octave slopes. Most importantly, each of these Poles is independently adjustable within each filter, allowing wide customization for the character of each filter. In the X1 range, each filter Pole can be varied from 20 Hz to 1,260 Hz in 20 Hz steps, and in the X10 range, from 200 to 12,600 in 200 Hz steps. These filter settings are adjusted by internal switches inside the B4.

A quick calculation shows that this gives approximately 270 million different permutations per filter. Most of these will not be of interest, but you never quite know what will do the job.

The Low Pass output levels of the B4 are variably and independently adjusted from the front panel, and the High Pass output levels are attenuated to calibrated levels by internal switches.

The active circuits are DC coupled JFET complementary followers operated without feedback.

The only capacitors in the signal path are the polypropylene filter elements. Filter portions which are unused can be bypassed, minimizing the active components in the signal path.

The B4 has a high input impedance of 100 Kohms and low output impedance of 150 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 0.5 dB down at 100 Khz.

Although DC coupled, each circuit has been trimmed for the very minimum of DC offset, and so you should expect to see about 1 mV of DC through the system. very low noise and distortion and wide bandwidth. It is important to note that any DC offset present at the input from source components will be passed through the B4 Low Pass circuit.

The distortion of the all the circuitry in series is about .003% at 1 volt, and the maximum output is about 10V peak. The noise in the unweighted audio band is a few microvolts, or about -110 dBV.

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