DACT CT2-50k-1 Audio Attenuator

The DACT CT2-50k-1 is a stepped attenuator, a volume control built from a rotary switch and a set of discrete fixed resistors rather than from a continuous carbon or conductive plastic track. The 50k in the name gives the input impedance, which suits it to line-level duty in a preamplifier or between a source and a power amplifier.

The reason for building a volume control this way comes down to what a conventional potentiometer does badly. A stereo potentiometer has two tracks that must match each other at every position, and inexpensive ones commonly deviate by two or three decibels, most of it at low settings where the tracks are thinnest. Because the stereo image is constructed from the level difference between channels, that mismatch does not merely change the volume, it moves the image sideways as the listener turns the knob. A stepped attenuator eliminates the problem by construction: each position is defined by resistors whose tolerance is known and can be made as tight as required.

There are two further consequences. The attenuation law is exactly what the designer specified rather than an approximation of a logarithmic curve, so the steps are evenly spaced in decibels across the whole range. And the signal at any given setting passes through fixed resistors and a switch contact instead of a wiper sliding on a track, which removes the usual sources of crackle and wear.

The trade-off is that the control moves in discrete steps rather than continuously. In practice, well-chosen step sizes are fine enough that this is quickly forgotten.