A stepped attenuator is an attempt to overcome the liabilities of the conventional potentiometer-based volume control. Even the best potentiometer compromises the quality of the signal that flows through it. The thin film of resistive paint and metal scraper that make up the average potentiometer seem to damage the signal almost as much as they attenuate it. Furthermore, finding a stereo potentiometer that follows a logarithmic taper and that can track from one channel to the other is tough.
With the stepped attenuator, on the other hand, high-quality fixed resistors replace the carbon track and, likewise, high quality switch contacts replace the metal scraper. Because tight-tolerance resistors are readily available, there is no problem in following the logarithmic taper accurately and consistently.
Like its unbalanced predecessor, this attenuator offers 36 steps of attenuation for two channels of signal. The center rotary switch controls both channels and presents coarse decrements, while the two flanking switches afford fine volume decrements for each channel. Two options are available: -2dB / -12dB steps for a maximum total attenuation of -70dB, which is perfect for active line stages; and -1dB / -6dB steps for a maximum total attenuation of -35dB, which is perfect for passive line stages or those setups that do not require lots of attenuation.
The balanced input signal sees an equal series resistor on each of its phases. Then a 6-position switch selects between no resistor or five resistors to shunt the balanced signal down in amplitude. Each input signal phase sees an identical load, so balance is maintained, no matter what the output attenuation is set. Next, a 2-pole, 6-position rotary switch presents the two phases their own series stepped attenuator, each of which terminates into ground. This arrangement incurs a -3dB insertion loss, but this is a small price to pay for such an elegant solution.
PCBs are 1.4 inches tall and 9 inches long and will fit within a 1U rack-mount enclosure. Each resistor position holds a redundant set of pads, so that radial and axial resistors can be used. The input resistance is nominally 28k.
The kit includes instructions and three high-quality, open-frame, rotary switches and one USA-made extra-thick, high-quality PCB, with 2oz copper traces and solder mask and silkscreen.