Why Use A Stepped Attenuator?
The 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 thin resistive 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.
A5 Stereo Stepped Attenuator
This cleverly designed stepped attenuator exploits both the series-attenuator and the shunt-attenuator configurations to yield the best compromise between flexibility, performance, and cost. The A5 attenuator uses two rotary switches and 24 resistors to yield a total of 36 positions (decrements) of attenuation in fine -1dB or -2dB and coarse -6dB or -12dB decrements. Had the attenuator been entirely of a series design, the attenuator would require 36-position rotary switch and 72 resistors; a purely ladder design, a two-deck, 2-pol, 66-position rotary switch and 140 resistors. In contrast, the A5 attenuator, from 0 to -5dB steps of attenuation, is solely a shunt attenuator, with no more than one series resistor in the signal path; thereafter, the attenuator uses both a shunt and series configurations, with never more than 7 resistors in the signal path.
The kit includes two shorting rotary switches (2-pol/6-pos) and one USA-made extra-thick (0.94"), high-quality PCB, with 2oz copper traces and two side solder mask and silkscreen. The kits are sold without resistors, so you, the purchaser, can use your favorite. In addition, packs of all the needed resistors are also for sale: 24 resistors for $10.
The A5 PCB is 1.4 inches (36mm) tall and 5.8 inch (147mm) long and will fit within a 1U rack-mount enclosure. Each resistor position on the PCB holds a redundant set of pads, so that either radial resistors, such as bulk-foil resistors, and axial resistors can be used. The spacing between switches is 3 inches (76.2mm) and the switches themselves hold the attenuator assembly to the front panel.
The A5 stereo attenuator presents a nominal load resistance of 28k to the signal source. It can be optimized for passive line stage use by using -1dB and -6dB steps, for a maximum of -35dB of attenuation.
Kit includes two rotary switches, PCB, and user guide.