Cascade Stepped Attenuator

With this 2-stage stepped attenuator volume control topology, one can enjoy the adjustment precision and channel balance of conventional long-string stepped attenuators while even outperforming them in convenience and cost. Coarse level is set by a 6-position rotary switch in 7.5 dB steps while the fine adjustment uses a similar switch in 1.5 dB steps, thus providing a total attenuation range of 45 dB.

Two design versions are presented: The 'unloaded' variant may be used when the load at the output can be deemed negligible, e.g. the noninverting input of an amplifier or anything above about 150 kΩ. The 'loaded' version is optimized for 70 kΩ output load, which can be e.g. a balance potentiometer stage.

Benefits include:

• 2-pole 6-position rotary switches are readily obtainable stuff at most sellers, without need of Chinese sources.

• Due to the multiplication of positions in the cascade connection, the step size can be smaller for a given total attenuation.

• The number of resistors and amount of solder work needed is less than half of that for a conventional attenuator of similar performance.

• The knob doesn't have to travel full south which is not so convenient.

• Less positions to go through during major adjustments.

The resistances shown are commonly available E24, 48, or 96 series values. Here, all calculated values fall within 1% of these standard values.

There is one step of overlap in the settings, as the major levels (-7.5 etc.) can be reached at two position combinations. This is to facilitate minor back-and-forth moves in case the preferred listening levels fall close to these major levels.