Good news for the those audiophiles who run balanced systems. The BM-1 is a new mono stepped attenuator that uses 24 resistors and two rotary switches: one 11-position, 1-pole, shorting switch; one 6-position, 2-pole, shorting switch. The configuration is interesting, as it consists of a shunt stepped attenuator cascading into two series stepped attenuators, one for each phase.
The BM-1 stepped attenuator presents five connections: two for the balanced input signal and two for the balanced output and one for the ground connection. This is a very clever design, being able to set 66 -1dB volume decrements for a balanced signal source is a huge undertaking; and being able to do it with just 24 resistors and two single-deck rotary switches is a miracle. Imagine how large and expensive a two-deck, 66-position rotary switch and 132 resistors would cost, if a conventional series attenuator were used instead. (And a ladder stepped attenuator would cost twice as much as the series attenuator.) The small price we pay is a -3dB insertion loss, because of the shunt attenuator. Since most balanced signal sources already provide far to much voltage output, hence the need for a volume control, the -3dB loss is not a problem.
The extra-thick, 2oz copper PCB is 1.4 by 5.8 inches big and the rotary switch spacing is 3 inches. The PCB are made in the USA. M1 mono stepped attenuators include 8-page user guide with assembly instructions and a resistor-value tables.