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Intrinsic to the shunt attenuator scheme is that bypass still leaves the series resistor in the signal path. Even fully open some attenuation remains. And the input resistance changes with volume setting, in the case of my loaner from 10KΩ to 110KΩ. This makes source matching more critical. It's why some will prefer a ladder or series type. In a series stepped attenuator a series of resistors forms a long voltage divider and the rotary switch determines where in this series the output is tapped. The input impedance remains constant and there's only one switch contact in the signal path. Pops and clicks are eliminated. In a ladder type the rotary switch selects an input/output plus output/ground resistor. The total resistance of any of these paired resistors is equal for constant input impedance. This involves two switch contacts in the signal path, twice as many resistors as the series and shunt types and often causes clicks and pops when the knob is turned. Commercial Khozmo competitors are Goldpoint and DACT. The former offer passive preamps but no remote, the latter attenuators and circuit modules. Bent Audio's module adds remote control to either's range of stepped attenuators. Whilst still on the subject of passives, we ought to in passing mention light-dependant resistor or LDR passives as proposed by Tortuga Audio in the commercial sector and Lightspeed, The Truth and Warpspeed in the semi DIY arena; and
transformer/autoformer-tapped solutions like Audio Consulting, Bent Audio, Music First Audio & Co. The latter's appeal lies in not burning up signal resistively as all traditional controls do.
Back to Khozmo, I was curious how Arek's motor dealt with the mechanical inertia of a stepped pot. How clunky would operation be? Would it involve mechanical or electrical noises?
These are common concerns. They're the reason why remote-enabled volume controls favor continuous wipers despite general consensus that discrete resistor solutions are superior especially at very low output voltages. Relay-switched resistor matrixes too often cause clicks. That's why my €10.700 Nagra Jazz uses high-quality Alps pots.
Here we get to the make-before-break concept of contact switching illustrated at right. This is a fancy name for bridged/non-gapped switching where the new connection is made before the previous contact is opened. This prevents the switched path from seeing any momentary open circuit between settings.
Adding remote to an indented control has to balance the mutually opposing needs for firm and not too loose manual operation and smooth easy motorization which doesn't struggle hard to overcome mechanical resistance. Just how slick or click would Khozmo's mechanics turn out to be by hand or wand? And what kind of wall wart would drive the motor?
Provisos. Any review of a passive preamp must invoke the usual boilerplate small print. Using a passive puts your source in charge of driving the interconnects to and the input stage of the power amp/s. A universal player with output opamps driving 10 meters of interconnect won't be ideal. Even so the less-is-more passive promise continues to seduce designers and listeners alike. Steve McCormack's very costly SMcAudio VRE-1c linestage and E.J. Sarmento's far more affordable Wyred4Sound STP-SE preamp in fact are just two which attempt to combine the virtues of passive operation with active buffering and available voltage gain. The basic thinking is that modern high-output sources plus amps of high input sensitivity make additional line-level gain redundant. Why add gain when you always listen below unity source gain? Why create and pay for something only to throw it all away again? This segues directly into the next question. Are active preamps additive? Are passives neutral then or subtractive? In the end only what sounds best to you matters. Theories and ideologies don't belong. Despite its name then, Khozmo isn't cosmic. It'll work for some but not others.
Can one make any generalized predictions on when and when not a passive might sonically work best?