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This is a schematic I extracted from the circuit of a large audio effect unit. This portion should be a digital-attenuator/crossfader between two audio sources.

In the front panel, there is a DRY/WET balance potentiometer that controls this section (not directly, but through a ADC+microcontroller that then feeds those CD4053 switches using 5 bits).

Also, not shown in the schematic, there is a simple inverting opamp used as unity gain mixer for DRY-OUT and WET-OUT signals.

At first look I thought it was pretty simple, the switches/resistors are arranged as a R2R ladder, so I understand how each signal gets attenuated..

but then I noticed two things:

1) why R40 and R61 are not at the end of the ladder (i.e. at the R38/R39 junction), but one step before?

2) how could this work as a crossfader between the two signals, if they share the same control bits on the same arrangement? This way the two signals would get attenuated at the same time, while I would expect that as one signal gets attenuated, the other would get boosted, and to get that you would need to rearrange the circuit around one pair of CD4053. Am I wrong?

I should note that I traced and checked (more than twice) this schematic from the actual unit, so there should be no errors..

obviously I am missing something

enter image description here

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  • \$\begingroup\$ Oop, never mind, looks like you're asking specifically why this differs from an ordinary R/2R ladder. Telling you to look up R/2R ladders won't be too useful there then. \$\endgroup\$ – Hearth Mar 14 '20 at 17:53

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