Analog Prairie Sumpost 16x2 Core Summing Matrix

Sumpost is a 16×2 passive "magnetic" summing mixer built around four NOS Triad A61-J transformer cores. Rather than simply combining sixteen channels through a conventional resistive summing network, Sumpost groups the incoming channels through dedicated transformer windings, allowing the magnetic cores themselves to become part of the summing process.

There are no op-amps, ICs or powered gain stages in the signal path. The result is a deliberately simple analog mix bus whose behavior changes naturally with program level, channel count and loading.

Each transformer uses a 1:2 turns ratio, providing approximately 6 dB of unloaded voltage step-up. Actual system level varies with channel loading, source impedance and the impedance of the receiving device.

Specifications

Inputs: 16 balanced line-level

Outputs: 2 balanced line-level

Input impedance: Load-dependent; varies with source/return impedance and active channel loading

Frequency range: 20 Hz–20 kHz

Maximum transformer input: +20 dBu @ 60 Hz / 1% THD+N

Designed, assembled and tested by Analog Prairie in Webster City, Iowa, Sumpost was developed as an alternative approach to passive summing: using vintage magnetic cores as an active part of the summing architecture rather than treating the transformer only as an output color stage.

Open to offers.