Analog Prairie Sumpost — 16×2 Passive Magnetic Core Summing Matrix

Sumpost is a 16-input, 2-output passive analog summing matrix built around magnetic core summing technology using four NOS Triad A-61J transformers. Send up to sixteen balanced outputs from your interface or D/A converter into Sumpost, combine them through the passive network and transformer cores, then return the finished stereo signal to your converter, preamp, or mix-bus chain.

Unlike conventional active summing designs, Sumpost uses the transformer windings and magnetic cores as an integral part of the summing architecture. The A-61Js are not simply output transformers added after the mix — each channel has its own dedicated winding to the transformer core, where the actual analog summing occurs.

At conservative operating levels, Sumpost can remain clean and controlled. Drive the network harder and the magnetic cores progressively contribute harmonic content, particularly with strong low-frequency material.

Features

• 16 balanced line-level inputs
• 2 balanced stereo outputs
• Passive magnetic core summing technology
• 4 × NOS Triad A-61J transformers
• 2 × DB25 analog input connectors
• 2 × Neutrik XLR outputs
• Eight fixed stereo pairs
• Odd channels feed Left / even channels feed Right
• Completely passive operation
• No external power supply
• 2U 19-inch rackmount chassis
• Shallow 4-inch chassis body
• Designed, assembled, and individually tested in Iowa

How It Works

Route stereo stems or individual channels from your DAW to the sixteen analog outputs of your converter.

1/2 → Stereo Pair 1
3/4 → Stereo Pair 2
5/6 → Stereo Pair 3
7/8 → Stereo Pair 4
9/10 → Stereo Pair 5
11/12 → Stereo Pair 6
13/14 → Stereo Pair 7
15/16 → Stereo Pair 8

Those sixteen analog signals are combined through Sumpost's passive magnetic core network and returned as a balanced Left/Right stereo pair.

Sumpost was built for engineers who want to keep the flexibility of a modern DAW while giving the final mix a real physical analog summing path through transformer iron.

Analog Prairie — designed and built in Iowa.