VINTAGE RELAY AMPLIFIER
1960s hydroelectric relays, re-engineered into a pair of 10-watt Class-A OTL amplifiers.
Building a power amp out of your microchip’s grandpa.
Ever heard a 1960s industrial relay sing?
These massive metal modules were once part of a hydroelectric power plant. After they were decommissioned, I opened them up and salvaged the metal-can transistors and Z11-core chokes inside, then redesigned those half-century-old industrial components around an entirely different purpose: audio.
At the heart of the build is an output-transformerless transistor amplifier with a CLC-filtered power supply. After recalculating the operating points, power supply, and thermal arrangement, the modules became a pair of independent mono Class-A power amplifiers, producing roughly 10 watts per channel.
The finished circuits are set into a carbon-steel base that I designed and fabricated from scratch. Components once used to control generating equipment now drive an Altec 299 compression driver or a pair of Dynaco A25s.
It is a way of giving an obsolete technology another life.
Build Highlights.
Original object — 1960s industrial relay modules retired from a hydroelectric power plant
Active devices — original metal-can transistors salvaged and reused from the modules
Power-supply chokes — original Z11-core chokes repurposed into a CLC filter
Circuit topology — output-transformerless solid-state power amplifier
Operating class — mono Class-A, approximately 10 W per channel
Mechanical design — custom carbon-steel base designed and fabricated around the original industrial modules
Reference systems — Altec 299 compression driver / Dynaco A25
Next iteration — a SET tube version built from the same industrial language
The complete build process and schematics are available on the YANKO AUDIO LAB Patreon.
