A practical guide to setting quiescent current on a Burmester 909 clone — and why its DC offset is not your problem.

The Transaudio T9.4 is one of the better-executed Chinese takes on the Burmester 909: a 36 kg, dual-mono, 225 W/8 Ω (450 W/4 Ω) class AB stereo amplifier that shows up in various guises — Transaudio T9.4, ASYPDZSW BL9, and a handful of unbranded “Reference X-Amp” boards from the same board house. They all share a circuit, and they all share the same two questions from new owners: what’s my DC offset supposed to be, and how do I set the bias.

A steady share of the vintage gear that lands on my bench arrives with the same request attached to it: “while you’re in there, can you add speaker protection and short circuit protection?” It gets asked as if it were one job. It isn’t. Those are two different problems, solved by two different circuits, sitting in two different places in the amplifier. The board pictured here does one of them very well and the other one not at all — and I’d rather explain that up front than have someone believe their receiver is now bulletproof.