Acoustical Manufacturing, Huntingdon

[thyratron]

We've had an M31 in our family from new. I was recently speaking to a prominent member of a highly regarded national wireless society who mentioned the output meter was a rarity. Here are some pictures and bumph that came with the equipment around 1946.

[Synchrodyne]

The Acoustical M31 was a significant amplifier, in that it was the first to use P.J. Walker’s partial cathode-loading output circuit.

That probably made it the first production amplifier with a distributed loading output. As far as I know, there was no commercial use of A.D. Blumlein’s earlier distributed loading circuit until it was developed by Hafler and Keroes in 1951 under the “ultralinear” name.

Thereafter – from say 1954 in UK practice – some form of distributed loading became de rigueur in valved hi-fi amplifiers.

So, you have an historically significant piece of equipment, and an example with clear provenance. Perhaps it qualifies for a glass museum-style case!


Cheers,

[Herald1360]

An interesting circuit. Strange to see a phase splitter driving a transformer, though. Maybe the ingrained idea of a transformer for driving p-p pairs was combined with a notion of getting dc field offset out of the core?

Until the driver transformer went, I guess that global NFB would only have offered a marginal improvement so wasn't used.

[Synchrodyne]

And to support that notion, the QA12 of 1947 had a valve phase-splitter and global feedback.

Cheers,

[Radio Wrangler]

That last circuit, posted by synchrodyne, is worth keeping prominently as an illustration (literally) of distributed sectional windings in an output transformer.

Usually these recipes are treated as company secrets, only discoverable by buying a unit and unwinding it.

David