UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

G6Tanuki · 3rd Oct 2019, 9:04 pm

Yes, the S27 was designed to feed a 500-Ohm line as standard; that was sort-of commonplace as a way to distribute audio around a site in times-past (just as the "100-Volt" line is used nowadays).

A small transformer can be used to match the 500-Ohm output to a modern low-impedance speaker. It doesn't need to be a perfect impedance-transformation, the S27 has a push-pull output stage so there's plenty of 'oomph' on-tap to override any matching-indiscretions.

[This thread reminds me that I've got a S27 in my to-do pile; it's had the original output-transformer replaced by a modern one to match 8Ohm speakers and the S-meter zero pot moved so the new transformer can fit; since I'll only ever use it on headphones I'm minded to strip-out the bogus transformer, remove the two push-pull output-bottles and wire the headphones direct to the driver stage]

3rd Oct 2019, 9:04 pm	#9
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,005	Re: Hallicrafters S27 Yes, the S27 was designed to feed a 500-Ohm line as standard; that was sort-of commonplace as a way to distribute audio around a site in times-past (just as the "100-Volt" line is used nowadays). A small transformer can be used to match the 500-Ohm output to a modern low-impedance speaker. It doesn't need to be a perfect impedance-transformation, the S27 has a push-pull output stage so there's plenty of 'oomph' on-tap to override any matching-indiscretions. [This thread reminds me that I've got a S27 in my to-do pile; it's had the original output-transformer replaced by a modern one to match 8Ohm speakers and the S-meter zero pot moved so the new transformer can fit; since I'll only ever use it on headphones I'm minded to strip-out the bogus transformer, remove the two push-pull output-bottles and wire the headphones direct to the driver stage]