Radio Shack sold PA amplifiers with an output transformer well past the age of the tube, like the MPA series, e.g. MPA-40, a 20 W mplifier. On that thing you can obtain the raw amplifier output using the "70V" terminal. Then it has a number of through-the-trafo outputs labelled with nominal ohmages of speakers.

The Owner's manual extols the advantages of using transformers for speakers and describes how to use the 70V output in conjunction with external transformers.

Quote:

For complex multiple-speaker arrangements that require many speakers and long runs of connecting wire, we recommend you use a line transformer (not supplied), available at your local RadioShack store.

[...]

There are several advantages to using transformers.

• You can connect speakers with different impedances without causing differences in output between the speakers.

• You can add or remove a speaker without having to recalculate the entire system’s impedance.

• You can reduce signal loss when you use speaker wire over 50 feet long.

LOL!

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quickthrowman 11 hours ago | parent [-]

Sound masking systems still use 70V audio output with output transformers at each speaker, voltage drop is rough when your signal is only a few volts and you’re using small conductors. Last time I sold a sound masking install we used 14/2 cable for the 70V audio signal.

https://www.atlasied.com/speech-privacy-speakers?srsltid=Afm...

	▲	sethhochberg 11 hours ago \| parent [-]
		They're quite popular for distributed audio systems in general (of which sound masking is one type). "Constant voltage audio" comes in a few flavors and 70v is very common in the US, other parts of the world often use 100v. Background music systems in retail, voice paging systems, etc use constant voltage hardware because its much better technology for very long cable runs, daisy-chained speakers, and centrally located amplifiers. The cost is fidelity. Full-range audio transformers aren't cheap, so these systems usually make some compromises because your announcements or smooth jazz over the pasta aisle don't need to be true hi-fi. Its cool technology. Most of the speakers have variable power taps, so you can run a bunch of them in parallel on a single line and control the actual volume as-needed based on where the speaker is deployed by varying the transformer tap on each speaker.