My Philips 123 has always had an extension speaker socket wired in parallel with the internal speaker. The output transformer is mounted on the internal speaker, and the extension speaker socket taps in at the connection lugs of the internal speaker.

I understand this arrangement will significantly reduce the output impedance of the radio when the second speaker is in parallel. It has not been any problem with the 8ohm speaker in the baffle-board. However, I have another vintage enclosure that I want to attach, and the speaker seems to be more like 4ohm impedance (judging only by measuring resistance across it, unconnected).

I've read that it is safer to connect unmatched speakers to an amplifier in series, to develop a higher overall impedance on the amplifier output. However, the output transformer secondary is also providing feedback to the audio circuit, so I am reluctant to change the parallel connection to series without getting some advice.

Anyone got some advice on adding unmatched speaker?

Maven

In terms of DC resistance, 8 + 4 in parallel = 2.67 ohms which may reflect too low an impedance back to the power output valve, but some valves have a pretty wide tolerance -- within reason.

8 + 4 in series is 12 ohms, which it may be happier with.

The feedback is just a sample of the output signal so a series speaker connection shouldn't matter. They key thing is to endure that it's negative (phase) feedback otherwise you'll create an oscillator. If it whistles, connect to the other side of the output transformer to get the other phase.

I made a connection where the extension speaker is switched into series between the output transformer and the internal speaker, on the side that feeds back to the audio amp valve.

It works quite well for an 8ohm extension speaker with about 5metres cable, though the change in impedance seems to cut some of the treble from the audio.

It doesn't work well for the other extension enclosure, possibly raising the overall impedance too far? The 2nd enclosure has a longer cable - maybe 8meteres. I get a lot of noise and reduced signal.

I'm guessing that the extended cable lengths are picking up some RFI that gets into the feedback signal. I'll need to open up the 2nd speaker enclosure to see exactly what kind of speaker is in there.

Maven