Can I make my stereo play mono by...

Playing CDs with optical cable into Integrated...

If your CD player come with DAC built-in, connect a ' Stereo to Mono Signal Converter ' from the CD player analog out to the integrated line input. For MONO playback, select line input instead of optical digital in.

https://www.newark.com/mcm-custom-audio/50-20795/stereo-to-mono-signal-converter/dp/25AC5154?gclid=E...#

Put the speakers next to each other if your pre doesn't have a mono switch, that will nearly be mono. Alternatively, if you have the freedom to set them up properly and enjoy the stereo, search for " Sumiko master set" this discribes a systematic approach to setting them up in the room and getting it right using nothing more than a CD with a good range of bass notes and your ears.

Buy a preamp with a mono function switch, if you don't have one already!

Only two-channel tube amps can be parallel-connected! This will give double the power for mono use. Never parallel the outputs of a two-channel transistor amp! Some of these amps have internal bridging circuits that will allow conversion to mono safely.

I find that using a pair of speakers this way for mono is much better than a single speaker.

Leave your pair of speakers in their normal location. Use the "mono" function on your preamp (if it has one). You will have a nicely centered mono image between the pair of speakers! Leave the amp connected as usual for stereo.

... thank you for your explainations - in this case I guess Electrical Engineering trumps common sense...

Amplifiers are high current, and they have an idea of what the signal should be.

If 2 amplifier channels are summed by a cable, they will usually fight each other, providing their maximum output current to win.  Imagine there's a trumpet on the left. but not on the right. The two amplifiers will move all their current to change the voltage to what they think is the correct signal. One will fight for the trumpet, the other will fight for silence.

That is why you can't just use a wire to connect the outputs.

Never allow 2 amplifier channels to touch via a conductor.

You may have 1 amplifier channel serially connected to any number of speakers:

Amp (+) ---> Speaker 1 (+) (-) ------ > Speaker 2 (+) and so on, until ----> Amp (-)
Do not short speaker + and -

You may connect speakers in parallel, but this has the usual impedance limits.

Of course, this is not really mono. This is 1 channel (L or R) outputting on mulitiple speakers.

The safe way to create a mono signal, if you don't have it in a switch is to use the RCA/XLR cables and sum them there. It's relatively safe only because at 1-2 V you are unlikely to create high enough currents to damage anything.

Technically, the ideal way to do this is via a high impedance summing circuit.