[Leifer]

I've always wondered....
Does a "mono" button on a preamp, change the sound of mono recordings ?
What about mono vinyl ?

[richidoo]

Nope.

Mono button sums left and right channels of a stereo recording together. Mono recordings are already summed together so no effect from the button.

Mono CDs are stereo with equal tracks left and right, and no noise, so no effect on CDs. But all vinyl has "stereo" noise so if you play a mono record with a stereo cartridge you will hear the music as mono but the noise is stereo, so mono button can sum the noise to the center to be less distracting. A mono cart playing a mono record should be no diff with mono button.

On modern recordings of popular genres bass is usually panned to the middle, so mono mode will mostly effect the wider panned mid/treble. But classical symphonies and some 60's early stereo music can have more separation in the bass that can sound muddy when summed to center. ymmv

[Elizabeth]

There are also many early 50's even 60's 'monaural' releases which include the info: This is a monaural recording, but if played on a stereo, will have more information.. etc... (Not exact words.. but what it means.)
So those sorts, even if transferred to a CD with no such statement. can sound better left playing in  stereo than mono.
I never use the mono switch.... Sounds fine.
And I play a lot of mono Jazz CDs... some Lps...

[S Clark]

Vinyl or digital?  If vinyl, the biggest improvement is to get a mono cartridge. 
That makes a big difference.