Mono switch on phono preamp vs Mono Cartridge

@lewm I see now what you meant. My bad.

Given that, a true mono cartridge really does not need a mono switch on the phono-stage or preamp. That will do nothing useful, since the mono cartridge already ignores any signal (i.e. dirt, noise) in the vertical (i.e. stereo) walls of the groove. My point for that scenario stands: why introduce circuitry into the signal path unnecessarily?

Strapped stereo cartridges are another matter in that regard, but they aren’t true mono cartridges anyway. They still try to reproduce the stereo walls of the record grove, which are just dirt and noise. In that case engaging the mono switch on the phono-stage may help cancel out noise.

I’ll throw this out there…just as an alternative approach…

If the funds are directed towards upgrading to a better stereo cartridge, say Umami Red or even Umami Blue - sell the ML and add that $ to was to be spent on second arm, base and a lower quality mono SL cartridge, wouldn’t it be an overall bigger improvement even in the mono playback? What’s the ratio of mono to stereo records in your collection? Hana ML is a great cartridge but it’s not as good as Umami Blue. Not even to mention how far beyond the ML’s performance Red will be…

dwette, you wrote, "Strapped stereo cartridges are another matter in that regard, but they aren't true mono cartridges anyway. They still try to reproduce the stereo walls of the record grove, which are just dirt and noise."  As I understand it, bridged stereo cartridges do cancel signal derived from vertical travel of the stylus tip because of phase difference.  The imperfection might arise where the two channels of the cartridge differ from one another in responsiveness by even a tiny amount and then cancellation would be imperfect.  But by and large, they work.  That is why for those bridged cartridges, and there are many if not most among mono cartridges, using the mono switch may help further to clean up noise, I think.

First of all, we need to understand that all mono cartridges send a mono signal to the phono stage.  In that sense, in the electrical sense if you will, they are all truly mono.  The difference and it is important is that cartridges like the Miyajima cartridges are that these later "true" mono cartridges are only compliant in the lateral plane.  That means they do not move in the vertical plane AT ALL.  So if one of these "true" mono cartridges is inadvertently used on a modern microgroove stereo record, said record will be damaged.  Actually it will be damaged severely.  Ruined is not an exaggeration.

@billstevenson That’s not correct about modern true mono cartridges. Many (most) of them are vertically compliant. The Lyra Atlas SL, AT MONO33, and others. 

@jcarr can probably clear this all up, but this is what the Lyra Atlas Lambda Mono manual says about it,

The Atlas Mono has a completely unique, genuine monaural cantilever/ coil/stylus assembly for true monaural playback.

On monaural records, the vertical axis of the groove contains no musical information, but it will frequently have noise, in the form of groove damage and dirt. The Atlas Mono is completely insensitive to the axis, which greatly improves the signal/noise ratio. The coil former is a square permeable plate which is oriented parallel/vertical to the record surface (rather than the 45-degree angle used for stereo cartridges), while the coils are wound so that they only generate a signal when there is horizontal movement of the stylus and cantilever.

The Atlas Mono has been designed with two electrically separate monaural coils directly on top of each other, since most monaural cartridges are in fact used with stereo amplifiers and stereo speakers. The separate coils also help avoid possible ground loops and hum problems that could otherwise be caused if a single coil (as with most other monaural cartridges) is fed into a stereo two-channel amplification system. Each channel of your stereo system will be fed an identical monaural signal, so each speaker will produce an equal monaural signal.

This will automatically result in a single mono "image" located centrally between your speakers. If your stereo preamp incorporates a balance control or separate attenuators for each channel, you may turn down either channel to hear the mono signal from only one speaker. And for pure monaural playback in a true monaural audio system with a single amplifier and a single speaker, only one of the two channels (one vertical direction pair of the cartridge’s output pins) needs to be connected to the preamp’s phono input. Feel free to experiment and use the connection method that provides the best sonic results in your system.

Your Atlas Mono is physically capable of tracking stereo records as well as mono records (stereo records will not be damaged by such playback), however stereo records will not be reproduced in stereo sound.