Mono switch on phono preamp vs Mono Cartridge

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.

Thank you @dwette 

I have previously addressed similar questions and concerns on the Steve Hoffman forum.

https://forums.stevehoffman.tv/posts/37109436/
https://forums.stevehoffman.tv/posts/37825839/
https://forums.stevehoffman.tv/posts/37851830/

To add some background to the above explanations, the reasons why mono cartridges exist in both true and ersatz forms have mainly to do with manufacturing and component costs.

A bridged or summed "mono" cartridge is far more likely to be encountered when the underlying design is fixed coil (MM, MI, MP, MF). The underlying reasons lie in how the fixed armatures (also called polepieces) of such cartridges are made; as stacked laminations punched out with high-hardness, high-strength dies (similar to how the laminations of small transformers are made).

The orientation of the fixed armatures to the moving magnet (or moving iron) determines whether the fundamental architecture is stereo or mono.
Altering a fixed-coil cartridge from stereo into _true_ mono demands a revision to the armature shape, which requires the fabrication of another punch die of different shape. Such dies are fairly expensive, and that cost will be multiplied by two; one set of dies for the stereo armatures and a second set for the mono armature. (Note that the high cost of the dies is justified because it makes the cost of the individual laminations cheap.)
Technically, dedicated stereo and mono armatures could be done, and would eliminate the need for signal bridging / summing. But due to the die tooling investment and component cost factors (1000 pieces of a single type of component is cheaper than 500 + 500 pieces of two types of components), I am not aware of any fixed coil (MM, MI, MP, MF) mono cartridge that has a dedicated armature of different shape from the stereo models.
It is far more economical for the cartridge manufacturer to simply sum the outputs of the stereo channels together, and label the result "mono".

The situation is different for MC mono cartridges, since with the fore-and-aft magnetic circuits that are typically used, the orientation of the polepieces has no bearing on whether the cartridge is fundamentally stereo or mono. What matters is the orientation of the moving coil bobbin to the stylus. If the bobbins and coils are oriented at 45º, the cartridge will be stereo, if the bobbins and coils are oriented horizontally, the cartridge will be mono.
Since the bobbins can be made as part of the (moving) cantilever assembly, the cartridge designer can simply orient the bobbins at 45º for the stereo models and twist them horizontally for the mono model. This will require separate cantilever and coil bobbin assemblies for the stereo and mono models, but as long as this is done, there is no need for bridging / summing.

Of course, if the cartridge manufacturer is insistent that their mono MC cartridge must use bridging or summing, nothing stops them from orienting the coil bobbins at 45º, and for the mono version, summing the two channel outputs together. But in practice, this would only be done if the component budget is so low as to permit only a single cantilever and coil assembly, which must be shared across both stereo and mono models.

hth, jonathan

Now I am confused.  I was under the impression that the Atlas was unique.  Are there other true mono cartridges that are vertically compliant?

All the Lyra Monos are (Kleos, Etna, Atlas). The AT and Hana monos are. The (now discontinued) Dynavector XV-1s Mono. These all have vertical compliance. Really, the majority of modern mono cartridges are. The ones that aren’t are those designed to play mono records mastered and pressed before the mid-60s (such as the Miyajima mentioned earlier). 

Here’s a Mono cartridge database I found that includes information about vertical compliance. I don’t think it’s comprehensive, but it does demonstrate the current prevalence of vertically compliant mono cartridges.  https://dgmono.com/mono-cartridge-database/