How are you playing your precious MONO Vinyl?

Elliot, You wrote,

"Everyone, 

the Aurorasound EQ-100 is uniquely different, from the manual:

"When a stereo cartridge is used, any vertical signal on a record is cancelled by an internal circuit of EQ-100. Only the R+L horizontal signal are detected to assure a superior sound stage with less noise."

Cancelling any vertical input seems to me FAR better than producing the noise and then summing the noise in both channels, that is my biggest objection and what I referred to as MUD on top of the Mono Content (Dual Mud?)."

First, the long sentence about how the Aurora works can be applied to any stereo preamplifier when the mono switch is activated. That is what they all do, and that is why HF noise from the LP surface irregularities is reduced, not enhanced, compared to playing a mono LP in stereo mode with a stereo cartridge.  Then in your second paragraph you revert back to this idea of "summing". Just forget it. It's a semantic twister. None of this is to say that I disagree with your contention that using a mono cartridge is superior to using a stereo cartridge plus mono switch. I am in no position to disagree, because I don't use a mono cartridge at all.  Although I do own a Shelter mono cartridge that has been sitting in a box for about 5 years.

For you guys who do use a mono cartridge to feed a stereo phono stage, consider that the two channels of your phono are unlikely to be absolutely identical in all measurable aspects of their performance. Thus the output from one channel may differ very slightly from that of the other channel, thus introducing, say, phase differences or slight differences in distortion or frequency response. Thus there could be an audible difference possibly between activating the mono switch and not activating it. And results might differ from one system to another, particularly because humans are reporting on what they hear. Nevertheless, I urge those of you with mono cartridges to try it both ways, mono switch on vs mono switch off, and report back.

Using a stereo cartridge on a mono LP is not going to harm the LP.  No way. You won’t get the best possible SQ, but there is no harm done.  However, many stereo LPs marketed at the dawn of the stereo era do say on the back of the cover, usually in small print, not to use a mono cartridge, for the reason mentioned already, the lack of vertical compliance of most mono cartridges of that era might damage the groove walls that carry stereo information.

For the umpteenth time, using a mono switch when playing a mono LP with a stereo cartridge is not per se going to increase noise because of "sum"-ing. Phase cancellation takes care of that, and reduction, not summing, of common mode noise is the principle benefit of using the mono switch.

Define “early”, because that’s where there is lots of controversy.

What is "EQ-100"?

If you think you have a true mono cartridge, do try with vs without the mono switch engaged and let us know what you hear. At least some of the Beatles LPs originally came out on Parlophone in the UK and were mono to begin with.  However, since the works are from the 60s, it is quite likely that modern RIAA equalization was used. I have never read otherwise.

I hope I did not say there are no "true" mono cartridges. I only refer to the type described. Still, the second caveat for comparing is valid; your particular mono cartridge may just be "better" or "worse" than your particular stereo cartridge. Therefore any conclusions are subject to that qualification. I do think even using only a mono switch when playing mono LPs with a stereo cartridge is superior to not using the mono switch.  At least in that comparison I am not using two different cartridges. If you want to get really purist, you can use one of those few mono cartridges that only has two pins and use it to drive only one channel of your stereo system. I've never tried that nor heard it on someone else's system.

This subject has been rehashed many times.  Many of the mono cartridges on the market are nothing but stereo cartridges in which the two channels have been bridged internally. The mono switch on a linestage also achieves mono output into both channels by virtue of bridging the two channels at the output.  So, when comparing this type of mono cartridge to the mono switch, the only difference is that in the one case the two channels were bridged before RIAA correction and amplification; whereas in the other case the mono signal is created after these processes take place. I could imagine reasons why either one is better than the other.  The other factor is that when you use a mono cartridge and compare it to your stereo cartridge, unless you are using the artificially created mono version of the very same stereo cartridge you are otherwise comparing two different cartridges.  Any two cartridges can differ in sonics by virtue of many factors, so it is hard to say in that case that "a mono cartridge is per se better".