Can I convert stereo to mono for a phono input

I am feeling intellectually lazy, but a superficial reading of this thread leads me to think that the OP may want to achieve EQUAL amounts of crosstalk, by the method described.  That's OK, but keep in mind that equal crosstalk of one channel into the other and vice-versa does not usually correlate with LEAST crosstalk of one channel into the other and vice-versa, unless you own the mythical perfectly constructed phono cartridge.  If your goal is least crosstalk possible, then this method may fail to get you there.

Dear Peter,
I have read several long treatises on azimuth and how to set it and why to set it.  Two such experts were Victor Khomenko of BAT and Brian Kearns, a "guy" who used to hold forth on the vinyl asylum and who seemed to know what he was talking about.  I printed out both of their long posts on VA. Those two disagreed on a few items, but they both agreed that azimuth adjustment should not be done in hopes of equalizing channel output.  (I actually wondered whether that was your goal, but I figured Almarg would have warned you off it before me, if that was the case.)  Since I own a Triplanar tonearm where it is a simple matter to alter azimuth, I actually conducted an experiment to determine for myself the effect of azimuth adjustment on channel output (i.e., not crosstalk).  I proved to myself that both Brian and Victor are correct.  At the most extreme angles off the perpendicular with respect to the LP surface, I was able to measure only a 1-2db change in channel output, for both the R and L channels.  Furthermore, you would never want to play LPs with the cartridge tilted so far away from horizontal (or perpendicular if you reference the LP surface).  Doing so would obviously damage both the LP and the stylus tip, over time.  The bottom line is that fiddling with azimuth is not the way to deal with a difference in signal voltage between R and L channels. Even if you could "fix" the problem that way, you would be sacrificing too much else, including in addition to increased stylus and LP wear, also minimizing crosstalk, to make it worthwhile. You should look elsewhere for your solution.

I would think that the read-out of a recording meter is very unreliable as to assessing channel balance.  First, the needle is bouncing around.  Second, you have no way of knowing how the material was encoded into stereo, meaning the fault may be in the recording.  Further, what do you know about the state of calibration of your meters?  IMO, if you don't hear the problem, why drive yourself nuts over a few dancing meter needles?  Try feeding each meter separately with a single signal; you don't even have to know for sure the voltage or db, so long as it is the same signal fed alternately to each channel.  This way you might determine if the meters are equally sensitive.

If the cartridge is fairly new, you could make a case with the dealer to replace it, if all else fails.  A decent cartridge should have inherent channel balance within less than 2db, but I don't know what bible that is written into.

By the way, as to your response to my earlier diatribe, getting the azimuth "right" is not going to cure your channel imbalance, if indeed there is one.  That was sort of my point.

Peter, Sorry for being so pedantic and possibly condescending.  I am sure you know what's up much better than I do, since you've got the stuff in front of you.  I speak as someone who endured for several years an evident channel imbalance (L>R) in my main listening room.  I never was able to figure it out.  Then one day I closed both halves of a large double door that connects the listening room (our living room) to the adjacent dining room.  Lo and behold, the perceived (by everyone) L channel bias went away.  Sometimes it's the room.  But in your case, you say you DON'T hear a problem; you're worried about what the meters tell you.  A certain old Borscht Belt comedian would say, "don't look at the meters".