Can I convert stereo to mono for a phono input


My preamp doesn't have a switch to convert stereo to mono.  I would like to switch a stereo signal to mono to set my cartridge azimuth (when using two channels out of phase, this method allows accurate balancing of the channels).  Is there a way to build a simple converter: two RCA female plugs taking a stereo signal and mixing it to mono, output as two RCA male plugs (mono signal)?


I have a test LP that provides a stereo track (test signal) with the two sides out of phase.  All I would need to do is feed that through the mono converter to set my azimuth. 

Thanks for any advice.  Peter

peter_s
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".

It remains a little odd to me that Phonograph cartridges which are inherently balanced devices, don't have XLR outputs.
Some do (well, some **arms** do anyway). Most tone arms have a balanced connection somewhere (even a lowly BSR or Garrard); its just not got the XLR connector.
Just play a mono LP with your stereo cartridge. For a long time, I used a stereo cartridge for everything. One time I was playing a mono Prestige repress of a Miles Davis album. I was sitting in the sweet spot, and the soundstage was emerging from the left wall, 90 deg. left of the speakers. 

My headshell has azimuth alignment, so I got out a bubble level and set it. Sure enough, the mono image returned to the phantom center channel where it belonged.

I also used a mono version of Sgt Pepper's "A Little Help from my Friends" to adjust the phase and crossover settings on my dual subwoofers. Paul's melodic bass line on that song dances back and forth above and below my 50 Hz crossover point, and having the same signal go to each subwoofer (one at a time) dialed in a seamless response that also eliminated a 100-200 Hz "hump".

Mono albums played w/stereo carts can be great tools for dialing in stereo image and phase issues.

A decent cartridge should have inherent channel balance within less than 2db,

In my system a 0.25dB imbalance moves the center of the sound stage by about a foot. Read manufacturers specs on their cart. channel balance. Like Lew wrote, most only guarantee ~2dB or better.

 If your system is all tube with no feedback like mine, there's another "can of worms". As tubes age their gain will change and without feedback to "steady" things balance can drift around a bit. Can be annoying.

One of the easiest ways to measure crosstalk with a test record like the Analog Productions record is with a calibrated 2-channel 'scope. A DMM will also be responding to noise and the reading will be bouncing around making an accurate reading more difficult. With a 'scope it's easy to see and compare the heights of the crosstalk peaks.
Adding on to johnnyb53’s comment I always found this to be a great explanation of how to set azimuth using a stereo cartridge on mono material -- certainly works for me (much better than using my Fozgometer)

http://www.durand-tonearms.com/Support/Support/azimuth.html  (click on the "tips to set azimuth" tab)