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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
john_tracy
341 posts
 
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.
folkfreak's avatar
folkfreak
1,349 posts
 
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)
peter_s OP
1,348 posts
 

Thanks again everyone.  I apologize for being away from this thread for a while...

Lew - I also have a channel imbalance in my room, and a preamp (VAC) that has no balance control.  I've been struggling with that for a while.  I think it could be my hearing (weaker L ear hearing), as some people don't notice it, but others do.  I would love to resolve this.  Anyhow, that is why I don't "hear" the imbalance effect of the cartridge, as the overall system already has the same issue!  But the meters, on multiple recordings, are all telling me the same thing.

Al - is switching the leads to one channel on the cartridge the same as me attaching a multimeter to the center pin on one channel and the ground sleeve on the other?  Or is the ground blended by the phono preamp?  I still haven't heard back from Einstein about my question.  But this pertains to your statement:

 keep in mind, though, that with the meter connected as you described it would read the difference between the two channels, not the sum. Your suggestion would work, when playing an out of phase track, if the connections to the two cartridge pins for one channel were interchanged. Although if that were done I would wonder if the proper connections could subsequently be restored without physically affecting the adjustment to some degree.

Johnny and Folkfreak - I will try working with a mono LP.

John_Tracy, that is a small difference in output for a significant difference in sweet spot.  Mine is around 2 feet to the left, in my estimation. I'm curious whether most balance/volume controls provide that high resolution of an adjustment.  I would guess 0.5 db is the lowest.


almarg's avatar
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almarg
9,670 posts
 
Is switching the leads to one channel on the cartridge the same as me attaching a multimeter to the center pin on one channel and the ground sleeve on the other? Or is the ground blended by the phono preamp?
Hi Peter,

While I suppose there may be a few exceptions, I would expect that in nearly all cases the ground sleeves of each of the RCA connectors provided at the outputs of phono stages and preamps are connected to the component’s circuit ground, and therefore to each other.

In the case of your Einstein phono stage, I see that the phono signals for the two channels are processed through circuitry in physically separate housings, so I would expect that the circuit grounds for the two channels are connected together via the external power supply which is connected to both housings.

So if my assumptions are correct a reading between the center pin of the RCA output connector for one channel and the ground sleeve of the RCA output connector for the other channel would simply indicate the signal voltage for the channel to which the center pin is connected.

Best regards,
-- Al

peter_s OP
1,348 posts
 
Thanks Al. I finally reached the manufacture via Skype. He told me that I could use a Y adapter, but only if I add some resistance to the output. For each channel,  I need to add a 500 or 1000 ohm resistor (0.6 W minimum)  to protect the circuitry. So off to RadioShack I go! I need simply insert this resistor on the hot (positive) leg  between a male and female RCA jack upstream of the Y adapter.  Then it becomes safe to sum the two channels, or should I say short the two channels together, and I can use the out of phase azimuth track to set my azimuth. Based on other  contributions to this thread, I will start there and then adjust by ear on one or two minor tracks.  

So I guess the upshot of this whole thing is that one can use the out of phase test track even if you don't have a mono switch if you take the proper precautions, and it's probably worth contacting your manufacturer, as  Al suggested early on.
Thank you everyone. 
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