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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

Showing 10 responses by peter_s

peter_s OP
1,313 posts
 

Thanks everyone so far.  I have the Hifi News test record, with the following track:

Band 5: Cartridge alignment (azimuth) test (300Hz L+R +6dB)

This track is designed for cartridge Azimuth adjustment. To check the stylus is absolutely vertical, play the track in stereo and you should get identical output from each speaker, but when the amplifier is switched to mono you will hear nothing.

I also have the Cardas Test Record, that has a 1K test tone.

It seems like my options are as follows:

  1. Swap the cartridge leads on one channel, use the out-of-phase band, and reduce to zero on a voltmeter (but I don't want to bother swapping leads)
  2. Leave the leads alone, use a voltmeter as Eric describes, and play an in-phase test tone to get to zero
  3. Attach a recording device to my preamp and adjust azimuth so an in-phase test tone yields equal values per channel
  4. Contact the manufacturer of my preamp and see if I can use Y adaptors to blend the two channels and then split the blended signal into two, downstream of my phono preamp, in order to get mono. Then use the out-of-phase track to zero the azimuth.

Any other thoughts about best solutions among these, or are there other good ideas?  I will email the manufacturer of the phono stage (Einstein) about blending the signal into mono.

Thanks, Peter

peter_s OP
1,313 posts
 
Al - what do you think of my suggestion #2 above?  If the signal is in phase (e.g. 1kz test tone), measuring the difference between channels with a voltmeter would give me an accurate zero, no???
peter_s OP
1,313 posts
 
My immediate goal is to get equal output out of both channels.  Right now the right channel is a bit louder than the left, and I can see this on the meters for my recording device.  There is no issue with me using a single in phase test tone for both channels at the same time, and using my meters to make the adjustment.  I would just rather do the "nulling" with the out of phase test tone - that is more definitive.
peter_s OP
1,313 posts
 
Thanks guys.  I get your point here. I think it's worth getting the azimuth right for its own sake.  If it solves the problem, great.  If not - hmmm - what ARE the other options?  This is definitely happening within the cartridge, as I can swap L&R inputs into my phono pre and it follows the swap.  Best, Peter
peter_s OP
1,313 posts
 
I haven't noticed it as audible, but it is visible on my recording meters.  It follows the cartridge leads, so it is upstream of the phono preamp. I'll have to look at the meters to see how many db off it is (they may be expressed as VU).
peter_s OP
1,313 posts
 
Lew.  The fact that the meters register differently when I switch the leads to the phone a preamp shows that it is not the meters. As for the signal, it is a test tone on a analog test record, I would think it is good, and I noticed this with multiple  recordings sources  during the process of recording.  I am away from home, but I need to look more closely at the meters to see what degree of resolution they have.
peter_s OP
1,313 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.


peter_s OP
1,313 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. 
peter_s OP
1,313 posts
 
 Wow! I think I just found a good way to electronically evaluate my cross talk. There is a free program called ispectrum available for the Mac, that provides an oscilloscope with a band pass. I can use my test record with test tones for left and right channels individually  and measure the output for each channel for each individual track.  Input to my Mac Mini can be either through the microphone input, or through my USB analog to digital converter. In either case, the oscilloscope measures both channels, stereo.
Based on prior posts, I doubt I will be able to counter at my channel imbalance this way. Perhaps the anti-skating is off a bit and maybe playing with that will help? 
peter_s OP
1,313 posts
 
Another interesting observation about setting channel balance.  I'm looking at the meters on vinyl studio to judge channel balance.  I started with a 1khz -14db test tone on the Cardas Test Record.  It showed a weak left channel.  Antiskating didn't help here at all.  Then I used the Hifi News Test Record with a -20db pink noise channel balance track, and the channels seem balanced in the meters.  Then a mono Ella recording and the left channel is just a tiny bit lower than the right, nothing really significant.  Which to believe?  It seems like my calibration sources are imperfect, particularly the Cardas. How to set azimuth if the source is not truly balanced b/t channels…  Not impossible, but adds a bit of additional variability, no?

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