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
Just use a multimeter. Measure the AC voltage across L and R. I’m assuming the test signal is < 1 kHz?

Attach one lead to L center pin and one to R center pin (after preamp) and adjust until AC voltage is minimal.
If you used a stereo test signal that is identical for both channels, wouldn't that be the same as a mono signal?
@mb1audio they're not the same. They are out of phase, so when added perfectly should be 0.

If one channel is louder than another, the AC output will be greater than zero. Tricky thing but it works. :)

Best,

Erik
OI should also point out that when doing azimuth alignments in other contexts, 0 may actually be impossible due to some noise or impossibly fine misalignment. The goal is to get as close to 0 as possible.
Erik, 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.

Regarding mb1audio's question, yes of course that would provide a mono signal, but I would wonder if the corresponding lateral movement of the stylus would allow as precise an adjustment of azimuth as the vertical movement that occurs while playing an out of phase track would allow. Maybe it would and maybe it wouldn't; I'm just not sure.

If the preamp or phono stage has a relatively high output impedance I suppose it would be reasonable to simply sum its L and R outputs together with a y-adapter. But I would be hesitant to do that if it has particularly low output impedance, especially for a significant length of time, at least without getting an ok from the designer.  The resulting current flow between the output stages could conceivably be more than they were designed to handle safely.

I suppose the best approach might be to purchase an inexpensive pro-oriented mixer, and connect it to the outputs of the preamp or phono stage. B&H Photo/Video (bhphotovideo.com) sells many such devices.

Regards,
-- Al
 
A combination of these y adapters will accomplish what you want to do.
http://www.audioadvisor.com/ssearch.asp?txtsearch=y+adapter
Azimuth adjustment on a test record like Analogue Productions, provides a test tone for each channel individually. No need to build or buy anything other than the right record.
Ops! You are right. You need the maximum AC voltage!! :) Same connection. My bad.

Best,

Erik


It remains a little odd to me that Phonograph cartridges which are inherently balanced devices, don't have XLR outputs.
It’s not the cartridge that needs XLR outputs, it’s the arm cable that does. Some arm makers, and arm cable makers as well, DO offer XLR termination, and some phono amp makers XLR inputs. An arm cable that is terminated in RCA plugs can have them replaced with XLR’s.

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

You need the maximum AC voltage!! :) Same connection.
I had thought about suggesting that, Erik. But I’m not certain that maximizing the voltage difference between out of phase signals on the two channels would allow as precise a determination as nulling out their difference by adding them together.

Envision that azimuth has been optimized precisely, by maximizing the measured difference between out of phase signals. Then envision that a small misadjustment of the azimuth is introduced. It seems to me that even though the amplitudes of the two signals might become significantly unequal, with one becoming larger and one becoming smaller, their difference might not change much if at all from the previously established maximum. Whereas if they were added together they would no longer sum to zero.

Best regards,
-- Al


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???
Hi Peter,

Regarding your question just above, the following comment I made earlier applies. In addition to option 2, it also applies to option 3. The bottom line is I don’t know :-)
I would wonder if the corresponding lateral movement of the stylus would allow as precise an adjustment of azimuth as the vertical movement that occurs while playing an out of phase track would allow. Maybe it would and maybe it wouldn’t; I’m just not sure.
Regards,
-- Al


It looks like the currently manufactured Einstein phono stage has a specified output impedance of 50 ohms. While it may very possibly be ok to short its two outputs together with a y-adapter while they are driving opposite polarity signals (even though that would cause far more current to flow in the two output stages than under normal circumstances when music is being played and the outputs aren’t shorted), and I suspect that a lot of audiophiles would just go ahead and do that, that is a low enough impedance that I personally would very definitely NOT take that risk without an ok from the manufacturer.

Best regards,
-- Al

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.

Why do things the easy way when you can do them the hard way and risk your equipment at the same time?

**Azimuth Adjustment

Track 2 - 1KHz reference level, left channel only                            Measure right channel output

Track 3 - 1KHz reference level, right channel only                            Measure left channel output **

Analogue Productions Test LP (about $40)

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.
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.
peter_s
My immediate goal is to get equal output out of both channels.
I agree with @lewm Uneven channel balance is not necessarily something that can or should be corrected by altering alignment.
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
... 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).
Thanks Lew, but you give me too much credit :-)

(Especially when it comes to non-electrical things like cartridge and tonearm adjustments).

Best regards,
-- Al
 
peter_s
If not - hmmm - what ARE the other options
First, I'd make certain the imbalance is in the phono cartridge and not in the phono stage itself. No cartridge is perfect, but a good cartridge really shouldn't have an audible channel imbalance, imo. If it were a new cartridge, I'd return it to the dealer. The only other solution is to just replace the cartridge or use the preamp's balance control.

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).
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.
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.
This can not be done.
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)

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.


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

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. 
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.


My experience has been that 0.25dB steps for a balance control is about as "wide" as one would want to go. My DIY line stage also doesn't have a balance control. So I provided my DIY power amps with input attenuators that can reduce their gain by ~2dB in 0.25dB steps.
 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? 
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?