Phono Question

[Bobbsy]

There are lots of theoretical reasons why this is a bad idea but I've done it hundreds of times (usually doing playback at rehearsals with a stereo backing track into single powered speaker) and not had a problem with the gear yet.

 

However, there is one theoretical problem which CAN be real, and that's how it sounds. Depending on how the stereo track is mixed, you can quite often get phase cancellation problems when you combine them to a single mono. The person who did the mix should be checking for mono compatibility but this doesn't always happen (read: I forgot to do it on several occasions) and you suddenly find your kick drum or something disappears when you combine to mono.

 

So long as you're aware of this aspect and watch for it, it shouldn't be a problem.

 

Bob

[Pete McCrea]

I must admit I was sitting there thinking that I've done this before plenty of times and it's been ok. Granted it's technically the correct way to do it, but the quality of the result is fine for the majority of times it is required.

[DangerMouse]

I had a feeling that the guy in the shop had no idea what he was talking about.

 

I only ever met one person in Maplin who did know what he was talking about! Needless to say it wasn't one of the staff members. :)

[KevinE]

The sound quality is usually awful doing this. Try it at home. You need to combine (or mix) the L&R together, not create a destructive interference pattern.

[paulears]

Come on - this has been done for years. The resistor version adds a level of security in that if one output dies, the other at least has something available. Both these systems can produce the odd phase cancellations spoken about, but 'destructive interference pattern'??

 

Sound quality is categorically NOT awful using either of these methods - even going into two mixer channels panned centre will have exactly the same problems with a non mono compatible signal. Most output designs have no electronic issues with combining that I have ever found. In fact, I have never ever had a problem with amp inputs that have been shorted out - far more 'brutal' than a resistor-less merge!

[Sound In Gloucestershire]

ive done this a few times for plugging into a single monitor for DJing purposes and its been fine

 

however I dont try and make a habit of it

[djmatthill]

I agree with Paul on this , Iv used this method for many years on various applications without any problems.

 

Using professional & domestuc equipment causing NO damage or real problems ,,

 

Can I ask what application the OP is using this adaptor for ???

 

 

Matt :)

[KevinE]

No Paul I must disagree with you. Putting stereo into a desk via two mono channels will mix the signal and give a proper mono effect, just joining them is not the same thing. By 'destructive interference' I mean that if one channel tries to drive the output high and the other is driving it low (for example with a track that has reverb) then the result with the L&R joined will be zero. This can lead to a 'hollow' or tinny sound or attenuated vocals or musical beats, similar to a lost earth. It's unpredictable, as it depends on the output impedance of the line output stage and the impedance of the input, not to mention the production of the musical track. With a proper mixing arrangement, the result will be correct and since a couple of simple in-line resistors will effect this, it's surely better to suggest this as the correct way in a forum which people often use as a means of reference.

 

I'm not being pedantic or snooty, I come across this problem alot in the cabaret, DJ and Karaoke business where inputs on PA desks are at a premium and amateurs have to use stereo sources eg Minidisc, CD, Karaoke, Laptop into a mono channel. It's not so bad on older mixes but modern productions just swim with reverb and phase effects and the result of incorrect connection is often unintelligible audio.

[djmatthill]

The use of such adaptor is perfectly acceptable for general PA and sound re-inforcement purposes. The quality of the sound if fine for all but the most fussy and trained ear.

 

If the OP would tell us his propsed purpose for this adaptor we could advise further.

 

Matt

[Bobbsy]

I'm afraid I have to disagree with you KevinE. The method of mixing the stereo tracks down to a single mono doesn't matter--it's the content that determines whether there's phase cancellations or not.

 

As paulears says, putting two stereo tracks together by using two faders on a mixer (both panned centrally) will have exactly the same effect as doing the mixing using a "Y" cable. As I said, if the person who mixed the material hasn't checked for mono compatibility and has the same material moving in a positive direction on one side and a negative direction on the other, you will indeed get destructive cancellations and they'll be there whether you mix with a console or a cable.

 

Fortunately, most recordings ARE "mono compatible" because they have to be able to be played on cheap transistor radios or whatever, but there CAN be problems.

 

Bob

[paulears]

I found this out the hard way with a Panasonic DAT recorder that had it's input XLRs connected via one with a reversed connection. For 6 months every single recording I did had no mono compatability.

 

Let's make sure that we separate electronic reasons for not doing the merge (there really aren't any) and the acoustic results that only rear their head when the source material is not mono compatible.

 

The fact that this nasty effect is revealed in the cabaret, DJ and karaoke markets is simply because many of their music sources have been produced in home studios where it sounds great in stereo, and my own experience is that very often it is reverb units that make his worse, the returns being out of phase - which sounds reat - but messes up the stereo image. Since my own problem, I have run a phase meter as a plug in and use it all the time. Mono and over wide mixes are very easy to see. Commercial recordings don't normally have this kind of error.

 

I realise others have already said it - but resistors do not work in the time domain, so using them will only result in level changes, and this has NO impact on mono compatibility whatsoever.

[jamesperrett]

Let's make sure that we separate electronic reasons for not doing the merge (there really aren't any) and the acoustic results that only rear their head when the source material is not mono compatible.

 

While I agree that there are two different criteria here, I'd disagree that there are no electronic reasons for not doing a simple Y merge. You can get away with it with most modern outputs because the outputs are protected - thanks to the ingenuity of modern IC op-amp designers. Less well protected outputs would still cause problems.

 

Cheers

 

James.

[thetechguru]

Hi,

 

This might sound like a silly question but I am trying to figure out how a connector I bought today works and if it is safe to use in the way that I want to. I have bought a phono 2 into 1 converter, so it allows me to place two mono phonos into one side and give me a single mono phono out. Now, I want to put a stereo signal in and get a mono signal out, will this do the job? Is there some circuitry inside to combine the two signals else surely you would short circuit the two live signals out?

 

Any help would be much appreciated.

Hi

 

Lots of going round the houses on this - joining two line outputs from a mini disc is not a problem, it used to be recognised that buffer resistors were needed but not nowadays as most outs and ins can handle it!( High Z bridging inputs).

 

Done all the time on pro installs!

[KevinE]

putting two stereo tracks together by using two faders on a mixer (both panned centrally) will have exactly the same effect as doing the mixing using a "Y" cable

 

But it won't. Within the innards of an audio mixer there are input buffer resistors and then gain restoration amps to achieve the mixing effect. Not all audio sources have output buffer resistors (save for maybe the odd 10-ohm here and there) and many are DC coupled right back to the final driver.

 

technical article here

 

 

My reasons for my argument were both electrical integrity and sonic quality based on 20 years experience, not on some theoretical whim I dreamt up. But I'm obviously banging my head against a wall here, so I'll withdraw.

[Rob_Beech]

I have to say, I do follow Kevin here to a point.

 

Whilst we know it's possible to do it without any significant audio quality loss on SOME new equipment, from experience, some gear, both old and new doesn't like this at all. Yes we can look at the mono compatibility of things, and yes it's an issue but most pro recorded stuff is fine with this (as has been mentioned previously). However there is alot of stuff out there, namely backing tracks where this IS a problem. A number of acts I work with use backing tracks on minidisc, for the most part I'll just use one channel of this as bringing them both in sounds awful* This is not a case of 1 channel being inverted polarity, it is simply certain parts of the track having differing phase / polarity.

 

 

Why don't you just put in some resistors and then it's a universal lead, able to work on any unit without the risk of shorting the outputs, you can get 947,002 resistors for about 16 and a half Yen ad it'll only take a minute per resistor to fix to the lead. That way you can say to yourself, whilst this might have been fine on most stuff, now I can be assured it'll be fine on it all. Of course, as we've already learnt, this wont help audio quality if its not mono compatible, or indeed just plain crap.

 

 

 

Rob

 

*one channel sounds bad enough, early 90s midi anyone?