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Mono/Stereo lines and adapters - a few questions...
Mono/Stereo lines and adapters - a few questions...
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Old 30th October 2016, 11:46 AM   #1
DominusAlchemi is offline DominusAlchemi  United Kingdom
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Question Mono/Stereo lines and adapters - a few questions...

Hello all,

This is a fairly basic question I think, and probably important for maintaining the quality of an audio signal. I would be grateful for any comments received.

I understand that a stereo (balanced) connector reverses the polarity of a mono signal, and runs two identical signals of opposite polarity down the end of the line, where each is merged so that interference cancels out.

I do not understand the following:
1) At what point are the signals merged, to cancel out interference? What confuses me is that a XLR cable has 3 connectors at both ends, which suggests to me that the two separate lines have not been merged?

2) If combine the two signals running through an XLR cable by means of an adapter at the end of the cable (e.g. XLR to 1/4" TS) then does this achieve the noise/interference cancellation, or is this doing something different?

3) For a signal running down a balanced line: If I insert the 1/4" TRS (stereo) plug into a 1/4" TS (mono) socket, am I losing one of the signals in the balanced line, so that only the signal at the plug tip is transferred?

4) Leading from 3 above, if only the tip signal is making it through, then should I use a stereo line jack adapter (i.e. 1/4" TRS socket to 1/4" TS plug) in order to merge the signals into one?

The above questions will assist me to understand how to run a guitar amplifier's balanced line (XLR) DI output into a BOSS equalizer pedal that only caters for MONO input. The output signal will then run through to a PA. The idea behind this is that I might find it necessary to adjust the EQ and/or input level when I run my Marshall AS100D acoustic amp signal (DI Out) into a house sound system.

I'd be very grateful to anyone who knows if you could clarify this for me!

Thanks to all.
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Old 30th October 2016, 05:36 PM   #2
Enzo is offline Enzo  United States
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First, do not confuse connectors with circuits. The connector doesn;t do anything to the signal, it just, well, connects it to something. it doesn;t create inverted signals or anything like that.

learn what balanced and unbalanced mean, I am sure if you google balanced line you will find many little explanations. An unbalanced line is simply a conductor - the hot - and ground. The signal runs through the conductor, and returns through ground. A balanced line has two signal leads and no ground connection. The two signal leads are just opposite ends of the source. So there is no merging to do, the two wires are the whole signal, it is one signal. Imagine your speaker, there wire two wires running to it, each side of the speaker gets a wire. At the speaker, they are not merged, they just put the signal across the speaker. By its nature, the current running through the one wire will be opposite polarity from the other wire.

An unbalanced line might pick up some interference. Like a noisy fluorescent light. The noise signal is picked up by the hot wire, and goes into your system. In a balanced line, both wires will pick up the same noise signal, but since they are opposite polarities, whatever is picked up by one wire wil be exactly the opposite of the second wire - they cancel. So at the end of the line, there will be no difference across the load. the noise cancels.

An XLR has two signal pins, pins 2 and 3. Pin 1 is used - or not used - as a ground, and usually connects to the shield. Only pins 2 and 3 are involved with the signal.

If you connect a balanced line to a TS plug, then you have grounded one side of the balanced pair, so it is no longer balanced. ANy polarity cancellation has been destroyed. You are not combining anything, you are simply grounding one side of the balanced signal.

Imagine you have a battery and a small motor. Two wires from the battery go to the two terminals on the motor. You could imagine that as a balanced line to the motor. The 9v battery has opposite polarities at its terminals, and the motor has as well. We don;t think of the 9v as being merged. We think of it as across the motor. it isn't two voltage sources, it is two ends of one voltage source. Notice the idea of ground is irrelevant here. Now I could wrap a shield around those wires, and ground it. That would be pin 1 of an XLR.

The balanced signal is a differential signal, the device receiving it uses the differential of the two wires as its source. The signal on the balanced line is like an AC equivalent to my battery. Some destinations have a transformer primary as the input, others use a differential circuit like an op amp for an input. They don't merge two lines, they simply accept the two ends of one signal.

If you ground one side with a TS plug, you are not losing a signal, you are just unbalancing the signal. If you connect both hot wires together, then you will have zero signal, as the exact same signal is on both wires, just opposite polarity.

Your floor toys are all unbalanced, and if you put a balanced signal into it, it will cease to be balanced. Why not run the DI down a balanced line to the mixer, then EQ it THERE? Isn't there someone running the house system?
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Old 30th October 2016, 05:48 PM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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1/ To cancel out interference a balanced line is used where one wire conducts the signal 180deg out of phase of the other wire. The easiest way to show an example is if you use a transformer with the secondary as the output. Doesn't matter what it is, a transformer is a transformer roughly speaking. Now, connect one secondary wire to ground, the other wire will conduct what is known as an unbalanced signal. The unbalanced receiver or pre amp will faithfully amplify all of the content, un screened noise and all.
If you send both output connections to a differential amplifier, XLR or TRS Jack, then that will amplify the difference between the wires and as any noise is on both wires and of the same polarity, there is nothing to amplify just the signal. That is a balanced line.
2/ It will achieve nothing of any use as it is unbalanced.
3/ It will become unbalanced but you will not lose any signal just increase the noise.
4/ No.
Most, if not all DI boxes, use an unbalanced input, instrument etc and convert it to balanced either with a transformer or a differential out put Op Amp. They run on phantom power or batter. Now there's another story!
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Old 30th October 2016, 06:08 PM   #4
DominusAlchemi is offline DominusAlchemi  United Kingdom
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Thanks to you both Enzo and JonSnell for your replies.

Thanks for supplementing my understanding of balanced signals, which I now see are not actually a combination, but that the differential is what provides the input (if I have understood correctly).

To Enzo's q: I am looking to connect to a house system where there is no mixing desk and competent soundman (e.g. little pubs where there is a Hifi system with strategically positions speakers). The input I expect to have available is a pair of phono L/R sockets. So I will have appropriate connections to match those.

I suppose the notion of inserting the balanced signal into a mono input of a BOSS EQ pedal is not the best idea in view of the fact that I lose any noise cancellation that would have occurred in the case of a balanced signal.

Do you have any alternative suggestions for how I might achieve the same? I.e. an ability to control EQ (to some degree) in the house by manipulating the feed signal going into the house system? A small mixer perhaps?

Thanks again!
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Old 30th October 2016, 06:32 PM   #5
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Yes a small 4 channel mixer will give you control and a balanced line out to keep mains borne noise to a minimum.
Fleabay from 20

http://www.ebay.co.uk/itm/Stageline-...-/181688018159
Brand new for 39.
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Last edited by JonSnell Electronic; 30th October 2016 at 06:34 PM. Reason: Include a fleabay link ...
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Old 31st October 2016, 01:26 AM   #6
DominusAlchemi is offline DominusAlchemi  United Kingdom
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I've looked at the mini mixer and it appears the primary advantage it offers is the led clip feature that would allow me to adjust the signal to the highest possible level without clipping. I.e. the amp DI out would be a mono signal into the mixer and the mixer output (with appropriate phono to XLR or phone connections) would permit a balanced out signal which I can extend for many meters as required.

Can I not achieve the same with the BOSS EQ pedal though? I.e. by feeding a mono signal from the amp DI out into the BOSS EQ pedal [mono] input and then at the BOSS EQ pedal output, by means of a TR to TRS adaptor, can I not convert the mono signal to a balanced signal?

If that were possible, then I forego the clipping function, but I should still be able to reduce interference using balanced output cables - either XLR or phone jack leads. Would that work?

Many Thanks!
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Old 31st October 2016, 05:32 AM   #7
Enzo is offline Enzo  United States
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By "mono" do you mean unbalanced? The XLR DI output is not stereo. That is why we call the plugs TRS rather than stereo plugs.

You can't convert an unbalanced signal into a balanced one with just a connector. They do make impedance adaptors though.

A balanced line has value running long lines. Short runs, they don't help you.
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Old 31st October 2016, 10:01 AM   #8
DominusAlchemi is offline DominusAlchemi  United Kingdom
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Hi Enzo, yes, where I used "mono" I meant "unbalanced".

Excuse my ignorance please and please would you explain why you can't convert an unbalanced signal into a balanced one with just a connector.

And how is the following of interest?: "They do make impedance adaptors though."

Thank you!
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Old 31st October 2016, 01:42 PM   #9
Enzo is offline Enzo  United States
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When you ground one side, it is no longer balanced. Using an adaptor doesn't unground the side.

Just google "impedance adaptor" and get information. This one is typical:
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Old 31st October 2016, 02:34 PM   #10
MitchMev is offline MitchMev
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Just to add some additional clarification, mono =/= unbalanced and stereo =/= balanced. A mono unbalanced signal would have one "hot" and one ground. A stereo unbalanced signal would have two separate hot signals (left and right) and and one or two grounds. Mono balanced would have one hot, one cold (a perfectly inverted copy of the hot signal) and one ground. Stereo balanced would have two separate hot, two separate cold, and one or two grounds.

In other words, the difference between stereo and balanced is that stereo has two different signals, whereas balanced has two identical signals in reverse phase. Reverse phase means that when one voltage goes positive the other one goes negative and vice versa. There's more to balanced signals than that, including impedance matching, but that's the basic idea. I can post some links if you're interested in learning more.

For your particular application, it seems like you are trying to run a mono signal into a stereo input. Running a balanced line into a stereo input does not cancel noise. You want to make sure the TRS input on the board is a differential input, not a stereo input. TRS is much more commonly used for stereo and XLR is usually differential (i.e. balanced line input).

Quote:
Originally Posted by DominusAlchemi View Post
Excuse my ignorance please and please would you explain why you can't convert an unbalanced signal into a balanced one with just a connector.
In order to get a balanced signal you need a device which inverts the signal for the "cold" line. The simplest device is a center tapped transformer, which is what most passive DI boxes and unbalanced to balanced adaptors use. Basically, the output ground is taken from the center of the transformer and then the opposite ends of the secondary winding are in reverse phase. This requires a solid understanding of basic electromagnetism and circuit analysis, but I can post a couple resources here:

An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.


This is generally what a passive DI box looks like:

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So actually slightly different than what I'm describing but has a similar effect.

Last edited by MitchMev; 31st October 2016 at 02:49 PM.
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