[kwkarth]

Quote:

Thanks for the responses guys. I think I get it now.
 
Headphones run on DC in that the polarity on the wires does not change. And the current is always only running in one direction. Up through the L + R wires and back down through the shared ground. But there is still "alternating" happening, in that the voltage rises and falls rapidly to create vibrations in the headphone drivers.
 
So does the same thing happen in conventional speakers too? (Meaning that I was wrong about them being AC.) I've read some comments about powerful headphone amps being able to drive some speakers directly.

No, all audio devices transmit, amplify, shape, and or transduce audio waveforms.  Headphones are transducers.  They convert an electrical audio waveform/signal into sound pressure level fluctuations in the atmosphere.  All audio waveforms/signals are AC.
 
Electrostatic headphones also need a DC bias voltage (typically 300-500vdc) present on the stators, which is analogous to the permanent magnets in dynamic headphones. The constantly fluctuating audio signal works against the constant magnetic or electrostatic force to move the diaphragm, and subsequently, the air to produce sound.
 
Common ground:
An AC signal, by definition fluctuates above and below 0 volts.  In order for current to flow, and subsequently work to be done, takes two conductors.  What is referred to as the "ground" circuit is really the return path for those electrons,  The common ground carries the return path for both left and right channels.  For the practical purposes of this discussion, it is not an issue to use a common return path.
 
Think of the whole affair as two water wheels, each one being turned independently by independent streams of water fron two tributaries.  As the water flows over the wheels, producing work, the run off from each wheel is dumped into a single larger stream that has the capacity to carry off the runoff from both of the smaller streams, without backing up.
 
If there were no place for the water to go from either stream, no work would be done and we would have a flooded mess on our hands.    

 

[Vitor Machado]

 
Quote:

If it does, then yes it could damage your speakers. Just try it out! Use a loopback cable to record the soundcard's output. If you're on windows ensure that the "DC offset cancellation" option is disabled for your recording device (line-in).

Let's keep it theoretical, I'm a bit lazy to try this out right now.

It is supposed to filter out DC after this stage (after the DAC, I believe?), correct? So it shouldn't reproduce it? Unless of course it's poorly designed and doesn't filter it at all...

 

[kwkarth]

A transducer cannot reproduce DC.  No such thing as a DC audio signal.
 
DC offset present in the output of an amplifier, if direct coupled, can destroy speakers, or at least offset the con position so as to create the possibility of non linear distortions, and greatly limit maximum dynamic range.  
 
If an amp was capable of producing a 30 volt swing across an output load (eg. speaker) with a 5v DC offset you would be throwing away 17% of the amplifier's power and limiting your dynamic range potential by the same.  On top of all of this, you could be damaging the speaker's voice coil both by heat and mechanical destruction of the voice coil former as it bottoms out against the magnet basket containment.

 

[kwkarth]

Quote:

Sorry I'm digging out this thread, but I have a simple question:
 
So, if I click "play" to this signal, am I gonna cause damage to my speakers?
 

(made this on Soundforge)
 
This is a DC current, right? Is my soundcard really going to output a DC signal if I play this?

Most likely not because most soundcard outputs are capacitively coupled to eliminate the possibility of DC offset in the output. 

 

[amcananey]

Quote:

 
But now my understanding is that only the red wire carries the active signal. During the + phase, it pushes electrons, and during the - phase, it pulls electrons. The black wire just goes with the flow.
 
Is that right?

 
I know this is an old thread, but can anyone tell me if the above is correct? I find it a very useful way to think about this, so I'm hoping it is....
 
Best,
Adam

 

[jaddie]

Quote:

 
I know this is an old thread, but can anyone tell me if the above is correct? I find it a very useful way to think about this, so I'm hoping it is....
 
Best,
Adam

No.  A circuit is a complete loop.  Both wires carry the entire signal.  In a headphone TRS circuit, you have a wire shared by both left and right, but it's still a complete loop for each.  Two hoses, one big drain.  The black wire and all others go with the flow.  "Push" is determined by your point of view, hole-flow or electron-flow.  The flow analysis is opposite, but they both have flow, and in AC it doesn't matter which direction, at least for very long.

 

[amcananey]

Let me rephrase. I know it is a circuit/complete loop. But the point that Yoga Flame was trying to make in the text I quoted is that an amp is never driving the flow through the black wire / ground. Instead, it is either pushing (+) or pulling (-) electrons through the red wire. In turn, that then pushes or pulls electrons through the ground / black wire. Is that right?

 

[brhfl]

Let me rephrase. I know it is a circuit/complete loop. But the point that Yoga Flame was trying to make in the text I quoted is that an amp is never driving the flow through the black wire / ground. Instead, it is either pushing (+) or pulling (-) electrons through the red wire. In turn, that then pushes or pulls electrons through the ground / black wire. Is that right?

I believe so. I misread that quite a few times myself. One cycle, so to speak, is a push and a pull, positive and negative voltage. Your 'black wire' is just the return, a safe comfy place for the current to keep flowing and finish that circuit. The water wheel analogy above was probably the best I've seen in this thread, very straightforward. And as has been mentioned, this is not unique to headphones - many (if not most) speaker amplifiers make use of common ground and work the same ol' way.

 

[mikeaj]

Quote:

Let me rephrase. I know it is a circuit/complete loop. But the point that Yoga Flame was trying to make in the text I quoted is that an amp is never driving the flow through the black wire / ground. Instead, it is either pushing (+) or pulling (-) electrons through the red wire. In turn, that then pushes or pulls electrons through the ground / black wire. Is that right?

 
From the perspective of the amp, sure. The output of the active electronics is tied to one side. Signal ground to the other.
 
But the headphone driver and wires don't really care either way. Anyway, there's a net movement one direction half the time and movement the other way the other half of the time. Negative is just the opposite of positive, which is arbitrarily chosen as just a reference. It's the potential difference that counts. It's just as valid to say that the difference is between the output and ground, as it is between the ground and output.
 
If you had a balanced amp, depending on its layout, it could be driving from both sides, so to say. The wires and headphones don't really care what the sides are connected to. The return has no special status different than the other wire.

 

[jaddie]

Quote:

Let me rephrase. I know it is a circuit/complete loop. But the point that Yoga Flame was trying to make in the text I quoted is that an amp is never driving the flow through the black wire / ground. Instead, it is either pushing (+) or pulling (-) electrons through the red wire. In turn, that then pushes or pulls electrons through the ground / black wire. Is that right?

Again, no.  You can't have the push without the pull.  The amp is doing both all the time.  Direction makes absolutely no difference.  Voltage (the "push") can be negative or positive.  Either way, it's still voltage, technically both push and pull, again, depending on your circuit analysis perspective. Here's how that works: considering a simple DC circuit with a + and - voltage source. Electrons flow from - to +, electric charge, or to look at it another way, holes left by electrons that move, moves from + to -.  It makes no difference, the current flow is the same, just a different way to look at it.
 
The only, and I mean ONLY concern anyone should have about the single ground wire idea is that it's carrying the total current from two headphone circuits, while each channel conductor carries only current for that channel.  That means that if you were playing an identical signal in both channels (never really happens in stereo), the current in the ground wire would be twice that of each channel conductor.  This might become an issue if the headphone cord was 500' long, but not at normal lengths unless the ground wire was extremely tiny.  Some headphone cords are 4 wires all the way back to the connector, some use shielded wire with the shield working as the ground, so it's huge compared to the channel wires.  It's just not really a problem.

 

[mikeaj]

By the way, most decent headphones—yeah, meaning all those normal ones with TRS terminations—have four conductors inside. They don't even share ground wires between the two sides, not that it'd really be fundamentally different if they did, or that the resistance of that little length is causing any significant crosstalk. The only shared point of the ground is at the jack... and inside the amp, if you count that.

 

[amcananey]

I think you are misreading my post. Either that, or I'm still not getting it. When I wrote "an amp is never driving the flow through the black wire / ground", I didn't mean that there is no current in the black wire. There obviously is. But the push or the pull (i.e., the "drive", as opposed to the flow) is coming from the positive/hot terminal on an amp, right? When in "+" mode, the amp is driving electrons through the red wire, through the headphones, and into the black wire. The push of the electrons, which originated in the red wire, then also pushes the electrons in the black wire, creating current.
 
When in "-" mode, the amp is pulling electrons out of the red wire. This creates a hole, that then pulls other electrons into that hole. The movement of those electrons leaves other holes that then pull other electrons, etc. This pulling in the red wire ultimately pulls electrons through the headphones and, in turn, through the black wire, again, creating current.
 
Is that right or should I just totally give up on this idea?

 

[mikeaj]

The push or pull is coming from both equally. Without either, there's nothing. Neither is doing something different than the other.  It's the arbitrary declaration of one side or the other as special (with respect to flowing, pushing, pulling, or whatever) and causing something to happen on the other that is being objected to.
 
In other words, as far as I can tell what you mean, you should give up on the idea.

 

[brhfl]

My understanding of the odd semantics happening here is that +/- voltage is being translated into the physical effect on the speaker - the push or pull with respect to the fixed magnet. Not some idea of pushing and pulling electrons.

 

[Zeleni Zec]

There is no difference in connecting speakers and headphones because most of amplifiers share common ground inside the chassis and it makes no big difference whether the three wires are carrying the signal or four. It is just matter of convenience splitting ground into two separate wires leading to the left and right speakers which are separated in the space anyway. As someone mentioned already, high end headphones have separate ground wires going from common ground in the 1/4 inch connector to the left and right ear cups and they are placed in the same jacket just for convenience. Only place where they share common ground is connector itself. 
 
Every amplifier needs two wires to deliver signal (voltage and current are just different presentation of the same electrical phenomenon) to the speakers or headphones. Most common definition is that ground is return path for the signal. There are some amplifier topologies that require separated ground wires (like Mjolnir) because the output is floating (not tied to the chassis or ground) and assigning polarities to the wires (ground and signal) serves only to keep the same phase relationship in both channels. If the amplifier is completely balanced from input to the output it usually means that the channels have separated grounds (inside as well as outside) and it does help to reduce crosstalk between channels, which usually gives better imaging than the other (unbalanced) solution.