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AC or DC - How headphones and amps work!

post #1 of 21
Thread Starter 

OT in another thread made me think about making a thread on this subject. Probably 99% of us know how drivers and amps work, but for those that are not clear on the subject I want to write a simple answer:

 

 

DC, modulated or not, will only move the cone into one direction. This doesn't make any sense, except for testing the polarity of drivers. Hook up a small battery to a driver and it will move either in out out. Lets say the way you connected it, it moves out. if you  reverse the poles of the battery, it will move in. If you do this 50 times a second, you have a 50Hz "hum". To this 10000 times per second and you have a high beep tone. You don't want DC near your headphones! Measure DC at the amps outputs and it should be only +-20mV or less, up to +-120mV on bigger speaker amps.

 

Music, as such a tone, which is an oscillation, moves the microphone diaphram. This is converted into current, + everythime the mics cone moves in from its resting point and - every time it goes outwards from its resting point. So, a 50 Hz vibration produces 50 Hz alternating current. Music is just many 10000s of various AC moving your headphones cones.

 

The amplifier makes the AC signal "stronger" thus moving the driver "louder". 

 

In essence this is why "directional" speaker wires are not actually directional. (EDIT: For clarification, the "system" positive and negative leads are "directional" in a sense. If you have to loudspeakers and wire one red to black and black to red by mistake then the sound waves that are the same will cancel each other out because one speaker is "out of phase". This is how noise cancelling works, it takes the microphone signal on the outside of the cup, inverts it 180° and plays it on the driver, effectively canceling out the sound from the outside. It is still AC on the red wire that is moving the driver, meaning the "power" moves backwards and forwards through the positive lead)

 

Below is a animated graphic on that page in the link that shows how a driver works.

 

http://electronics.howstuffworks.com/speaker5.htm


Edited by ev13wt - 2/1/13 at 3:03am
post #2 of 21

Just found the Phillips thread that started this... I wouldn't try to win the argument, does it help you enjoy music more :)?

post #3 of 21
Thread Starter 

This brought up the discussion, and we don't want to muddy up a otherwise fun thread about Philips headphones now do we.

 

 

Quote:
Originally Posted by ruffra View Post

Not sure who jcx is in your post but the modulation is on a dc voltage .  No AC at the output of you amplifier for goodness sake!!! Thats just mains hum!!!  The audio signal modulates the dc voltage rail of you amplifiers output system. .  If you have a Scope put it across an amp terminals and see if I'm wrong.  I Think jcx is describing the effect of modulation not AC.  AC is an Alternating Current/Voltage which swings around a Voltage null at full+/-100% variation and normally known as Sinusoidal.  Music is anything but sinusoidal.  

 

Anyway we seem to be yet again drifting from the original thread idea of reviewing X1''s.  So I will not respond to anymore posts on things not associated with X1's . There I've said it!!!

 

Quote:
Originally Posted by ruffra View Post

 

OK the current varies in accordance with the music/sound but it is applied via a DC voltage across the speakers. I didn't say it was a flat DC voltage/current otherwise you probably would melt the speaker coil if you didn't dissipate the heat that effect would create.

Google Images for  a typical power amp schematic.  Not a lot of AC anything applied across the speakers I think. 

 

  Here is an example   there are many    http://electroschematics.com/653/200w-audio-amplifier/

 

As you say the Basics need to be understood even as a hobby.   I have built enough audio amps years ago to know these things.  

 

Think I will stay clear of cables with directionality at this point, not going there tonight. Likely needs another thread on a different forum for that debate.

 

 

Lets discuss this and get to the bottom of it. First off, you can explain to me AC and DC coupled amplifiers.

 

AC Coupling

  • An amplifier with reactive components in the signal path, such as transformers or capacitors, blocks DC and permits only AC to pass through. Amplifiers with these components are called AC-coupled.

post #4 of 21
Thread Starter 
Quote:
Originally Posted by atomix View Post

Just found the Phillips thread that started this... I wouldn't try to win the argument, does it help you enjoy music more :)?

 

It is not even about enjoying music. That is a given. It is about understanding how things in this hobby work. If it does not interest you why are you trolling? :)

 

It is not an argument, there is a simple fact on how things work. There is no "grey" area here like talks about sensitive things like high end cable sound or burn in. And I sure hope this thread does not drift there.

 

It is about how drivers and amps work fundamentally. And this I am very interested in.

post #5 of 21

OK here is something from Audioholics which might help

 

3. How Does an Amplifier “Amplify”?

Perhaps one of the easiest ways to understanhow an analog audio amplifier works is to think of it as a kind of servo-controlled “valve” (the latter is what the Brits call vacuum tubes) that regulates stored up energy from the wall outlet and then releases it in measured amounts to your loudspeakers. The amount being discharged is synchronized to the rapid variations of the incoming audiosignal.

In effect, an analog amplifier is comprised of two separate circuits, one of which, the output circuit, generates an entirely new and powerful electrical output signal (for your speakers) based on the incoming audio signal. The latter is a signal of perhaps 1 volt that represents the rapidly varying waveforms of sounds (both their frequencies and amplitudes). This weak  signal is used to modulate a circuit that releases power (voltage and amperage) stored up by the big capacitors and transformer in the amplifier’s power supply, power that is discharged in a way that exactly parallels the tiny modulations of the incoming audio signal. This signal in the amplifier’s input stage applies a varying conductivity to the output circuit’s transistors, which release power from the amplifier’s power supply to move your loudspeaker’s cones and domes. It’s almost as though you were rapidly turning on a faucet (you turning the faucet is the audio signal), which releases all the stored up water pressure—the water tower or reservoir are the storage capacitors-- in a particular pattern, a kind of liquid code.

 

So the signal in can be anything form below 10 hz to above 20000 hz and usually a full spectrum in between.  I imagine people might call this AC but its a modulation of my different waves not AC sinusoidal like the main supply.    All the power to amplifier and drive speakers is done in a DC domain and the signal is applied to the DC amplification circuits.  DC is normally derived from Recitifying and smoothing the AC power from your wall socket.  So lets just clear up that the Signal is a waveform ,Amplification is done in DC with the signal applied to it and Power for the DC is by converting AC to DC .  Now have we cleared it up or is there still some problems with the concept

post #6 of 21
Thread Starter 
Amps that use an output coupling capacitor, usually, do this because they do not used split power supplies. There's usually a positive power supply (rail, or B+) and ground. To make this amplifier produce AC signals, the amplifier has to sit at half of the full power supply voltage.
If your amplifier has a 50V power supply, the amplifier output has to sit at 50/2, or 25V. Now the AC signal can swing above and below this 25V reference voltage. This means that the amplifier will clip evenly as the output approaches max.
 
So now we have +25 volts DC at the output of the amplifier. That would not be any good for speakers. The woofers would just push out, stay there and bake.
 
This design requires an output capacitor to block the DC and allow the remaining AC to be delivered to the speaker.
 
Electrolytic capacitors are required to couple the speaker to the amplifier, because high capacitance values are required.
 
 
 
 
DC on you amp means direct coupled, and not direct current. AC is capacitor coupled^^
 
So, your typical amp uses DC to amplify AC.

Edited by ev13wt - 1/31/13 at 5:24pm
post #7 of 21
Thread Starter 

When it comes to Volts and Amperes (Amps), we have alternating current and direct current (AC and DC respectively). The power from a wall outlet is AC, as is the output from a CD or tape machine. The mains from the wall outlet is at a high voltage and is capable of high current, and is used to power the amplifying circuits. The signal from your audio source is at a low voltage and can supply only a small current, and must be amplified so that it can drive a loudspeaker.

post #8 of 21

Uhm... increase the power to the coil and it pushes away from the magnet, decrease it and the magnet pulls the diaphragm back? No need to alternate the polarity of the current to make that work.

post #9 of 21
Great post! Thanks guys for the info and input! Very appreciated.
post #10 of 21
Thread Starter 
Quote:
Originally Posted by anodyne View Post

Uhm... increase the power to the coil and it pushes away from the magnet, decrease it and the magnet pulls the diaphragm back? No need to alternate the polarity of the current to make that work.

 

While this is basically correct, it is not how an amplifier works. The movement back (magnet pulling the wires back) is not strong enough and it is always the same. You would have no control over the driver, it would just flutter through the 0 point as soon as you take away the "power".

 

You use AC on the "red" positive wire and move the driver actively forwards and backwards. The "black" is the ground.


Edited by ev13wt - 2/1/13 at 3:06am
post #11 of 21
Thread Starter 

Some more input from this thread here: http://www.head-fi.org/t/502827/do-headphones-run-on-ac-or-dc 

 

 

 

 

 


Quote:
Originally Posted by mikeaj View Post

JaZZ, I don't know anything about your amp, but the fact that it uses bipolar junction transistors (BJTs) is nothing special in itself indicating that it uses a balanced drive output.  BJTs are very versatile semiconductor devices used in all kinds of devices including many op amps.  Bipolar refers to the configuration of the three doped regions inside.

 

OP, as said by others, headphones and speakers do work the same way, and they're wired the same too.  (Well, to be more precise, powered speakers have their own amplifiers internally, etc., but the main difference is just the level of the power input and output.)

 

I think you should just consider the voltages in the system rather than the currents because the voltages are easier to understand.  In the end, the current through the headphone drivers is just the voltage across them divided by their impedance anyway.

 

What the headphone drivers do is convert the electrical voltage across them into vibrations matching that pattern.  The voltage across them is the audio signal (which is whatever the source is sending), and it changes rapidly over time.  The audio signal is mathematically just the sum of sinusoids at different frequencies with different amplitudes, changing over time.  Therefore, half of the signal will be a positive voltage, and half will be a negative voltage.  As others have said, the average of the signal, aka the DC component, should be zero or else the headphone drivers will be damaged.  Since this signal is changing positive to negative, you could say that it is alternating.

 

Here is what some music looks like, as taken from a clip I loaded in Audacity.  The top is the left channel output, and the bottom is the right channel output.  Note that the whole thing may be scaled up or down by multiplication by the amplifier, so this doesn't completely correspond to the voltage seen across your headphone drivers.  However, the shape of the voltage will be the same as what you see here:

Audio waveform

 

Again, this signal is the voltage seen across the headphone drivers.  In other words, the top line represents the difference between the L audio signal and ground, while the bottom line represents the difference between the R audio signal and ground.  Ground is at a 0 signal level, so the difference between the signal and ground is just the signal itself.  The ground can be shared and physically connected together because it's just 0.  It's the same reference point for both L and R audio signals.

 

The current from point A to B is the voltage difference between A and B divided by the impedance between the two points.  When the signal and thus the voltage is positive, the flow of electrons will be in one direction.  When the voltage is negative, the flow of electrons will be in the opposite direction.

 

Crosstalk is the phenomenon of what's on one channel being reproduced on the other channel.  So what one channel sees with crosstalk is the correct signal added to an attenuated copy of the other channel's signal.

 

Hopefully this explanation is more clear to you.

 

Quote:
Originally Posted by jcx View Post

can't you guys posting beyond your knowledge find a elementary "how things work" site to check your "facts" [edit: the post's got better even as I wrote so recent posters needn't get bent out of shape by this comment]

 

 

while "AC" literally stands for alternating current it is used more generally - alternating voltage is of course implied, and any signal that averages to zero may be described as "AC" - even when voltage and current aren't involved

 

sound is "AC" - sound waves propagate as alternating compression and rarefaction of the local air pressure - the average of the air pressure is not detected as sound - is not recorded and is not reproduced on playback - only the the "alternating" part of the air pressure (and velocity) is "audio signal" - usually only the AC components with frequency > 20 Hz (and less than ~ 20 KHz) are considered "audible"

 

dynamic transducers (most headphone drivers) create "alternating" sound pressure as their diaphragms move back and forth in response to the alternating audio frequency current in their voice coils - the average of the current and voltage should be as close to zero as possible - any small "DC" component to the drive signal forces the voice coil/diaphragm off center and may increase distortion; large DC signal may over heat the voice coil, melting plastic or glue and destroy the headphone

 

electronic amplifying devices amplify small audio input signals by modulating a DC, constant polarity power source in response the control signal - some circuits/devices have a DC component on their output that must be blocked/removed before the amplified alternating current/voltage signal reaches the headphone

 

the amplifier circuitry may get its power from a "DC" battery source or it may use the wall outlet "AC" mains line current and convert that into the DC needed by its internal signal amplifying components

but the signal reaching your headphone drivers is "AC"

 

 

a common gnd wire (and the standard TRS jack plug) does cause some L/R channel crosstalk - typically it is very low since the wire resistance is a small fraction of an Ohm and the headphone drivers many 10s of Ohms

 
post #12 of 21
Quote:
Originally Posted by ruffra View Post

So the signal in can be anything form below 10 hz to above 20000 hz and usually a full spectrum in between.  I imagine people might call this AC but its a modulation of my different waves not AC sinusoidal like the main supply.    All the power to amplifier and drive speakers is done in a DC domain and the signal is applied to the DC amplification circuits.  DC is normally derived from Recitifying and smoothing the AC power from your wall socket.  So lets just clear up that the Signal is a waveform ,Amplification is done in DC with the signal applied to it and Power for the DC is by converting AC to DC .  Now have we cleared it up or is there still some problems with the concept

 

I think your misconception is that AC implies sine waves (and possibly a certain voltage) -- which is (both) not part of the definition of 'Alternating Current': the wave form can be random, but in case of music signals the integral over the parts above and below zero is equal. And in most cases its form won't look like mains power.

 

In many amplifier designs (though not necessarily in all) the signal processing is powered by a certain DC voltage. Nevertheless the output is AC as defined above.

post #13 of 21
Quote:
Originally Posted by plakat View Post

 

I think your misconception is that AC implies sine waves (and possibly a certain voltage) -- which is (both) not part of the definition of 'Alternating Current': the wave form can be random, but in case of music signals the integral over the parts above and below zero is equal. And in most cases its form won't look like mains power.

 

In many amplifier designs (though not necessarily in all) the signal processing is powered by a certain DC voltage. Nevertheless the output is AC as defined above.

 

It would seem that input signals including audio waveforms are currently descibed as AC . However when I was a boy they were just that input signals with variable wave forms.  AC was to do with Mains and DC was not AC !!!

 

Regarding definitions  Here is a selection   http://www.thefreedictionary.com/alternating+current

 

As you can see My simplified definitions features a number of times.  However I have no problem with what people define AC as ( I know what I was talking about which is Mains AC) However I have not seen a Audio Amplifier circuit which just uses Mains AC in some form of direct manner . I know about switch mode power supplies but that end up as looking like a DC output as well  Do you have an example you can point me towards???

post #14 of 21
Quote:

Originally Posted by ruffra View Post
 

It would seem that input signals including audio waveforms are currently descibed as AC . However when I was a boy they were just that input signals with variable wave forms.  AC was to do with Mains and DC was not AC !!!

 

Regarding definitions  Here is a selection   http://www.thefreedictionary.com/alternating+current

 

As you can see My simplified definitions features a number of times.

[...]

 

The definition of alternating current has always been something like those Free Dictionary quote. "Mains" power is an example of A/C, but it hasn't ever been the definition.

post #15 of 21

Mmmmmmm dat coupling 

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