I’m trying to bone up on basic electronics, specifically audio amplifiers, and I understand how the power supply operates, but do not understand what kind of ‘power’ (i.e. dc, ac, alternating dc, etc) comes out of the amplifiers output stage to the speakers...

A buddy of mine says ‘alternating current’ but since the output stage gets its power from the power supply (VDC) what is the source of the alternating current??....and IF this is correct, why does the power supply bother to provide dc only to the output stage?

My thinking is that what actually feeds the speakers is a form of alternating DC current from the output stage, but need ‘basic clarification’ only, thanks

 

There is some confusion as to what is meant by DC and AC. The terms have different meanings in different applications.

In order to understand how an audio amplifier works let us begin by examining how a loudspeaker works.

A loudspeaker is basically a paper cone with a coil of wire called the voice coil. The coil is surrounded by a very powerful magnet.

When we pass a current through the coil, the coil moves out, pushing the cone with it. When we pass a current through the coil in the opposite direction, the coil moves in, pulling the cone with it. When we make the current oscillate back and forth, the coil and cone will do the same producing an acoustic wave which is what we hear as sound.

The oscillating current through the coil can be appropriately be described as an alternating current.
(Don't bother to call it an alternating DC current because that would be oxymoronic.)

Electronics within an audio amplifier require a power source that is at a constant voltage. This can be labelled as a "DC supply". The output of the power supply is at a constant voltage. The design of the electronics in the amplifier is able to make the output current go in either direction, forward and reverse. That is what power amplifiers are designed to do. In other words, the output stage can take DC power and convert it to AC power in order to move the loudspeaker cone back and forth. Hence, in a way, the power amplifier turns DC power into AC power.

The power amplifier cannot use AC power directly because it is at one frequency only (50Hz or 60Hz) which has nothing to do with the frequencies that the loudspeaker needs, 20Hz to 20kHz, in order to produce music sound.

 

As above, there is a difference between AC or DC power to a system, and an AC or DC signal within that system. The various stages in an audio system are powered by 1 or 2 DC power supplies. The circuit uses that energy to amplify and modify an AC signal. My house has 90 psi static water pressure. That doesn't change when I turn the kitchen sink water on and off rapidly (like a square wave signal) or slowly (like a sine wave signal).

ak

 

Ok, so lets see how well I understand your points - please correct me where I’m lost:

1) So the voltage output from the power supply is a constant VDC (relatively speaking)...

2) The power supply VDC supplies both the amplifyer’s input stage and output stage...

3) The output stage takes the VDC provided by the power supply and makes it go back and forth in an ac or alternating pattern, and this output stage current is thereby provided to the speaker voice coil...

Assuming this is correct, then can you explain (in simple terms please - lol) how exactly this current alternating action takes place within the output stage??

Greatfully,

Phil P.

 

There are various power amplifier output designs that can make this happen.

1) You can use a bipolar supply to begin with. That is, you actually have two supplies, a positive supply and a negative supply.

2) You can use a single unipolar supply. The output stage can make the current increase and decrease into the primary winding of a transformer. This is a transformer-coupled amplifier. The secondary winding drives the alternating current into the loudspeaker coil.

3) You connect the output of the amplifier to the loudspeaker coil via a very large capacitor.

4) You can use pulse-width modulation (PWM) to feed the loudspeaker coil after proper filtering.

 

I’m trying to bone up on basic electronics, specifically audio amplifiers, and I understand how the power supply operates, but do not understand what kind of ‘power’ (i.e. dc, ac, alternating dc, etc) comes out of the amplifiers output stage to the speakers...

A buddy of mine says ‘alternating current’ but since the output stage gets its power from the power supply (VDC) what is the source of the alternating current??....and IF this is correct, why does the power supply bother to provide dc only to the output stage?

My thinking is that what actually feeds the speakers is a form of alternating DC current from the output stage, but need ‘basic clarification’ only, thanks

It's AC, you can't have "alternating DC" as that's a contradiction in terms, like Mr chips's post with the diagram of the speaker coil, to reproduce the original sound, the coil has to go "in and out," so the only way it can achieve this is with an AC signal.
Best way is to study circuit diagrams of amplifier output stages with transistors.

 

Still churning this problem .... So at this juncture I believe this is the string of events that cause the ‘ac’ supplying the speaker voice coils:

1) A very low voltage signal is born, say from a microphone, which is of a oscillating or synodal (alternating) nature...

2) This signal enters the amplifiers input stage where it’s very low voltage is amplified to a more usable value (called pre-amplification) - this is also where tone and volume controls are...

3) This now voltage amplified signal then goes to the amplifiers output stage where current is amplified to match the output signal to the speakers impedance -

Thus, the ‘alternating nature’ of this wave was born with the original sound and merely amplified (first voltage, then current) along it’s pathway to the speaker ....

As to the capacitor you mention, I believe one is located within the power supply but that you’re referring to another within the output stage (??)... (Not counting speaker crossover cap)

I’m getting pretty sure that this understanding is correct ...

 

As to the capacitor you mention, I believe one is located within the power supply but that you’re referring to another within the output stage (??)

That capacitor is only required if the output stage is powered from a single supply voltage.
To generate a symmetrical AC signal, the output stage must thus be biased at near 1/2 the supply voltage, Vs.
The large capacitor at the output to the speaker is then just to block the 1/2Vs DC voltage from the speaker (which it can't tolerate) and let through only the AC audio signal.

 

Still churning this problem .... So at this juncture I believe this is the string of events that cause the ‘ac’ supplying the speaker voice coils:

1) A very low voltage signal is born, say from a microphone, which is of a oscillating or synodal (alternating) nature...

2) This signal enters the amplifiers input stage where it’s very low voltage is amplified to a more usable value (called pre-amplification) - this is also where tone and volume controls are...

3) This now voltage amplified signal then goes to the amplifiers output stage where current is amplified to match the output signal to the speakers impedance -

Thus, the ‘alternating nature’ of this wave was born with the original sound and merely amplified (first voltage, then current) along it’s pathway to the speaker ....

As to the capacitor you mention, I believe one is located within the power supply but that you’re referring to another within the output stage (??)... (Not counting speaker crossover cap)

I’m getting pretty sure that this understanding is correct ...

Think of voltage as a graph. Time along the horizontal, instantaneous voltage on the vertical. The horizontal is only about 5/60 of one second.

The power line supplies 60Hz AC voltage so it makes a sine wave through our graph. 5 Full cycles centered on zero volts. Negative peaks going down to -172 and positive peaks going to +172 v

That signal is fed through a transformer to bring voltage to + / - 50 V. Still centered on 0 V. (Still AC).

That power is then Rectified (negative half cycles flipped to positive half cycles). It looks like a row of camels.

Then a big capacitor is added and the peaks no longer look like half cycles, they are just a slight ripple about 50 v above ground (0v) (DC).

Now, your audio signal is pushed into your preamp. That is an AC-like signal too but voice, instruments, noise is ranging from 20 to 20,000hz. We don't want the ripple 60Hz from the wall power to influence our amp so our amp must be powered with pretty nice and clean DC. A well designed amp can put up with some power supply variation, some.

The power amp amplifies the signal and then feeds the power out, through a capacitor, to the speaker. This is alternating. Up we usually call it an amplified audio signal, we don't call it AC at this point. AC is nicely alternating, out output signal looks like a noisy mess Because voice, instruments, noise does not look like a ringing bell sine wave.

 

we don't call it AC at this point

I do.

 

Me, too.

 

Thanks very much for these very thoughtful and descriptive responses!...Sorry for ‘Me Dummy’ Yet I’m still a little fuzzy as to (I believe what is)

1) the function of the stated capacitor in the output stage, just prior to the speaker (single power supply only??).

2) single power supply vs dual power supply - is dual supply for stereo??....

3) and, capacitor is NOT required where dual p/s are present.... Please clairfy

Your continued tutelage is most appreciated - Many thanks!

 

A picture is worth 1000 words. Understand the schematic at the top of this article, and it will all become clear. I picked this article at random, just because it had a nice picture at the top.

http://www.electronics-tutorials.ws/amplifier/class-ab-amplifier.html

 

1) the function of the stated capacitor in the output stage, just prior to the speaker (single power supply only??)

What about my post #8 did you not understand?

2) single power supply vs dual power supply - is dual supply for stereo??....

Nothing to do with that.
Its determines whether or not you need a capacitor to the speaker.

3) and, capacitor is NOT required where dual p/s are present.... Please clairfy

With a dual supply the amp output can be designed to be at 0V DC, thus no capacitor is required to the speaker.

 

Thanks very much for these very thoughtful and descriptive responses!...Sorry for ‘Me Dummy’ Yet I’m still a little fuzzy as to (I believe what is)

1) the function of the stated capacitor in the output stage, just prior to the speaker (single power supply only??).

2) single power supply vs dual power supply - is dual supply for stereo??....

3) and, capacitor is NOT required where dual p/s are present.... Please clairfy

Your continued tutelage is most appreciated - Many thanks!

Single versus dual supply has nothing to do with stereo. It's just a design choice. With single supply you're basically adding a constant DC offset to the signal so that you can work with positive voltages. At the output, the capacitor basically removes that offset so that the signal fluctuates naturally above and below zero volts. Otherwise the speaker wouldn't produce a proper sound wave. Dual supplies don't have that offset added, so no capacitor at the output is needed.

 

I purposely stated that the labels DC and AC have different meanings depending on context, in particular when you consider the INPUT coupling options DC/AC/GND on the INPUT channel of an oscilloscope.

A signal can be DC + AC at the same time.
AC does not always mean that the current has to reverse direction.
A 10V signal with a 1mV ripple can be viewed as DC + AC.
From a frequency analysis perspective, DC means 0Hz while AC means anything that is not 0Hz.

 

Single or dual supply, if the output is transformer coupled, you do not need a capacitor on the output.

 

Many modern audio amplifiers are "bridged". They use two amplifiers, each driving one wire of the speaker differentially and with opposite phase. Then a single polarity supply can be used and each wire of the speaker has the same DC offset voltage then an output coupling capacitor is not needed.
The voltage swing across the speaker is almost doubled and the current in the speaker is also almost doubled resulting in about 3.5 times the power with the same speaker and supply voltage as an ordinary single amplifier.

 

A lot (most?) automotive amplifiers sold these days are Class D.

 

Thanks, that helps to fill in another blank, but from the ‘60,000 ft’ level, I dont yet get why a single or dual power supply (I believe in the output stage - ?........) will determine whether the necessity of a capacitor in the output stage.... I know that caps help smooth things out and remove the ripple - is there ripple in the output stage - or am I confusing the output stage with the power supply cap??

Thanks to ALL