Audio signal to speakers

[JFS-Chatt]

Hi all,

Working on better understanding audio signals here. I've read quite a bit that varied voltage is used to send output to speakers, but why is voltage used instead of current? It seems that there is no way to completely ignore current, as changing voltage will change current and thus vibrate the speaker coil.

Secondly, what generates the changing voltages in the audio equipment (iPod, perhaps)? It seems like there would need to be a very wide range of voltages to produce the necessary frequencies.

Any guidance on my understanding is much appreciated.

 

[Gryd3]

Well, you opened a can of worms.

You can control current and voltage the exact same way... the only difference is the feedback... ie, how you confirm you are outputing the appropriate amount.
When doing this with voltage, you can simply measure the voltage at the output and adjust accordingly.
When doing this with current, you need a 'shunt resistor' or other component with a known resistance. You measure the voltage across this component and do a little math to determine how much current is flowing through it. (and thus... anything it is connected to)
The voltage or current output can be adjusted in a linear fashion or digital fashion.
Components like transistors can be turned half-way on (or anywhere in-between on and off) so by dynamically varying how on or off the transistor is, you can limit the provided output. The draw back here is that to limit the power, the transistor itself has to waste the unwanted power... which is done as heat. This requires heat-sinks and larger components.
You can do the same thing by only turning the transistor all the way on and off... but in order to get a value in-between, these components are used with something called PWM (Pulse width modulation).
This rapidly turns the transistor on and off... but this happens so fast, that the speaker or any other load sees the 'average' voltage. The You can simply adjust the pulses so it stays off longer than it stays on to decrease the output to the load. (Additional components like capacitors and inductors can be used to help smooth out the output to make it much close the analogue output mentioned above)
This digital control method does not give off nearly as much heat and can be much smaller.

Now as far as "Why use voltage instead of current for an amplifier?" That I won't give you an answer for.
What you should do, is look up, and build some simple amplifiers.
Some classes are as follows:
Class-A
Class-B
Class-AB
Class-D

At the vary least, look them up and see the schematic on how they were built.

 

[KrisBlueNZ]

The basic reason is the complex nature of a loudspeaker.

I will try to explain, but I'm at the limit of my knowledge here, so some details may be wrong or incomplete. I hope you get the general picture though.

Loudspeakers are inductive, because they are constructed from a coil of wire, but they also have a property called motional inductance, which is one way of looking at the effect caused by the inertia of the cone, which is very significant, especially for large speakers.

A speaker driver has its own self-resonant frequency, and a more or less complicated and uneven frequency response, which are a result of that inertia, and which can be greatly affected by the speaker enclosure. Its impedance and the phase relationship between applied voltage and current flow also vary wildly over the frequency spectrum.

An audio amplifier is really a voltage amplifier with high output current capability. The input signal is a voltage, and the amplifier amplifies it enough that when it is applied across the loudspeaker, significant current flows. The current waveform is related to the voltage waveform according to the speaker's complex characteristics.

The output stage of the amplifier is supplied from one or two power supply rails - for example, +30V and -30V. The output stage is able to drive its output over a certain voltage range, say +25V to -25V, to cause current to flow in the speaker.

Because of the nature of impedance, and especially motional impedance, in various situations the amount of voltage that would have to be applied to an inductance in order to cause a specific desired amount of current to flow could be almost infinite. It is not feasible for an amplifier to be able to provide a current signal that follows the input voltage signal into an inductive load or a loudspeaker.

For a simple resistive load this is no problem, since voltage and current are proportional, but for an inductive load, it is not feasible.

People have experimented with current output amplifiers for audio use. The amplifier has to be designed to gracefully handle the situation where producing the correct output current is not possible given the limited supply voltage available.

There's a lot more to this subject. I've only told you the little I know about it. You can find more information by Googling some of the keywords I've mentioned, and other keywords you find in your search.

Beware of audiophile claims about sonic quality; these are often not backed up by as little as a double-blind test, may vary greatly from one self-proclaimed "golden-eared expert" to another, and ultimately may mean nothing at all.

Edit: In answer to your second question, an audio signal is a single voltage that varies in a complex way over time. In a stereo system, two signals (channels) are used, and more channels are used for 5.1 etc. Each channel signal contains all of the sounds that will be reproduced by the relevant speaker or earphone. In digital devices such as CD players, DVD players, iPods etc, the data is obtained from a digital source, decompressed if necessary, and converted to a digital data stream with a certain bit width (typically 16 bits) and sample rate (typically 44,100 samples per second) and this data is fed through a digital-to-analogue converter (DAC) which turns it into a voltage signal that represents the sound for one channel. Search PCM with Google or on Wikipedia for more information.

 

[BobK]

It seems like there would need to be a very wide range of voltages to produce the necessary frequencies.

The range of voltages is not related to the frequencies being reproduced. The range is related to the loudness. The audio frequency is produced by a voltage that varies at that frequency. A 1KHz sound will be produced by an alternating voltage at 1KHz. This might be at 1V for a soft sound or 10V for a louder sound.

Bob

 

[Arouse1973]

The basic reason is the complex nature of a loudspeaker.

I will try to explain, but I'm at the limit of my knowledge here, so some details may be wrong or incomplete. I hope you get the general picture though.

Loudspeakers are inductive, because they are constructed from a coil of wire, but they also have a property called motional inductance, which is one way of looking at the effect caused by the inertia of the cone, which is very significant, especially for large speakers.

A speaker driver has its own self-resonant frequency, and a more or less complicated and uneven frequency response, which are a result of that inertia, and which can be greatly affected by the speaker enclosure. Its impedance and the phase relationship between applied voltage and current flow also vary wildly over the frequency spectrum.

An audio amplifier is really a voltage amplifier with high output current capability. The input signal is a voltage, and the amplifier amplifies it enough that when it is applied across the loudspeaker, significant current flows. The current waveform is related to the voltage waveform according to the speaker's complex characteristics.

The output stage of the amplifier is supplied from one or two power supply rails - for example, +30V and -30V. The output stage is able to drive its output over a certain voltage range, say +25V to -25V, to cause current to flow in the speaker.

Because of the nature of impedance, and especially motional impedance, in various situations the amount of voltage that would have to be applied to an inductance in order to cause a specific desired amount of current to flow could be almost infinite. It is not feasible for an amplifier to be able to provide a current signal that follows the input voltage signal into an inductive load or a loudspeaker.

For a simple resistive load this is no problem, since voltage and current are proportional, but for an inductive load, it is not feasible.

People have experimented with current output amplifiers for audio use. The amplifier has to be designed to gracefully handle the situation where producing the correct output current is not possible given the limited supply voltage available.

There's a lot more to this subject. I've only told you the little I know about it. You can find more information by Googling some of the keywords I've mentioned, and other keywords you find in your search.

Beware of audiophile claims about sonic quality; these are often not backed up by as little as a double-blind test, may vary greatly from one self-proclaimed "golden-eared expert" to another, and ultimately may mean nothing at all.

Edit: In answer to your second question, an audio signal is a single voltage that varies in a complex way over time. In a stereo system, two signals (channels) are used, and more channels are used for 5.1 etc. Each channel signal contains all of the sounds that will be reproduced by the relevant speaker or earphone. In digital devices such as CD players, DVD players, iPods etc, the data is obtained from a digital source, decompressed if necessary, and converted to a digital data stream with a certain bit width (typically 16 bits) and sample rate (typically 44,100 samples per second) and this data is fed through a digital-to-analogue converter (DAC) which turns it into a voltage signal that represents the sound for one channel. Search PCM with Google or on Wikipedia for more information.

The only speakers I aware of that can benefit from a transconductance amplifier are tweeters and full range drivers. They have to be mounted in a enclosure to benefit from the a sort of air suspension you get which supplies the damping to the speaker.

This is needed because you don't get this from the low impedance voltage amplifier anymore. The enclosure damping is usually enough because of the light weight internal structure of the speakers. People have noted that you get a better bass and treble response using this approach but I have yet to try it.

Adam

 

[Merlin3189]

This looks like one of those very muddy pools that innocent questions lead us into, but I'll risk dipping my toe in!

"It seems that there is no way to completely ignore current, as changing voltage will change current and thus vibrate the speaker coil. " - Absolutely! You have it spot on.

As far as I'm concerned, it's the current that matters. The coil in the speaker is wire in a magnetic field and the force depends on the current x magnetic field. (And movement of the speaker cone depends on the force. And sound is generated by the movement of the cone.)

So back to your first point, "varied voltage is used to send output to speakers, but why is voltage used instead of current?" Your second point, above, answers it: Voltages cause currents and currents cause voltages. You could look at it either way and, as (I think) others have said, the idea of voltage output has more to do with the way we design amplifiers than the properties of the speakers.
In my experience of this area, amplifiers and speakers are usually specified not in voltage, nor in current, but in power (Watts) and resistance (Ohms.) This of course can be interpreted as a Voltage or as a current! V= sqrt( PxR) or I = sqrt( P/R) so for eg. my amplifier claims to output 30W into 8 Ohms, so it is saying it can output about 2A at about 15V into an 8 Ohm speaker. (It lies, of course! Nearly All amplifier manufacturers do.) But that's fortunate because my speakers say 20W at 8 Ohms, so they want no more than 1.6A and 12.6V. But the point is, all these parameters are related and any two define the rest. And just to be clear, these are the maximum values for volts and amps: most of the time the output will be very much less. The max values are only reached on the loudest peaks of sound.

Kris quite rightly points out that speakers are not pure resistances, but that is an unfortunate consequence of their construction & operation. Life would generally be a lot easier for amplifier designers if they were pure resistances. Unless, unusually, an amplifier is designed for a specific speaker (and enclosure and environment), the designer can only assume that the load is going to approximate to a pure resistance of the nominal value (but of course have in mind that it might not be and try to protect his circuit from possible variations.)

Others have explained the "varying voltages" point, but I wonder whether you have seen the wiggly lines of sounds on an oscilloscope? Most people have, even if just on TV or in films. If you think of those, the size (height) of the wiggles represents the Voltage OR the current (it doesn't matter which) and thus the loudness of the sound. The frequencies are represented by the wiggliness. Slow, long waves are low pitched sounds and fast, short wiggles are high pitched sounds. Most sounds are mixtures of lots of frequencies, so you get lots of fast wiggles on top of longer slower waves that vary in length from moment to moment. If you want to see some, have a look at Audacity* and zoom in on the display.

*Audacity is a great free program for recording and editing sound. You can open a sound file or record some sounds using the mike on your computer (if it has one - most laptops do) or feeding something into your sound card. If you want to understand what sounds "look" like in electronic circuits, just play with Audacity for a while.