# Audio amplification and general confusion

Discussion in 'General Electronics Chat' started by bachevelle, Sep 5, 2008.

1. ### bachevelle Thread Starter New Member

Sep 5, 2008
3
0
I've spent alot of time recently reading and then rereading all of the books I have on hand trying to figure out what is actually going on inside an audio amp. Perhaps someone here can help. As a note, i'm concerned less with the math that give us correct values for components for the time being than i am with the actual theory of what is happening. I guess voltage is just as good a place to start as any..... I always assumed that voltage was something "static." It was explained to me as electrical POTENTIAL. So, in my feeble mind, voltage at different points in a circuit could vary, with respect to "ground" but was static in the sense that it doesn't move from one place to the next. Obviously, I don't have the whole story... My interest in audio leans toward production so my interest naturally is mic preamps and signal processing. As an example, the output of a dynamic microphone...what is it? Current or voltage, or both? I understand AC signal with respect to audio signals so i'm okay there, but in following my previously stated assumption, I thought voltage was potential and current did the work. Would that not mean that the result of a coil moving in close proximity to a magnet would produce a current? I read a book about power amps that stated the 3 stage topology as- Input stage= voltage to current amp. Voltage amp stage= current to voltage. Output stage=unity gain voltage/current amp. I take this to mean the the input of this theoretical amp as an AC voltage signal. I guess i'm assuming the what flows between the mic and the amp is current. Forgive me for rambling, i'm even irritating myself. I feel like the more i read the dumber i get. Any ideas? Ben.

2. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
1,440
368
With a moving coil transducer, a current is produced. When the coil is terminated at a resistance, such as the input to the amplifier, then you get a voltage proportional to the current.

Voltage sources have low impedance and will usually maintain a near constant voltage across a range of loads, whereas, a current source won't. Usuallly, if the output of "something" results in a voltage that is dependant on the load it feeds then its output is a current. I suppose one way to decide if something is a voltage-source is to ask "what is its output voltage?" The answer is easy if you have a voltage-source.

In a power amplifier, the input signal may be a volt or two at very low current but in order to feed power into a loudspeaker then more volts are required (Voltage amplification stage) and to maintain volts across the load, more current is required (current amplification stage).

Most audio power amplifies are voltage-sources where the output is a voltage, proportional to the input, which the amplifier maintains almost regardless of load. Through negative-feedback the amplifier compares a sample of the output signal with the input signal and sources or sinks current via the output transistors, as necessary, to make the ouput voltage track the input.

Last edited: Sep 5, 2008
3. ### iamspook Member

Aug 6, 2008
27
0
It might help you to find parallels with mechanical systems. Dynamics of moving objects Mass, Spring, friction have parallels with Inductance, Capacitance and Resistance. The maths is the same for the most part.

Power is Volts x Current and the mechanical analog to this is force x velocity.

Clearly, velocity and force together have some "oomph".
So you need both, but some systems maintain a high voltage
without a lot of current capability, and some systems provide a
lot of current without a lot of voltage.

Some devices will drop a certain voltage across them, more or
less independently of the available current (e.g. an LED), and some
devices consume very little current but are controlled by a voltage -
like a Field-Effect transistor. But in reality, you need both current
and voltage in some finite ratio to do any work. In those situations
where your devices are influenced mainly by voltage, then it is
a high-impedance input, while those requiring a high current will
be low-impedance input. Devices that can supply a lot of current
will have low output impedance.

The function of a typical amplifier is twofold. The first is to respond
to tiny changes in a transducer - like a microphone and convert it's
impedance and power into a stronger signal (More current and voltage
The pre-amp stage drives another stage(s) which in turn boosts the
gain and current to give a very low output impedance to
drive the speakers. Ideally, the amp's output impedance will be zero
but that's not practical. A good amp will drive 1 ohm load and still
maintain a high voltage. This means a lot of current flows. Resistance
consumes power, and the output transistors will ideally have no resistance
in series with the speakers - but again, this is unrealistic, and the
resistances cause losses which produces heat. This is a bit simplistic,
but nevertheless correct. You might have heard of a class A and class
B amp. The class A's output transistors are always conducting which
means those imperfect devices produce a LOT of heat. (power is
volts x amps, and both of these quantities are available at the same time). An ideal class B amp puts one transistor in conduction while the
other is turned off. This dramatically reduces the losses at the expense
of design problems to maintain linearity caused by device imperfections.

The following link discusses the mechanical analogs:

http://www.swarthmore.edu/NatSci/echeeve1/Ref/Analogs/ElectricalMechanicalAnalogs.html

I hope this has given you one more bit of the puzzle.

4. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
Coming from the mic, there are elements of both, voltage and current. Speakers are transducers, which means they can create sound from an AC signal, and an AC signal from sound.

Most amplifiers just increase the level of current, or voltage, or both. They are designed to ignore DC (though DC amplification is useful for other jobs) because it isn't important. If it is sound, then when it is converted to an electrical signal it is AC.