What audio amp gain setting does?

[fonograph]

I mean what happens inside the amplifier,I am not asking what gain setting does to output signal,that would be like asking what color does blue have.For example Violectric headphone amp can set gain to 5 different levels that are spaced 6db apart.

When I flick the gain switch,what changes inside the amp? Can class D/switch mode amplifiers have gain settings too or only class A/AB/linear amplifiers have it?

[spec]

+ fonograph

An audio power amplifier of any type, class A, A/B, D, in essence converts a low level voltage signal into a higher voltage signal that has sufficient current capability to drive a speaker or headphones (see attached image below).

So, for example, say you had an MP3 player. The digital to analog converter (DAC) will convert the digital MP3 files to a 1V RMS analog signal. But that signal would not be able to supply any current worth mentioning.

If you take an 8 Ohm speaker running at 2 watts, it will need a signal of 4V @ 0.5A. This means that you will require an audio power amplifier between the DAC output and the speaker. The audio power amp will need a voltage gain of 2 and be able to supply at least 0.5A (power in watts = I*V or I squared * R).

To change the audio volume from the speaker you need to change the speaker power and you do this by changing the voltage of the signal driving the speaker. And this is done by a restive potentiometer, which can be continuously variable or stepped as in your case. The potentiometer, or volume control as it is called, connects across the output from the DAC, so that the voltage of the signal being fed to the input of the audio power amp can be varied.

If you are wondering about dBs, they are based on a logarithmic power scale to match the way that humans hear sound.

Just look up decibels on the internet if you need more information. Or if you want another lecture, just ask.

 

[BrianHG]

Use the table at the bottom right to find the Voltage or Current Ratio/dB from this site: http://www.learnabout-electronics.org/Amplifiers/amplifiers13.php

Example, a gain of 6db means the input voltage p-p will come out of the amplifier at 2x the voltage p-p
a gain of 20db means the voltage coming out is 10x Vp-p compared to the input voltage.

As for what the gain does, most of the time it can be 1 of 2 things:

A) At the audio input, there is a trim-pot or resistor divider to lower the signal going into the amplifying electronics which may have a 1 designed huge gain.  Altering the input level, usually called a pre-amp attenuator, changes to overall signal gain from input to speaker output.

B) Within the core of the amplifier electronics, the gain of the amplifier may be changed by altering the feedback circuit.  Today, typically, an amplifier has what's called a negative feedback design.  This means a portion of the output signal being fed to your speakers is sent back to the beginning of the amplifier's input circuitry to control how much voltage should be sent out to the speaker compared to the source audio coming from the audio input source.  Shrinking this fed back portion of the speaker's output will increase the overall gain of the amplifier since the amp purpose is to match this signal level with the source signal level.  This is known as a negative feedback portion of the amp.  To learn more about this, look up on op-amp theory.

[boB]

I mean what happens inside the amplifier,

When I flick the gain switch,what changes inside the amp? Can class D/switch mode amplifiers have gain settings too or only class A/AB/linear amplifiers have it?

Probably changes a resistor value.  Most likely the amount of negative feedback from output to input in a voltage divider.

Even a Class D amplifier will have an analog feedback network since it's analog in and analog out.

boB

[Dave]

Most likely the amount of negative feedback from output to input in a voltage divider.

No, it would be a bad idea to do that, as it would also change the frequency response of the amplifier and potentially make it unstable.

It's usually done as spec's image above shows - the attenuation of the input signal is varied, while the gain of the amplifier remains fixed.

[Audioguru]

Most likely the amount of negative feedback from output to input in a voltage divider.

No, it would be a bad idea to do that, as it would also change the frequency response of the amplifier and potentially make it unstable.

Oh yeah? The LM386 audio amplifier IC does that with a capacitor to bypass a negative feedback resistor which has no effect on its audio frequency response and it remains stable. A gain switch can have resistors in series with the added capacitor to change the gain in steps with a switch.

[fonograph]

Why would it attenuate input signal? Sounds like bad way to do it,it will degrade the signal SNR and there is attenuator at the output of amp anyway.

I also saw many measurements of amps and often the amp have lower THD+N at same output power when set to lower gain,that suggest to my noob brain that something more complex is happening inside the amp when changing the gain setttings.

Doesnt it change the internal power supply voltage to the output stage transistors?

[Dave]

Oh yeah? The LM386 audio amplifier IC does that with a capacitor to bypass a negative feedback resistor which has no effect on its audio frequency response and it remains stable. A gain switch can have resistors in series with the added capacitor to change the gain in steps with a switch.

Could you please provide a schematic of the circuit in question?

Why would it attenuate input signal? Sounds like bad way to do it,it will degrade the signal SNR...

That much is true. But if the output amplitude is lower, you're not going to hear the degraded SNR anyway - the noise on the output is going to remain more or less constant.

... and there is attenuator at the output of amp anyway.

Huh?

I also saw many measurements of amps and often the amp have lower THD+N at same output power when set to lower gain,that suggest to my noob brain that something more complex is happening inside the amp when changing the gain setttings.

If a class AB amplifier is driven with low amplitudes, it might stay purely in the class A region, keeping the distortion lower than when it's driven hard. This is just the first thing that popped up in my head, there might be many other reasons.

Doesnt it change the internal power supply voltage to the output stage transistors?

Fancier amplifiers might do that. They are known as class G and H amplifiers, but you don't really see them often.

[spec]

Think we posted at more or less the same time Dave 

Why would it attenuate input signal? Sounds like bad way to do it,it will degrade the signal SNR and there is attenuator at the output of amp anyway.

The first part is true and your observation is correct, but it is all a mater of degree. If you use a volume control on an audio signal of 1V, as I said, the noise on quiet volume settings is acceptable.

But there is another aspect and that is called headroom. An audio signal, with music, not speech, has a tiny average amplitude, interspersed with huge peaks. If you have the level set too high the amplifier will not be able to reproduce these peaks with the result that your music will loose some of it's brightness and sparkle.

Not sure why you think there is an attenuator on the output of the amplifier.

I also saw many measurements of amps and often the amp have lower THD+N at same output power when set to lower gain,that suggest to my noob brain that something more complex is happening inside the amp when changing the gain setttings.

Nothing is altered inside the amp. The volume is changed by a pot just like I said. An amplifier nearly always produces less distortion  at low levels (this is not absolutely true for all amps but in principle it is) because the components in the amplifier are not been stressed so much. Think of it like a car, at slow speeds it makes little noise but at high speeds it makes a lot of noise. But nothing is changed inside the car.

Doesnt it change the internal power supply voltage to the output stage transistors?

Unless the amplifier has a stabilized power supply, or it is a class A type, the voltage power rails are changing all the time. They drop when the amplifier is producing a loud sound because the amplifier takes more current from the supply line. Then when the current demand is reduced for a quite passage of music the supply line raises again.

The power lines have no effect on the volume from the amplifier, because amplifiers use negative feed back and that keeps the gain constant. The only time that the supply lines will affect the sound is if the amplifier is being driven with too large a signal and it is clipping (not reproducing the top and bottom extremities of the output waveform).

[ArthurDent]

Audioguru is correct that the LM386 does use a capacitor to control gain but it does this in an interesting way by varying the value of a 1.35K ohm resistor between pins 1 and 8 at audio frequencies without changing the D.C. voltages across the resistor. The way this works is that by placing a 10 mfd capacitor across the 1.35K ohm resistor you have a short circuit for all audio frequencies, but not at D.C.. This is similar to coupling between stages in an audio amplifier where there is a coupling capacitor to allow the audio to pass but blocks the D.C. between stages. With the LM386, placing a resistor in series with the 10 mfd capacitor doesn’t change the D.C. across the 1.35K ohm resistor but does work as parallel resistors at audio frequencies to vary the gain.
 
With the LM386 if you connect a capacitor from the output (pin 5) and feed it back to the 1.35K ohm resistor (pin 1), you can affect the frequency response to get bass boost as described below.

Here is what the internal schematic of the LM386 looks like and a copied description of how this works from their application data.

[fonograph]

There is attenuator at the output of amplifier,the "pot" like Alps RK27.It makes no sense to me to attenuate the input before amplifing it back,thats like intentionaly stabbing yourself in the leg with dirty knife and then spending two hours meticulously desinfecting and stitching yourself.

[boB]

Audio power amplifiers don't normally have their "gain" adjusted....  They sit at a preset gain, usually somewhere in the 20dB to 30dB area and the input signal level is adjusted by a volume control which is just a voltage divider.   The output is what it is and like EEVblog's KRK video would like to show, the output noise is hopefully low enough not to hear unless you stick your ear right up to the speaker.  The best way to change the gain using feedback would be with a switch rather than a pot so that when the pot gets dirty or has DC passing through it, there is no crackling or "opening" of the feedback loop which could be very bad.  Reducing the gain in this way could also reduce the residual amplifier output noise if wanted too.

The only places I have seen an output attenuator or "pad"  is where their are multiple small speakers and you want to individually vary the zone volume.

Changing supply rail voltage won't do much except to make the audio clip earlier.  Not to be confused with the amplifier topology that switches in different output transistors dynamically to reduce heat.  Is that Class H ?

boB