All audio equipment use discrete components, passive and active ones, like resistors, capacitors, BJTs, JFETs and OpAmps. Each of these components creates a thermal noise that adds up to the audio signal. The creation and the parameters of the noise and how it correlates with the sequence of the components has been in the interests of science for more than 100 years.
The main law we are interrested in and plays a major role in audio electronics design is the Signal-to-Noise ratio, that tells us how much contaminated with noise is our signal. $$ SNR=\frac{P_{signal}}{P_{noise}}=\left ( \frac{A_{signal}}{A_{noise}} \right )^{2} $$
Looking at the above equation and we realize that the more powerfull our signal is the less contaminated with noise is. Thermal noise from components is around -110 dBuV (in terms of voltages). It looks just easy; all we have to do is to increase the distance between our signal and the
noise floor of -110dBu. Unfortunately, this is not true, because the term of clipping comes into our design and can mess up our signal and dramatically reduce the headroom of a device.
Practical components like discrete amplifiers have a maximum Aplification Factor they can achive. For most OpAmps this is +22dBu. Except that the available power from our Power Supply Unit restricts us to amplify inside specific limits.
A signal voltage cannot cannot go above +22dBu or the available voltage from power rails and this means it will be clipped and the output waveform will be distorted with extra harmonic content in it.
Headroom is the maximum available voltage swing without clipping. A signal is amplified near the maximum limit has increased probability of becoming clipped and thus our design has reduced headroom.
So, what's the best way to avoid high noise contamination and maintain a nice headroom?
It seems the best method is to keep enough distance from the noise floor and also enough distance from the maximum limit as well. It is the optimal solution between noise contamination and headroom.
Lets take a look at a common gain stages sequence of a device. The signal would probably be amplified or attenuated in several stages to achieve the best output in terms of headroom, noise and in/out impedances. (We want go into operating point details in this text).
In Fig.1 The signal is amplified and then attenuated with a voltage divider to pass through Stage 3.
Stage 2 is supposed to be our main amplification stage.
Fig. 1: Amplify then attenuate
This configuration has increased probability of clipping at Stage 2 and thus has reduced headroom.
In Fig.2 the signal is first attenuated the restored and amplified in Stage 2.
Fig.2 Attenuate then amplify
This configuration does not suffer from clipping and has and increased headroom available compared to the previous configuration, but our signal is much more noise contaminated. This is because it is difficult for Stage 2 to restore the attenuated signal to some level. Amplifying an attenuated input signal adds up noise because of:
1. The signal magnitude is decreased and gets closer to the noise floor (so the SNR becomes low).
2. Using a voltage divider, such as a potentiometer, adds up some more noise to the total noise from the componentes, and this total noise is then amplified along with our signal. As a result the SNR at the output signal will be also low, because of the amplified noise.
It is not good to amplify attenuated signals without amplifying them first to an optimal level for processing by next stages. This level is called the nominal level and is an internal signal level for every device, even IC operational amplifiers chips. It is selected to optimize further signal processing
without noise contamination and without reducing headroom too much.
The best place to bring the signal to the nominal level is a soon as possible it gets into the input of the device. Then we have a signal that is amplified to a nominal level right at the input and is then further processed with reduced risk of clipping and noise contamination.
The main law we are interrested in and plays a major role in audio electronics design is the Signal-to-Noise ratio, that tells us how much contaminated with noise is our signal. $$ SNR=\frac{P_{signal}}{P_{noise}}=\left ( \frac{A_{signal}}{A_{noise}} \right )^{2} $$
Looking at the above equation and we realize that the more powerfull our signal is the less contaminated with noise is. Thermal noise from components is around -110 dBuV (in terms of voltages). It looks just easy; all we have to do is to increase the distance between our signal and the
noise floor of -110dBu. Unfortunately, this is not true, because the term of clipping comes into our design and can mess up our signal and dramatically reduce the headroom of a device.
Practical components like discrete amplifiers have a maximum Aplification Factor they can achive. For most OpAmps this is +22dBu. Except that the available power from our Power Supply Unit restricts us to amplify inside specific limits.
A signal voltage cannot cannot go above +22dBu or the available voltage from power rails and this means it will be clipped and the output waveform will be distorted with extra harmonic content in it.
Headroom is the maximum available voltage swing without clipping. A signal is amplified near the maximum limit has increased probability of becoming clipped and thus our design has reduced headroom.
So, what's the best way to avoid high noise contamination and maintain a nice headroom?
It seems the best method is to keep enough distance from the noise floor and also enough distance from the maximum limit as well. It is the optimal solution between noise contamination and headroom.
Lets take a look at a common gain stages sequence of a device. The signal would probably be amplified or attenuated in several stages to achieve the best output in terms of headroom, noise and in/out impedances. (We want go into operating point details in this text).
In Fig.1 The signal is amplified and then attenuated with a voltage divider to pass through Stage 3.
Stage 2 is supposed to be our main amplification stage.
This configuration has increased probability of clipping at Stage 2 and thus has reduced headroom.
In Fig.2 the signal is first attenuated the restored and amplified in Stage 2.
This configuration does not suffer from clipping and has and increased headroom available compared to the previous configuration, but our signal is much more noise contaminated. This is because it is difficult for Stage 2 to restore the attenuated signal to some level. Amplifying an attenuated input signal adds up noise because of:
1. The signal magnitude is decreased and gets closer to the noise floor (so the SNR becomes low).
2. Using a voltage divider, such as a potentiometer, adds up some more noise to the total noise from the componentes, and this total noise is then amplified along with our signal. As a result the SNR at the output signal will be also low, because of the amplified noise.
It is not good to amplify attenuated signals without amplifying them first to an optimal level for processing by next stages. This level is called the nominal level and is an internal signal level for every device, even IC operational amplifiers chips. It is selected to optimize further signal processing
without noise contamination and without reducing headroom too much.
The best place to bring the signal to the nominal level is a soon as possible it gets into the input of the device. Then we have a signal that is amplified to a nominal level right at the input and is then further processed with reduced risk of clipping and noise contamination.
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