Tips eliminate audio Noise. NOISE Reduction
I will give the reader some tips on how to eliminate the noise in electronic circuits, why it happened, and how to read Noise Reduction specifications. Usually does not make sense to not know, so it was expected that this paper will help those who are trying to make sense of it all.
Noise has a value greater interference with the sensitive (high gain) circuit, but the information provided by most IC manufacturers and the transistor does not always make choices that best matches the device. Of course, there are a lot of information, but an explanation of what it means and how to apply the appropriate bit.
This short article hopefully will help some confusion.
In this context, noise refers only to the circuit noise, and no hum, buzz or any other foreign outside influences. This is usually due to bad (or wrong) earthing practices, or signal cables running close to or a harmonic magnetic field to produce items such as transformers and bridge rectifiers. Also can blame the radio frequency interference, which will often cause problems if prevention ignored.
Even without doing any calculations, we can see that the voice of a resistor is proportional to the resistance and temperature. Operation at high temperature resistors in the input level clearly undesirable, such as high resistance value. This also applies to other resistive device, such as dynamic voice coil (magnetic) microphone, a guitar pickup coils.
Noise Reduction was also the semiconductor device. This is mainly caused by changes in molecular energy levels as a semiconductor device.
Noise Reduction due to low frequency effects, and therefore not so much a problem with the audio circuitry. Become worse as the frequency is reduced, and this can be seen in some data sheets. At the lower frequency extreme, the Noise Reduction level increases. since the beginning of the incoming audio on the audio end of the less powerful.
In audio electronics term ‘dBv’ refers to decibels relative to 1V RMS, and “DBU” means decibels relative to 775mV. It is also known as dBm, and the old conventions relating to 1mW into 600 Ohm load. This usually happens on the phone, but not much relevance for audio applications. However, we are stuck with it. 0dBv equivalent to 2.2 DBU.
Noise Reduction is often called the input of the amplifier. Because, This allows the calculation of the sound output instantly by simply subtracting the numbers dB. So an amplifier with the ‘Equivalent Input Noise’ (EIN) from-40db gain 120dBu have to have a voice-output 80dBu (120 – 40). This is equivalent to 80dB Signal to Noise ratio (S / N) relative to 0dBu.
According to the theoretical minimum input noise (EIN) for each amplifier is-129dBu. Although not explicitly stated, this indicates that the input will be terminated with a resistance.
Usually, the source 200 ohm resistor will give this number at 25 ° C. Sometimes, a short circuit is used as a substitute, and this provides clear voice performance better. because, although A short circuit is significant, because there is no real world signal source has zero impedance.
This means that the perfect amplifier with a gain of 40db and the 600 ohm source impedance will have an output noise level of-89dBu, and if benefits must be increased to 60dB, the output will be noise-69dBu.
If the noise at the output voltage will increase only 3dB, not 6dB as expected. As a result, we can be fairly sure that it is the input noise is the most sensitive part of a preamp that will set a deadline for the signal to noise ratio of the entire unit.
How to overcome this noise op-amp normally requires little explanation, because this is almost not defined in terms that most constructors will be able to relate to. Data sheet tells you that “noise figure is 5nV / √ Hz”. To get this into something that we can understand, first we need to take ‘the square root of Hz’ and make sense of it. Taken as the audio bandwidth 20Hz to 20kHz, so that the square root of this is …
√ 20,000 = 141 (it was not worth the effort subtracting 20Hz, 141 so close enough)
With a sound figure 5nV / √ Hz, equivalent input noise (EIN), therefore …
5nV x 141 = 707nV
If we assume a typical gain of a phase sensitive microphone 100 (40db) and the output level of 1V (0dBv), this means that the sound output is the same as voice input, multiplied by the benefits. Signal to noise ratio can then be calculated …
707nV x 100 = 70.7uV (EIN =- 120.8dBu)
Signal to noise (dB) = 20 x log (1V / 70.7uV) = 20 x log (14,144) = 83dB
We also can calculate by using dB alone.
EIN =- 120.8dBu
Gain = 40db
S / N = 120,8-40 = 80.8 (ref 0dBu), or 80.8 + 2.2 = 83dB (ref 0dBv)
For low-level such as microphone pre-amps or moving coil Phone pre-amplifier, usually to determine the EIN, which allows users to calculate the votes for each gain setting, due to changes in the gain varies. The same amplifier as above with the theoretical gain will have a signal to noise ratio of 123dB. All this assumes that the passive components (especially resistors) do not contribute any noise. This is false, Anyway, the components that operate at temperatures above 0k (zero Kelvin, absolute zero, or about -273 ° Celsius) to produce noise, but the contribution is relatively small passive components with a resistance given the quality of the device kept as low as possible, and minimize stress .
If the amplifier will have equal input-129dBu sound. This means that with a gain of 60dB, the best signal to noise ratio of 69dB relative to 775mV (or 0dBv ref 71.2).
Some examples of a microphone preamp op-amp using 1458 op-amps (equivalent to uA741). This has a voice input rate of about 4uV – this translates to about 30 to 35nV / √ Hz, or almost 20db worse than NE5534A. With 46dB gain (200), the circuit successfully 65dB signal to noise ratio.
I think a good ratio of about 80dB to 46dB gain using LM833 op-amps (NE5534 dual version), but according Theoretical 85.2dB.
Look carefully for devices with low noise sensitive circuitry, and make sure they also have the bandwidth necessary to achieve high profits. LM833 or NE5532 dual op-amps is an excellent choice for low noise, but they also have a wide bandwidth. Do not be tempted to use a smaller device, because their bandwidth is too limited to 1458 was only 3dB down at 8kHz, and died very quickly after that.
Usually found in the performance of better sound can be obtained by using discrete op-amps – built using individual components. General techniques for low noise transistors to choose based on their noise data, which will show the optimal collector current for a given source impedance.
Use the device in parallel, further decrease noise. Two transistors in parallel will have a 3dB noise level is better than one device. Will reduce the use of four other 3dB, and eight will provide further 3dB reduction.
For each bit of digital, the noise floor relative to 6dB. A little bit 1 system used, and need to go to a minimum of 8 bits even before the ordinary speech can be understood – is not acceptable, but understandable. Once it was good, I go ……….
Prior to digital to analog conversion, signal back to the 16-bit format. This has nothing to do with sound, I guess I just wanted to mention it, ha ha ha ……………….. easy hopefully useful to friends of the profession in the world. thanks