Preamplifier : Circuit, Working, Types, Differences, How to Choose, & Its Applications

The preamplifier or preamp was first invented in the early 20th century as a main element of radio & phonograph technology development. Later, transistor-based preamplifiers & finally digital preamps emerged by offering higher reliability, lower cost & lower noise. These amplifiers brought more functions & features like; balance and filters additionally to surround sound, equalization, wireless connectivity & room correction. Preamps are significant components within the audio system. They provide the audio signal to the power amplifier & the loudspeaker. They also adjust the signal level & the signal routing to suit different conditions. This article briefly explains a preamplifier, its working, types, and applications.

What is a Preamplifier?

A preamplifier is an electronic amplifier that removes the signal from the detector without considerably degrading the intrinsic S/N ratio. The preamplifier features mainly include; tone control, digital inputs of versatility & balance adjustment.

These amplifiers are critical components in home theatre & home stereo systems. So these are mainly designed to use an audio signal from an audio source device & amplify it to a level that can be further processed through a power amplifier to play through speakers. These types of amplifiers are also responsible for processing the audio signal like; adjusting the tone, balance & volume.

Preamplifier Working Principle

The preamplifier works by changing a weak electrical signal to a strong output signal which is noise-tolerant & strong enough for transmitting to a power amplifier as well a loudspeaker is known as a preamplifier or preamp. The final output signal would be distorted or noisy without using this amplifier.
Preamps are used in home stereo systems and home theatre systems but the choice of this amplifier mainly depends on various factors like; the number of required channels, the preferred features & sound quality.

Preamplifier Circuit

The preamplifier circuit with a single transistor is shown below. This simple circuit is used to provide about 3 – 30 voltage gain & is depends on the source as well as load impedance. This preamplifier circuit has low input impedance.

The required components to make this preamplifier circuit mainly include; resistors R1-2.2 MΩ, R2 – 4.7 KΩ, capacitors C1 & C2 electrolytic capacitors – 10 µF, 10V and NPN transistor T1 = BC148B. Connect the circuit as per the diagram shown below.

Working

The simple preamplifier circuit is designed with a single low-power amplifier transistor. This transistor is configured in CE (common emitter) mode. Whenever the input signal is provided to the base terminal of the T1 transistor throughout the C1 coupling capacitor then it couples the input signal to the BE terminals of the T1 transistor.

The 12V Vcc is provided to the collector terminal of the transistor and it serves the extra function by giving the base current. The C1 capacitor in zero signal condition works as an open circuit since the capacitor reactance is infinite at zero frequency. Therefore, the ‘C1’ capacitor works as a blocking capacitor. Likewise, a C2 capacitor serves a similar function. So C2 capacitor works as a coupling capacitor & feeds the amplified signal to form the output voltage. This simple preamplifier circuit utilizes fixed bias. So in the above circuit, the zero signal process can be established by choosing R1 resistance.

Preamplifier Types

There are three types of preamplifiers current-sensitive, Parasitic-capacitance, and charge-sensitive preamplifier which are discussed below.

Current-sensitive Preamplifier

The current-sensitive preamplifier with 50Ω input impedance provides the correct termination of the 50Ω coaxial cable & changes the current pulse to a voltage pulse from the detector. If the increased time of the amplifier is small as compared to the detector increase time, the preamplifier’s voltage gain is ‘A’, and the voltage pulse amplitude at the preamplifier o/p will be given as;

Vout = 50 Iin A

where ‘Iin’ is the current pulse amplitude from the detector. So this signal is fed to a fast discriminator for counting applications whose o/p is recorded through a counter/timer.

The dominant limitation for timing applications on timing resolution through photomultiplier tubes & microchannel plates is variation within the transit times of the electrons when they cascade throughout the detector. So this can cause a jitter within the pulse arrival time at the output of the detector. But, if the detector signals are sufficient to need a current-sensitive preamplifier, then the preamplifier input noise effect on time resolution should also be considered. These types of preamplifiers are designed mainly for timing applications that have AC-coupled time constants within a few hundred nsec range.

Parasitic-capacitance Preamplifier

The preamplifier which has a high (~5 MΩ) input impedance is known as a parasitic capacitance preamplifier. So, the generated current pulse by the detector is incorporated into the combined parasitic capacitance available at the preamplifier input & detector output. Generally, this combined parasitic capacitance ranges from 10 – 50 pF. The resultant signal is a voltage pulse with amplitude that is proportional to the whole charge within the detector pulse & a rise time equivalent to the period of the detector current signal.

The input capacitance and a resistor are connected in parallel which causes an exponential decay of the signal with a ~50 µs time constant. An amplifier with a unity gain & high input impedance is incorporated as a buffer to drive the coaxial cable’s low impedance at the output. Here, the 93-Ω resistor with the output in series can absorb reflected signals within long cables by ending the cable in its characteristic impedance.

These types of preamplifiers are not utilized with semi-conductor detectors due to the sensitive gain to little changes within the parasitic capacitance. The detector capacitance for partially depleted semiconductor detectors changes with the applied bias voltage to the detector diode. Additionally, the interconnecting cable’s small movements can adjust the input capacitance with a few tenths of a pF.

Charge-sensitive Preamplifier

Charge charge-sensitive preamplifier is a type of preamplifier that is frequently used while designing readout circuitry mainly for pulse detectors. Their design provides stability, and low noise & their integrating nature offers an output. So this output is proportional to the whole charge supplied from the detector throughout the pulse event.

As compared to other preamplifiers, the gain of this preamplifier is stable. It does not depend on the amplifier bandwidth or the input capacitance. These types of preamplifiers are normally used within radiation detection applications, wherever individual detection signals need to be calculated with high precision. These can be installed within the home audio system & can be combined with other audio equipment. These amplifiers are very cheap and generate a natural sound.

How to use a Preamplifier with an Amplifier?

Generally, the preamplifier is used before the amplifier or power amplifier in an audio system. We cannot supply signals to the power amplifier from a microphone (or) any analog audio sensor. So we should use the preamplifier before the amplifier.

Amplifier simply focuses on amplifying the electronic signal at a high range they are not capable of removing noise and also interference from the signal. For the high-rate signal amplification, a strong and noise-free electronic signal needs to be given to the amplifier. However the output of the microphones, transducers, analog sensors, etc not appropriate for the input of the amplifier as they create only noise, weak and interference embedded signals.

So preamplifier is used to prepare the signal for the i/p of the power amplifier. If the preamp is not allied before the amplifier, the power amplifier output will be noisy & distorted. In addition, the preamplifier assists in balancing the gain.

Difference B/W Preamplifier Vs Amplifier

The main differences between a preamplifier and an amplifier are discussed below.

How to Choose the Right Preamplifier?

We know that a preamp boosts the microphone signal before it achieves the main mixer or audio interface. This amplifier can have an impact on the quality & character of your recordings, thus selecting the correct one for your system is essential. So while selecting a preamplifier, many factors need to be considered which are discussed below.

Gain & Headroom

The amount of amplification that a preamplifier applies to the input signal is known as gain whereas the headroom is the highest level that a preamplifier handles without distorting. So a preamp should have a minimum of gain – 60 dB & headroom of – 20 dB.

Color & Tone

Color & tone refer to the sonic features that a preamplifier imparts to the signal like; clarity, warmth, saturation, brightness, etc. The option of color & tone mainly depends on your personal preference & the style and type of music you are recording.

Features & Functions

These are the extra options that control that a preamplifier offers like; a polarity switch, phantom power, low-cut filter, pad, phase alignment, or metering. So all these features as well as functions help you in shaping & optimizing the signal before it achieves the next phase of the signal chain. So before selecting a preamplifier, you must look for features & functions that suit your needs as well as workflow.

Compatibility & Connectivity

It refers to how fine a preamplifier functions with other studio equipment & software like a mixer, audio interface, monitors, or DAW. If you want a preamp for your current setup then you must look for a preamp that has suitable input & output connectors like; TRS, RCA, or XLR. You must also verify the preamp compatibility with your operating system & DAW,

Quality & Budget

This refers mainly to how much you are ready to spend on a preamplifier & what level of reliability & performance you anticipate from it. If you need a preamplifier based on your budget as well as the quality you require for your recordings, then must check its reviews, warranty & support from customers.

Advantages & Disadvantages

The advantages of the preamplifier include the following.

• This amplifier boosts signals from source equipment to a range that a power amplifier can function with efficiently.
• These amplifiers provide a technique to switch between different source components.
• It has a better quality of sound.
• This amplifier has less noise and more gain.
• Preamplifier has a higher bandwidth.
• This amplifier has a high dynamic range.
• These are not expensive.
• This amplifier Installation and troubleshooting are very simple.
• These are available readily & responsive highly.
• Preamplifiers are linear.
• It has high input and low output impedances.
• These are normally charge-sensitive and current-sensitive.
• This amplifier provides a particular characteristic/ attitude for your sound.
• Low-voltage audio signals are transformed very quickly.

The disadvantages of the preamplifier include the following.

• This amplifier has less number of inputs and outputs.
• These are heavier.
• It does not allow extra control.
• It is a critical component within gyro meter electronics.

Applications

The applications of preamplifiers include the following.

• Preamplifier identifies very low amplitude AE signal generated from AE sensors & changes into usable and amplified forms.
• This amplifier is used wherever the input signal is very small & a power amplifier is not capable of detecting this small signal devoid of a preamplifier stage.
• Usually, it precedes one more amplifier to arrange an electronic signal for further processing or amplification.
• This is used in communications systems to amplify the extremely small signals obtained by the antenna & is frequently placed very near to the antenna.
• It is used to modify a signal from low-voltage and high-impedance to a high-voltage and low-impedance signal which is susceptible to signal degradation.
• This amplifier amplifies the electronic signals by simply improving the S/N ratios before they go into a cable to an amplifier.
• It is used mainly for high-fidelity audio to support stereo channels as well as switches among audio inputs.
• This kind of amplifier offers RIAA equalization mainly for the recording and playback of phonograph records.
• This amplifier is used to support different audio channels in home theaters.
• This amplifier is used in any audio system to prepare the input audio signal mainly for amplification within the next stage.
• These are used in Sound cards, Equilizer Machines, DJ Mixers, and many more.
• These amplifiers are used with analog sensors, transducers such as proximity sensors as well as microphones to enhance their output.
• Preamplifiers are used at the receiver end of the communication system to get rid of noise & interference from the acquiring signals.

Thus, this is an overview of a preamplifier, working, types, circuit, and their applications. A preamplifier plays a key role within high-quality audio systems in boosting and refining the signal from source equipment. Whenever selecting a preamplifier some crucial factors need to be considered like compatibility with other audio equipment, preferred sound quality, etc. So, understanding the main role & also its types can improve your audio experience significantly. Here is a question for you, what is an amplifier?