What is a Preamplifier Circuit?

A preamplifier (preamp) is an electronic amplifier that prepares a small electrical signal for further amplification or processing. In an audio system, it is typically used to amplify signals from audio sources to line-level.

Key characteristics of preamps include:

• Gain: Ability to increase signal amplitude, usually adjustable
• Noise: Unwanted random fluctuations added to the signal
• Frequency response: Uniformity of gain across the audio frequency range
• Distortion: Undesired alteration of the signal waveform
• Input impedance: Resistance a source sees looking into preamp input
• Output impedance: Resistance a load sees looking back into preamp output

A good preamp should have high gain, low noise, flat response, low distortion, high input impedance, and low output impedance for optimal performance. Let’s examine each characteristic in more detail.

Gain

Gain is a preamp’s ability to increase the amplitude of an input signal. It is usually expressed in decibels (dB):

Gain (dB) = 20 * log10(Vout / Vin)

Where Vout is the output voltage and Vin is the input voltage.

Preamp gains typically range from 10-60 dB depending on the application. Microphone preamps need more gain (40-60 dB) to amplify mic-level signals to line level. Phono preamps for turntables have 30-50 dB gain and RIAA equalization to compensate for the bass attenuation and treble boost used in vinyl record production.

Noise

Noise refers to unwanted random fluctuations added to the audio signal by the preamp circuitry. It is usually measured in terms of Equivalent Input Noise (EIN) – the amount of noise at the preamp’s input that would produce the same output noise level if the preamp had zero noise itself.

EIN = 20 * log10(Vno / Vs)

Where Vno is the preamp’s output noise voltage and Vs is the source voltage required to produce an output equal to the noise floor. A lower EIN indicates better noise performance.

EIN is often specified as an rms voltage (uV or nV/√Hz) over a given bandwidth (20-20kHz for full audio range) at a specific source impedance (typically 150Ω for mics). Noise can also be specified as a Signal-to-Noise Ratio (SNR):

SNR = 20 * log10(Vs / Vn)

Where Vs is the source voltage and Vn is the equivalent input noise voltage. A higher SNR indicates better noise performance.

Noise sources in preamps include:

• Thermal noise from resistors (Johnson noise)
• Shot noise from current flow in transistors
• Flicker (1/f) noise in transistors and ICs
• Power supply noise coupling into signal path
• Electromagnetic interference (EMI) and radio frequency interference (RFI)

Careful selection of low-noise components, proper circuit design and layout, and effective power supply filtering and decoupling can minimize preamp noise.

Frequency response

Frequency response refers to a preamp’s uniformity of gain across the audio frequency range (20Hz – 20kHz). Ideally, a preamp should have a flat response, providing equal gain at all frequencies.

Frequency response can be visualized using a Bode plot – a graph of gain (in dB) vs. frequency. The bandwidth of the preamp is defined as the range of frequencies where the gain is within 3 dB of the midband value. At the upper and lower frequency limits, the response is 3 dB down from midband, called the half-power or -3 dB points.

Factors affecting a preamp’s frequency response include:

• Coupling and bypass capacitor values
• Transistor and op-amp gain-bandwidth product (GBP)
• Stray capacitances and inductances in circuit layout
• Impedance interactions between stages

Careful component selection and circuit design can extend a preamp’s frequency response to cover the entire audio range with minimal variation.

Distortion

Distortion refers to any undesired alteration of the signal waveform caused by the preamp. The two main types are harmonic distortion and intermodulation distortion (IMD).

Harmonic distortion occurs when the preamp adds frequency components that are integer multiples of the input frequencies. It is usually expressed as a percentage of the fundamental:

THD = 100 * √(V2^2 + V3^2 + V4^2 + …) / V1

Where V1 is the fundamental voltage and V2, V3, V4 are the voltages of the 2nd, 3rd, 4th, etc. harmonic frequencies.

Intermodulation distortion happens when two input frequencies interact in a nonlinear system to produce sum and difference frequencies. IMD is usually measured by applying two test tones to the preamp and measuring the resulting sum and difference frequency amplitudes relative to the original tones.

A preamp with low distortion will have THD and IMD values well below 1% at normal operating levels. Distortion can be minimized by using high-quality linear components, providing adequate voltage headroom, and avoiding clipping.

Input and output impedance

Input impedance refers to the resistance a source sees looking into the preamp input. A preamp should have a high input impedance (>10 kΩ) to avoid loading the source and affecting its response.

Output impedance refers to the resistance a load sees looking back into the preamp output. A preamp should have a low output impedance (<100 Ω) to drive subsequent stages or long cables without loss of signal level or high frequency response.

Preamp Topologies

There are several common preamp circuit topologies, each with its own benefits and drawbacks. The choice depends on the specific application, desired features, and performance requirements.

Discrete transistor preamps

Discrete designs use individual transistors (BJTs or FETs) to implement the gain stages. They offer flexibility in customizing the circuit for a particular need. Discrete preamps are known for their potential for low noise and distortion when designed properly. However, they require more components and design effort compared to op-amp based preamps.

Op-amp preamps

Op-amp preamps utilize integrated circuit operational amplifiers to provide gain. Op-amps have differential input stages and both plus and minus supplies allowing simple AC-coupling if required so they can operate across the full output voltage swing. They offer:

• Easy design with few external components
• Built-in stable gain and high input impedance

• 

  Low distortion and noise with proper component selection

  However, their frequency response is limited by the op-amp’s gain-bandwidth product, and they may have slew rate limitations for high level high frequency signals.

Transformer-coupled preamps

Transformer input preamps use a transformer to convert the low-impedance balanced input signal to a high-impedance unbalanced signal for the preamp stage. Transformers provide:

• Galvanic isolation between source and preamp
• EMI/RFI noise immunity
• Voltage step-up for better noise performance

However, they add cost, weight, and potential frequency response and distortion issues if not designed properly. Transformerless designs are preferred for most modern applications.

Circuit Diagrams

Here are some example low-noise preamp circuit diagrams for various applications:

Microphone Preamp

This is a simple high-gain preamp for dynamic or ribbon microphones using an NE5534 low-noise op-amp:

The NE5534 input stage provides up to 60 dB gain adjustable via the R2 trim pot. C1 and C2 provide AC coupling and DC blocking. R4 and C4 create a low-pass filter to reduce RF interference. The TL072 output stage provides a low-impedance balanced output to drive long cables.

RIAA Phono Preamp

This preamp provides RIAA equalization and gain for moving-magnet (MM) phono cartridges:

The RIAA equalization network (R3, R4, C2, C3) provides the necessary frequency shaping. The NE5532 dual op-amp implements a two-stage amplifier with passive equalization in between. The gain is set by R5 and R6 to provide 40 dB at 1 kHz. C6 and R9 form a rumble filter to attenuate subsonic frequencies.

Instrument Preamp

This preamp is designed for high-impedance sources like electric guitar or bass:

The input stage (Q1, Q2) is a low-noise bipolar differential pair with high input impedance. R4 sets the gain to 20 dB. The second stage (Q3, Q4) is a common-emitter amplifier with another 20 dB of gain. The output stage is a unity-gain buffer to provide a low output impedance. The preamp runs on a ±15V supply for maximum headroom.

FAQ

Q: What is the difference between a preamp and a power amp?
A: A preamp amplifies low-level signals to line-level (about 1V rms) while a power amp further amplifies the line-level signal to drive speakers (tens to hundreds of watts).

Q: Do I need a preamp if my audio interface has built-in preamps?
A: If your audio interface has good quality preamps with enough gain and features for your needs, you may not need an external preamp. However, a dedicated preamp may offer better noise performance, coloration, or functionality.

Q: What is phantom power and do I need it for my microphone?
A: Phantom power is a +48V DC supply provided by the preamp to power condenser microphones. Dynamic and ribbon mics don’t need it. Check your microphone’s specifications to see if it requires phantom power.

Q: Can I use a line-level source with a mic preamp?
A: Yes, but you may need to attenuate the line-level signal first to avoid overloading the preamp input. Some preamps have a pad switch for this purpose.

Q: What is the difference between balanced and unbalanced connections?
A: Balanced connections use three conductors (hot, cold, and ground) to cancel noise induced in the cable. Unbalanced connections use two conductors (signal and ground) and are more susceptible to noise. Use balanced connections for long cable runs or noisy environments when possible.

Conclusion

Designing a low-noise preamp involves careful consideration of gain, noise, frequency response, distortion, and impedance requirements. By understanding these key characteristics and common circuit topologies, you can select or design the appropriate preamp for your audio application. Always use quality components, proper layout techniques, and adequate power supply decoupling for best performance. With the right preamp, you’ll be able to capture and amplify your audio sources with transparency and fidelity.