Audio low noise preamplifier circuit diagram

Introduction to Audio preamplifiers

An audio preamplifier, or preamp, is an electronic amplifier that prepares a small electrical signal for further amplification or processing. In an audio system, they are used to boost the signal from a microphone or instrument to a level that can be used by a power amplifier to drive speakers.

The main purpose of a preamp is to amplify low level signals without significantly degrading the signal-to-noise ratio. It provides voltage gain but no significant current gain. A preamp may also include controls for adjusting tone, volume, balance and other sound properties.

Key Characteristics of Audio Preamps

Some of the key characteristics and features to look for in an audio preamplifier include:

• Low noise: Preamps should add minimal noise to the original signal
• High gain: Ability to amplify weak signals to usable levels
• Wide frequency response: Flat response across the audible frequency range
• Low distortion: Minimal harmonic and intermodulation distortion
• High Input Impedance: To avoid loading down the signal source
• Low output impedance: For driving cables and subsequent stages
• Sufficient headroom: Ability to handle signal peaks without clipping

Types of Audio Preamplifiers

Audio preamps can be categorized based on their circuit topology, the active devices used, and their application. Some common types include:

1. Solid-state preamps

These use transistors and integrated circuits and are known for their reliability, low cost and compact size. They can be made with bipolar junction transistors (BJTs) or field-effect transistors (FETs).

2. Vacuum tube preamps

Also known as valve preamps, these use vacuum tubes as the amplifying devices. They are valued for their warm, musical sound but require higher voltages and are less reliable than solid-state designs.

3. Hybrid preamps

These combine tubes and solid-state devices, usually with tubes in the input stage and transistors in the output, aiming to get the best of both.

4. Microphone preamps

These are designed to work with the low signal levels from microphones. They provide high gain and very low noise. Mic preamps may be standalone units or built into mixing consoles.

5. Phono preamps

Also called an RIAA preamp, these amplify the signal from a turntable cartridge and apply the RIAA equalization curve. They require very high gain and low noise.

6. Instrument preamps

These are optimized for the output of electric musical instruments like guitar and bass. They often include tone controls and overdrive effects.

Audio PreAmp Circuit Design Considerations

Designing a high-quality, low-noise audio preamplifier requires careful consideration of the circuit topology, component selection, layout, and grounding. Here are some key factors:

Gain and Noise

The preamp should provide enough gain to bring the source signal up to a usable level, typically around 30-60 dB, while adding minimal noise. Low-noise transistors or tubes and high-quality resistors can help minimize noise.

Frequency Response

The preamp should have a flat frequency response across the audio range, typically 20 Hz to 20 kHz. This requires careful selection of coupling and bypass capacitors and setting appropriate RC time constants.

Distortion

Harmonic distortion and intermodulation distortion should be minimized. This can be achieved through proper biasing of active devices, using feedback to linearize the response, and ensuring adequate headroom.

Input and Output Impedance

The input impedance should be high enough to avoid loading the source, typically 10 kilohms or higher. The output impedance should be low to drive cables and subsequent stages without high-frequency loss, typically below 100 ohms.

RIAA Equalization

For phono preamps, the RIAA equalization curve must be accurately implemented using a combination of RC networks in the feedback loop of the amplifier.

Power Supply

The preamp circuitry requires clean, stable power at the correct voltages. Careful design of the power supply, including regulation, filtering, and decoupling, is critical to achieve low noise and hum.

Physical Layout

Proper physical layout of the circuit board and components is important to minimize noise pickup, crosstalk, and stray capacitance. Ground planes, shielding, and careful routing of sensitive traces are essential.

Example Preamp Circuit Diagrams

Here are a couple examples of audio preamplifier circuit diagrams with different design approaches:

Single-ended JFET Preamp

This is a simple, low-noise preamp using a junction field-effect transistor (JFET) in a common-source configuration with source degeneration for improved linearity.

Component	Value
J1	2N5457 or similar N-channel JFET
R1	1 megohm
R2	1.5 kilohm
R3	100 kilohm
R4	10 kilohm potentiometer (volume control)
C1	10 uF electrolytic
C2	0.1 uF film

The input signal is AC-coupled via C1 to the gate of J1. R1 sets the input impedance and R2 provides source degeneration. The drain load is R3 and the output is taken from the drain via the volume control pot R4 and output coupling cap C2. The JFET is biased by the Voltage Divider R1/R2.

Differential BJT Preamp

This preamp uses a differential pair of bipolar junction transistors (BJTs) for balanced operation and good common-mode rejection. The constant current source Q3 and R5/R6 set the tail current.

Component	Value
Q1, Q2	Matched NPN pair e.g. BCM61C
Q3	PNP transistor e.g. BC557C
R1, R2	10 kilohm
R3, R4	100 kilohm
R5	1 kilohm
R6	500 ohm trim pot
R7, R8	10 kilohm
R9, R10	1 kilohm
C1, C2	10 uF electrolytic
C3, C4	0.1 uF film
C5	100 uF electrolytic

The balanced input signal is AC-coupled via C1/C2 to the bases of Q1/Q2. R1/R2 set the input impedance. The collector loads are R3/R4 and the output is taken via R7/R8. R9/R10 and C3/C4 form a low-pass filter to remove RF interference. C5 is the power supply bypass.

PCB Layout for Low-Noise Preamps

Proper printed circuit board (PCB) layout is crucial for achieving optimal performance in a low-noise preamp. Here are some guidelines:

1. Use a ground plane on one or both sides of the board to provide a low-impedance return path and shield against interference.

2. Keep signal traces as short as possible and away from power supply and high-current traces.

3. Use symmetric layout for differential signals to maintain good common-mode rejection.

4. Provide ample power supply bypassing with local decoupling capacitors near each active device.

5. Physically separate input and output sections to minimize crosstalk.

6. Use shielding around sensitive components like input jacks and transformers.

7. Pay attention to trace width and spacing to maintain controlled impedance and minimize stray capacitance.

Testing and Measuring Preamp Performance

After constructing a preamp, it’s important to test and measure its performance to ensure it meets the design goals and specs. Some key parameters to test include:

Gain

Measure the output voltage for a given input voltage at 1 kHz to determine the voltage gain in dB. Check that it matches the design value.

Frequency Response

Apply a sine wave at different frequencies from 20 Hz to 20 kHz and measure the output level. Plot the response and check for any unexpected variations.

Noise

With the inputs terminated, measure the output noise voltage over a specified bandwidth (e.g. 20 Hz to 20 kHz) with an RMS voltmeter or spectrum analyzer. Express the result in terms of equivalent input noise (EIN) in dBu.

Distortion

Apply a sine wave at a level below clipping and measure the total harmonic distortion (THD) at the output with a distortion analyzer. Repeat at several frequencies.

Common-Mode Rejection (for balanced circuits)

Apply a common-mode sine wave to both inputs and measure the output. The CMRR in dB is the ratio of differential gain to common-mode gain.

Clipping Level

Increase the input signal until the output starts to clip and note the input and output levels. This determines the maximum input level and headroom.

Troubleshooting Audio Preamps

If a preamp is not performing as expected, here are some troubleshooting steps:

1. Check the power supply voltages for the correct levels and low ripple/noise.

2. Check for faulty or incorrect value components, cold solder joints, or shorts.

3. Check the DC operating points of the transistors against the design values.

4. Isolate sections of the circuit by lifting components and injecting test signals.

5. Compare the measured performance against the design specs and look for discrepancies.

6. Substitute known good components for potentially faulty ones.

7. Check for oscillation or instability with an oscilloscope, especially at high frequencies.

Preamp Projects and Kits

For those interested in building their own preamps, there are many schematics, PCB layouts, and complete kits available. Some popular projects include:

• The Hairball Audio JFET Preamp: A simple, low-noise design using 2SK170 JFETs.
• The AMB Labs α10 and α20: High-performance differential preamps with stepped attenuators.
• The Jensen Twin Servo 990: A transformerless preamp with 990-style discrete op-amps.
• The Alice Microphone Preamplifier: A 1970s-inspired transistor preamp with transformer I/O.

There are also many preamp kits available from companies like Seventh Circle Audio, DIYRE, and Hairball Audio that provide all the necessary parts and instructions.

FAQ

What is the difference between a preamp and a power amp?

A preamp provides voltage gain to bring low-level signals up to line level, while a power amp provides current gain and drives speakers. Preamps work with smaller signals and have higher input impedance.

Do I need a preamp if I have an audio interface with built-in mic preamps?

If your audio interface has high-quality, low-noise preamps with enough gain, you may not need an external preamp. However, a dedicated preamp can provide better sound quality, more control, and the ability to drive long cable runs.

Can I use a preamp with line-level signals?

Yes, most preamps have enough headroom to handle line-level signals without clipping. You can use the preamp’s gain control to adjust the level as needed.

What is phantom power and do I need it for my preamp?

Phantom power is a method of providing DC power to microphones that require it, such as condenser mics. It is a 48V supply that is sent over the same wires as the audio signal. If you use mics that need phantom power, your preamp will need to provide it.

How do I choose the right preamp for my needs?

Consider the type of sources you will be amplifying (microphones, instruments, etc.), the desired sound character (clean, colored, vintage, etc.), the required features (EQ, DI, etc.), and your budget. Read reviews and compare specs to find the best match.

Conclusion

Audio preamplifiers are a critical component in any sound system, providing the necessary gain and buffering to interface low-level sources with subsequent processing and amplification stages. Designing and building a high-quality, low-noise preamp requires careful consideration of circuit topology, component selection, layout, and testing.

By understanding the key characteristics and design principles of preamps, as well as following best practices for construction and troubleshooting, it is possible to achieve excellent audio performance for a variety of applications. Whether starting from scratch or working with a kit or existing design, a well-crafted preamp can provide years of reliable service and sonic satisfaction.