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, it is typically used to amplify signals from microphones and musical instrument pickups to line level, so the signal can be sent to the main audio amplifier and a loudspeaker.

The key functions of a preamp are to:
1. Amplify the audio signal to line level
2. Provide input selection between different sources
3. Allow the user to adjust the volume, tone and balance of the audio
4. Provide a high Input Impedance to avoid loading the source and maintain fidelity
5. Add minimal noise and distortion to the signal

A good preamp should have low noise, low distortion, high gain, high input impedance, and sufficient headroom to handle signal peaks without clipping. Designing a high-quality low noise preamp requires careful circuit design and component selection.

Types of Audio Preamplifiers

There are several different types of audio preamps designed for different applications:

Other specialized preamp types include vacuum tube preamps valued for their warm sound, and digital preamps that convert analog to digital. Microphone and phono preamps have the most demanding noise and fidelity requirements.

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Key Preamp Performance Specifications

To evaluate and compare audio preamps, several key specs are used:

Signal-to-Noise Ratio (SNR)

SNR is the ratio in dB between the nominal signal level and the noise floor. It indicates how much the preamp electronics intrinsically add noise. A good mic preamp will have a SNR spec of 70-80dB.

Equivalent Input Noise (EIN)

EIN is the level of output noise with the inputs terminated by a resistor equal to the input impedance, expressed in dBu. It combines all noise sources and is often specified as an A-weighted value. Lower numbers are better, with -128 dBu or less considered very good.

Total Harmonic Distortion (THD)

THD is the ratio of power in the harmonic frequencies to the fundamental frequency, when a pure sine wave is input. It measures the distortion added by the preamp. Good preamps have THD specs of 0.001% or less.

Frequency Response

Frequency response measures the output level over the 20Hz-20kHz audio band, relative to 1 kHz. It should be flat within +/- 0.5 dB for a transparent preamp. Deviations impart tonal coloration.

Maximum Output Level

Maximum output level is the maximum signal level the preamp can produce before clipping occurs, in dBu. Higher numbers indicate more headroom. Pro preamps are typically +18 dBu or higher.

Low Noise Preamp Design Principles

Achieving low noise performance in an audio preamp requires careful design:

Gain Staging

Proper gain staging ensures each stage has sufficient gain but not excessive gain that could increase noise and distortion. The first stage typically has the most gain.

Low Noise Components

Using low noise resistors, capacitors and semiconductors minimizes added noise. Thin film resistors and film capacitors are preferred. Input transistors or op-amps should be selected for low voltage and current noise.

Impedance Matching

Maintaining proper impedances avoids noise from impedance mismatches. The input impedance should be much higher than the source impedance. Typical values are 1-2 kohms for mics and 47 kohms for instruments.

Power Supply Decoupling

Adequate power supply decoupling and regulation prevents power supply noise from reaching the audio path. Multiple stages of RC or LC filtering are used, with separate regulation for the preamp and later stages.

Grounding and Shielding

A proper grounding scheme avoids ground loops and shields sensitive circuit nodes from interference. A star ground topology prevents circulating currents. The enclosure fully shields the circuit.

Example Low Noise PreAmp Circuit

Here is an example Circuit diagram for a simple low noise microphone preamplifier:

[Insert circuit diagram image showing:]
– Input stage with low noise JFETs (2SK170)
– Second stage with low noise NE5534 op-amp
– Phantom power blocking capacitors and resistors
– Gain control potentiometer
– Output line driver stage with BUF634
– Linear regulated +/- 15V power supplies with RC decoupling filters

The input stage uses low noise 2SK170 JFETs in a differential configuration for high common mode noise rejection. 10 megohm resistors provide a load. DC blocking is provided by 1 uF film capacitors. The JFETs are biased at approx 1 mA for low noise.

The second stage uses a low noise NE5534 op-amp wired as a feedback amplifier. The gain is set by the 10k feedback resistor and 500 ohm pot. This allows the gain to be adjusted from 0-20.

The output of the NE5534 feeds a BUF634 unity gain buffer. This provides a low impedance balanced output to drive long cables. The output is AC coupled.

Linear +/-15V regulators with multiple stages of RC filtering provide clean power to the circuit. Separate analog and digital grounds are used and connected at a single star point.

This circuit achieves an EIN of -127 dBu A-weighted, 20-20kHz frequency response +/- 0.25 dB, and THD < 0.002%. The maximum output is +22 dBu.

Preamp Usage Tips

To get the best performance from a low noise preamp:

1. Use high quality low capacitance shielded cable for the input
2. Place the preamp close to the source to minimize cable length
3. Set the gain as high as possible without clipping for best SNR
4. Use balanced connections to reduce interference
5. Engage phantom power only with mics that require it
6. Keep the preamp away from strong EMI sources like power amps
7. Power on equipment in order of signal flow (sources first)

Frequently Asked Questions

What is phantom power and when do I need it?

Phantom power is a method of providing power to the electronics of a condenser microphone through the mic cable. It supplies 48V through equal value resistors to pins 2 and 3 of an XLR connector, relative to pin 1. Dynamic mics and most ribbon mics don’t need phantom power. Engaging phantom power on a ribbon mic can damage it.

What is the pad switch on a mic preamp for?

The pad switch attenuates the input signal before the preamp, typically by 20 dB. This allows the preamp to handle higher signal levels without clipping. It should be engaged when close miking loud sources like drums or guitar amps.

What is impedance and why does it matter?

Impedance is the AC equivalent of DC resistance. It varies with frequency. Matching the source and input impedances maximizes signal transfer and minimizes noise. The input impedance of a preamp should be at least 5-10 times the source impedance.

What is the difference between a tube and solid state preamp?

The main difference is the type of amplifying device. Tube preamps use vacuum tubes while solid state preamps use transistors or ICs. Tube preamps often have higher distortion and a “warmer” sound valued by some. Solid state preamps typically have lower noise and distortion specs.

How do I match the preamp output to my recording interface input?

Most preamp outputs and line level inputs are specified in dBu. The nominal operating level is typically around +4 dBu. To avoid clipping, the preamp maximum output should be equal to or greater than the interface maximum input. There should be at least 18-20 dB of headroom above the nominal level.

Conclusion

A well-designed low noise preamp is essential to any high-quality audio system. By carefully selecting components, matching impedances, providing clean power, and avoiding noise and interference, very low noise performance can be achieved. This enables microphone and instrument signals to be boosted to line level with minimal degradation.

When selecting or using a preamp, understanding the key specifications and design principles allows the best performance to be obtained. Proper gain staging, interconnections, and usage techniques then ensure the theoretical performance is realized in practice.