Understanding superior professional audio design: A block-by-block approach

Today's high-end professional audio market has morphed from being the reserve of a few recording and broadcasting companies to enabling every musician to record his or her own music, making it easy to move up the chain to the higher-end esoteric, "It doesn't matter how much it costs," type of equipment.

Even though the actual product requirements in categories such as musical instrument, "prosumer" (midpoint between professional and consumer audio) and professional audio (recording, broadcast and large-scale live venues) differ, the technologies that drive the functionality of these products are all based on the same types of circuits.

Figure 1 illustrates the various parts of the professional audio signal chain. This article walks you through the main blocks, provides desirable parameters, and identifies the benefits designers should consider when designing a professional audio product.

Figure 1. A block diagram of a typical professional audio system. (Click on image to enlarge)

Analog Inputs
Typically, there are three different types of analog inputs into an audio system - microphone, instrument and line level. The majority of external microphones have a balanced, differential output. These microphone signals are usually in the millivolt range and require some gain to bring them up to line levels, suitable for processing or storing digitally.

Line inputs for products can be both single-ended and differential, and are in the 0.7 to 3.4-V range. Additionally, in some broadcasting environments, analog I/Os can swing up to 43.72 Vpp.

For microphone preamplifiers, there are a few key specifications you should look for, regardless of whether they are single IC amplifiers, discrete transistor amplifiers, or programmable gain amplifiers (PGA).

One of the key specifications is the equivalent input noise (EIN). This is a measure of the internal pre-gain noise of the amplifier circuit. Usually, this is given at different gain levels and can be expressed in nV/√Hz or in dBu, but always at specific gain values. As an example, Figure 2 shows the EIN curve for the PGA2500 digitally controlled, analog microphone preamplifier.

Because EIN is pre-gain, any gain that the microphone preamplifier applies will amplify the EIN as well as the input signal. Therefore, low EIN is desired in a microphone preamplifier to make sure that when a designer gains a signal, only the performer is heard, not the noise of the amp.

Figure 2. The EIN curve of an amplifier circuit (in this example, the PGA2500 microphone preamplifier) is a measure of the internal pre-gain noise of the amplifier. (Click on image to enlarge)

The next specification to look for is the total harmonic distortion and noise (THD+N). This is a measure of the harmonics created by the device, at a certain gain, when a single frequency is input to the device (for example, 1 kHz) at almost full scale. This is usually given as a percentage (see Figure 3).

A high level of THD will sound distorted, metallic and "tinny" as harmonics of the inputs are created. Therefore, in a high-performance professional audio system, the focus is to keep THD+N as low as possible to keep the sound as clean as possible.

Figure 3. THD+N is a measure of the harmonics created by the device (in this case, the PGA25000 (Vcc = +5 V)) at a certain gain, for a single input frequency. (Click on image to enlarge)