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What’s Under The Hood? Power Amplifier Sections, Connectors & Classes

A power amplifier is not just a black box that makes signals stronger...
This article is provided by Bartlett Audio.

As audio professionals, the more we understand what’s under the hood of modern power amplifiers, the better we can make a wise buying decision.

What are the main sections or parts of a power amp?

Every power amplifier includes a power supply, an input stage, and an output stage. Most amps also have protection mechanisms; some have DSP, and a few have networking capability.

Let’s explain each feature…

Power Supply
Basically, a power amplifier uses the input signal to modulate DC from its power supply. This supply receives 120 volts AC from the mains outlet and converts it to DC to operate the transistors, FETs and MOSFETs and so on in the amp circuitry.

Two types of power supply are analog and switching. A typical analog power supply rectifies the incoming 50 or 60 Hz AC and low-pass filters it to create DC for the power amplifier circuitry.

A switching power supply converts the incoming AC to DC, switches it on and off at an ultrasonic rate, runs those pulses through a small, lightweight transformer, then rectifies and filters the waveform to produce DC. The switch-mode supply can be smaller and lighter than the analog supply, but is more complex.

Some amps have a separate power supply for each channel so that high demands on one channel don’t affect the other. A few also have a separate power supply for the input stage, which is the part of the amplifier that does not drive the loudspeakers.

It’s important that the power supply have enough power reserve to supply power for transients or signal peaks. That happens when the supply uses big filter capacitors that store energy, and releases it when needed.

If the amp is heavily loaded down (that is, it is driving a low output impedance), the power supply voltage may drop or “sag,” causing distortion. Using a separate power supply for the input stage prevents distortion in the input stage caused by the output stage’s supply voltage sagging.

Amplifiers of very high power draw lots of current through the power cable from the AC outlet. To avoid limiting the current that can be drawn, the AC power cable has to be heavy gauge and short.

And the circuit breakers feeding the amplifier’s AC outlet need to be 20 A or higher rather than 15 A. Low-current AC outlets can prevent the amplifier from reaching its maximum power output.

Input Stage
The input stage or “front end” accepts the input signals and feeds them to the output stage to be amplified. Here you’ll find connectors that mate with the input cables.

Level controls and any plug-in modules are part of the input stage as well. The level controls do not affect the gain of the amp; rather, they affect the input sensitivity – the input voltage required to drive the amp to full power.

Turning down an amp’s level controls does not make it less powerful or reduce its wattage rating. Instead, this requires the amp to have higher input signal to drive it to full power.

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Put another way, turning down the level controls reduces the level to the output stage of the power amplifier. If you send the amp a high enough signal level, you can drive the amp to its full rated power even with the level controls turned down from maximum.

In fact, it’s standard practice to set the amp’s level controls for proper gain staging. Set up the sound system’s mixer so that signals peak around 0, then gradually turn up the power amp’s level controls until the sound is as loud as you want it. This results in the best system signal-to-noise ratio and headroom.

If you turn up the amp’s level controls to maximum, you’ll often hear mixer noise through the system loudspeakers because the mixer will have to be run at levels well below 0 on its meters.

Let’s look at other parts of the input stage. LED’s on the front panel indicate signal level, clipping, and overheating, so they can be used for diagnostics if you hear no sound or distorted sound.

Connectors in the input stage are on the back panel of the amp. You’ll see these types of connectors:

• 1/4-inch phone jacks: These are most often seen in portable PA or small band PA systems. TS (tip-sleeve) is unbalanced; TRS (tip-ring-sleeve) is balanced and is preferred for its rejection of hum and noise.
• Female XLR: This three-socket locking connector mates with a male XLR and provides a balanced connection. It’s used in portable PA and touring sound applications.
• Terminal block (terminal strip, screw terminals). This type is intended for permanent installations. It lets you eliminate connectors and their cost because the input cable is hard-wired to the terminal block.
• RCA or phono connectors: These are used for background music systems and home stereos.

All XLR inputs, terminal blocks and most phone jacks are wired balanced which rejects hum and noise on the input cable.

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