Output Stage
This stage amplifies, or increases the power of, the input signal up to a level sufficient to drive the loudspeakers.
In this stage are the power transistors (output devices), which tend to generate a lot of heat. Also in this stage are the output connectors which are wired to the loudspeakers.
Four types of output or loudspeaker connectors are phone jacks, five-way binding posts (banana jacks), Speakon connectors and terminal blocks (screw terminals).
• Phone jacks are inexpensive connectors for low-power applications. They are often seen in portable PA systems.
• Five-way binding posts provide a temporary or permanent, high-power connection to banana plugs, spade lugs or stripped wires. You’ll find these connectors in amps for touring sound.
• Speakons are a high-power, locking, cylindrical connectors used in touring sound.
• Terminal blocks are mainly used in installed sound applications to eliminate connectors and their cost.
Figure 1 is the back panel of a Crown I-Tech HD power amplifier, showing XLR, Speakon and five-way binding post connectors.
The best power connectors have low contact resistance. As contact pressure and contact area increase, contact resistance goes down. High-pressure contacts increase current flow by helping the current to penetrate through the surface films. They also increase contact area by flattening out the contact surfaces.
So when you use banana plugs, it helps to “stretch out” the ribs in each pin to increase contact pressure. Use a small screwdriver to bend the ribs.
Protection
The better power amps include circuitry that protects the loudspeakers and the amp itself from overheating and burning out. Some include a limiter to prevent the output power from getting too high and causing clipping, which can destroy tweeters.
Others prevent DC and ultrasonic signals from reaching the loudspeakers in the event of amp failure. Low-end units just blow a fuse or trip a circuit breaker if the current draw is too high, while high-end amps limit the output power so that the music doesn’t stop.
Cooling
The main cause of amplifier failure is overheating, so most amps include heatsinks and fans to keep the amp cool. In some units the fans come on only when needed. Some Class D amps tend not to get hot, so they don’t need fans.
