Power Amplifier Topics:
» Matching Amps to Speakers
» Damping Factor De-Mystified
» It's all in the Ohms
» Slew Rate, Does it Really Matter?
» What to Look For In a Power Amp

The right amount of power, and the right features, that's what marks the right power amp for your live PA system. This Sweetwater Buying Guide includes information that can help you choose a Power Amp for your needs. Since there's so much to consider when purchasing a Power Amp, don't hesitate to call us at 1-800-222-4700 for more information.

Matching Amps to Speakers

Tapco's J-1400 offers 1400W @ 4 ohms (bridged)

When you're matching a Power Amp to a PA Speaker, a good rule of thumb is to pick an amplifier that can deliver power equal to twice the speaker's continuous IEC power rating. This means that a speaker with a "nominal impedance" of 8 Ohms and a continuous IEC power rating of 350 watts will require an amplifier that can produce 700 watts into an 8 Ohm load. For a stereo pair of speakers, the amplifier should be rated at 700 watts per channel into 8 Ohms. A quality professional loudspeaker can handle transient peaks in excess of its rated power if the amplifier can deliver those peaks without distortion.

Using an amp with some extra "headroom" will help assure that only clean, undistorted power gets to your speakers. Some professional amplifiers are designed so they have additional headroom. These amps can cleanly reproduce transient peaks that exceed their rated power. In this case select a model with an output power rating equal to the continuous IEC power rating of the speaker. Consult the amplifier manufacturer or owner's manual to learn more.

In some applications, such as critical listening in a studio environment, it is important to maintain peak transient capability. For these applications, use an amplifier that can deliver 6db (or four times as much) more power than the continuous IEC power rating.

If budget restraints or legacy equipment force you to use an amplifier with less power, extreme care should be taken to see that the amplifier is not driven into clipping. It may surprise you to learn that low power can result in damage to your speaker or system.

Damping Factor De-Mystified

Loudspeakers have a mind of their own. You send them a signal and they add their own twist to it. They keep on vibrating after the signal has stopped, due to inertia. That ’s called "ringing" or "time smearing." In other words, the speaker produces sound waves that are not part of the original signal. Suppose the incoming signal is a "tight" kick drum with a short attack and decay in its signal envelope. When the kick-drum signal stops, the speaker continues to vibrate. The cone bounces back and forth in its suspension. So that nice, snappy kick drum turns into a booming throb. Fortunately, a power amplifier can exert control over the loudspeaker and reduce ringing. Damping is the ability of a power amplifier to control loudspeaker motion. It’s measured in Damping Factor, which is load impedance divided by amplifier output impedance. Let’s explain. If the speaker impedance is 8 Ohms, and the amplifier output impedance is 0.01 Ohms, the damping factor is 800. That’s a simplification. Since the speaker impedance and amplifier output impedance vary with frequency, so does the damping factor. Also, the impedance of the speaker cable affects damping. Thick cables (with low AWG) allow more damping than thin cables with (high AWG). The lower the amplifier’s output impedance, the higher the damping factor, and the tighter the sound is. A damping factor of 1000 or greater is considered high. As you might suspect, damping factor is most important at low frequencies, say 10 Hz to 400Hz. High damping factor equals tight bass.

- How It Works
How does an amplifier control speaker motion? When the loudspeaker cone vibrates, it acts like a microphone, generating a signal from its voice coil. This signal generated by the speaker is called back EMF (back Electro Motive Force). It creates a current, which travels through the speaker cable back into the amplifier output, then returns to the speaker. Since back EMF is in opposite polarity with the speaker’s motion, back EMF impedes or damps the speaker’s ringing. The smaller the amplifier output impedance, the greater is the effect of back EMF on the speaker’s motion. An amplifier with low output impedance short-circuits the back EMF, so the back EMF drives the loudspeaker with a relatively strong current that works against the speaker’s motion. When the speaker cone moves out, the back EMF pulls the speaker in, and vice versa

In short, the loudspeaker damps itself through the amplifier output circuitry. The lower the impedance of that output circuitry, the more the back EMF can control the speaker ’s ringing.

It’s All in the Ohms

Ohms, is a measure of resistance. Audio amplifiers are commonly designed to work with 4, 8 or 16 Ohms of resistance, and optimum system performance will be obtained if the total resistive load (or impedance) of the loudspeaker or set of speakers is exactly correct for the amplifier. If the total loudspeaker impedance is too high, the power delivered to the loudspeakers will be reduced. If the total loudspeaker impedance is too low, the power delivered to the loudspeakers will be increased, which can result in speaker overload and damage to the amplifier.

You can connect any amount of speakers to one amplifier provided that they are correctly wired and do not collectively fall below the specified output impedance of the amp. Multiples of loudspeakers can be connected together by three different methods, termed Series, Parallel, and a combination of the two, Series/Parallel.

In the case of PA sound, calculating parallel loads is an important capability for two main reasons; first, because dual speaker connections whether on an amplifier, a mixer/amplifier or a speaker enclosure are all wired in parallel. Some people think that if you run separate speaker cables from each speaker output on the amp or mixer/amp to the enclosures you somehow "avoid" putting the speakers in a parallel circuit. Others think that if you run a speaker cable from one cabinet to another you put the cabinets in "series" and that just adds the two loads together (e.g., two 4-ohm speakers in series = 8 ohms). But the truth is that everything gets put in parallel. In fact it's quite difficult to put speaker enclosures in series - you need a special wiring harness.

The following equations help you match the impedance of PA Speakers to Power Amplifiers for optimized performance (avoiding overloads and other issues). Impedance (Z) is how much a device resists the flow of an AC signal, such as audio. Impedance is similar to resistance, which is how much a device resists the flow of a DC signal. Both impedance and resistance are measured in ohms

Slew Rate, Does it Really Matter?

Slew rate is a measure of an amplifier's ability to follow its input signal. The term is used to define the maximum rate of change of an amplifier's output voltage with respect to its input voltage. The unit of measure is volts per microsecond. To put it in more technical terms, Slew Rate is nothing more than a term used to describe how quickly the potential on a circuit node must change with respect to time. As far as slew rate having an effect on perceived sound, the real issue is slew rate limiting, which relates to an amplifier’s ability to pass complex waveforms without clipping them, resulting in an open musical sound. Slew rate is measured by feeding an input signal that is too fast for the amplifier to cope with. So slew rate is an overload condition, and it should not happen at all for an audio amplifier. Therefore, being proud of a slew rate is very strange indeed

In fact, the following is taken from a lab assignment at MIT and is the only reference to slew rate regarding the overall design and building of an audio amplifier:

6.101 Introductory Analog Electronics Laboratory No. 5
Objective: "Build a small audio power amp and play loud music to retaliate against the 6002 students!"

Slew rate: No visible slewing allowable within the frequency range of 10 Hz to 20 kHz at full output into 100 Ω."

Many mic amp specifications play a purely objective role. These include voltage gain (usually just called "gain"), input noise, common mode rejection, and so forth. Other objective specifications, such as THD, slewing, frequency response, and phase response curiously do not always translate into specific predictable sound quality. Specifications should be treated with respect, but your ears should be the final judge of any preamp’s performance.