Amplifier Power Class: Not All Ratings Are Similar

October 01, 2012, by Bruce Bartlett

In designing a live sound system, you’ll come up with all sorts of questions, such as: One power amplifier is rated at 1,000 watts, and a different brand is also rated at 1,000 watts. Do they actually produce the same power?

Suppose you add more loudspeakers in parallel to a power amp output. Does the amp produce more power, less power, or the same? Does each loudspeaker have to handle more or less power as you add more loudspeakers?

To answer these and other questions, we need to explore the finer points of amplifier power ratings and loudspeaker loads.

(1) How loudspeaker loads affect amplifier power output
(2) How to compare power ratings of two different amp models
(3) Why to avoid 2-ohm operation
(4) Using bridge-mono operation to get more power

As amplifier data sheets show, an amp’s power output tends to increase as the load impedance in ohms decreases. You might see a spec of 400 watts into 8 ohms, 600 watts into 4 ohms, and 800 watts into 2 ohms. Why does that happen?

To explain, let’s start with the basics. As Ohm’s Law states, Current = Voltage divided by Resistance. The smaller the resistance, the higher the current. So low-impedance loads draw more current than high-impedance loads.

Here’s an example. When you play a 1 kHz tone through a power amp at a low level, it puts out a constant voltage, say, 20 volts. What is the current drawn by an 8-ohm, 4-ohm, or 2-ohm loudspeaker?

Volts / Resistance = Current
• 20 volts / 8 ohms = 2.5 amps
• 20 volts / 4 ohms = 5 amps
• 20 volts / 2 ohms = 10 amps (assuming the amplifier can provide 10 amperes)

Power = Voltage x Current. So the three loads would receive this amount of power:
• 8-ohm load: 20 volts x 2.5 amps = 50 watts.
• 4-ohm load: 20 volts x 5.0 amps = 100 watts.
• 2-ohm load: 20 volts x 10.0 amps = 200 watts.

As you can see, the amplifier power doubles every time the load impedance is halved. That’s at low levels, where the power supply is not forced to produce much current.

What happens when you add more loudspeakers in parallel to a single amplifier channel?

See Figure 1. If you connect one 8-ohm loudspeaker to a power amp output, the load impedance is 8 ohms. In the example above, that loudspeaker receives 50 watts.

Now suppose you connect two 8-ohm loudspeakers in parallel to the same power amp. The total impedance seen by the amp is 4 ohms, so the amp puts out 100 watts, and each loudspeaker receives half that power, or 50 watts.

Figure 1. How adding loudspeakers affects amplifier power and the power applied to each loudspeaker. (click to enlarge)

Now wire four 8-ohm loudspeakers in parallel and connect them to the same amp. The total impedance is 2 ohms, so the amp puts out 200 watts, and each loudspeaker receives 50 watts.

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So each loudspeaker gets 50 watts no matter how many loudspeakers you add in parallel. That means the SPL goes up as you add more loudspeakers to one power amp. Again, we’re talking about low levels where the amplifier doesn’t have to produce much current.

Imagine an ideal amplifier connected to an AC mains supply with unlimited current. This amp’s power rating doubles whenever the load impedance is halved. If an amp produces 400 watts at 8 ohms, it could do 800 watts at 4 ohms and 1,600 watts at 2 ohms.

In this ideal amplifier, the power supply’s DC voltage is constant no matter how much current is drawn from it. The supply could drive hundreds of loudspeakers in parallel with a total impedance near zero ohms. But real-word supplies can’t deliver infinite current. Nor can the AC mains or the output transistors – they would melt.

In a real-world power amp with limited current capability, the typical 4-ohm power rating is about 1.5 times the 8-ohm rating, not 2 times. And the 2-ohm rating is about 2 times the 8-ohm rating, not 4 times.

Most power amplifiers are rated for 4-ohm and 8-ohm loads, and high-end models can handle 2-ohm loads as well. A low-impedance load (like 2 ohms) is harder to drive – it draws more current – than a higher-impedance load (like 8 ohms). A “heavy load” means “a low-impedance load.”

Amps with puny power supplies run out of current when they drive low-Z loads. Amps with hefty power supplies can produce more current, hence more power, with low-Z loads.

Check the amplifier data sheet. It should specify how much power the unit can deliver into different loads. If the data sheet provides an 8-ohm and 4-ohm spec, but no 2-ohm spec, that means the amplifier probably can’t drive a 2-ohm load continuously without overheating or failing.

Comparing Amplifier Power Ratings

Let’s move on. Do two amplifiers with the same power rating produce the same power? Not always. Specifying amplifier power only in watts is insufficient.

A complete power rating includes several factors:
• Power per channel
• Number of channels driven simultaneously
• THD
• Frequency range
• Continuous (long-term) power or peak (short-term) power

Here are a few examples. Suppose one amp produces 500 watts per channel at 0.01% THD. Another amp produces 500 watts per channel at 1% THD. Although both amplifiers could be called “500 watt amplifiers,” the first one will produce more power than the second when both are turned up to the same distortion level.

Or suppose one amplifier puts out 500 watts continuous, and another puts out 500 watts peak (but 250 watts continuous). The first one will sound clean at 500 watts continuous, but the second one will clip if forced to produce 500 watts continuously.

You need to compare apples to apples. 500 watts in one amp is not necessarily the same as 500 watts in another amp. Compare both amps at the same THD, at the same load impedance, at the same continuous power, and over the same frequency range.

Also, manufacturers vary in how conservatively they rate their products. Some companies underrate a unit’s power to allow for manufacturing tolerances – to ensure that all production units will meet the power spec.

Most amps produce a little less power when all channels are driven. That’s because the power supply voltage tends to sag when more channels are in use. Amps with dual independent power supplies don’t have that problem.

Avoid 2-Ohm Operation
Although some amps can handle 2-ohm loads continuously, it’s best not to operate the amplifier that way.

When a loudspeaker’s impedance is specified as 8 ohms or 4 ohms, this is the nominal or average rating. Actually, the impedance varies with frequency, so an 8-ohm rated loudspeaker might be 6 ohms at a certain frequency.

Four 8-ohm loudspeakers combined in parallel might be 1.5 ohms at that frequency, which is almost a short-circuit across the amp! The result is overheating and possible failure. That’s one reason not to apply a 2-ohm load.

Also, loudspeakers are reactive loads, not just resistive. They have inductance and capacitance. That makes some loudspeaker loads especially difficult to drive, causing more heat in the power amp. Top-quality amplifiers can drive these “difficult” loads with less distortion than lesser amps.

Running an amplifier into a low-impedance load makes the amp run hotter because low-Z loads draw more current. More current causes more heat dissipation in the amp’s devices. High heat tends to destroy transistors and should be avoided.

Because 2-ohm loads draw a lot of current through the loudspeaker cables, more power is lost by heating those cables than if you used a 4-ohm load. A 2-ohm load requires very thick loudspeaker cables to prevent losing power via cable heating.

What’s more, a 2-ohm load reduces damping factor compared to a 4-ohm load, so the bass and kick drum may sound less tight. That’s because damping factor = load impedance divided by amplifier output impedance. As the load impedance decreases, so does the damping factor.

Even if a power amp is claimed to drive a 2-ohm load at high power, I don’t recommend using a lot of loudspeakers to take advantage of that power.

Instead, consider a 2-ohm rating as the ability to handle 4-ohm loudspeakers whose impedance might dip to 2 ohms at certain frequencies. If you need more power, I suggest getting a second amp and more loudspeakers, which will also provide redundancy.

Bridging Power Amps
One way to get more power from an amp is to run it in bridge-mono mode. When you bridge a stereo power amp, you wire the load across the amp terminals in such a way to get more power than the two channels combined.

For example, a 200-watt stereo amp might produce 600 watts mono when bridged. Note that bridging is not the same as bi-amping.

Caution: Make sure the amp’s data sheet says that the unit can be run in bridge mono mode. If not, you may damage the amplifier. Some amps require a switch to be set to bridge mode before you apply power.

To bridge a stereo amp, enable its bridge-mono switch and connect one loudspeaker across the two red terminals of the two channels.

Figure 2. Top: Dual-mode speaker wiring. Bottom: Bridge-mode speaker wiring. (click to enlarge)

Figure 2 shows how to do it. The 2-channel signals are opposite in polarity in bridge-mono mode, and so the total voltage across the loudspeaker is twice the voltage of a single channel.

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In bridge mode, one red terminal becomes the positive polarity signal, and the other red terminal becomes the negative polarity signal. The manufacturer’s manual tells which terminal is which.

A bridged amp provides double the voltage of a single channel. That double voltage forces twice the current through the load.

So in theory, with 2X voltage and 2X current, you get 4X the power in bridge mode compared to a single channel in stereo mode.

Real-world amps provide something like 3X the power in bridge mode because the amount of current available is limited.

Bridged amps should be used with loudspeaker impedances that are twice the minimum rated impedance in non-bridged mode.

For example, if an amp is rated at 4 ohms minimum in stereo mode, the load for bridge-mono operation should be 8 ohms minimum. Avoid driving a 4-ohm load with a bridged amp unless the specs say that the amp can handle it.

Bridging a stereo amp effectively turns it into a mono amp. So if you want to use two loudspeakers in a system, you’ll need two bridged stereo amplifiers.

Feed each amp a single mono input signal. In a stereo system, you’d feed the left-channel signal to amp 1 and feed the right-channel signal to amp 2.

Because the output signals of a bridged amplifier are floating, you should not connect an amplifier output terminal to a grounded device. That shorts one channel and may blow the amp.

Bi-Amping
Here’s our final topic. A single amplifier-loudspeaker system uses one amplifier channel driving a loudspeaker with an internal passive crossover.

Operating at high power, that crossover filter sends the lows to the woofer and the highs to the high-frequency driver.

In a bi-amplified sound system, you use one amplifier for the woofer(s) and another for the high frequency driver(s). An active crossover sends the lows to one amp channel and the highs to the other amp channel.

Each channel covers a specific frequency band, not the entire audio range. With a single-amp system, one amplifier handles the complete audio range (Figure 3).

Figure 3. Passive and active crossover systems, single-amp and bi-amped systems. (click to enlarge)

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Some systems can be tri-amplified, and some touring systems are four-way or five-way. They are the norm for high-power systems.

• Distortion frequencies caused by clipping the woofer power amplifier will not reach the tweeter, so there is less likelihood of tweeter burnout if the amplifier clips. In addition, clipping distortion in the woofer amplifier is made less audible because the woofer can’t reproduce high frequencies.
• Intermodulation distortion is reduced.
• Direct coupling of amplifiers to loudspeakers improves transient response – especially at low frequencies.
• Bi-amping reduces the inductive and capacitive loading of the power amplifier by a passive crossover.
• The full power of the tweeter amp is available regardless of the power required by the woofer amp.
• The active crossover can be variable frequency, so it adapts to a wide range of loudspeaker systems. It also lets you control the level and polarity of various drivers.
• The active crossover can have steeper roll-offs. This protects loudspeakers by keeping out unwanted frequencies.
• You omit the non-linear passive LF crossover used in passive loudspeakers. The result is higher sound quality.
• Peak power output is greater than that of a single amplifier of equivalent power.

Let’s explain that last advantage in more detail. Suppose you have an 800-watt single amplifier, and also a 600-watt LF amp and 200-watt HF amp as part of a bi-amped setup.

In general, you can get more volume – higher peak levels –  from the bi-amped system even though the two systems have the same total power. A loud bass note and a loud high-frequency note sounding simultaneously might clip the 800-watt single amp.

But since the bi-amped system is split into low and high bands, each amp is more likely to pass the signal without clipping.

As we’ve seen, there are many aspects of amplifier power to understand. The more you know about the nuances, the higher your skill level as an audio professional.

Bruce Bartlett is a microphone engineer (http://www.bartlettmics.com),  an audio journalist, and a recording enginee (http://www.bartlettrecording.com). He is the author of “Practical Recording Techniques 5th Edition” and “Recording Music On Location”.