How To Set Up a Power Amp

So you've taken the leap and opted for separates, and now you're wondering how to set up the power amp properly with you're A/V preamp/processor. Relax, it's not difficult at all.

There are two types of connections on any power amp—inputs that accept line-level signals from the preamp/processor and outputs that send the amplified signal to the speakers. Let's start with the inputs.

Amp Inputs
Does your amp have balanced and/or unbalanced inputs? An unbalanced connection uses an RCA jack, and you would use cables with RCA jacks on both ends to connect your pre/pro to the amp.

A balanced connection typically uses XLR connectors, which are relatively large round connectors with three pins (male) or three holes (female) in a triangular formation as shown above. This same type of cable used with professional microphones.

An unbalanced audio connection has two conductors—one carries the signal and the other is the shield/ground. With a balanced connection, there are three conductors—one is the shield/ground, while the other two carry the audio signal, and one is out of phase with the other. A balanced connection reduces the risk of induced noise from power cords, radio-frequency interference (RFI), and electromagnetic interference (EMI). By the way, if you are going to the expense of a balanced system, make sure it is balanced throughout the amp and not just at the inputs.

In a nutshell, here is how a balanced connection works. As I mentioned above, the two signal conductors carry the same signal, but one is out of phase with the other. Any induced noise that is picked up along the cable will be at the same phase in both conductors. At the amp's input, a differential amplifier recognizes and rejects any signal that is in phase in both conductors, leaving only the intended audio signal.

Whether your amp uses balanced or unbalanced connections, connect each output channel from the pre/pro to the corresponding input on the amp. If your pre/pro and amp are very close to each other, unbalanced connections are usually fine, since the cable lengths are short, so there's not much chance of induced noise.

On the other hand, if both units offer balanced connections, you might as well use them—if not for noise rejection, then to possibly reduce the chance of ground loops, which can cause an annoying buzzing or humming sound. Ground loops occur when the grounds of two connected devices are connected along two different paths—the audio connection and something else, such as the AC cords—and those two ground connections are not at the same voltage. Using balanced connections can help reduce the risk of ground loops, but only if each device's ground is properly implemented.

If your pre/pro has balanced outputs but your power amp has unbalanced inputs (or vice versa), you might be tempted to connect them using some sort of adaptor. However, this is generally not recommended, because you lose any noise rejection and ground-loop isolation.

If you must connect balanced to unbalanced, you can use a transformer to isolate all unbalanced connections, as shown above. Various manufacturers, such as Rane, make boxes for this purpose. Otherwise, you need to make special cables as described in this excellent article about audio-system connections from Rane.

Speaker Outputs
Because most speaker cables are not shielded, they should be as short as possible to minimize the chance of picking up noise along the way. However, some argue that they should all be the same length so their electrical characteristics—impedance, capacitance, and inductance—are equal, minimizing the chance that these characteristics will affect the sound differently from different speakers.

If you subscribe to this argument, we suggest using cables of equal length for the front left, center, and right and cables of longer but equal length for the surrounds, since the surrounds are likely to be farther away from the amp. On the other hand, this means that some of the cables will be longer than they need to be, and the excess length could lead you to coil them, which can change the overall capacitance and inductance, not to mention increasing the chance of picking up induced noise. In reality, the difference in sound from cables of different lengths is probably negligible, so we believe it's best to use cables of just the right length from the amp to each speaker.

Depending on the type of connectors used for the amp's outputs and the speakers' inputs, the speaker cables can be terminated with banana plugs, spade lugs, or simply bare wire. If the cables have spade lugs, make sure they fit on the amp's binding posts—some spade lugs are too small for some binding posts.

Most importantly, make sure the polarity is the same for all speaker connections. All speaker cables have two conductors that are clearly distinguishable from each other—in many cases, one is marked with a red tag and the other is marked with a black tag at both ends. Similarly, the speaker outputs on most amps and the inputs on most speakers are color-coded red and black.

Make sure that the color-coding of all connections is consistent—red is connected to red and black is connected to black. It really doesn't matter if the colors match, only that they are all the same orientation, though matching the colors is much easier to keep track of. (On the amp pictured at the top of this article, each output is coded with a different color for each positive terminal, but the negative terminals are all black; in such a case, connect the red-coded speaker-cable conductors to the colored terminals and the black-coded conductors to the black terminals. Then, at the other end of the cable, connect the red conductor to the red terminal of each speaker and the black conductor to the black terminal.)

Gain Structuring
Does your amp have individual trim controls—essentially small volume knobs—for each channel? If not, don't worry about it; this is a feature that's losing favor, especially among more mainstream manufacturers.

If your amp does have trim controls as shown above, one common approach is to crank them all the way up, since you'll be using the volume control on your pre/pro to regulate amplitude. However, depending on the amp, this can introduce a lot of noise into the audio signal.

Here is a quick procedure to ensure the trim controls are set optimally. This process is called gain structuring, which maximizes the gain (amplification) while minimizing noise.

First, set all the trim controls in their lowest or minimum position (usually, that's fully counterclockwise). Make sure the volume on your pre/pro is at its lowest setting, or better yet, just turn it off. In fact, you can do this even before you connect the amp to your pre/pro. However, you will need to connect your speakers to the amp and turn it on.

One by one, turn up the trim controls until you hear hiss in that channel's speaker. At this point, the gain is high enough that you can hear the amp's internal self-noise. Back off the trim until the hiss is gone. After doing this for all channels, the trim controls should be in roughly the same position; if one is very different from the others, there's probably something wrong with that channel.

That's all there is to it. Once you have set up your amp, it's time to kick back and enjoy the sound of your new separates.

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COMMENTS
K.Reid's picture

Scott/Kim:

What about RCA and speaker cable direction? If I recall, many manufacturers of cables include arrows for which direction the cable should be connected a given component to maximize signal transfer. Any comments or thoughts on this...

Scott Wilkinson's picture
I don't believe it makes any difference which direction the cable is connected. Audio signals are AC—alternating current—so electrons travel in both directions along the conductive pathway. Also, nothing physically moves from an output to an input; the alternating voltage at the output causes the electrons in the conductor to jiggle back and forth around a stationary point. If there were an inherent directionality in the cable, the electrons would have an easier time moving in one direction than the other, which, I imagine, would degrade the audio quality.

I have no idea why some manufacturers specify a direction on their cable jackets. Normally, such things are marketing hype, but I see no marketing benefit to this.

loop7's picture

Great article Scott. I had been wondering about the trim controls on my Parasound and if they should be tweaked. Now I know how to do it and why.

K.Reid's picture

Good to know, Scott. I guess Audioquest must be one of those manufacturers among others because I'm looking at my new Syndney cables I brought and sure enough there are those arrows. I may make an inquiry to Shane Buettner if he is still at Audioquest for comments as to why the company opts to do this. Your reasoning certainly sounds logical to me.

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