Choosing a studio power amplifier

We all need an audio amplifier to drive our loudspeakers so we are going to discuss the different types available. This is a music site so we will keep this focused on the needs of a project studio. For a typical home studio I recommend an amplifier with around 150W per channel into 4 or 8 Ohms (speaker impedance) 150 Watts per channel will be able to drive most 6.5 inch driver near field monitor systems.When you check out an amplifier try and avoid fan cooled ones as they will add to the ambient noise floor of your studio and this is not desireable. Amplifiers up to 150 watts are typically using passive heatsink cooling without cooling fans.

The easiest solution would be to go to a music technology shop and you will find such amplifiers for around $300.00 There are a couple of standard models which will suit your needs. Most of these amplifiers have separate left and right volume controls and also have a variety of input connectors. Typically on the input side you will find balanced XLR inputs (male) and also TRS Jack connectors. Some may even have a set of phono (RCA) connectors for use with domestic equipment such as CD players. Most people will be connecting their main sound card (audio interface) outputs to the inputs of the amplifier or alternatively an analog mixer output. (usually control room monitor output connections)

The output of the amplifier may have binding posts which accept copper speaker cables directly (as found on the rear of loudspeakers), banana plugs, or possibly Speakon (TM) connectors. The Speakon is a design of connector especially for loudspeaker connectivity.

The specifications of an amplifier are many… it is a good idea to familiarize yourself with some of the basic ones:

Power output : Look for amplifiers with at least 150Watts per channel RMS - RMS is important as it provides a good standard to measure the continuous output power of an amplifier. You often see other ratings such as PMPO but these are not good standards to rely on for power output.

Distortion figures on amplifiers can vary wildly because there are numerous ways of measuring distortion. Some use a 1kHz tone at full rated power others provide a figure based on some other given output power. To confuse things further the rating maybe given as a THD + N (noise) figure so you may have to do some comparative and investigative work when comparing specs.

Two other important specifications to check are the slew rate and damping factor. The slew rate of a power amplifier relates to it’s capability to track (accurately follow) transients in the input signal at the amplified output. The slew rate will be shown as volts/uS (Microseconds) The higher the better in general.This relates to the “punch” an amplfier can deliver. You want an amplifier that can repreduce drum “attacks” well, if it does not it is a form of distortion, such amplifiers can be described as “smooth” sounding and that is not what we want for music production, we require accuracy.

Damping factor relates to how well the amplifier can control the woofer speaker (at it’s natural resonant frequency) cone after it has moved it. Good damping factor specifications mean better controlled speakers and more accurate reproduction. (less distortion)

Ultimately it’s in the listening, try and listen to your new amplifier on the speakers you will be using. I like to do a few tests of my own with amplifiers.

1) I like to hear virtual silence with my ear close to the tweeter with the amplifier on without any music playing. I do not like to hear noise (hiss) or any power supply hums.

2) I do not like to hear clicks and thumps as the amplifier switches on (so check the amplifier has anti-thump relays) These mute the amplifier output when the power switches on, it stops the initial power supply surges reaching your speakers. Also listen to the amplifiers physical casing for any acoustic hums some amplifiers make and annoying humming sound so listen to it’s case.

3)When evaluating audio fidelity listen to clarity, high frequency detail, ability to reproduce transients (drum attacks etc.) and overall tonal balance and presentation.

Choosing an amplifier is good fun and you should try and listen to a few if possible so you can hear the differences for yourself. Loudspeakers tend to vary much more in terms of sonic presentation but amplifiers do shows some differences to. Happy listening.

 
Written by Barry Gardner, professional mastering engineer at SafeandSound mastering based in London UK.