The Ultimate Power Amplifier Buyer’s Guide 2021


So you’re wondering which power amplifier you should buy, rent or otherwise try out. In this comprehensive buyer’s guide, we’ll go through everything worth considering before you make any decisions about a power amp.

If you’ve found yourself asking, “which power amplifier should I buy?” this extensive resource is for you.

Please feel free to jump around this article and read all additional resources I have provided links to.

With that, let’s get into this comprehensive power amplifier buyer’s guide to help you in your next amp purchase!

Related article:
Top 11 Best Power Amplifier Brands In The World


Table Of Contents


What Is Your Power Amplifier Budget?

The first thing to consider when making any purchase is your budget. Money can be a touchy subject for some, and so I’ll keep this section brief.

I would never advise anyone to overspend on any audio equipment. Know what you can realistically afford, and do your best to stay within those limitations, whatever they may be.

Power amplifiers, like many audio devices, range significantly in price. The market is rather large, and so there should be a good selection for any budget.

Note that some retailers offer payment plans, which could be an option.

Consider the cost to benefit ratio of the purchase of the power amplifier. For example, if the power amp is needed for business, perhaps stretching the budget is more appropriate. On the other hand, if you don’t plan on making money with the amp, perhaps a more conservative budget is appropriate.

Also, consider any additional accessories or upkeep that may be required for your power amplifier.

Only you can determine your budget. All I’m here to say is that you should consider it.

Related My New Microphone article:
How Much Do Audio Amplifiers Cost? (With Pricing Examples)

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What Is The Intended Application Of The Power Amplifier?

There are plenty of audio systems that require power amplifiers. In fact, any system that utilizes speakers to produce sound will require a power amplifier. Though many speakers will have built-in power amps (these are known as active or powered speakers), this article will focus on external power amplifiers.

Thinking of sound systems that utilize speakers, we can list out a variety of different applications for power amplifiers. Notable examples include:

  • Home theatre and entertainment systems
  • Live venue PA systems
  • Car audio

Knowing what the intended application(s) are will help you in your search for the ideal power amplifier. Many power amplifiers on the market are designed for specific applications. We’ll discuss these topics further in the upcoming section Common Power Amplifier Applications.

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Understanding Power Amplifier Impedance

Impedance is a term that’s thrown around quite a bit in audio technology, and for a good reason: it’s an important specification. When it comes to power amplifiers, we should understand how the amplifier’s output impedance works with the input impedance of the connected loudspeakers. It’s also worth comprehending the input impedance of a power amplifier for a more complete picture.

As a physics primer, impedance is the measurement of a system’s opposition to alternating current when a voltage is applied. It’s made up of electrical resistance (DC portion) and reactance (AC portion) and is measured in ohms (Ω).

Though impedance is frequency-dependent, speaker impedance specifications are generally given as nominal values rather than as detailed graphs. Nominal impedance is the “average” designed impedance of a given audio device.

A power amplifier’s output impedance rating is generally given as a “rated impedance” that will match whatever speaker impedance rating the amplifier can drive safely. If the loudspeaker impedance is too high, the power delivered from the amp will drop. Conversely, if the loudspeaker impedance is too low, the higher power transfer may overload the speakers, causing damage to the speakers and even the amplifier itself.

The typical rated output impedance ratings are:

  • 1 ohm (common for car audio subwoofers)
  • 2 ohms (common for car audio subwoofers)
  • 4 ohms (common for subwoofers and fullrange speakers)
  • 8 ohms (common for subwoofers and fullrange speakers)
  • 16 ohms (common for instrument cabinet speakers)

The power output ratings of a power amplifier will often be relative to the connected load impedance of the speaker(s). More power is generally available for lower impedances, though we should be wary of overheating when too much power is demanded from low-impedance speakers.

As for the true output impedance of the power amplifier, it will be much lower than the rated impedance (generally one-tenth or less). For optimal signal/power transfer between the amp and the connected speakers, we need the load impedance (the speaker input impedance) to be much higher than the source impedance (the amplifier’s output impedance).

This is known as impedance bridging and can be understood with the following diagram and equations:

This image has an empty alt attribute; its file name is mnm_SourceLoad_Impedance.jpg

VL = VS • ZL / (ZL + ZS)

As for the input impedances of the power amplifier, we must consider the input types of the amp. Power amplifiers may feature line, instrument, phono or even mic level inputs through a variety of connectors (especially if it’s an integrated amplifier with a built-in preamplifier section). The impedances of each of these inputs should be optimized for whatever signal types they expect to receive. These impedances are much more standardized and shouldn’t be of concern to us.

To recap, the amplifier impedance ratings we’re concerned with are the rated output impedance values. Match these to the speaker(s), and we’ll achieve proper performance. Keep this in mind, as we’ll get to the critical section on Matching The Power Amplifier To Its Loudspeakers soon.

What Is Amplifier Impedance? (Actual Vs. Rated Impedance)

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Understanding Power Amplifier Output Power

The audio signal strength a power amplifier will output is measured in power (watts). This is perhaps the most important specification of a power amplifier (it’s even in the name) but is one of the most confusing. I’ll try to simplify power amp power specs as much as possible in this section.

Let’s start with the easy part. A greater output power specification means that the amplifier can output more signal power and, all else being equal, a signal that will translate to greater perceived volume.

But the way in which the power/wattage is given may vary. Some manufacturers give peak values, while others give RMS (root mean square), continuous, or other “types” of power ratings.

Let’s consider the differences:

  • Peak power: the maximum peak power output of the amplifier. This spec is not overly useful but does give an impressive wattage number.
  • Continuous power: the maximum continuous power the amplifier can output without overheating. Continuous here means an extended period of time, covering practical usage times for the amp.
  • RMS power: technically RMS only applies to voltage and current. RMS power generally refers to the continuous power.

Power output ratings will also generally be listed according to the rated load impedance. Power amplifiers will be able to drive lower load/speaker impedance with greater power, as is often shown in the power/wattage specifications.

Power amplifiers with two or more channels may also offer bridging. In bridge/bridging mode, two channels are configured into a single channel to drive a loudspeaker with more power than either of the individual channels alone.

As we’ll get to in the section on Matching The Power Amplifier To Its Loudspeakers, it’s important to have compatible amplifier power output specs and speaker power handling specs (along with impedance specs).

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How Much Power Do You Need?

To calculate how much power you need for your specific applications, it’s best to look at the speakers first and then choose a compatible power amplifier capable of driving them.

This can be a bit confusing, but let’s discuss it anyway to further our understanding.

It makes sense that an outdoor 100,000-person festival is going to need magnitudes more power than your local coffee shop open-mic night.

How much power you need for your application depends on several things, including the listening environment. A small room will need less power than a large room, and outdoor venues will need even more. Rooms with more acoustic treatment (acoustic deadening) or people will sound better but require more power to get to the same perceived volume.

The listening position(s) relative to the speakers is also important.

Now let’s dive into a few speaker specifications to understand why.

Speakers will have their own power handling and sensitivity rating. The power handling rating refers to how much power (peak, continuous, program, etc.) a speaker can handle without getting damaged. This power rating, as we’ve discussed, should be compatible with the amplifier.

Sensitivity ratings refer to how well a speaker converts amplifier power (electrical energy) into acoustic (mechanical wave) energy. Sensitivity specifications are generally given as decibels of sound pressure level per 1 watt of power at 1 measurement distance of 1 meter.

For every doubling of distance, the sound pressure level drops by half (-3 dB). For every identical speaker emitting the same level of sound at the same distance from the listener (assuming little to no phase cancellation), the sound pressure level will increase by +3 dB.

Take that information and combine it with the following table to gather a rough understanding of how much power you’ll likely need.

Here’s a table to suggest the safe listening times at defined sound pressure levels according to the NIOSH (National Institute for Occupational Safety and Health) and the OSHA (Occupational Safety and Health Administration):

NIOSH Standard (dBA)Equivalent Sound Pressure Level (at 1 kHz)Maximum Exposure Time LimitOSHA Standard (dBA)Equivalent Sound Pressure Level (at 1 kHz)
127 dBA127 dB SPL
44.8 Pa
1 second160 dBA160 dB SPL
2.00 kPa
124 dBA124 dB SPL
31.7 Pa
3 seconds155 dBA155 dB SPL
1.12 kPa
121 dBA121 dB SPL
22.4 Pa
7 seconds150 dBA150 dB SPL
632 Pa
118 dBA118 dB SPL
12.6 Pa
14 seconds145 dBA145 dB SPL
356 Pa
115 dBA115 dB SPL
11.2 Pa
28 seconds140 dBA140 dB SPL
200 Pa
112 dBA112 dB SPL
7.96 Pa
56 seconds135 dBA135 dB SPL
112 Pa
109 dBA109 dB SPL
5.64 Pa
1 minute 52 seconds130 dBA130 dB SPL
63.2 Pa
106 dBA106 dB SPL
3.99 Pa
3 minutes 45 seconds125 dBA125 dB SPL
35.6 Pa
103 dBA103 dB SPL
2.83 Pa
7 minutes 30 seconds120 dBA120 dB SPL
20.0 Pa
100 dBA100 dB SPL
2.00 Pa
15 minutes115 dBA115 dB SPL
11.2 Pa
97 dBA97 dB SPL
1.42 Pa
30 minutes110 dBA110 dB SPL
6.32 Pa
94 dBA94 dB SPL
1.00 Pa
1 hour105 dBA105 dB SPL
3.56 Pa
91 dBA91 dB SPL
0.71 Pa
2 hours100 dBA100 dB SPL
2.00 Pa
88 dBA88 dB SPL
0.50 Pa
4 hours95 dBA95 dB SPL
1.12 Pa
85 dBA85 dB SPL
0.36 Pa
8 hours90 dBA90 dB SPL
0.63 Pa
82 dBA82 dB SPL
0.25 Pa
16 hours85 dBA85 dB SPL
0.36 Pa

Here’s a table with common sound pressure levels to get a better idea of how loud certain dB SPL values are:

dB SPLPascalSound Source Example
0 dB SPL0.00002 PaThreshold of hearing
10 dB SPL0.000063 PaLeaves rustling in the distance
20 dB SPL0.0002 PaBackground of a soundproof studio
30 dB SPL0.00063 PaQuiet bedroom at night
40 dB SPL0.002 PaQuiet library
50 dB SPL0.0063 PaAverage household with no talking
60 dB SPL0.02 PaNormal conversational level (1 meter distance)
70 dB SPL0.063 PaVacuum cleaner (1 meter distance)
80 dB SPL0.2 PaAverage city traffic
90 dB SPL0.63 PaTransport truck (10 meters)
100 dB SPL2 PaJackhammer
110 dB SPL6.3 PaThreshold of discomfort
120 dB SPL20 PaAmbulance siren
130 dB SPL63 PaJet engine taking off
140 dB SPL200 PaThreshold of pain

It’s also critical that a power amplifier has enough headroom to perform without distorting the output signal. Headroom is effectively the difference between the absolute maximum output and the nominal, continuous output of a signal. Choosing an amplifier with more power than is necessary will allow the peaks of the signal to be amplified without distortion.

To learn more about the complex topic of decibels, check out my article What Are Decibels? The Ultimate dB Guide For Audio & Sound.

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Power Amplifier Number Of Channels

The number of channels a power amplifier has refers to the number of independent signal outputs. Each channel will have its own amplifier circuit and will act to amplify its own audio signal.

The two most common types of audio are mono and stereo. Therefore, we’ll most commonly find monoblock amplifiers (a single channel) or stereo amplifiers (two channels).

Monoblock amplifiers are the most simple and are often used for passive subwoofers in car, home, PA and studio applications. The components of a monoblock amplifier amplify a single channel rather than being shared between multiple channels. Monoblocks, therefore, are larger, heavier and more expensive (per channel).

A stereo amplifier has two independent channels for left and right audio within a single unit/chassis. Because the stereo amp signal paths share the same chassis, transformers and power supply, they are not as well isolated as monoblocks. This lack of isolation often causes some amount of crosstalk, resulting in more noise, interference and distortion in the signal path.

Additional channels are commonly added to car audio amplifiers. In car audio systems, each channel is often dedicated to its own speaker. This is also true of home sound systems, though multiple speakers may be connected to a single channel. In live PA systems, it’s common to have a single amp channel power multiple speakers.

Finally, we should discuss the power amplifiers capable of surround sound (this is common in AV receivers). These amps will have enough channels to accommodate certain surround sound formats (2.1, 5.1, 7.1, 7.2, 9.1, and so on). It’s important to note that, in most cases, the power available in these multi-channel amps is limited, and the fewer channels used, the more power will be available per channel. This drop-off in power as more channels are added is critical to note, though many manufacturers omit the exact drop-off.

What Is A Monoblock Amplifier? (Monoblock Vs. Stereo Amps)

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Power Amplifier I/O

Now that we’ve discussed the number of channels (each channel will have its own output) let’s focus on the input/output specifications of power amplifiers more generally.

Remember that the most basic power amplifier will be a monoblock circuit with a line level input and a speaker level output.

Power Amplifier Inputs

The line (or speaker) inputs of power amplifiers can come in a variety of connectors, including:

  • XLR 3-pin (balanced)
  • 1/4″ TRS (balanced)
  • XLR/TRS combo (balanced)
  • Terminal block, including Euroblock (balanced or unbalanced)
  • RCA coaxial (unbalanced)

Power Amplifier Outputs

The speaker (or line) outputs of power amplifiers can come in a variety of connectors, including:

  • Loose speaker wire
  • Neutrik speakON
  • Binding post pair
  • Terminal block, including Euroblock
  • RCA (unbalanced line level)

Other Power Amplifier I/O

Power amplifiers will also have a power input, which connects the amp to the power mains via an adapter.

If the amplifier has a preamplifier circuit as well, it is referred to as an integrated amp. These amplifiers will have additional I/O for mic, instrument, phono, and other inputs along with headphone, line and other outputs.

Integrated amplifiers will also oftentimes offer digital I/O, which is less common in strict power amplifiers. These digital connections include coaxial, optical, USB, and more.

Additionally, a power amp may have remote control capabilities with some sort of input for the remote.

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Matching The Power Amplifier To Its Loudspeakers

Now it’s time for perhaps the most important part of this article. Let’s discuss the process of connecting a power amplifier to its loudspeakers.

We’ve discussed impedance, power and channels. Now let’s put it all together to understand how to connect the power amplifier to its loudspeakers optimally.

Let’s begin with the channels. If we want to utilize all the amplifier channels, we’ll need at least as many connected speakers as we do channels.

Next, consider the amplifier’s power output (per impedance rating) and double-check that it’s compatible with the speaker(s) it’ll be driving. Note that the power ratings don’t have to match perfectly, but care should be given to avoid overloading either when they do not match.

For example, a 300W amplifier can drive a 200W speaker so long as the amp isn’t cranked. Similarly, a 200W amplifier can drive a 300W speaker so long as the impedance of that speaker, so long as the speaker’s impedance isn’t so low that it draws too much current out of the amplifier at a given gain value.

That brings us to impedance.

Check the impedance of the speaker and the power specs of the amplifier at the given impedance. If the speaker impedance is lower than the amp is rated for, consider another amplifier or another speaker. You could run into the situation where the low impedance speaker draws too much power from the amp, causing it to overheat. An overheated amplifier will either shut down and go into protection mode or, worse, have its components fried.

I should mention here that multiple speakers can be connected to a single amplifier channel. The speakers can be connected in series or parallel or a combination of the two. The overall load impedance the amplifier will drive changes depending on how the speakers are connected. Let’s discuss.

Wiring speakers in series requires connecting the positive output of the amp channel to the positive terminal of speaker A, and the negative terminal or speaker A to the positive terminal of speaker B (and so on for more speakers), before connecting the negative terminal of the last speaker to the negative terminal of the amp channel.

Two Loudspeakers Wired In Series

The combined resistance of the series speakers is as follows:

RT = R1 + R2 + … + Rn

Where n is the number of resistors in series.

The more speakers added in series, the greater the load impedance and the less power will be transferred from the amplifier.

Moving onto parallel, the positive output of the amp channel is connected to the positive terminal of speakers A, B, and so on. The negative terminals of the speakers and then connected to the negative terminal of the amplifier channel.

Two Loudspeakers Wired In Parallel

The combined resistance of the parallel speakers is as follows:

1 / RT = (1 / R1) + (1 / R2) + … + (1 / Rn)

Where n is the number of resistors in parallel.

The more speakers added in parallel, the lower the load impedance and the more power will be transferred from the amplifier. This can be dangerous if the impedance drops below what the amplifier can handle.

Speakers can be connected in a combination of series and parallel. The calculation becomes a bit more difficult, but more speakers can be added so long as the resulting impedance stays within the rated range of the amplifier.

Ideally, all speakers should have the same impedance specification. Start by calculating the impedance of the groups of speakers connected in series. With these impedances, use the formula to find the overall load impedance with the parallel formula.

Why Do Speakers Need Amplifiers? (And How To Match Them)
Complete Guide To Power Amplifier Specifications & Data

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Tube Vs. Solid State Power Amplifiers

The gain stage of a power amplifier could be based around a tube, a solid-state circuit, or a hybrid combination of the two.

Tubes tend to saturate the more you push them (the more gain that’s applied). This can be incredibly pleasing to the ear when done right. Tube power amps are often more colourful and smooth. Note that tubes are also relatively fragile and are sensitive to temperature.

Solid-state gain circuits are often more clean, clear or clinical, though they can also have their own sonic character. These circuits are often preferred when less sonic colouration is wanted. Solid-state circuits are much less sensitive to temperature.

Tube power amplifiers are pretty rare outside guitar amps. However, the distinction is worth mentioning.

Solid-State Vs. Tube Amplifiers (Pre, Power & Guitar Amps)

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Power Amplifier Classes

Power amplifiers come in a variety of designs regarding the actual gain/amplification circuits. The different “types” of power amplifiers are typically denoted by class.

Let’s talk about the popular and not-so-popular amplifier classes in this section:

Class A Power Amplifiers

Class A amplifiers use one output switching transistor (Bipolar, FET, IGBT, etc.) in their circuit design, which conducts current over the full 360 degrees of the input cycle.

When designed properly, class A amps offer superb linearity, high gain and low signal distortion. The drawback is that class A amplifiers lose a lot of their power as heat and have very low efficiency ratings around 30%. This class is more popular with preamps than power amps.

Class B Power Amplifiers

Class B amplifiers use two complementary transistors (either bipolar or FET) in their circuit design. Each transistor amplifies half of the input cycle with a “push-pull” configuration. When the signal polarity is positive, one transistor conducts, and the other is off. When the signal polarity is negative, the other transistor conducts while the first is off.

Class B amps offer superb linearity and good gain when designed properly but do produce distortion at the zero-crossing point. Class B amplifiers lose some power as heat and have efficiency ratings around 50%. This class is also more popular with preamps than power amps.

Class AB Power Amplifiers

Class AB amplifiers use two complementary transistors (either bipolar or FET) in their circuit design. Each transistor is mostly responsible for its own half of the input cycle. However, to reduce distortion, both transistors are biased so that they both conduct near the zero-crossing point.

When designed properly, class AB amps offer superb linearity and good gain, and low signal distortion. Class AB amps typically have efficiency ratings between 50% to 60%.

Class C Power Amplifiers

Class C amplifiers have high efficiency ratings around 80% at the expense of distortion. These circuits are not used in audio power amplifiers.

Class D Power Amplifiers

While the amplifier types mentioned above are considered linear designs, class D is the most popular type of “non-linear” switching amplifier. It utilizes pulse-width modulation, pulse-density modulation, or delta-sigma modulation, which utilize the transistors’ on/off switching capabilities.

The time average power value is effectively tied to the input signal. As power is applied, the class D amplifier can effectively control a higher output voltage with the input voltage.

Class D amplifiers can theoretically reach 100% efficiency and do not produce nearly as much heat as their linear counterparts. They also benefit from reduced size and weight while maintaining superb audio quality.

Lesser-known switching amplifier types include:

  • Class F
  • Class G
  • Class I
  • Class S
  • Class T

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Built-In Amplifier Limiters, Fuses & Safety Mode

A power amplifier may be designed with a built-in limiter, fuse and/or safety mode feature.

Limiters effectively set a maximum signal level that will not be exceeded. A power amp limiter will keep the output from surpassing this limit, thereby protecting the connected loudspeakers from overload. This helps prevent clipping but may also affect the audio. It will certainly help save amplifier and speaker components from overloading.

A fuse is an electrical component that also acts as a safety feature. It’s essentially a metal wire or strip that melts when too much current flows through it, thereby stopping or interrupting the current. If a power amplifier outputs a signal beyond its limits or starts overheating, its fuse will blow, and the amp will stop working. This has the potential to save the amp and even the speakers.

Safety mode or protection mode extends the idea of a fuse even further, offering more triggers to safely shut the amplifier down if it’s in danger. DC, infrasonic, thermal overload and short circuit protection are common safety features to look for.

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Common Power Amplifier Applications

Let’s consider a few common applications for power amplifiers and discuss what we should look for in each:

Home Audio Systems

Home audio systems definitely need power amplifiers to drive their speakers.

From simple stereos to Hi-Fidelity playback systems to home theatres with surround sound, a power amp will be required.

These power amplifiers are often part of integrated amps, which have preamps to amplify mic, instrument, phono, and other low signals to line level before reaching the power amp.

They are also commonly included in audio-video receivers, which process audio and video for entertainment systems. AV receivers are often capable of surround sound and have output channels to allow for these immersive formats.

Live Sound & PA Systems

Live sound and PA systems (commonly referred to as “pro audio”) utilize power amplifiers to drive their speakers. In larger setups, it’s often best to use passive speakers, which means external power amps will be required.

With large PA systems, careful planning is often used to connect several speakers to individual power amplifier outputs and achieve more output. That being said, large venues will rely on a great number of power amplifiers. Mains, subwoofers, and monitors tend to have their own sets of power amps.

Car Audio

Car audio utilizes power amplifiers to drive the speakers. In car audio, it’s not uncommon to have multi-channel amps driving the separate speakers of the car. This helps to control the front/back, left/right parameters with the car stereo. It’s also typically the case the subwoofer(s) each have their own monoblock amp.

Top 11 Best Car Audio Amplifier Brands In The World

Instrument Amplifiers

Instrument amplifiers are often designed with preamps and power amps, which technically makes them integrated amplifiers.

However, instrument power amps are useful to know about. They often drive separate cabinets or are part of an amp/cabinet combo, known aptly as “combo amps.”

These amps generally drive a single speaker, which is usually rated at 4, 8 or 16 ohms.

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A Note On Powered Speakers

Note that powered and active speakers are also very common. These speakers have built-in power amps and do not require external power amplifiers. Common examples include studio monitors (though there are certainly passive studio monitors), powered PA speakers, powered subwoofers, computer speakers, portable Bluetooth speakers, soundbars, and more.

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Know The Additional Costs Of Power Amplifier Accessories

Speaker Cables

Power amplifiers output strong speaker level signals. These signals require thicker gauge speaker cables to safely transfer them from the amplifier output to the speaker input.

The specific gauge of the speaker cable/wire is dependent on the intended speaker level signals. Use thicker gauge (lower number) cable for long wire runs, high power applications, and low-impedance speakers. Thinner gauge (higher number) cable is typically fine for short runs, low power applications and high-impedance speakers.

Consider the speaker cable connectors as well, ensuring the cable will connect to the amplifier and speaker properly.

Rackmount Cases

If you opt for a rackmount power amp (or power amps), you may want to purchase a designated rack to host it/them.

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This article has been approved in accordance with the My New Microphone Editorial Policy.

Arthur

Arthur is the owner of Fox Media Tech and author of My New Microphone. He's an audio engineer by trade and works on contract in his home country of Canada. When not blogging on MNM, he's likely hiking outdoors and blogging at Hikers' Movement (hikersmovement.com) or composing music for media. Check out his Pond5 and AudioJungle accounts.

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