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By Gunter Fellbaum, September 15, 2011 12:51 pm

In this article, I will explain the term “total harmonic distortion”, often also called “THD” which is frequently used to describe the performance of wireless outdoor speakers.

Looking for the perfect model from the huge amount of products, you may have a hard time understanding some of the technical language and terms that you will find in the specifications of today’s wireless speakers. THD is usually not as easily understood as some other commonly used terms such as “signal-to-noise ratio” or “frequency response”.

In short, “harmonic distortion” describes how much the audio signal is being degraded as a result of the speaker or in other words how much the signal differs from the original signal. There are two common ways to express harmonic distortion, either in percent (%) or in decibel (dB). If a speaker specifies a distortion of 10% for example then one tenth of the energy radiated by the speaker is distortion. A distortion of 10% can also be expressed as -20 dB. 1% distortion equals -40dB.

A wireless speaker actually has several components which contribute to harmonic distortion. One of these is the built-in power amplifier. This amplifier is driving the speaker element. Usually the higher the amplifier is driven the higher the amount of amplifier distortion. For this reason, some manufacturers will list amplifier distortion depending on amplifier power.

Distortion specs for different power levels are usually given for several power levels or as a diagram showing distortion versus output power. Both of these methods allow to better evaluate the quality of the amp.Normally distortion is measured with a 1 kHz test tone. However, amplifier distortion will usually increase with increasing frequency, especially in digital class-D models.

The second contributing factor is the loudspeaker element itself. Most speakers use a diaphragm type driver which is driven by a coil that is suspended in a magnetic field. The coil will follow the magnetic field which is controlled by the audio signal to move the diaphragm. However, this movement is not perfectly linear. This results in the signal being distorted by the speaker element itself. Also, the larger to power level with which the speaker is driven, the larger the distortion. Often speaker manufacturers will show distortion for small to moderate power levels only.

As such both the amplifier as well as the speaker element itself contribute to distortion. In addition, there are other factors which also contribute to distortion. The total amount of distortion is the sum of all of these factors. Depending on the material used to build the speaker enclosure, there will be vibrations or box resonances. These usually depend on the sound pressure level, the box shape, the enclosure material and audio frequency. Therefore additional sound distortion will be caused by the box itself.

Total distortion is best determined by measurement. A signal generator is used which provides a highly linear sine tone to the speaker. The sound is picked up by a measurement microphone. The microphone signal is then analyzed by an audio analyzer. The audio analyzer will compute the amount of higher harmonics or distortion. Intermodulation distortion analysis is another method which gives better picture of the speaker distortion performance with real-world signals by using a test signal with two harmonics and measuring how many harmonics at different frequencies are produced by the speaker.

Another factor contributing to distortion is the signal transmission of wireless speakers, especially with models that transmit an analog signal at 900 MHz. More advanced models use digital audio transmission. Typically these transmitters work at 2.4 GHz or 5.8 GHz.

You can get further details about wireless speakers as well as wireless surround sound speakers from Amphony’s website.

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By Gunter Fellbaum, September 10, 2011 12:47 pm

Demands regarding audio power and audio fidelity of today’s speakers and home theater systems are constantly growing. At the heart of these systems is the audio amplifier. Today’s audio amps have perform well enough to meet these ever growing demands. With the ever increasing amount of models and design topologies, such as “tube amps”, “class-A”, “class-D” and “t-amp” designs, it is becoming more and more difficult to pick the amp which is ideal for a particular application. This article will explain some of the most common terms and clarify some of the technical jargon which amplifier manufacturers often use.

An audio amp will convert a low-level audio signal which often comes from a high-impedance source into a high-level signal which can drive a loudspeaker with a low impedance. Depending on the type of amp, one of several types of elements are used to amplify the signal such as tubes and transistors.

Tube amps were commonly used a few decades ago and utilize a vacuum tube which controls a high-voltage signal in accordance to a low-voltage control signal. Tubes, however, are nonlinear in their behavior and will introduce a fairly large amount of higher harmonics or distortion. A lot of people prefer tube amps because these higher harmonics are often perceived as the tube amp sounding “warm” or “pleasant”.

One drawback of tube amps is their low power efficiency. In other words, most of the energy consumed by the amp is wasted as heat rather than being converted into audio. Therefore tube amps will run hot and need sufficient cooling. Tube amps, however, a fairly expensive to make and therefore tube amps have mostly been replaced with amps using transistor elements which are less expensive to manufacture.

Solid-state amps use a semiconductor elements, such as a bipolar transistor or FET in place of the tube and the earliest type is known as “class-A” amps. The working principle of class-A amps is very similar to that of tube amps. The main difference is that a transistor is being used in place of the tube for amplifying the audio signal. The amplified high-level signal is sometimes fed back in order to minimize harmonic distortion. If you require an ultra-low distortion amplifier then you might want to investigate class-A amps since they offer amongst the lowest distortion of any audio amps. However, similar to tube amps, class-A amps have very low power efficiency and most of the energy is wasted.

To improve on the low efficiency of class-A amps, class-AB amps employ a series of transistors which each amplify a separate area, each of which being more efficient than class-A amps. Due to the higher efficiency, class-AB amps do not require the same amount of heat sinks as class-A amps. Therefore they can be made lighter and cheaper. When the signal transitions between the two separate regions, however, some amount of distortion is being generated, thus class-AB amps will not achieve the same audio fidelity as class-A amps.

Class-D amps improve on the efficiency of class-AB amps even further by using a switching transistor which is constantly being switched on or off. Thereby this switching stage hardly dissipates any power and thus the power efficiency of class-D amps usually exceeds 90%. The switching transistor is being controlled by a pulse-width modulator. The switched large-level signal has to be lowpass filtered in order to remove the switching signal and recover the audio signal. Due to non-linearities of the pulse-width modulator and the switching transistor itself, class-D amps by nature have amongst the highest audio distortion of any audio amplifier.

More recent audio amps incorporate some sort of mechanism to minimize distortion. One approach is to feed back the amplified audio signal to the input of the amp to compare with the amplified signal. The difference signal is then used to correct the switching stage and compensate for the nonlinearity. “Class-T” amps (also called “t-amp”) use this type of feedback mechanism and therefore can be made extremely small while achieving low audio distortion.

You can find further information regarding t amps as well as stereo amplifiers from Amphony’s website.

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By Gunter Fellbaum, August 26, 2011 12:59 pm

Modern amplifiers come in all shapes and sizes. Finding the perfect model for your application can often be tricky. There is a flood of different names and terms describing audio amps, such as “Class D” or “T-amp“. Also, each manufacturer publishes a large number of specifications, including “SNR”, “dynamic distortion” etc. In this article, I will have a closer look at one of the most fundamental of these terms: amplifier output power. This term is also known as “amplifier wattage”.

Amplifier specifications are sometimes hard to interpret since they are not fully standardized. As such it may be difficult to figure out the true performance of the amp simply by looking at the specs. One good method though is to do a listening test before purchasing your amp. During this test you ideally want to set up the amp in a similar environment as your application. Now I will give some details about “amplifier wattage”. This spec is often misunderstood. It is important to look fairly closely at how the manufacturer shows this parameter.

“Wattage” is sometimes also known as “Power” or “amplifier output power”. To put it in a nutshell, “wattage” relates to how high the amp can drive your speakers. The higher this number the louder your speakers. You want to pick the amplifier wattage based on how large your listening environment is. For best audio quality, you may want to go with an amp that has higher power than you need since many amps will show increasing distortion as the audio power goes up.

Wattage is either given as “Watts peak” which means the amp can produce short burst of this amount of power or “Watts rms” which show how much power the amp can deliver for a longer period of time. In the past, vendors have usually preferred listing the “peak power”. This number is higher than the average or “rms” power. However, “peak power” can often be misleading since there is no standard showing the amount of time that the amplifier has to be able to deliver this amount of power.

Today most amps will specify rms power which gives a better indication of the amps true performance. However, please ensure that your amp has enough headroom to avoid clipping of the audio. Having enough headroom is essential since music signals differ a great deal from sine wave signals which are used to measure rms power. Short bursts of high power are often found in music signals. These bursts will drive the amp into high distortion unless the peak power is high enough.

Please note that often the peak power of the amp will depend on the impedance of your speakers which is typically between 4 and 8 Ohms. Due to the limited amplifier supply voltage, the maximum output power will be half if you connect an 8-Ohm speaker of the peak power that the amp can deliver to a 4-Ohm speaker. Usually maximum power is specified for a 4-Ohm speaker impedance. However, ideally the manufacturer will tell which speaker impedance the power rating is being referenced to. Please note that some amplifiers cannot drive speakers with very low speaker impedance.

You can find more information about modern t-amps in addition to stereo amplifiers at Amphony’s website.

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By Gunter Fellbaum, August 18, 2011 7:29 am

Choosing the perfect audio amplifier can be overwhelming given the large number of different types and technologies. I will help you understand some of the basic amplifier jargon terms. This knowledge will help you choose the perfect amplifier.

Some audio sources such as surround-sound receivers already come with a built-in audio amplifier. Using a separate amplifier, however, offers some advantages. One advantage is having more flexibility and being able to pick an amplifier that is best suited to drive a particular speaker. Some of today’s mini amplifier models can be hidden virtually anywhere due to their small size.

Output wattage is one of the factors that most people will consider first when selecting an amplifier since driving low-sensitivity speakers or driving speakers in large rooms or outdoors requires fairly high wattage. However, be careful not to overdrive your speakers. This can damage your speakers beyond repair.

But don’t be overly concerned about wattage. An amp supplying 20 to 50 Watts will give you more than plenty of power to sufficiently drive a speaker in any but the largest room.

Amplifier wattage is given either as rms or continuous power which shows how much power the amplifier can deliver continuously or as maximum or peak power which shows how much power the amp can output for short periods of time. You should choose an amplifier that has sufficient rms power, although 10 to 30 Watts should be plenty for most applications but has enough headroom for peaks that occur in music signals.

Audio quality is just as important as having enough audio power and therefore you should also look at figures such as total harmonic distortion or THD. No amplifier is perfect. There will be some amount of signal distortion which is caused by the amplifier. The amount of distortion is expressed in percent or dB. A smaller distortion number means a lower distortion of the audio signal by the amplifier. The amount of distortion will be less than 0.05% for a high-quality amplifier. Consumer type amplifiers have higher distortion figures. Typical figures are up to 10% depending on the output power.

Another specification is called signal-to-noise ratio. This number specifies how much undesired components such as hum and noise the amplifier will add to the audio signal. Consumer-grade amps will have a signal-to-noise ratio (SNR) of at least 80 dB. Higher-end amps will have an SNR of 100 dB or more. Audiophile amps will have an SNR of close to 120 dB.

The majority of today’s amplifiers are based on a technology called Class-D which offers higher power efficiency than amplifiers based an Class-A or Class-AB technologies. High efficiency means that little power is wasted as heat which can make the amplifier fairly small. Some of today’s mini amplifiers are no larger than a deck of cards. However, when picking a Class-D (digital) amplifier, be sure that the amplifier has low distortion and a high signal-to-noise ratio.

Gunter Fellbaum has been developing audio and electronic products for over 10 years. You can find further information about t-amp models as well as stereo amplifiers from Amphony’s website.

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By les scammell, April 10, 2009 3:51 am

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