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Amplifiers, subs, enclosures and such.

Amplifier Gain

Setting the amplifier gain (level) is the most important aspect of any audio system and there are a few different methods for doing so. The absolute BEST method is to use an oscilloscope or spectrum analyzer. Because I am particularly lazy at the moment and don't really want to spend the next couple of hours typing instructions, I will leave you with this excellent link I found that explains everything you need to know to setup an amp with an o-scope.

http://www.bcae1.com/settinggainswithscope.htm.

Now since most people don't own or want to buy an o-scope, there is an alternative. But I must stress that this is ONLY a fallback and you should really try to seek out a professional to setup your amplifier.

You will need:

    ·   A multi-meter capable of measuring AC volts

    ·   A test tone CD (40-60Hz for bass systems, 1KHz for mid-range systems, link below)

    ·   A small screwdriver

Just for an example, I will use a mono amp that's 500 watts RMS @ 2 ohm with two 225 watt RMS, 4 ohm subs wired parallel. In the chart below, the number across the top are possible impedances (final ohms of sub(s) when wired together) and the number down the left side represent watts RMS. The numbers in the intersect of each is the expected AC voltage at the rated power.

1) Determine the RMS power output of one channel on your amplifier and the rated RMS input of your speakers. You will use the lower of the two in the chart below. Using the example, you would use  450 watts RMS (225 watts each sub) as the target setting for output of the amp since 450 is less than 500.

2) Match the final ohms of the connected sub(s) with the ohms in the chart.  Note the voltage at the intersect of your watts and ohms.

3) Turn the gain/sensitivity all the way down (to the highest number, usually counter clockwise)

4) Insert leads of multi-meter into one of the channels you’ll be using with the sub(s) connected (if you’re bridging the amplifier, use those terminals).

5) Set multi-meter to measure AC volts

6) Turn head unit on, turn off all filters (low-pass and high-pass), set your EQ settings to 0 (i.e. Bass, Treble, Mid) and turn off loudness.

7) Insert the test tone CD, set track to repeat (if available)

8) Set to the volume at 75/80% of your max volume on your deck.

9) Slowly turn the gain up until you obtain the voltage from the chart that you obtained in step 2.

10) Turn the HU off, disconnect the voltmeter and readjust your filters, EQ and other controls to your liking and you're done. I would recommend to not turn the bass boost up past +3dB.

 

Test tones: 

60Hz

1KHz 

 


 

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 Amplifier Class
 
 

Abstract: With the ever changing requirements in the audio market, there have been many advancements in audio amplifier topologies. Knowing the types of audio amplifiers available and the characteristics associated with them is essential in selecting the best audio amp IC for your application. 

Introduction 

An audio amplifier increases the amplitude of a small signal to a useful level, all the while maintaining the smaller signal's detail. This is known as linearity. The greater the amplifier linearity, the more the output signal is a true representation of the input.


With the ever-changing performance requirements for amplifiers in the audio market, there have been many advances in audio amplifier topologies. Consequently, designers must know the types of audio amplifiers available and the characteristics associated with each. This is the only way to ensure that you select the best audio amp for an application. In this tutorial, we examine the most important characteristics of each class of audio amp available today: Class A, Class B, Class AB, Class D, Class G, Class DG, and Class H.

Class A Amplifiers

The simplest type of audio amplifiers is Class A. Class A amps have output transistors (Figure 1) that conduct (i.e., do not fully turn off), irrespective of the output signal waveform. Class A is the most linear type of audio amp, but it has low efficiency. Consequently, these amps are used in applications that require high linearity and have ample power available.
Figure 1. A Class A audio amp is typically associated with high linearity but low efficiency.
Figure 1. A Class A audio amp is typically associated with high linearity but low efficiency.

Class B Amplifiers

Class B amplifiers use a push-pull amplifier topology. The output of a Class B amp incorporates a positive and negative transistor. To replicate the input, each transistor only conducts during half (180°) of the signal waveform (Figure 2). This allows the amp to idle with zero current, thereby increasing efficiency compared to a Class A amp.
There is a trade-off that comes with a Class B amp: the increased efficiency degrades audio quality. This happens because there is a crossover point at which the two transistors transition from the on state to the off state. Class B audio amps are also known to have crossover distortion when handling low-level signals. They are not a good choice for low-power applications.
Figure 2. With a Class B audio amp, the output transistors only conduct during half (180°) of the signal waveform. To amplify the entire signal, two transistors are used, one conducting for positive output signals and the other conducting for negative outputs signals.
Figure 2. With a Class B audio amp, the output transistors only conduct during half (180°) of the signal waveform. To amplify the entire signal, two transistors are used, one conducting for positive output signals and the other conducting for negative outputs signals.

Class AB Amplifiers

A compromise between Class A and Class B amplifier topologies is the Class AB audio amp. A Class AB amp provides the sound quality of the Class A topology with the efficiency of Class B. This performance is achieved by biasing both transistors to conduct a near zero signal output, i.e., the point where Class B amps introduce nonlinearities (Figure 3). For small signals, both transistors are active, thus functioning like a Class A amp. For large-signal excursions, only one transistor is active for each half of the waveform, thereby operating like a Class B amp.
Class AB speaker amps offer high signal-to-noise (SNR), low THD+N, and typically up to 65% efficiency. This makes them ideal choices as high-fidelity speaker drivers. 
Figure 3. A Class AB amp biases both transistors so that they conduct when the signal is close to zero. Thus, these amps provide more efficiency than Class A, with lower distortion than Class B.
Figure 3. A Class AB amp biases both transistors so that they conduct when the signal is close to zero. Thus, these amps provide more efficiency than Class A, with lower distortion than Class B.

Class D Amplifiers

 Class D amplifiers use pulse-width modulation (PWM) to produce a rail-to-rail digital output signal with a variable duty cycle to approximate the analog input signal (Figure 4). These amps are highly efficient (often up to 90% or higher) because the output transistors are either fully turned on or fully turned off during operation. This approach completely eliminates the use of the linear region of the transistor that is responsible for the inefficiency of other amplifier types. Modern Class D amps also achieve fidelity comparable to Class AB amps. 
Figure 4. A Class D audio amp outputs a switching waveform at a frequency far higher than the highest audio signal that needs to be reproduced. These amps are highly efficient because the output transistors are either fully turned on or fully turned off during operation.
Figure 4. A Class D audio amp outputs a switching waveform at a frequency far higher than the highest audio signal that needs to be reproduced. These amps are highly efficient because the output transistors are either fully turned on or fully turned off during operation.

Class G Amplifiers

Class G amplifiers are similar to Class AB amps, except that they use two or more supply voltages. When operating at low signal levels, Class G amps select a low supply voltage. As the signal level increases, these amps automatically select the appropriate supply voltage (Figure 5). Class G amps are more efficient than Class AB amps because they use the maximum supply voltage only when required; in contrast, Class AB amps always use the maximum supply voltage.
Figure 5. A Class G amp is more efficient than a Class AB amp because it uses the maximum supply voltage only when required.
Figure 5. A Class G amp is more efficient than a Class AB amp because it uses the maximum supply voltage only when required.

Class DG Amplifiers

The Class DG amplifier uses PWM to produce a rail-to-rail digital output signal with a variable duty cycle. In this respect, a Class DG amp is the same as a Class D amp. The Class DG amp, however, also uses a multilevel output stage to sense the magnitude of the output signal (Figure 6). It then switches the supply rails, as needed, to supply the required signal power more efficiently.
Figure 6. A Class DG amplifier senses the magnitude of the output signal and then switches the supply rails, as needed, to supply the required signal power more efficiently.
Figure 6. A Class DG amplifier senses the magnitude of the output signal and then switches the supply rails, as needed, to supply the required signal power more efficiently.

Class H Amplifiers

Class H amplifiers modulate their supply voltage to minimize the voltage drop across the output stage. Implementations range from using multiple discrete voltages to an infinitely adjustable supply. Though similar to the Class G technique of reducing dissipation across output devices, the Class H topology does not require multiple power supplies (Figure 7).
Class H amps are generally more complex than other audio amplifier designs. These amps require extra control circuitry to predict and control the supply voltage. 
Figure 7. A Class H audio amp reduces dissipation across the output devices connected to that supply. This allows the amp to operate with an optimized Class AB efficiency, regardless of output power level.
Figure 7. A Class H audio amp reduces dissipation across the output devices connected to that supply. This allows the amp to operate with an optimized Class AB efficiency, regardless of output power level.

Summary

This has been a brief survey of the many types of audio amplifiers commonly used in designs today. Clearly, when designing a system, care should be taken in determining the audio amplifier topology best suited to the application. A good understanding of these different classes of audio amps will help you choose the best audio amp for your design. 

 

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