Updated: 10-12-2017
Whether you want to buy a class D amplifier board to be used for a particular purpose or you just want to test a class D amplifier chip, the easiest and cheapest is often to buy an amplifier board with that chip and then modify the board as needed. Today almost all class D amplifier chips are supplied in packages for SMD mounting. SMD mounting is intended for robot assembly and then the production costs of the board are lowered considerably. But, SMD package outlines are often so small hat you need to make a PCB with good (rather professional) tools because the tolerances are very small. So, traditional breadboarding is very difficult.
Choosing an amplifier board is normally a matter of understanding the information you may find for the amplifier chip and the implementation on the board from the Internet.
First, decide what are the most important qualities you want: High quality sound (low THD)? Output power level? For DIY modification or use "as is"? Cooling possibilities? Price level? Find a realistic compromise and do not just want all to be the best.
Then understand that there are certain components the manufacturers try to save on:
- The output filter is often of less good quality and with insufficient capacity. Good output filter chokes are costly and may take up a lot of space on the PCB. Small chokes may work fine at lower output levels when their limits are not yet reached. For proper design the output filter chokes must be able to handle the full peak output current without saturation. Further, the more current margin an output filter choke has, the less it is driven into non-linearity. Chokes with a current ratings of between 7 and 20 ampere are normally what is needed. Toroidal core chokes are suited as they have a high magnetic storage capacity. Chokes with enclosed magnetic design are better due to the reduced magnetic leakage field and shielding abilities but high capacity ones are expensive.
- The output filter capacitors should be of a foil type with much better linearity than ceramic capacitors. Ceramic SMD capacitors are, however, often used because they can be mounted by robot and require no manual work.
- Likewise, the input coupling capacitors should be of the foil type for better linearity and in particular to avoid piezoelectric noise. The efficient dielectrics used for SMD capacitors, such as X7R, are piezoelectric and work like a microphone. A microphone picking up ambient noise is not needed at the input of an amplifier for audiophile use.
- The power supply line decoupling capacitors are used to store the energy needed by the amplifier to handle powerful transients in the bass. More decoupling capacitors put in parallel improve performance as the resulting ESR is lowered. The capacitance needed depends on how quickly the power supply reacts when the voltage on the decoupling capacitors starts to drop. Ideally the voltage for an amplifier should not drop but some hundreds of millivolts are acceptable. If the decoupling capacitors are supplied directly from an ordinary net-transformer the demands are higher as the decoupling capacitors are only recharged during the peaks of the sine-wave. For a 50 Hertz net and with double rectification the decoupling capacitors are charged every 10ms. The voltage across a 10000uF capacitor drops 1 Volt in 10ms with a load current of 1 Ampere. For such net-transformer feed power supplies the 100Hz ripple must be accepted to be larger. With symmetric power supplies generating both a positive and negative voltage, current is only drawn from either the positive or negative line at a time. Therefore, the net-transformer is only dimensioned to handle the average of the load current and rely on the decoupling capacitors to compensate for temporary needs. As a rule of thumb, some 4700uF to 30000uF are needed for decoupling of the power supply line(s) for amplifiers up to 2x60 Watt.
- Finally cooling of the amplifier chip. A number of class D amplifier boards do not provide sufficient cooling of the amplifier chips. Good heat-sinks take up a lot of space, are heavy and costly. Also, for class D amplifiers there seems to be a mantra that they do not need cooling and have thermal protection anyway. Relying on the thermal protection circuit is not proper design and influences both immediate performance and reliability. High temperatures affect also the other components on the board and their performance is normally temperature dependent as well. The temperature should be kept under control and normally not exceed some 50-60 degrees Celsius. Account has to be taken of if the amplifier board will be enclosed in a casing where ventilation may be limited and the air temperature can build up.
Amplifier boards, in particular class D amplifier boards, are sold with the same amplifier chip but at different price levels. The differences in price normally relate to the quality of the remaining components used on the board. Which one you should choose depends largely on how much you are willing to do yourself in modifying the board for good operation and how much money you are ready to spend. Even if you are prepared to do modifications yourself there are situations where a well equipped board is available and not so much more expensive, compared to a cheap board, that the difference is obviously worth the higher cost. It also depends on your level of perfectionism.
You can get a lot of information about an amplifier board sold on the Internet from an else rather sparse description. In particular the photos are useful.
For a start find out what chip is used (normally specified for an amplifier board). Look for the data-sheet of that chip on the Internet and get an idea about its potential performance. In particular THD level, maximum supply voltage for operation, quiescent current, impedance of power switches and cooling features are important. Then you can get an idea about what performance level can be achieved by the chip and what quality of the other components in the circuit are needed.
Look at the photo(s) of the amplifier board and try to estimate:
- if the output filter chokes are sufficiently large (depends on value and physical size; you can often find similar chokes on the Internet and from that information estimate their ratings) and does it appear possible to change them with larger ones if needed,
- are the output filter capacitors of the foil type (normally possible to see as they are large) or are they likely to be ceramic SMD types,
- are the input coupling capacitors of the foil type (normally located close to the input connector and pretty large) or are they likely to be ceramic SMD types,
- what type of power supply line decoupling capacitors are found on the board and their voltage rating and quality (the largest capacitors on the board; you can often read their value and brand from the photo; does their voltage rating allow full use of the chip? if they are just ordinary black they are typically of a standard quality),
- does the cooling facilities implemented allow for the chip to remain at a reasonable temperature if it is operated close to maximum output power (make a quick estimate of the power losses in the chip from the data-sheet information) and does it seem physically possible eventually to improve the cooling,
- does the board in general appear to be well designed (connectors, arrangement of components, PCB quality (fiberglass) etc.).
With such an evaluation prepared you have a good idea about the effort the board is going to require. There are evidently issues you may only notice once you have the board for test.
Whenever receiving a board, always test it for a start and make sure it seems to work “as is”. The moment you start modifying anything your guarantee is normally lost. But, that risk is inherent for DIY activities.
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