Published:2011/8/4 22:00:00 Author:Phyllis From:SeekIC
By Ton Giesberts
Coming up
All that’s left now is the power supply and the wiring diagram. We’ll deal with them in next month’s issue, when we’ll also present some measured results for the fully assembled amplifier. Items such as input and output filters and EMC problems will be handled in a separate article, which will also appear next month.
Kelvin connections
Kelvin connections (which are also called four-point measurements, depending on the application) are used at three locations in the amplifier to eliminate the effects of contact resistance and parasitic self-inductance.
A Kelvin connection is a connection to the terminals of a specific component for accurately measuring the voltage across the component without using extra taps. Here such connections are used for the voltages across the sense resistors for over current detection, the feedback from the loudspeaker terminals and the input ground.
For current detection, it should be evident that four-point measurement is necessary, since the resistance of the sense resistor is only 10mQ. The overcurrent sense leads are tapped off directly from the resistor leads. The resistors ore upright models in ceramic packages (MPC75 from Fukushima Futaba Electric Co. Ltd.), and they have practically zero self-inductance. If you cannot obtain these resistors, you will have to look for other low-inductance resistors with the same form. Resistors with axial leads are truly unsuitable, since fitting them vertically is by itself enough to produce too much self-inductance.
Each pair of signal leads is routed close together to the pins of the IC. The same is true of the feedback from the loudspeaker terminals (to the corresponding resistors). For the input ground, using a Kelvin connection means that all ground connections are individually routed to the common ground pin of the IC (the ’0V terminal for the analogue +5-V supply). This can be clearly seen on the solder side of the amplifier circuit board. Quite a few circuit board tracks join together here, with the result that this junction is necessarily somewhat broadened.
Prefitted circuit board
Due to the special nature of this amplifier (particularly the high switching frequencies), the choice of components is especially important and the use of SMDs in various locations is unavoidable.
Most amplifier builders have little or no experience with soldering these miniature components, and for this reason we provide this circuit board with all of the SMDs already fitted. All you have to do is to fit the IC and the ’normal’ components. In addition, two cores for the output inductors are provided with the board, since they are made from a rather special material.
The price of the circuit board and inductors is only £34.50 / $55.70 / €49. This is a two-channel design, so you only need one circuit board for a stereo amplifier.
Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/04/Clarity_2x300W_Class_T_Amplifier_Part_2__building_amplifier_board__(5).html
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