Clarity 2x300W: Class-T amplifier （5）

Published:2011/8/3 2:55:00 Author:Li xiao na From:SeekIC

By Ton Giesberts

Power supply

The supply voltages for the amplifier board are provided by a second printed circuit board. This board includes, among other things, the +5V and VN10 supplies, as well as fuses for the main supply voltages. It also supplies a delayed ’un-mute’ signal that prevents switch-on ’plopping’. To avoid creating an earth loop and prevent ripple currents from flowing though the input stage ground path, the mute signal is fed to the IC via an optocoupler. It is located on the amplifier board. The input of the optocoupler is thus fully isolated from the amplifier, but an active signal is required to switch the amplifier Mute input.

The main supply voltages (VPP and VNN) for the TA3020 are decoupled as well as possible using special electrolytic capacitors (C30, C31, C34 and C35) and MKT capacitors (C32, C33, C36 and C37). A simple decoupling network is used for the 5-V supply voltage for the input amplifiers.

The suppress possible interference from the output circuit as well as possible, analogue ground and modulator ground (which is also the ground for the rest of the circuit) are kept separate and coupled on the solder side of the board at a single location using an SMD inductor.

Layout

As already mentioned at the beginning of the article, the amplifier relies on a carefully designed layout. The layout thus forms an essential part of the overall amplifier. Tripath emphatically recommends copying the reference layout, since otherwise the large high-frequency currents could give rise to unexpected effects. Naturally, some of the components we have selected differ from those used on the reference board, primarily with regard to their dimensions. This is because we have given special attention to the availability of the components (preferably in single quantities). Some of the tracks have been shifted slightly in some places, and a few components have been added, but by and large we still succeeded in maintaining the recommended layout. If you take the trouble to look at the photo of the reference board in the data sheet, you will see the resemblance to the photos of the prototype. For some of the components shown in the schematic that are fitted on the solder side of the board in the Tripath layout, we have put them on the component side instead. This is why the circuit board is placed parallel to the heat sink in our version, with the transistors mounted below the board. This produces an attractively compact and robust module, but we’ll save further comments for the construction description in the second part of the article. What we can tell you already is that although the module looks very simple at first glance, on closer examination you will notice that compared with the schematic, a few things seem to be missing. Many of the components are SMD types, and they are fitted on the solder side of the board. This yields the lowest likelihood of interference problems and results in an amplifier board with very modest dimensions for a 2 ¥ 300 W amplifier. Most of these SMD devices come in 0603 ’shapes’, which in all honesty are nasty little things to work with. To make things easier for you, we will try to supply the circuit board in the near future with the SMD components already fitted.

In the second part of this article (September 2004 issue), we give detailed attention to the construction of this unusual amplifier.

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