Clarity 2x300W Class-T Amplifier-Part 2: building amplifier board （3）

Published:2011/8/4 21:38:00 Author:Phyllis From:SeekIC

By Ton Giesberts

Power supply and ground

The main supply voltage is connected to the circuit board using flat (car-type) terminals. This allows very large currents to flow and makes it easy to connect the board to the power supply. Special electrolytic capacitors are fitted across the power supply terminals to decouple the worst RF current spikes. We have done our best to implement these connections as star points, but we had to spread them out a bit to keep the distance between the IC and the MOSFET leads reasonably short.

For decoupling capacitors C30. C31, C34 and C35, we selected a Nichicon family that combines a very good capacity/size ratio with low series resistance and low self-inductance. The optimum choice from this family is the 470-uF type with a working voltage of 63 V. In our case, this determines the maximum allowable supply voltage for the final amp.

From the star point, a track runs to the inductor (L3) that isolates the analogue and digital grounds of the IC. This inductor comes in an SMD package (1812A) and is located on the solder side under the IC (Figure 2). It is a member of the Epcos SLMID family and has a value of 10 uH, with a series resistance of less than 1 LI and a current rating of more than 300 mA.





The ground connections for the loudspeaker outputs are also tapped off from the star point, so the currents from the loudspeakers are conducted back to the main power supply as directly as possible. This avoids imerference to other parts of the amplifier. The ground planes on the circuit board are exclusively intended to provide protection against interference. They are only connected to the AGND pin for the analogue power supply (pin 28). This means that no other connections to the amplifier are tapped off from the ground planes.

A star point is also used for the connections between the analogue +5-V supply and the various components, including the trimpots, jumpers and resistor bridges for the modulator settings. This can be seen on the component side of the layout in Figure 1.





Construction

Despite all the attention given to the layout, the level of interference generated by the output stage is rather high. This means that additional measures must be taken to minimise the negative effects on the analogue portion. This is done by keeping the surface area needed to fit the passive components as small as possible. The only way to achieve this is to implement practically everything using SMD components, and to place them as close together as possible underneath and beside the IC on the solder side of the board (see Figure 2).

The only components that depart from this rule are the two trimpots for DC offset adjustment and the input capacitors. Any interference picked up by the trimpots is filtered out by C3 and C16. Interference picked up by CI and C14 is filtered out by C2 and CIS. Some of the SMDs must have a voltage rating greater than 50 V, due to their location in the circuit. This applies to R15, R16, R36, R37 and R51. SMD components are also used for R8, RIO, R29, R31 and several decoupling capacitors, since this gives better functional results and takes up less space.

For readers who wish to take a soldering iron to the SMDs, despite the fact that the board is provided with the SMDs already fitted (for example, to change the input sensitivity or modify the board for a lower supply voltage), the solder lands are designed to allow either 0805 or 0603 packages to be used for all SMD capacitors and resistors. They are thus somewhat larger than the standard size for reflow or wave soldering. You can thus touch a soldering iron with a very fine tip on the solder lands while soldering, which considerably simplifies soldering SMDs.

For the capacitors in the output filters, we have chosen 400-V polypropylene types that are especially suitable for applications with extreme pulse loading. Here we also use a compact version. These capacitors have a pitch of ’only’ 15 mm, which results in a low value of parasitic self-inductance here as well. There is enough room here for somewhat wider components if you wish to use a different type (such as polyester or capacitors from a different manufacturer).

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