The switch SW2 controls the dual/bridge mode. I wanted to be able to use the two amplifiers separately so a switch (SW2) for switching between stereo and bridge was required. The small (and free) application "Rescalc.exe" can help you with resistance calculations (serial and parallel) The inverting gain is 179,1 = 179, acceptable! It reduces the R1 value to get four times higher gain. I introduced a gain switch (SW1) to be able to increase the gain. Using mostly standard resistors (some metal film resistors) and measuring the exact resistance I was able to find combinations that worked. For a bridge design the trick is to get the resistor values so that both circuits have the same gain. The lower amp is inverting with the gain of R2/R1 (where R2 is the feedback resistor). The upper circuit is the non-inverting amplifier with the gain of 1 + R2/R1. If you use a thermal washer then you can delete one thermal compound interface.The design of the power amp is really straight forward, and follows the datasheet example in the absolutely excellent application note AN-1192 from Texas Instruments, which should be your bible if you want to use the LM3886. If you use mica then both sides of the mica need thermal compound. To reduce this risk, insert a thermally conductive, electrically insulated space betwee one of the interfaces. This is at risk of being shorted to to ground or to +ve supply with careless handling. If the chipamp back face is live then the spacer and heatsink will also be live. This is to keep the metal surfaces as close together as possible and to eliminate any and all air from the thermal interfaces. Use thermally conductive compound spread very thinly on both sides of the metal spacer. Make that hole big enough to pass the whole chipamp area through.ġ.) bolt the chipamp direct to the heatsink face, if the legs of the chip are long enough to reach all the way through.Ģ.) insert a copper or aluminium spacer plate that slightly exceeds the thickness of the plastic case.īolt the heatsink to the chipamp using that highly conductive metal spacer. That pic looks like it shows the heatsink separated from the chipamp thermal interface with a layer of sticky goo and the wall of the plastic casing.Ĭut a hole through the plastic case.
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