TPA3118 - The DNA of the TPA3116 but in mono outline and also with fine sound.
Key specs: 4.5V-26V supply, 2x30W in 8 Ohm, 0.1% THD, 30mA quiescent current.
DC offset measured at outputs: 1mV / 2mV (19V supply).
The TPA3118 is a limited version of the TDA3116D2. The limitation seems obtained by different packages for the two chips. TPA3118 is supplied in a plastic package (HTSSOP32) with a cooling pad turned downward on the PCB while the TPA3116D2 package has a cooling pad on the top for a heat-sink. The TPA3118 is described in the common data-sheet as able to operate "without heat sink", which is no doubt correct for up to 19V supply voltage but may be arguable at full supply voltage.
The TPA3118 has a wide supply voltage range, even down to below 5 Volt. Being so versatile, it can be used for most audio applications in ordinary consumer products. For higher power levels TPA3118 is limited by the maximum in supply voltage of 26V and the possibility to cool the chip.
Low-cost TDA3118 modules in mono configuration (PBTL) were briefly tried out individually. That left the impression of clear and undistorted sound with no apparent flaws.
A stereo amplifier
It was tempting to build a stereo amplifier from two small boards and appreciate particular features of the chip. Based on the same chip as the TPA3116, the TPA3118 has output switches with an impedance of only 120mOhm (typ.). When the TPA3118 is used in a parallel coupling (mono), with the output switches operated synchronously, the resulting switch impedance becomes as low as 60mOhm. For comparison, a TPA3255 allowing up to 260W in 4 Ohm offers switch impedances of 85mOhm. Though restricted by supply voltage and the switch impedance not being the only characteristic of importance, the low switch impedance invites low impedance loads to be used and less heating of the chip. Loudspeakers with importantly varying impedance can be handled. Quite some output power can be provided and the amplifier is suited for automotive use, moderate PA equipment and guitar amplifiers.
If needed, the TPA3118 can be used with a supply voltage below 5V. Further, the modulation frequency may be set as high as 1.2MHz in return for higher idle losses.
Taking the attractive price into account the limitations are fair. A maximum supply voltage of 26 Volt makes the parallel-coupled TPA3118 less suited for 8 Ohm loudspeakers simply because its potential cannot be used. The chip cooling is generally adapted to the moderate supply voltage and can be somewhat improved with a heat-sink on top of the housing. The THD is not among the best in the group with the TDA8932 claiming 0.007%, but still it sounds fine. A new TPA325x series is launched by TI, that is at the top for monolithic class D amplifiers, but unfortunately at a somewhat higher price.
The implementation with the two low-cost boards put a few further constraints.
Four 330uF/25V electrolytic capacitors are used for decoupling of the power supply line. The total of about 1300uF on-board is a good start but not sufficient for an amplifier suited for low impedance loudspeakers. More importantly, the 25V rating means that a 24V supply voltage will use almost the full voltage rating of the capacitors and leave a moderate reliability. Electrolytic capacitors should be left with around 20% of their voltage rating unused, if possible. For the low-cost boards the supply voltage should not exceed 22V and 19V is suggested. That should allow 20-25 Watt in 8 Ohm and the double in 4 Ohm.
The initial LC output filters do not live up to the amplifier potential and should be replaced with better filters external of the board.
The input signal coupling capacitors are of a tiny ceramic SMD type. It is possible to identify the capacitors on the boards and even remove them, but it is very difficult to insert anything much better with the miniature size. Generally, the initial capacitors should be left in place.
Cooling of the TPA3118 chip is to the PCB and it is difficult to add a heat-sink on top of the housing because adjacent SMD components have a profile as high as that of the TPA3118. If implemented, a heat conductive spacer plate of 2mm thickness and the size of the TPA3118 housing should be put below the heat-sink, such that the heat-sink is elevated above the adjacent components. The TPA3118 chip housing is, luckily, bigger than the TPA3110 housing but somewhat smaller than for instance the TDA8932 housing.
On each of the two low-cost boards, an AWG 16 wire was connected on the rear side to the ground plane. This is needed for the external output filter. The initial output filter chokes were removed and left terminal points for external filters.
The polarity protection diodes were short-circuited.
The boards were mounted on a bigger board that serves as a mother-board. On the bigger board more power line decoupling was added. For each amplifier board, another 5500uF were added which leaves some 6800uF total for each channel.
A 3.5mm jack connector was used for the input signal.
It could be interesting to test the TPA3118 boards with different output filters as the boards can supply quite some current. Therefore, the wires for the external filters were taken to screw terminals on the bigger board.
Should the output filter be powered without loudspeakers, the TPA3118 chips may be permanently damaged. Therefore, protection diodes in the form of catch-diodes were included on the bigger board, next to the screw terminals for the external filters. These diodes are of a 1A Schottky type and prevent the voltage of any chip output to go much above or much below the supply voltage rails. 1A is sufficient because Schottky diodes have much higher surge ratings and the loading, if invoked, will be surge only.
As a first output filter, a 22uH(5A)/680uF LC filter was connected to the screw terminals. For prudence reasons, the amplifier was first tried with a 12V supply and worked. Also with a 5V supply it functioned. Then, the supply voltage was increased to 19V. Even with 19V supply, the TPA3118 surface temperatures did not go above 40 degrees Celsius without heat-sink.
The sound was without obvious flaws and disturbing hiss. A characteristic was a powerful bass that made bass drums and synthesized deep bass be heard as on more powerful amplifiers. In general the sound was, as could be expected, very similar to that of the TPA3116. In particular if the cost of the amplifier is taken into account, the TPA3118 stereo amplifier construction is largely worth every penny spent.
Comparative tests have not yet been performed.
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