Intrigued by the high interest that the compact amplifier boards based on the family of Texas Instruments' TPA 3118/3116/3110 of Class D amplifier chips have been generating among the enthusiasts community worldwide, I decided to find out for myself what these boards sound like stock, and if the many modifications others have tried really do improve performance to a higher level.
This is the board I got, though mine was from eBay India. Each is a mono board so one needs two for stereo.
Below is an actual photo of one of my boards. They're very compact and measure roughly the same size as a credit card.
There are many vendors selling similar boards on Amazon, eBay, Alibaba, Taobao, etc and there are quite a few variants to choose from. Just to give an idea of the variety, you will find stereo boards, stereo boards with volume pot, stereo boards with Bluetooth, and plain PBTL mono boards with double the power output of the stereo boards. The TPA3116 boards usually have some form of heat sink placed on top of the chip itself. Due to its higher heat dissipation ability, it can make more power compared to TPA3118 based boards. TPA3118 has heat transfer pad at the bottom of the chip and is supposed to transfer the heat to the circuit board itself, with the circuit board itself acting as the heat sink. Typical stereo TPA3118 boards produce about 30 Watts into 8 Ohm load with 24V supply.
The stereo boards typically come with a gain of 36 dB, making it quite suitable for plugging in portable players or smart phones directly. My boards came with a slightly lower gain of 32 dB. My understanding is this gain of 32 dB is still higher than the more typical 26-28 dB gain on most power amplifiers. The downside of higher gain is the higher noise floor.
If one wants to understand more about this chip amp, it is essential to go through the datasheet here. It gives all information needed by board designers, including example schematics on which most of the commercially available boards are based on. There is also information on how to change the gain of the amplifier.
The stock board is actually a very nice sounding amplifier. I drove it with my Kuartlotron buffer, the buffer acting as the volume control. The output of the TPA3118 board drives my 8 Ohm speakers. One can almost live with the sound of the stock board, except for a hardness in the midrange. This leads to a harshness and makes it difficult to listen for extended time. Though the datasheet example circuit specifies a load of 4 to 8 Ohms, it is my understanding from reading many hundred pages of forum posts that the output filter inductor of 10 uH is actually more suited to 4 Ohm load, and can be stretched to 6 Ohms. So if you want to drive an 8 Ohm load like me, the most fundamental change one must make is to swap out the inductors to the more correct value. I have variously come across 15 uH, 22 uH and 33 uH as being the best value for 8 Ohm speakers. I have not undertaken this mod as I am not sure about the value, and also because the cost of replacement inductors is going to be more than the cost of the board itself.
There are cheaper mods that will bring substantial improvements. The first and foremost is to change the four 330 uF/25V power decoupling capacitors to something better. I used 470 uF/35V Keltron caps. The replacement caps must be 10 mm diameter or less to fit within the available space, and rated for at least 25V. Changing to better capacitors completely removed the harshness that I heard. The stock caps are surface mount type. Care must be exercised when desoldering so as not to lift the pads. Caps with regular leads can be used as replacement by bending the leads carefully to match the pads on the board. This mod is the most important mod, in my opinion.
Next on the agenda was to decrease the gain from 32 dB to about 26 dB, mainly because I can hear a whining noise from one channel, and also because I was not happy with such a high gain. There are two resistors whose values can be changed as per the gain table given in the datasheet for setting the gain to 36, 32, 26 and 20 dB. The easiest to set is 20 dB because it involves removing one of these resistors from the circuit board. I wanted to set mine to 26 dB as that is closest to what I understood to be the gain of a conventional power amplifier (of course there are low gain power amplifiers like Firstwatt F5). This involved removing one of the two resistors mentioned above and replacing it with appropriate value. I am avoiding going into details like which R, which value, etc as it gets fairly technical, but if someone is interested, I can provide more info on how to go about it. But be warned that desoldering SMD resistors is not easy. Soldering back the replacement resistor is tougher, especially for someone like me with no prior experience of working with SMD parts. It's perfectly possible to brick your board. I very nearly rendered one board useless while desoldering the resistor as the PCB trace got lifted and fell off. Choosing your gain depends on how much gain you have at the source-preamplifier stages. If you have a 10-12 dB preamp stage, even 20 dB at the power amplifier stage (which this TPA3118 board is) ought to be sufficient. I would not use the highest gain setting of 36 dB as the noise also gets amplified by that amount. The next lower gain of 32 dB is also high.
The third mod one can do (with some caveat) is to beef up the power supply rail. I use 19.5V/4.7A laptop (one per board). I used a 6800 uF/50V capacitor across the 19.5V DC supply into the board (+ve terminal of capacitor on +ve terminal of the supply, -ve terminal of capacitor on -ve terminal of the supply). I use two laptop bricks from two different brands, though they have exactly the same voltage and current ratings. One channel powered by brand X adapter had been consistently having a high frequency whine audible even when the volume control is zero. The volume of the whining noise does not increase even when volume knob is turned up. I tried a simple R as well RC filters across the source RCA socket but neither had any effect on the whining noise. A couple of days back I noticed that one channel is sounding lower than the other side. I rechecked the values of the resistors I used on both channels and discovered that they were not closely matched. Later, I used closely matched resistors but still had channel imbalance. I reflowed at solder points on the board with the lower gain, in the hope that it could cure a poor solder but that didn't help either. Finally, I took off the power rail capacitor (6800 uF that I had fitted) on both boards, and that instantly cured both the channel imbalance and the whining noise. So in effect I had introduced two problems when trying to beef up the power supply rail. This is very likely because of a degraded capacitor as the other channel worked perfectly. Hence the caveat at the beginning of this para. I may re-use proper CRC at the supply rail when I can buy fresh capacitors. For now I am running without them.
The last mod that I did was to use bootstrap snubbers for the output inductor filters to reduce EMI that can arise by the switching of the output stage (sudden transients into inductive loads produce EMI). The "output" side of the inductors already have snubbers by way of the zobel network. What is added is a C and R before the signal reaches the inductor. This additional snubber cleans up the sound further.
There are further mods like playing around with different switching frequencies, but I don't plan to play around with these boards any more. I am afraid I will brick them as the thickness of the copper pads is thin and cannot withstand multiple soldering/desoldering.
In a coming post, I will post about my impression of the sound after all these mods.
This is the board I got, though mine was from eBay India. Each is a mono board so one needs two for stereo.
Below is an actual photo of one of my boards. They're very compact and measure roughly the same size as a credit card.
There are many vendors selling similar boards on Amazon, eBay, Alibaba, Taobao, etc and there are quite a few variants to choose from. Just to give an idea of the variety, you will find stereo boards, stereo boards with volume pot, stereo boards with Bluetooth, and plain PBTL mono boards with double the power output of the stereo boards. The TPA3116 boards usually have some form of heat sink placed on top of the chip itself. Due to its higher heat dissipation ability, it can make more power compared to TPA3118 based boards. TPA3118 has heat transfer pad at the bottom of the chip and is supposed to transfer the heat to the circuit board itself, with the circuit board itself acting as the heat sink. Typical stereo TPA3118 boards produce about 30 Watts into 8 Ohm load with 24V supply.
The stereo boards typically come with a gain of 36 dB, making it quite suitable for plugging in portable players or smart phones directly. My boards came with a slightly lower gain of 32 dB. My understanding is this gain of 32 dB is still higher than the more typical 26-28 dB gain on most power amplifiers. The downside of higher gain is the higher noise floor.
If one wants to understand more about this chip amp, it is essential to go through the datasheet here. It gives all information needed by board designers, including example schematics on which most of the commercially available boards are based on. There is also information on how to change the gain of the amplifier.
The stock board is actually a very nice sounding amplifier. I drove it with my Kuartlotron buffer, the buffer acting as the volume control. The output of the TPA3118 board drives my 8 Ohm speakers. One can almost live with the sound of the stock board, except for a hardness in the midrange. This leads to a harshness and makes it difficult to listen for extended time. Though the datasheet example circuit specifies a load of 4 to 8 Ohms, it is my understanding from reading many hundred pages of forum posts that the output filter inductor of 10 uH is actually more suited to 4 Ohm load, and can be stretched to 6 Ohms. So if you want to drive an 8 Ohm load like me, the most fundamental change one must make is to swap out the inductors to the more correct value. I have variously come across 15 uH, 22 uH and 33 uH as being the best value for 8 Ohm speakers. I have not undertaken this mod as I am not sure about the value, and also because the cost of replacement inductors is going to be more than the cost of the board itself.
There are cheaper mods that will bring substantial improvements. The first and foremost is to change the four 330 uF/25V power decoupling capacitors to something better. I used 470 uF/35V Keltron caps. The replacement caps must be 10 mm diameter or less to fit within the available space, and rated for at least 25V. Changing to better capacitors completely removed the harshness that I heard. The stock caps are surface mount type. Care must be exercised when desoldering so as not to lift the pads. Caps with regular leads can be used as replacement by bending the leads carefully to match the pads on the board. This mod is the most important mod, in my opinion.
Next on the agenda was to decrease the gain from 32 dB to about 26 dB, mainly because I can hear a whining noise from one channel, and also because I was not happy with such a high gain. There are two resistors whose values can be changed as per the gain table given in the datasheet for setting the gain to 36, 32, 26 and 20 dB. The easiest to set is 20 dB because it involves removing one of these resistors from the circuit board. I wanted to set mine to 26 dB as that is closest to what I understood to be the gain of a conventional power amplifier (of course there are low gain power amplifiers like Firstwatt F5). This involved removing one of the two resistors mentioned above and replacing it with appropriate value. I am avoiding going into details like which R, which value, etc as it gets fairly technical, but if someone is interested, I can provide more info on how to go about it. But be warned that desoldering SMD resistors is not easy. Soldering back the replacement resistor is tougher, especially for someone like me with no prior experience of working with SMD parts. It's perfectly possible to brick your board. I very nearly rendered one board useless while desoldering the resistor as the PCB trace got lifted and fell off. Choosing your gain depends on how much gain you have at the source-preamplifier stages. If you have a 10-12 dB preamp stage, even 20 dB at the power amplifier stage (which this TPA3118 board is) ought to be sufficient. I would not use the highest gain setting of 36 dB as the noise also gets amplified by that amount. The next lower gain of 32 dB is also high.
The third mod one can do (with some caveat) is to beef up the power supply rail. I use 19.5V/4.7A laptop (one per board). I used a 6800 uF/50V capacitor across the 19.5V DC supply into the board (+ve terminal of capacitor on +ve terminal of the supply, -ve terminal of capacitor on -ve terminal of the supply). I use two laptop bricks from two different brands, though they have exactly the same voltage and current ratings. One channel powered by brand X adapter had been consistently having a high frequency whine audible even when the volume control is zero. The volume of the whining noise does not increase even when volume knob is turned up. I tried a simple R as well RC filters across the source RCA socket but neither had any effect on the whining noise. A couple of days back I noticed that one channel is sounding lower than the other side. I rechecked the values of the resistors I used on both channels and discovered that they were not closely matched. Later, I used closely matched resistors but still had channel imbalance. I reflowed at solder points on the board with the lower gain, in the hope that it could cure a poor solder but that didn't help either. Finally, I took off the power rail capacitor (6800 uF that I had fitted) on both boards, and that instantly cured both the channel imbalance and the whining noise. So in effect I had introduced two problems when trying to beef up the power supply rail. This is very likely because of a degraded capacitor as the other channel worked perfectly. Hence the caveat at the beginning of this para. I may re-use proper CRC at the supply rail when I can buy fresh capacitors. For now I am running without them.
The last mod that I did was to use bootstrap snubbers for the output inductor filters to reduce EMI that can arise by the switching of the output stage (sudden transients into inductive loads produce EMI). The "output" side of the inductors already have snubbers by way of the zobel network. What is added is a C and R before the signal reaches the inductor. This additional snubber cleans up the sound further.
There are further mods like playing around with different switching frequencies, but I don't plan to play around with these boards any more. I am afraid I will brick them as the thickness of the copper pads is thin and cannot withstand multiple soldering/desoldering.
In a coming post, I will post about my impression of the sound after all these mods.
hyeah:
