Dual-Mono TPA3122D2 AMP
For $25 I purchased two SURE Electronics TPA3122D2-based amp modules from eBay (link).
It is an extremely simple configuration: input DC blocking capacitor, output low pass filter and power, and a few control lines. I suspect sound quality is overwhelmingly determined by the chip and not so much by implementation. Of course some minimum level of quality is required for all the components. The kit is also suitable for beginners because all the components are through-hole and there is ample space.
There is a discussion on these Ti TPA31xx amps over at diyaudio [link]. The reports are good.
I plan on using them in BTL configuration (balanced input) with the $99 ES9018 DAC in voltage mode out. With Hifiduino S/W control, there is no need for external volume control resulting in a compact and “straight-through” DAC-AMP configuration. 20~30W per channel is plenty power for the Alpair 12p.
Here are some photos:
Very nice quality board. (Nowadays almost any PCB from China is top-notch in quality). Just the board itself is worth the price of the kit
Hmmm, not a solid ground plane. May need to add some bridges for the ground current as I did with the $99 ES9018 board. Easy to do mod if needed…
For the power ground currents, there is nothing to do; there is a straight path to the power ground connection in the board.
For the signal ground currents,
Here is the circuit diagram of the board implementation
Here is the circuit diagram of the TPA3122D2 Evaluation Board [link]
And here is a photo of a stock implementation (courtesy of PartsExpress -the Amp is also available from PE)
The plastic storage case for the components is actually pretty nice (at least worth $.99 at the local 99cents store :-))
No-name components… replace with your favorite parts
The output inductor is pretty beefy. Should work pretty well if not that the value is listed as 10 uH (For BTL into 8 ohm, it requires 22 uF according to the data sheet. For a replacement, people at diyaudio have recommended the WURTH inductors (not specifically for this amp, but other class D) such as the Wurth WE-PD (sheilded) series inductors [link] at $3.66 each. Toroidal inductors are also a good choice because the magnetic field is confined within the toroid. This seems a good candidate at at $2 each: [link]
The TPA3122D2, current version of the chip [link]
Have some film-foil caps I plan to use for the input DC blocking capacitor (Multicap PPFX). These are an improvement to the including capacitors in the kit. (These are pricy caps, but they have been sitting in my drawer for years :-))
Other low cost are:
Film and Foil
- Dayton film and foil: [link] Parallel two .47uF capacitors to get the desired value. These are the most cost effective film and foil capacitors.
The amp has better performance if configured as mono, balanced input (BTL mode). Notice that at 20 W output, the BTL mode outperforms the SE at 1 W output:
Here is comparing with a newest generation TPA3116D2. All of these chips have similar performance, the newer chips have better numbers at the frequency extremes…
PRETTY GOOD PERFORMANCE
If we compare the two devices (5W trace), we get the following plot. The older TPA3122D2 is comparable to the latest devices. One thing to note is that the spec sheet uses a gain of 20 db for the 3122 (and 3123) and a gain of 26 db for the 3116 part. If we use the same gain for both devices, the 3116 will perform markedly better.
OUTPUT DC BLOCKING CAPACITOR
There is another advantage of of configuring this Amp in BTL: the output DC-blocking capacitor is not needed.
The circuit is built to support SE configuration. As such there is space for two 220uF electrolytic capacitors in series with the speaker output leads. If configured for BTL, these capacitors which can negatively impact the sound, can be omitted.
The circuit diagram for BTL configuration is shown here (taken from the datasheet). Notice how simple is the circuit: input DC blocking capacitor, power filter and bypass caps, output low pass LC filter. The few other capacitors are clearly explained in the datasheet.
Input and bypass caps
Put high quality film caps on LIN, RIN, and BYPASS (C1, C2, C14). This will be the biggest factor in improving sound quality.
[Rule of thumb — make the BYPASS cap value the same as the input cap value. The input cap value should be based on your required bass cut off, which is determined by the input impedance, which is controlled by the amp gain. The minimal input impedance at the 36dB gain selection is 9Kohm, while the maximum impedance is 60Kohm at 20dB gain (all this is in the TPA3122D2 data sheet). Use the standard filter formula to determine this value: C = 1 / (2 * 3.14 * frequency * impedance)]
The two input caps in the BOM appear to be mylar film caps. Replacing these with good polypropylene caps will improve the sound quality. The BYPASS cap (C14) is a mono ceramic. The BYPASS pin is the feedback loop bypass of the analog first stage. It definitely needs a quality polypropylene cap. I’ve found the change in audio quality to be noticeable.
Analog supply isolation
Replace C22 with a larger value electrolytic (100uF or so) and a small resistor (100-220 ohms) between VCC and C22. This will isolate the internal opamp stage from the switching stage.
The analog stage power supply isolation mod is a more subtle mod. The internal analog stage opamps theoretically should cancel out 100% of the power supply noise via common mode rejection. I’ve found this to be rarely the case. Further, the majority of the power supply noise is coming from the switching frequency of the digital output stage, which means the input stage opamps are doing the greatest amount of feedback at the output stage’s switching frequency. If you think about it, the analog stage is doing the adjustment right at the point the digital stage is switching — it’s not a clean path for an audio signal. Adding a filter for the analog stage power supply reduces this potential for distortion.
Power supply capacitors
For more bass, replace the C7/C8 and C10/C11 pairs with a 2200 uF (8 ohm speaker) or 4700 uF (4 ohm speaker) and make sure the power supply you plug into the board has at least double that value.
POWER SUPPLY CAPACITORS MOD
The datasheet recommends the use of low ESR capacitors. A good candidate is the new Panasonic FR, which is the successor of the popular FM. The board can fit capacitors of 3.5 mm lead spacing and 8 mm in diameter. There are space for 4 PS capacitors. Since we are not using the output DC blocking capacitors, we can potentially use this space to fit more capacitors (mod the connections) or larger capacitors
Panasonic is introducing the FR-Series, new Aluminium Electrolytic Capacitors in Radial Construction. This capacitor is the perfect solution for applications, which require ultra low ESR – very high ripple current – very long life in a small mounting form.
In comparison to our current products e.g. FM series, we could achieve a lifetime upgrade of up to 100% (up to 10000h at 105°C) and a capacitance increase of up to 30% by improving the material technology.
The largest 35V 3.5mm lead spacing capacitor is 470 uF [link]
A simple configuration is to use a single 2200 uF capacitor (or two, one above and one below) and install it horizontally.
POWER BYPASS CAPACITOR MOD
Solder .1uF mono caps right across Pin1-Pin20, Pin10-Pin11, and Pin7/8-Pin16/17. This will put these bypass capacitors as close to the chip as possible
POWER CONNECTION MODS
Input protection diode
The power input line has a diode for reverse-polarity protection. The evaluation board does not have such diode. I think I will omit the diode and just put a jumper.
The power line snakes around from one side to the other. There is opportunity to shorten these lines and make the supply lines in a more symmetric manner.
ANALOG SUPPLY ISOLATION MOD
The analog supply is tied to the digital supply. Internally, there is a regulator providing a regulated (and lower) voltage to the analog section as shown in the block diagram:
It is simple to isolate this supply from the digital supply. The following are potential methods:
- RC filter between VCC and AVCC. The resistor will drop some voltage, but full voltage is not requires as there is an internal voltage regulator
- Pi filter CLR or CRC for improved filtering
OUTPUT FILTER INDUCTOR MOD
As indicated above, the output inductor needs to be changed to 22 uF for BTL mode into 8 ohm according to the spec sheet. According to “Design Considerations for Class D Amps” [link]
The output inductors are the key elements in the performance of the class-D audio power amplifier system. The most important specifications for the inductor are the dc resistance and the dc and peak current ratings. The dc resistance directly impacts the efficiency by adding to the total load resistance seen by the power supply.
The inductor current ratings must be high enough to avoid magnetic saturation, which will cause an increase in audio signal distortion or, if completely saturated, will cause the inductor to appear as a short rather than an open circuit to the PWM output. This could potentially damage the device or speakers from the resulting high current surge that may occur during turn on, or the increased quiescent current during normal operation. It would seem best, then, to choose an inductor that has a much higher current rating. The tradeoff is that the size and cost increase as the current capability increases. Shielded inductors will also help reduce distortion and EMI, minimizing crosstalk in the process.
Of the two inductor options considered above:
- Wurth WE-PD (sheilded) series inductors [link] at $3.66 each.
- Bourns 2100 series inductor [link] at $2 each
The Bourns will perform better (according to the TI document) due to its lower resistance (15 vs 43 mOhm) and higher current rating (7 vs 4.1 Amp), plus it is cheaper!
OUTPUT FILTER CAPACITOR MOD
According to “Design Considerations for Class D Amps” [link]
The filter capacitors should be ceramic capacitors with X7R characteristics for stability over voltage and temperature, and can be found in common
surface-mount packages as small as 0805. The values of capacitance calculated in the example above are readily available in ceramic chip and metal film
capacitor product lines. Measurements have shown little difference between the performance of these two types of capacitors, though some audiophiles will strongly recommend the metal film.
ADDITIONAL WISDOM FROM THE FORUMS
The only difference between the various TI TPA31xx class-D medium power chips is their thermal rating. They are actually all the same piece of silicon, just in different packages. You can glue a heatsink to any of them and get the same thermal derating.
The TPA3122D2 can take a recommended maximum VCC of 30V and will put out ~47W into a bridged 8 ohm load. It has built in thermal protection (rated 150C trip point), so you couldn’t overheat it if you wanted to.
A really easy heat sink is to mount the DIP chip underneath your PCB with all the rest of the components on top, then mount the PCB on a chassis so the DIP package contacts it (an aluminum plate in a project box will do nicely).
Your 10% THD figures are totally dependent on VCC. As long as the peak output doesn’t saturate the rails, the distortion figures will remain well below 1%. If we assume a 90% efficiency, then 12V VCC gives 10.8V P-P across a load. That’s approximately 3.6W peak/2.6W RMS for 8 ohms single ended mode. At 24V VCC, you get 21.6V P-P and approx. 14.6W peak/10.3W RMS. If you look at the TPA3122D2 data sheet, figure 6., you’ll see that the 10% distortion figure pretty much matches these calculations. Of course the efficiency level changes with power output (figure 14.), so you’d have to take that into account when making final calculations.
Even at peak power in BTL mode, these chips won’t dissipate more than a few watts, so I wouldn’t worry too much about heatsinking.
The switching frequency of the TPA3112D1 is at 310KHz median vs. the TPA3122D2 250KHz median which isn’t a whole lot of difference. The use of bead filters is merely an economical trade off for low cost applications. If you want good filtering with proper loading, you’re going to have to go with a standard LC filter.
Effect of output filter: [link]