This is a small stereo amplifier based on the STMicroelectronics TDA7266. I built it to mate with a Raspberry Pi running Moode Audio Player to stream music in my office. I’ve provided schematics and other information below for those adventurous souls that decide to make one of these, or something similar.
This version is built inside of a BUD AC-431 aluminum chassis. The front panel is bamboo that was laser cut at ponoko.com. It’s attached to the front of the chassis by the retaining nuts for the power switch and volume pot.
The amp is powered by a 12V DC wall plug power supply. According to the TDA7622 data sheet, this amplifier (powered by 12 VDC) should be capable of supplying approximately 3 Watts per channel into an 8 ohm load at around .03% THD at 1kHz. I’ve not made any measurements myself, but it sounds very good through an old pair of Infinity Reference bookshelf speakers.
The controls and connectors are what I had on hand. They come from eBay and other sources so I don’t have specific part numbers for them. If you build this amplifier you’ll have to adjust the size of the holes in the chassis to accommodate what you are using.
The bottom cover is made from a Bud Industries BPA-1505 Chassis Bottom Cover. The only modifications to the bottom cover are the ventilation holes. They theoretically work with the ventilation holes in the chassis to allow some airflow across the TDA7266 heat sink. However, when playing music at moderate levels I’ve never noticed the heat sink to get warm.
The chassis and bottom cover are painted with Krylon Satin Black spray paint from Lowes.
There is nothing especially critical concerning the wiring inside the chassis. In general, the wires are routed as they would be for any amplifier. The signal wires are kept separate from the power and speaker wiring and the length of all wires are no longer than they have to be.
The wire pairs carrying the input signals are twisted together. I’ve read it’s a good idea to twist the power wiring pairs and the speaker output wire pairs also. I didn’t do that for this amp and so far I’ve not noticed any issues.
The schematic of the amplifier PCB closely matches the application circuit from the ST datasheet. I added an additional 100nF decoupling capacitor and increased the value of the input coupling/DC blocking capacitors from .22uF to 2.2uF. According to the datasheet, the input impedance of the TDA7266 is 30k ohms. The .22uF capacitors specified on the datasheet would give a 3dB corner frequency of around 24Hz. I had stock of some WIMA 2.2uF PET capacitors so I made use of them to reduce the corner to around 2.4 Hz. I doubt there is a human alive that can perceive the difference, but it was an easy change.
A PDF of the schematic can be downloaded here:
Schematic for the TDA7266 Stereo Amplifier PCB
The TDA7622 is an easy IC to use. It requires few external components and should perform well given a reasonable PC layout.
It’s IMPORTANT to note that this amplifier has bridged outputs. Do not allow either conductor of either of the speaker wires to make contact with ground/common. Doing so will damage the TDA7266.
The gain is internally set to 26dB (approximately 20x Voltage gain) and I’m not aware of any way to change that. However, in my opinion, 26dB is a perfectly reasonable gain setting for the intended use of this amplifier.
Note that the 100nF decoupling capacitors (C1 and C2) are placed close to the power input pins of the TDA7266. If you produce your own PCB for this amplifier keep that placement in mind.
The ground planes for power common and signal common on the PCB are kept separate except for the link that is just below the negative lead of C3 on the bottom side of the PCB.
I used ground planes here instead of the generally accepted practice of running independent traces to a star ground. For this application, the ground planes appear to work well.
The PCB is relatively easy to assemble. The toughest part is soldering the two mounting pins on the heat sink. It takes a lot of heat.
To solder the heat sink pins I set my Hakko FX888D to 800 °F. It’s not necessary to apply solder all the way around the pins. I covered approximately 30% to 50% of the diameter of each pin when I assembled the prototype.
U1, the TDA7266, is installed after the heat sink pins are soldered. Use thermal compound between the TDA7266 and the heat sink.
The data sheet doesn’t specify (unless I missed it) what potential the tab of the TDA7266 is tied to. My probing around indicates it is tied to ground/common. In any event, the heat sink on my PCB is isolated and is not tied to ground/common or any other part of the circuit. It also does not touch the chassis at any point.
As previously mentioned, the amplifier is built into a BUD Industries AC-431 aluminum chassis. The cutouts I made to the chassis are detailed on a couple of drawings:
The diameters of the holes in the front and rear of the chassis conform to the components I used. If you decide to recreate this chassis then you will need to adjust the size of the holes to match your parts.
I drilled the larger holes with a metric step drill (examples).
PDFs of the chassis modification drawings that contain full size templates are here:
I found that deburring the holes on the inside of the chassis was a difficult and nightmarish experience. The chassis is small and cramped inside and I undoubtedly don’t have the proper tools for the job. I don’t have any tips for that process other than to recommend to be very careful. There are a lot of sharp edges.
Building this amplifier was a fun project (not withstanding the deburring process) and, in my opinion, produced a nice looking, and nice sounding little amplifier.