LM3886 Stereo Amplifier

This stereo amplifier is based on the LM3886 Audio Amplifier IC from Texas Instruments.

amp-lm3886In its current configuration it produces 48 Watts per channel (before clipping) when connected to an 8 Ohm dummy load with a 1kHz sine wave for the input signal.

There are provisions for 3 switched audio inputs. 115VAC power is provided at the fused IEC connector.

amp-backInside, the amp is configured in a stacked configuration with the transformer (Antek AS-2225) centered on the bottom plate of the enclosure.  The power supply filter and amplifier PC boards are mounted on a shelf just above the transformer.

am-lm3886-insideThe source inputs are carried from the back panel to the selector switch by twisted pairs. Those long leads running from back to front look trouble prone and I was concerned they would pick up noise. So far I’ve not noticed anything.

The input signal for each amp is fed by a twisted pair from the dual gang, volume control, potentiometer on the front panel.

The amplifier PC board circuit closely follows the examples shown in the LM3886 data sheet.

lm3886_amplifier_schematicA PDF of the LM3886 Amplifier PCB schematic is available here: LM3886 Amplifier Schematic

The values chosen for R1, L1, and C3 in the output networks are different than what is typically specified. The values here were arrived at experimentally and were chosen to provide what was, subjectively, determined to sound best through my speakers (a pair of Klipsch KG4s). I plan to do some more experimenting. The roll-off of the Zobel network might be too low.

The sharp eyed reader may notice that the LM3886 specified in the schematic is the ‘T’ package but the photo of the amp’s insides show the “TF” isolated package.  Originally I had the “T” style packages installed along with shoulder washers and insulators. I replaced those with the “TF” package shortly before taking the above photo.

The same perceptive reader has probably already noticed that the construction of the output inductor, L1, as seen in the above photo, doesn’t match the description for L1 given on the schematic. L1 was modified after the photo was taken. I’m always tinkering with the things I build.

For the amplifier PC board layout I opted for two ground planes (one for the signal common and one for the power common) instead of the more typical practice of snaking traces around to a common star point. The two planes converge at the COM terminal on the PCB (I suppose that’s a 2 point star). I’ve not experienced any issues with this configuration (yet).

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Render of LM3886 Amplifier PCB – Top
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Render of LM3886 Amplifier PCB – Bottom

The Power Supply Filter PC board provides the smoothing capacitors and a fuse for each power supply rail. It has a total of 20,000uF per rail. The rectified DC is provided by a 35A bridge bolted to the base plate of the enclosure.

power_supply_filter_pcbA PDF of the Power Supply Filter schematic is here: Power Supply Filter Schematic

The Power Supply Filter PCB:

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Render of Power Supply Filter PCB – Top
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Render of Power Supply Filter PCB – Bottom

The 25VRMS secondary of the power transformer provides just over +/- 34VDC supply rails after rectification and filtering. This value can vary up or down depending on the exact mains voltage available. Household mains voltage is nominally 110VAC in the U.S.

For an 8 Ohm load, +/- 34VDC is OK per the LM3886 data sheet. Although I’m using this amplifier with my 6 Ohm impedance KG4s, I believe a saner option would be to use a power transformer with a lower secondary voltage. Something such as the Antek AS-2218 might be a better choice. I will probably make the change the next time I’m inside the amp.

The custom enclosure is constructed from individual parts. The overall look and size was arrived at with the desire to have a unique appearance in a compact envelope while still allowing enough room inside to make the necessary connections. The heat sinks on either side are from Heatsinkusa.com. They are much larger than they need to be but they provide the side walls of the enclosure and reduce the number of parts needed.

The front panel and most of the other parts were fabricated at a local machine shop. The Walnut panel pieces on the front were laser cut at ponoko.com.

I’ve been happy with this amplifier. It is dead quiet with no inputs connected. There is no hum or any other perceptible noise when my Raspberry Pi running Moode Audio is connected. I haven’t tried it with any other sources.

 

TDA7266 Low Power Stereo Amplifier

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.

Front View of SA1 Amplifier

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.

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Rear View

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.

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Bottom View

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.

tda7266_amp_inside_fronttda7266_amp_inside_rearThe 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.

Schematic of the TDA7266 Amplifier
TDA7266 Stereo Amplifier Schematic

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.

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Render of Top of PCB
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Render of Bottom of PCB

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.

tda7266a_pcb_assy
Assembled TDA7266 Amplifier PCB

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.

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Soldering the Heat Sink Mounting Pin

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:

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TDA7622 Amplifier Chassis Modifications
tda7266_amp_bottom_cover_mods
TDA7622 Amplifier Bottom Cover

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:

TDA7266 Stereo Amplifier Chassis Modifications

TDA7266 Amplifier Bottom Cover Modifications

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.