LM386 Amplifier For The Bench

Render of the Front of the Amplifier

Presented here is a small monophonic audio amplifier with an integrated speaker. It uses the LM386N-4 amplifier IC from Texas Instruments.

This unit will find some use as a general purpose audio amplifier on my work bench, but mainly I built it as a learning experience and for the fun of it. I thought others might want to build something similar so I have documented it here.

The renders shown above and below were originally intended as place holders until I got photos of the completed amplifier. Ends up I like how the renders look, so I am leaving them in place. Photos of the actual amplifier are at the bottom of this post.

The amplifier is housed in a 3D printed enclosure which was designed in SolidWorks. A PCB mounted power on/off switch and volume control protrude through the front panel, along with a headphone jack, and a high efficiency LED that illuminates when the amplifier is powered on.

Render of the Rear of the Amplifier

An RCA jack is located on the back panel and accepts the audio signal to be amplified. Text under the jack reminds that the maximum peak voltage that the LM386 can tolerate at its inputs is 800mV.

The unit is powered by a 9V battery which is accessed by removing the three screws and the back panel.

Schematic

The schematic of the LM386 amplifier is shown below. Leaving the power supply portion out of the discussion (that’s the bit at the top of the schematic, beginning with the “Battery Snap Connector”), this schematic shows what I consider to be the minimum requirements for a functional and robust audio amplifier using the LM386 IC.

A PDF of the schematic is here:
PDF – LM386 Amplifier Schematic

Amplifier Gain

In this circuit, the LM386 is using its default gain of 20X.  The LM386 datasheet shows the circuits and components that can be used to increase the gain up to 200x.  Increasing the gain has drawbacks such as increased sensitivity to noise, instability, and higher distortion. Unless there is a specific circumstance for increasing the gain past the 20x default, I see no reason to do so.

Audio Input Circuit

The audio input circuit consists of everything between the audio in jack (J4) and Pin 3 of the LM386N-4 IC (U1).

My calculations indicate  that the input impedance seen at J4 will be approximately 9k. This figure was arrived at by taking the resistance of potentiometer R5 (10k) in parallel with the stated input resistance of U1 (50k)  and adding the resistance of R6 (1k).

The amplifier’s input impedance was calculated like this:

    \[ R_i_n = 10k   //   50k + 1k = {\underline{9,333\hspace{1 mm}\Omega}} \approx 9k \]

Whether or not that’s the correct way to calculate it, I don’t know.  That’s the way I did it.

I selected a value of 4.7µF  for the input coupling capacitor (C8). That capacitor, along with the assumed U1 input impedance of 9k, form a high pass filter with a 3dB cutoff off of approximately 4 Hz.

The 3dB cutoff frequency for the high pass filter is calculated using this formula:

    \[ F_c = \frac{1}{2 \pi R C} \]

Adding the values from the text:

    \[ F_c = \frac{1}{2 \pi  * 9,000 * .0000047} = {\underline{3.8\hspace{1 mm}Hz}} \approx 4Hz \]

After the high pass filter, potentiometer R5 acts as an input signal attenuator, more commonly known as a volume control.

A RC low pass filter with a 3dB cutoff of approximately 340 kHz,  composed of R6 (1k) and C6 (470pF), follows the potentiometer to filter out high frequency signals.

The equation for calculating the 3dB cutoff for a RC low pass filter is the same one used for calculating the cutoff of a high pass filter. Here we use our new values:

    \[ F_c = \frac{1}{2 \pi  * 1,000 * 4.7x10^{-10}} = {\underline{339\hspace{1 mm}kHz}} \]

Power Supply Decoupling

Proper power supply decoupling is critical for the LM386 if one wishes the amplifier to perform to its specifications.  Here I’ve used a 220µF electrolytic (C3) and a 100nF MLCC (C4) in parallel. It is good practice to place these decoupling capacitors right at pin 6, not several inches away.

With local battery power it is likely that the 220µF is not required. However, adding it gives me the option of running this circuit from other power sources.

A 100µF Electrolytic (C7) is connected to the “bypass” pin (pin 7) of the LM386 (U1).  This capacitor isolates the input stage of the amplifier from power supply noise.  Actually, a 10µF capacitor connected to pin 7 would probably be sufficient.  I’ve not noticed an issue with the value at 100µF so I am leaving it in place for now.

Bass Boost

The “Bass Boost” circuit from the datasheet, R2 (10k) and C5 (33nF), is included to help compensate for the small (77mm) speaker’s lack of bass. It also appears to quiet the hiss that is prevalent with the LM386. One drawback of adding this circuit is that the bass is a bit overwhelming when using headphones. I suppose a switch could be added to switch the bass boost components in and out of circuit but I’ve decided to live with the consequences of my decision.

Audio Output

Signal output from the LM386 is via pin 5 of the IC.  A Zobel network composed of C1 (100nF) and R1 (10Ω) provides a load for high frequency components of the amplified signal and quells high frequency oscillation.

The output capacitor C2 (1000µF) is necessary since the LM386, by its nature, uses a single polarity power supply. This results in the amplifier internally adding a DC bias to the input signal. The output capacitor blocks that DC from getting to the speaker.

In addition to blocking DC from the speaker, C2 (1000µF), along with the speaker’s impedance (in this case 8 ohms), form a high pass filter. The value of C2 needs to be of sufficient value to pass the lowest desired frequency to the listener. In this circuit, the chosen value of 1000µF results in a low end cutoff of approximately 20 Hz.

Once again we use the equation for calculating the 3dB cutoff for RC filters with the new values:

    \[ F_c = \frac{1}{2 \pi  * 8 * .001} = {\underline{19.9\hspace{1 mm}Hz}} \approx 20 Hz \]

A 330Ω resistor (R3) is placed in series with the headphones (when they are plugged in).  This resistor reduces the audio level in the headphones and will help prevent damage to the LM386 IC should the headphone wiring produce a short circuit, or if a monophonic plug is inserted into the stereo jack.

The PC Board

The PCB holds all the components (except for a speaker) necessary for the amplifier to be fully functional.

Connections are provided for SPEAKER OUT, PWR IN, AND AUDIO IN. The 9V battery snap connector is soldered to the PWR IN pads

The three connection points will accept 2.54mm, 2 pin, Molex KK headers. However, I don’t have the proper crimping dies for the mating connectors so I don’t use the headers and, instead, solder the wires directly to the pads.

A dimensional drawing of the assembled PCB (PDF) is available here:
PDF – PCB Dimensions, LM386 Amplifier

The Enclosure

The enclosure for the amplifier consists of three 3D printed parts. Renders of the housing and back panel are shown at the top of this post. Inside the case, there is a 3D printed retainer to keep the speaker in place. It fits a CUI Inc. Speaker, P/N GF0771. I ordered the speaker from mouser.com.

That speaker retainer makes for a chunky looking creature. However, it is going to be under constant stress since it will be clamping the speaker in place. I don’t know how the PLA will hold up to that. It is likely chunkier than it needs to be.

Threaded brass inserts (M3) are installed into the case with a soldering iron. The fasteners for the project consists of 2x M2 x4 screws and companion nuts that hold the battery bracket to the PCB, 2x M3 x 12 screws that clamp the speaker retainer in place, and 3x M3 x 6 screws that hold the back panel on.

Inside, the amplifier looks something like this:

The PCB assembly slides in from the back and is held in place by the back panel. A nut on the potentiometer can also be added but it is not shown in the render above.

STL files for the enclosure parts are available at the bottom of this post. Use at your own risk.

The Completed Amplifier

The 3D printed case got a couple of coats of filler primer, a small amount of filler putty and pitiful little sanding. The finish coat is Krylon Gloss White enamel.

The raised lettering on the back panel did not come out too well.


Here are the STL files for the Case, Back Panel, and Speaker Retainer. If you want to 3D print these parts you’re on your own. The same goes for using any of the information presented here. There are no guarantees or warranties that any of the files and/or information is correct, or is of any use at all.

ZIP of STL Files – Speaker Housing Parts

AM/FM Stereo Receiver Using the Si4831 and TDA7266

This project is an AM/FM stereo receiver. It incorporates an AM, FM, FM Stereo tuner for the commercial broadcast bands along with an integrated stereo amplifier.

There is an LED indicator for power, one showing when a station is tuned in, and another indicating when a stereo signal is being received. An auxiliary (external) audio input is provided that is currently connected to a Raspberry Pi running Moode Audio.

photo of the model sr1 stereo receiver
Model SR1 Stereo Receiver
The AM/FM Stereo Receiver Circuit

The radio receiver circuit is centered around the Si4831 from Silicon Labs. This small, 24 pin, SSOP IC contains all the RF circuitry for an AM/FM stereo tuner. Broadcast stations are mechanically tuned by means of an inexpensive 100k linear potentiometer. The tuning range is set by selecting resistors based on the part of the world you are in. These details are explained in the Si4831 datasheet and Silicon Labs Application Note AN555.

The majority of the remaining required external components are inexpensive resistors, capacitors, and one inductor.  A 5:1 RF transformer is required if an external air loop antenna is chosen (as I did) for AM broadcast reception.

The Si4831 provides the necessary connections for the tuning and stereo front panel indicators, the inputs for AM and FM antennas, and stereo audio output.

photo of AM/FM stereo tuner pcb
AM/FM Stereo Tuner PCB

It’s worth noting that the tuner PCB, with the exception of connectors, uses all surface mount devices. The Si4831 is only available in a SMD package and the application note (AN555) has some specific recommendations on component placement and grounding. I stayed with SMD for all of the parts in an attempt follow those guidelines.

Although I’ve hand soldered small surface mount devices such as the Si4831, it’s not something I enjoy. Some time prior to envisioning this receiver I had invested in a small Qinsi QS-5100 reflow oven. I chose the QS-5100 over the less expensive T962 models based on online reviews such as this one at Dangerous Prototypes. Unlike the T962 models, which apparently require rework prior to use, the QS-5100 is ready to use right out of the box. However, it is necessary to characterize the soldering/heating profile. I intend to post a blog entry on that process at some point.

The schematic for the tuner PCB was developed while studying the Si4831 datasheet and the AN555 Application Note.

graphic showing the AM/FM Tuner schematic
AM/FM Stereo Tuner Schematic

I chose to use an external air loop antenna for AM reception since an internal ferrite antenna would most likely be ineffective as it would be mounted inside the steel chassis. As it turns out, AM reception is poor even with the external air loop antenna. I suspect that the transformer (T1 on the schematic) I selected for coupling the antenna to the Si4831 is not ideal. However, the only AM broadcast I ever listen to comes in nice and clear – Wheels with Ed Wallace on 570kHz. It’s on every Saturday morning here in the Fort Worth/Dallas area.

The Stereo Amplifier

As noted in the title of this post, the amplifier is based on the TDA7266 from STMicroelectronics. Originally, I had used the TDA7297 which is capable of delivering approximately twice the output power of the TDA7266. I had success with that device in a previous project (see TDA7297 Amplifier).

However, in this application, the TDA7297 presented very high distortion (enough that it was easily audible) at all but the lowest volume levels.  The first thing I discovered was that the steady 15VDC power I was expecting was fluctuating between 2 to 3 Volts, dropping as low as 12V at times.  It’s my belief that the 50VA transformer I was using (the largest I could fit in the case) had insufficient regulation for the TDA7297 power requirements. I was also using two 4700uF filter capacitors which could have been a bit much for that little transformer to keep up with. Looking back, I think that using just one 47oouF, 3300uF or even a 2200uF filter capacitor would have been more that adequate. I still have that completed PCB with the TDA7297 installed and one day I may return to it to understand exactly what was going on.

So, instead of investigating the exact cause of the power fluctuations with the TDA7297, I built another amplifier PCB using the lower power TDA7266 (the pin outs of the two devices are identical). This time I used a single 2200uF capacitor for the DC filter and left everything else in the circuit the same. DC power remained steady at all volume levels and the amplifier sounded great. I stayed with the TDA7266.

photo of the TDA7266 amplifier PCB installed in the radio
TDA7266 Amplifier PCB as Installed

The amplifier PCB contains the DC rectifier, DC filtering, and power distribution for the receiver. In the above photo one can see the transformer secondaries coming in at the bottom of the PCB feeding the bridge rectifier and the filter capacitor locations to the right of it. At the left of the rectifier is a 5V regulator circuit that supplies power to the tuner PCB.

The schematic for the amplifier portion of the amplifier PCB follows the TDA7266/TDA7297 datasheet example with some added filtering at the inputs and an additional 100nF decoupling cap at the device power input pins. The additional circuitry required for power input and power distribution is also included.

graphic of the amplifier pcb schematic
Amplifier PCB Schematic

The receiver is powered by 110VAC mains through the rear IEC connector. An Antek AN-0512 transformer provides 12VAC which is rectified and filtered on the amplifier PCB to provide approximately 15VDC to power the tuner and amplifier. The 15VDC is tapped by a 5V regulator to supply power to the AM/FM tuner PCB.

The internal wiring is shown in the following photos.

photo of internal parts and wiring from the front perspective
Internals from Rear
photo of internal wiring of the receiver from the rear
Internals from Front

The front panel is laser cut / engraved by Pololu and follows the silver face trend I’ve been on lately. I used their brushed aluminum surface/black core, 1.5mm thick acrylic. The artwork was created in Inkscape.

photo of the front of the receiver powered on
Front of Receiver Powered On

The calibration of the radio dial is linear and spans 240 degrees. While working on the dial calibration for this receiver, I now understand why commercial radios with analog tuning (knobs) don’t have very precise tuning readouts. My strategy for aligning the knob marker with the frequency markings on the dial quickly developed into “it’s good enough.” The photo above shows the receiver tuned to WRR 101.1 FM, broadcasting out of Dallas, Texas.

The rear panel was also manufactured by Pololu. For it I chose their 1.5mm thick black face/white core acrylic. Again, Inkscape was used to create the artwork for the laser.

photo of rear view of the SR1 receiver
Rear View

I had originally intended to provide two auxiliary inputs. That intention didn’t account for the limited number of switchable sections available on the 3 position, 4 pole rotary switch I was using for source selection. I had neglected to account for the fact that I needed to power off the tuner PCB when an auxiliary input was selected (to silence the front panel TUNE and STEREO LEDs).  So one of the external Aux outputs had to be sacrificed so I could switch the 5V power lead to the Tuner PCB. The local ACE Hardware store had the correct size hole plugs to fill the voids left by the now missing auxiliary input #2 RCA jacks.

I never did produce an overall wiring diagram for this project. If I had done so, the rotary switch issue mentioned above would have been discovered much sooner and definitely before the rear panel was made.

Although this is the second receiver I’ve built I still gave it the SR1 model number (Stereo Receiver #1).

 

 

TDA7297 Amplifier

This is a small integrated amplifier project based on the TDA7297 from STMicroelectronics.

The TDA7297 is a dual bridge amplifier that is powered by a single polarity power supply.  It appears to be a big brother of sorts to the TDA7266 which I used in another project (Low Power Stereo Amplifier). In fact, both chips have the same pin out and can use the same PC board layout. The TDA7297 can dissipate more heat than the TDA7266 so a new PCB layout was created for this project that doesn’t rely on a board mounted heat sink.

The schematic for the amplifier PCB is based on the data sheet application diagram. Since I was doing a new layout for this amp, input filtering was added that was missing on the TDA7266 version.

Notice that the schematic above has two distinct return paths, Signal Common (S) and Power Common (P). As can be seen in the PCB layout renderings below, these paths are kept separate on the PC board and connected together at one point at the negative terminal of the power input jack, J5.

Two separate copper pours are used for each of the return paths. Some audio amplifier PC board designs use a “star ground” consisting of several separate paths snaking to a common point. I don’t have any data to show which method is better for an audio amplifier. I’ve settled on using copper pours and have not had any issues. It does seem to me that the fewer “antenna loops” snaking around a PC board, the better.

R1 and R4, the 1M resistors, are placed between the input lines and Signal Common and provide a return reference for the inputs when there is no source connected to the amplifier.

R2/C4 and R3/C7 provide a low pass (sometimes referred to as a HF filter) filter at each input with a 3dB cutoff at approximately 59kHz.

C5 and C6 are the 2.2uF coupling/DC blocking capacitors. Along with the 30k input impedance of the TDA7297 they provide a high pass filter at each input with a 3dB cutoff at approximately 2Hz.

R5, R6, and C8 comprise the mute/standby delay circuit used to eliminate thumps when the amplifier powers on.  The circuit is from page 5 of the TDA7297 data sheet.  With 15VDC power, the 10uF capacitor shown in the data sheet allowed a slight, audible tun on noise in the speakers. I had a 100uF electrolytic with the same footprint so I used that instead. The 100uF capacitor increases the time constant of the RC circuit and completely mutes the amplifier during power up.

The data sheet shows only one 100nF decoupling capacitor shared between the two Vcc input pins (pins 3 and 13). Another 100nF was added to provide decoupling right at each Vcc pin. The locations of C2 and C3 are shown in the PCB layout renderings below.

Top view of TDA7297 PCB render
TDA7297 PCB Top View

The PCB renderings show how the Power Common and Signal Common copper pours are configured as mentioned above.

Bottom view of TDA7297 pcb rendering
TDA7297 PCB Bottom View

Below are photos of the amplifier right after it had gone through final assembly on the work bench. It is housed in a model EM-01 cabinet from circuitspecialists.com. The EM-01 is a small cabinet, but this is a small amplifier, and I think the components fit in there comfortably.

Inside top view of the completed amplifier as viewed from the front
Inside Top View from Front

The heat sink is made from 2.079 inch wide extrusion from heatsinkusa.com.  I had them cut it 60mm long and then added a total of three M3 tapped holes. One for mounting the TDA7297 chip and two on either side to mount it to the support shelf using a couple of L-brackets. The heat sink extends below the support shelf a few millimeters. The support shelf was made by a local machine shop.

Inside View from Rear

The source selector switch and volume control are from Mouser. The power switch is a Ulincos U16F2SW latching push button with a white LED indicator ring wired to illuminate when the amplifier is powered on. A 22k resistor was added at the LED power terminal to soften the intensity of the LED.

The rear panel connectors are what I had in inventory.

Picture of rear of SA2 amplifier
Rear View

 

Picture of front view of SA2 amplifier
Front View

The front and rear panels/labels are 1.5mm thick acrylic that were laser cut and engraved at pololu.com.  The artwork was created in Inkscape.

As of this writing the SA2 is powered by a Mean Well GSM60A15-P1J switching power supply which supplies 15VDC at 4A.

I named this amplifier the SA2 because it is the second amplifier I’ve built after I started naming them. It’s currently earning its pay in my living room.

 

 

 

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).

render-lm3886_amplifier_pcb_top
Render of LM3886 Amplifier PCB – Top
render-lm3886_amplifier_pcb_bottom
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:

render-power_supply_filter_pcb-top
Render of Power Supply Filter PCB – Top
render-power_supply_filter_pcb-bottom
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.

tda7266_amp_rear
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.

tda7266_amp_bottom
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.

1000574a-render_top
Render of Top of PCB
1000574a-render_bottom
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.

heatsink_pin_soldered
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:

tda7600_amp_chassis_mods
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.