TDA1521A Stereo Amp

Few years ago I built a simple stereo amp based on LM1877 to drive my budget home studio monitors. It was better than the built-in Genius amplifier but the sound was still not clear, especially in the highs. Pretty high THD distortion (10%) even at low volume could be one of the reasons. And the chip itself is not intended for high quality audio. So I decided to do it again, but this time better.

There’s many chip-amp options these days and they are all easy to put together with minimal part count. While browsing catalogs of local stores I came across some NOS Philips TDA1521A chips from the 90s and they piqued my interest. They are designed as “Hi-Fi” power amplifiers and on paper sound perfect for what I need. The basic specs are – 2x6W into 8ohm load with +-12V DC power supply and 4V peak-to-peak input voltage. 0.15% THD at 4W, 0.5% THD at 6W (click here to download the datasheet). That’s more than enough power for my application and THD numbers look great compare to 10% I’m getting now.

The Circuit

My build uses pretty much the same circuit from the datasheet with few minor tweaks. I reduced input capacitors from 220nF to 150nF. This brings the bass cutoff frequency to about 55Hz. My drivers are barely able to reproduce anything below 100Hz, so there’s no point in amplifying very low frequencies. The second modification was to bump up filter capacitor values. Data sheet calls for a pair of 680uF capacitors. In addition to those I added a pair of 820uF and a pair of 220uF capacitors for a total of 1720uF per rail.

The layout is drawn in my DIY Layout Creator software, as usual. I tied to keep it tight and logically organized. Filter capacitors are strategically put between the power transformers and the rest of the circuit to act as a noise shield. Click here to download the layout in PDF format.

I had the board made (semi-professionally), with solder mask and component placement printed on the board.

Part Selection

For filter capacitors I used two pairs of Panasonic FM and a pair of Nichicon FW capacitors that I had in my bin. All capacitors in the nF range are Wima film. For power transformer I was able to find a cool Holden&Fisher epoxy potted toroidal transformer with 2x10V and one 20V secondary, pulled from an old hi-fi device. I seemed to be beefy enough to provide power for this build. I used the 2x10V secondary and terminated the 20V secondary with heat-shrink. The two resistors in the Zobel network are 2W metal oxide. Rectifier diodes are ultra-fast UF4007 and I put a 0.2A slo-blo fuse before the power switch.

I tried to keep input leads (green) as far as possible from output leads (purple) and AC leads by routing output leads and AC leads below the board and soldering from below.
That should reduce the risk of noise and feedback.

For the enclosure I got a nice powder coated aluminum box that has four sides removable independently. It’s very convenient to have access to the circuit from pretty much any angle. It allowed me to pack everything tightly in a relatively small box with no noise issues.

For input and output connects I used standard 1/4″ jacks, so I can use this amplifier as a guitar power amplifier if need be. The top two jacks are inputs, the bottom two are outputs. I put them on the back to make the wiring look neater. The front panel hosts only the power button.

One thing I dislike about this chip is that it’s not very easy to mount on the heatsink. Most chips have flat backs, so you can attach them to pretty much any flat surface. But this SIL-9 package has the metal tab sticking out of the center of the chip. There’s almost 2mm of the body sticking back from the tab, so care must be taking when selecting a heat-sink that will fit the package. Between four local electronic supply stores, only one had a heat-sink that could be installed in this space without too much modifications to the heat-sink (had to drill a mounting hole). The bigger brother – TDA1521 has flat back and does not have this issue. Detailed drawings and dimensions can be found in the data sheet.

The Result

It turned out great. Much better than my previous stereo amp build. More volume on tap, more clean headroom and sounds clear. It’s also pretty good as a clean guitar power amp. Guitar pickups are not able to drive it properly on their own, but if using a preamp or a pedal to drive it with higher level, lower impedance signal, it sounds pretty good though a guitar speaker. Very clean and without any harshness in the sound.

Leave A Comment

  • About

    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.