Low Budget Special - modding cheap computer speakers

[Th3_uN1Qu3]

We all like big, flashy, and well-paid long-term projects. But sometimes it's the cheap azz mods that give you the most satisfaction.

So, for your viewing pleasure, a Low Budget Special tonight. Excuse the picture quality please, i don't have my camera with me. With that being said, let's see what we have here...

Oh, it's a pair of plain white computer speakers. I'm sure you remember those. This particular pair was advertised with a whopping 120W output power, PMPO of course...

Let's see what we have inside. The speakers are rated for 3 (three) watts, while the amplifier is a TEA2025 IC, which delivers a maximum of 2.5W per channel in stereo mode, and that's under ideal conditions...

Now, on the right speakers and with the right enclosure, 2.5W can make a bit of noise. But it is pretty informative to look at the loaded output voltage of the power supply (with the amplifier reproducing a sine wave at maximum undistorted output). If i allow the output to distort the supply voltage falls even further. Now if you take a look at the datasheet again, we're left with little more than 1W per channel... One watt. On real music material, the power LED on the front flickers along with the kick drum. That's some serious supply sagging there folks. No wonder - that transformer over there will do 500mA, and that's on a good day. The bridge rectifier is made of 1A diodes. Also the main filtering capacitor is just 1000uF. That's not even enough to get rid of mains hum when using the headphone output.

Anyway, with this lack of headroom, it's pretty obvious that the amp will run into clipping most of the time, distorting the output to various degrees. I thought this was a pair of amplified speakers not a fart machine!

Now, since they cut so many corners on the amplifier, maybe they put that spare cash into the enclosures, right? Wrong. They're made from cheap, thin plastic, with no bracing. The internal volume of the boxes is roughly 1 liter, which is way too small for any decent bass reproduction. Also, as you notice, the boxes are ported. And they're ported wrong. The port is way too short, giving a tuning frequency of roughly 160Hz. Not only does that basically guarantee there will be no bass, also because of the small enclosure you also get a huge peak in the low midrange.

The peak on this particular pair is at ~340Hz, and it's over 8dB. Overall, the range between 200Hz and 450Hz gets a huge boost, while under 120Hz there's basically zero output. The peak is bad enough that it makes the plastic boxes ring on male voices and guitar. It also gives a "muddy" quality to the sound.

Using pink noise and my ears i was able to EQ out the peak, and they sound much cleaner, but that's hardly an ideal solution. Since there is no tweeter, the high end is also lacking.

So, i present you:
Low budget mods.

• Use an external 12-15v brick able of at least 2A output. This will help them make some more noise.
• Line the inside of the plastic enclosures with whatever sound absorbing material you have on hand. Will help in taming box resonances.
• Make the port longer. Will both make them put out more bass AND reduce the midrange peak. Two birds with one stone.
• Modify the highpass filter at the input of the amplifier so that it doesn't try to reproduce low bass frequencies anymore. This will lighten the strain on the power supply. However, this will also affect the headphone output, so if you use that leave it alone. Alternatively, since this is a single supply amp, you can replace the output coupling caps with smaller ones, which will accomplish the same thing.
• Add a pre-emphasis network to the amplifier to correct the treble response. Again, this will also affect the headphone output.

I left my measurement mic at home so all numbers were arrived at with my ears only, but i'll take the speakers with me and do some real measurements before and after every mod. This is gonna be entertaining.

[lti]

Re: Low Budget Special - modding cheap computer speakers

The TEA2025 isn't rated to power 4 ohm speakers with a 12V power supply. If you get a good chip, it should still handle 12V.

I have a 9V 700mA unregulated power supply powering a TEA2025 and a tone control circuit, and the output voltage never drops during operation. The transformer on your speakers is even smaller than the transformer that came with my cheap computer speakers.

Look at the port on the cheap speakers I modified. It is about 1/4" long.

Finally, make sure the SVR cap has a high enough voltage rating. The TEA2025 amp I have used a 10V cap, but the 9V unregulated power supply produces 12V with such a small load. The cap had 11.5V across it. Those speakers appear to have a 9V unregulated power supply, so the designer might have put a 10V cap in that amp as well.

[Th3_uN1Qu3]

Re: Low Budget Special - modding cheap computer speakers

The TEA2025 isn't rated to power 4 ohm speakers with a 12V power supply. If you get a good chip, it should still handle 12V.

If it's a good chip it'll even handle 15 volts. My dad had a little 2.1 system where the TEA2025 was run to absolute maximum for the "subwoofer". That system was properly made with good quality speakers and crossovers. The original chip lasted like 7 years... then i burned three replacements one after another. I eventually settled for 12v and it worked fine since.

I'll check all the caps but i ran those for a while on a 15v supply and they worked no problem.