(Re)building a 20W subwoofer amplifier

[CalcProgrammer1]

Well, my friend gave me an old PC speaker set. It was a 2.1 set, but the digital controller chip that controlled EQ and volume was useless. So, I took out the motherboard and looked to see what parts were available. I desoldered some chips.

There were 3 main ones:
TDA7370B (Quad amplifier for car stereo)
TL074CN (Quad op-amp)
KIA324P (Quad op-amp)

I decided (after reading the datasheets) that I'd try building a working amplifier out of these parts. I went for the TDA7370B and wired it up on a circuit board. I basically built the "Double Bridging" circuit that the datasheet described, but using a plain old standard board from RadioShack. This does work, but it isn't great for subwoofer purposes. All frequencies come through (I knew this would happen as there's no EQ or crossover) but when I play a low note (using my PDA and a tone generator), the volume is pretty quiet.

I'm wondering if and how I could use the remaining chips to make a preamplifier to boost the low signal from my PDA (or PC or other device) to make the subwoofer louder. Also, when no song or tone is playing, the amplifier will make random clicks, static, and buzzing sounds, but once I start playing music, it stops. Why would it do this, and what can I add to stop it?

 

[audioguru]

The TDA7370 has four amplifiers. Two are a few Watts each for laft and right channels and two are bridged to supply 11W to a sub-woofer.

20W is when the supply is 14.4V, the speaker is 4 ohms and it is driven too loud so the output is severely distorted square-waves.

The datasheet shows a recommended pcb. Use it.

 

[CalcProgrammer1]

Ok, my power supply is 12V. I've bridged both sides for two channels at 11W. Hooking up the speaker to just one side with one input, I get a pretty powerful subwoofer if the input is right. With my computer, I was able to get a powerful bass output without any kind of extra amplifier. My speaker is 4 ohms (Altec Lansing 6" subwoofer).

I used the schematic that the datasheet provided, but I don't have any way of etching PCB's, so I built it on a RS project board. It seems to work fine, except I get a loud buzzing sound when it is plugged into a device that isn't playing music. I also get a "popping" sound at a seemingly fixed interval, about once every second. This popping sound occurs even if it isn't connected to anything, I'm suspecting it's a capacitor charging and discharging, but I'm not sure.

Is there any way you can use a capacitor to filter noise out of an input signal? The datasheet recommended a 4.7uF capacitor between the input line and the chip, but did not specify how to wire the ground pin from the input. I hooked the ground from the input signal to the common ground of the circuit, but I'm not sure if that adds any noise or not.

Also, the circuit works fine with my 12V 800mA RadioShack transformer plug (well, except for the noise), but when I use the "13.5V 4A" transformer that was inside the subwoofer box (I rewired it, with the 110V AC input connected directly to the transformer using wire nuts and the 13.5V AC output going through the rectifier that was on the main board, it tested 12.5V or so on my multimeter), it plays louder with more noise and more static until it just stops playing and makes crackling sounds and then nothing (I'm thinking it's some kind of thermal overload protection).

 

[audioguru]

If you do not use the recommended pcb for an IC power amplifier then it might oscillate at a few hundred kHz which causes severe distortion and heating.
Of course the input cables to any amplifier must be shielded audio cables to avoid mains hum and interference..