My megafone with 2n3904 electret and TDA2005 circuit makes constant noise

[Pendorcho]

Hi guys,
I'm learning electronics on my own and wanted to make a megafone using a plastic loudspeaker taken apart from an alarm, with 8 ohms coil capable of 20W

I built the following pre amplifier:
**broken link removed**
And also built the bridged TDA 2005 amplifier from the datasheet and connected both circuits, and used 9v as power supply.

It makes a strong hum noise which frequency varies in different med-high frequencies for no evident reason. However if I speak on the electret I can hear my voice, (although i'd expect a bigger amplification)

I took the speaker apart and used a multimeter in cc to analyze the output of both circuits. I got around -6v in the pre output and 3.75v in the amp output. (according to the multimeter).

This voltages change +- 0.5v when I speak close to the electret.

I would expect 0v when I don't speak to the mic, I'd like to understand why it has a continuous current component in the output.

Both circuits were assembled following the schemmatics.

I'd like to make this design work fine, without the noise component, and removing the continuous current from the output, which I suppose is not good for the speaker.

I hope you guys can shed some light to this.
Thanks,
Pendorcho.

 

[audioguru]

Your link does not work so we cannot see your schematics. I found a preamp for an electret mic there that used only one 2N3904 transistor.

The noise is probably acoustical feedback howling because the microphone can hear the speaker and the amplified sound goes around and around. The microphone can be held in foam rubber so that vibrations from the speaker along the case of the megaphone are reduced. Also turn down the gain and speak louder and closer to the microphone.

The voltage at the outputs of the TDA2005 bridged amplifier are normally half the supply voltage so they can swing equally up and down.

The TDA2005 barely works when its supply is only 9V. A little 9V battery will quickly drop to less than its 8V minimum supply voltage.
With a regulated 9V supply its power into an 8 ohm speaker is only about 1.5 Watts which is not very loud.