Hiss in simple amp circuit

[mid_life_crisis]

http://www.techlib.com/electronics/audioamps.html

I built the first circuit on this page without the extra cap for higher gain and it works, but there is really bad background hiss. The circuit is usable, but noisy.
Any ideas from the electronics experts if I may have a bad component or miswired something? Or is this just normal for a circuit like this?

[luzceloffan]

That's a shielding issue, where do you have the board? You need to set it in a shielded box , or box in metal like the effect pedals and solder a ground wire to the box, so it will protect the signal from static and interference.

[mid_life_crisis]

It's in a plastic project box. I don't have a metal one readily available. I'll pick one up this weekend. Thanks.

This is going to sound nuts, but can I test this theory first by wrapping the plastic box in aluminum foil?

[luzceloffan]

Originally Posted by mid_life_crisis

It's in a plastic project box. I don't have a metal one readily available. I'll pick one up this weekend. Thanks.

This is going to sound nuts, but can I test this theory first by wrapping the plastic box in aluminum foil?

If You want to keep the plastic box, You could use copper Tape that is used to shield the cavities in basses and guitars and will function the same.

http://www.guitarfetish.com/Copper-G...Kit_p_176.html

Aluminium should works too ... but rememer to solder the ground to the foil

[Rune Bivrin]

If what you're hearing is hiss - as if there's a tiny water fall the amp - then it's not a shielding issue. Hum or radio reception might be cured with a metal box, and wrapping it in aluminium foil connected to the ground of the circuit is a cheap way of testing it. But don't even bother trying to solder it. Aluminium will not let itself be soldered other than with special solder. Just use a crocodile clip for testing.

However, that circuit is NOT suitable as a gain box for a passive bass. The input impedance is much too low. It's set by the resistance of the potentiometer which is 10 kOhms. You need at least 500k, particularly if you're trying to get the signal level up. 10k will in itself reduce the level substantially.

Also, the chip used - 386 - is a low grade, cheap power amplifier (2W out, if I remember correctly), and not known for low noise. You want a circuit with something like a 5534, TL072 or any other low-noise, low power operational amplifier.

[mid_life_crisis]

So the basic chip and design are inappropriate for the purpose.
Thank you.