Noise in Amplifier.

"L.A.T."

** OK - if that has not helped, then the " hiss " is being generated inside the electret microphone.

Such mics have a fet pre-amp, so it generates hiss.

BTW:

is the same level of hiss evident when monitoring on headphones??

....... Phil

On Wed, 19 Mar 2008 03:39:43 GMT, "L.A.T." put finger to keyboard and composed:

You stated in your OP that the hiss was present even with the microphone disconnected. Try shorting the microphone input and then check for hiss. Next short pins 2 and 3 of the LM386 amp and check for hiss. That should help you locate the source of the noise.

Looking at the PCB layout, I'm wondering whether there is any potential for a low input signal to be affected by any noise current flowing along the outside trace between pins 3&4 of the amp and the negative terminal of the battery. This noise current could give rise to a small noise voltage which would be added to the mike voltage, and this combination would then be amplified by the transistor stage which has a gain of about 20. You could try adding an additional ground wire between the MIC- and Headphone- points on the PCB.

- Franc Zabkar

--
Please remove one \'i\' from my address when replying by email.

Hi, I'm a little confused, you've got a microphone (the electret) and without the amp it's too low in volume? (even with the laptop mixer gain maximised?)

Then with the amp it's too high in volume and high in noise? and without the electret the noise is still there.

Well the overall amp has again of some 300 odd and it's going into a mic input. How about totally bypassing the 386, take the output from the pot wiper.

A sound card max input for microphone is around 100mv. the electret should output from 10uV up to 2-5mv. depending on its sensitivity and the sound pressure.

I'd take the ouput from the wiper and perhaps even increase the 220ohm in the emitter circuit up to 470.

If there is still noise then i'd start looking elsewhere including psu coupled noise, or earth loops in cable grounds, installing twisted pairs, reducing noise bandwidth etc.

Greg

I haven't tried. I will be off the air for a few days, and when I come back I will try that first thing.

The plot thickens. I have just found a couple of files of recordings that we made when we were first testing the thing and the noise was nowhere near what it is now. It was, in fact, acceptable.

For the record: I have found some more of the original recordings. Recorded with the original circuit with the volume control advanced very slightly, through the microphone input on the laptop, and they are good. Signal to noise is acceptable, any noise from the amplifier masked by the noise of the breeze. I think it is evident that something has changed in the original amplifier circuit.

>

snip

Thank you for your recommendation. Mr. Lyrebirdman

has bought this Creative soundcard and it works well. I have reset the little Oatley amp to its previous specs and the output at full volume is now enough to drive headphones and/or to drive the Creative soundcard but only just. The quality of the recording is much improved as you promised. With Audacity we can amplify the track after we get it home. I am thinking of mounting the dish and its microphone in a little eggcrate-lined box, firstly to diminish unwanted sound but more crucially to protect it from knocks and mishaps in the pre-dawn, pre-birdcall darkness. Is there a practical limit to the length of the cable from the mic to the amp? At the moment it is about 4cm. Would I suffer any adverse effects from making it, say, 10cm? Thank you all, once again.