Noise in Amplifier. - Page 2

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Re: Noise in Amplifier.

"L.A.T."

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**  Still not visible to me, but FZ has posted a link to the schem now.

OK,  all you gotta do is modify the headphone output circuit.

1.  Replace the 22 ohm with a 2k2 ohms.

2.  Fit the 22 ohm in parallel with one of the 1N5817 diodes.

Then crank up the VOL pot to compensate for the 40 dB loss of signal.

Should improve the signal to noise by lots.




.......   Phil








Re: Noise in Amplifier.

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Curious. I didn't have a 2k2 so I put together a couple of resistors to
achieve about 2k6. Not enough gain. So I tried 1k5. Too much gain. Drawers
full of resistors but nothing closer. So off to Dick Smith (which is a town
or two north of us) to spend twenty or thirty cents on resistors.
Trouble is, I don't think the hiss has decreased much, so I guess there is
something basically wrong with the way I made the amp in the first place.
But it is hard to imagine what  I could do wrong with such a simple little
circuit.



Re: Noise in Amplifier.

"L.A.T."
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 ** 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



Re: Noise in Amplifier.

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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.



Re: Noise in Amplifier.

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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.



Re: Noise in Amplifier.

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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.
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Re: Noise in Amplifier.

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Re: Noise in Amplifier.
finger to keyboard and composed:

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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.

Re: Noise in Amplifier.
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



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