Buzz issue with electret microphone/preamp

[JN]

Hi guys, 

Ok, here's a totally noob question but I'm going crazy here.

For a project I'm using an electret microphone (you know, one of the small standard ones) which I am powering and amplifying with the very very simple standard transistor circuit (schematic attached). I'm using the BC547 transistor.

Everything is working fine, sort of. Audio signal from the electret microphone is being amplified as it is supposed to. It doesn't sound good but it gets the job done.

However, I have to touch the ground connection with my finger in order to avoid a monster buzz sound. This only happens when using the electret microphone, though. If I'm using my laptop or phone to provide an audio source there's no buzzing sounds at all. I recorded a snippet using the electret microphone so you can hear the issue (https://www.dropbox.com/s/z06vdtkpsuwojbh/electret-mic-test.wav). Surely this must be related to a grounding issue but I just can't figure out what's wrong and how to fix it.

At the moment I'm using a standard 9V battery, however I am going to use an external power supply for the final project.

I really hope somebody can point me to a way of fixing this one. I'v been searching the web for days now to find out what's wrong and I know this is an absolutely rookie issue ... but I just can't figure it out.

Any help would be deeply appreciated! Sorry for the noob question...  :-[

 

[radardoug]

You are obviously not connecting the ground to ground. All circuits require a two wire interface, send and return if you like for the electron flow.

 

[JN]

Thanks for replying. 

At first, this was my reasoning as well. I have the "ground" point connected to the zero/minus side of the battery which is the only thing that seems to make any sense to me. If this is not right, what is my ground then?

 

[gyraf]

You need to shield the parts inside a grounded, conducting shield.

 

[ricardo]

How about you show how you connect to the next stage.

What is the next stage?

What sort of connectors?

How do you connect the 'ground' connection to the next stage?

 

[JN]

Hi gyraf,

Stuffing everything into a metal box including the wires that goes to the electret microphone and connecting the box to the ground point in the circuit did the trick! 8) No more buzzing sound! Amazing! I don't really understand entirely why this makes such a big difference but it does work so thank you very much for helping me out. 


Hi ricardo,

In my case, the next stage is an unbalanced input on an audio interface. I'm connecting the "ground" from the preamp circuit to the "ring" on the cable that goes to the interface input. 

 

[PRR]

> understand entirely why this makes such a big difference

I'm recording a Children's Choir. On my lawn. In the recording I hear my neighbor mowing his lawn, and the fish-trucks roaring past.

If I move the choir inside, I hear much less mower and truck noise. Even better with the windows closed.

High-gain electronics will pick up electrical noise the same as the choir-mikes pick up sound noise. What electrical noise? If you go way out in the woods you still get "static" from far-away lighting. When you go anywhere with 120/230V 50/60Hz power you get a LOT of "hum", plus some buzz from electronics and lamps distorting the power wave. But our ears don't hear this, so it seems "odd" at first.

The world is full of electrical noise.

Sensitive electronics MUST be inside closed metal boxes.

Now that you have "sensitive electronics", take it out of the box, connect headphones, and walk around. Usually worse near fluorescent lamps. A refrigerator or air-conditioner may be silent when resting and big-hum when cooling. Power transformers can be strong hum sources.

 

[JN]

Hi PRR,

thanks a lot for the explanation, makes a lot of sense! Amazing that there's so much electrical noise around us