microphone interference

[UltraRC]

Hello, I have made myself an external microphone for my computer (schematic included).
I am getting some interference from our electric fence as I live in a rural area with animals, so whenever the electric fence unit sends out a pulse of electricity, I'm thinking that the whole fence-line which would be around 1km or so surrounding our house and property is acting as a massive antenna. The preamplifier section is picking up the interference which then gets amplified around 80X and you end up with an annoying ticking noise in my recordings.
The electret mic its self is connected to the end of some coax that I use for making antennas, the positive connected to the signal and the negative to the outside of the coax. The coax is about 20cm long and is then connected to a circuit board for amplification.

So what I'm asking is how I could get rid of this. Would this be a form of high-frequency interference in which case I could simply use a low pass filter? Or would I need some form of physical shielding around the circuit board made from some sort of metal? And also while I'm at it there is a lot of static in the background of my recordings, would that maybe for the same reason? I don't know.

Any help would be good, thankyou.

[DimitriP]

If it's not battery powered, try batteries. The noise could be coming in from the power supply.
Use a  100nf, 10nF and 1-100uF where power enters the preamp. 
Minimize the distance of the electret connection to the junction of R1-C1.
 

[Audioguru]

A BC547 is not low noise. A BC549 is a BC547 that is selected for low noise.
The electret mic draws only 0.5mA so the value of your resistor feeding it is much too low. Try 16k ohms and the level will be higher too.

[Cerebus]

A BC547 is not low noise. A BC549 is a BC547 that is selected for low noise.
The electret mic draws only 0.5mA so the value of your resistor feeding it is much too low. Try 16k ohms and the level will be higher too.

Moreover the internal JFETs in those mike capsules tend to have a V_DS breakdown voltage of only around 6V so that 1k on a 12V supply is probably already causing breakdown. The 'standard' feeder resistor from a 5V supply for most electret capsules is 2k2 with, as you say, 500uA typical current so a DC bias point of 1.1V. With that in mind 16k seems a bit low (4V bias point). A bias point of 3V or less would seem wiser, so I'd plump for around 18k as a starting point, possibly higher, and measure the resultant bias voltage before settling on a final value.

[radiogeek381]

In general, the first resort in these cases is some decoupling cap.

1. As DimitriP says, a few caps (at least a 0.1 uF) from the power rail
to ground is a good start.

2. I'd also add a 0.1 from the "hot" terminal of the mic to ground.

2a. It is worth looking at the OUTPUT of the circuit as well -- the impulse
noise could be coming in on any wires.  Perhaps a little decoupling
on the output terminal wouldn't be all that bad.  0.01 uF from Q2's collector
to ground at 20kHz is about 800 ohms reactive.  That might be too much of a load
or present too much of a roll-off for you, but this adventure might
require some compromise....

2b. The voltage divider providing the base bias is really kind-of soft --
the base is looking back into 151Kohms for its bias supply.  A 0.01 uF
cap at a minimum would bypass some of the RF noise.  You might also
consider reducing the bias resistors by a factor of 10 or more.  It may not
be important in this circuit, but those two bias resistors are contributing
about 2.5 uV of noise in a 10kHz bandwidth.  (Vn = sqrt(kTBR) )

3. Lots of folks like ferrite beads.  Sometimes they help, sometimes they
don't.  The idea is to add some inductance in the paths that might be
contributing interference.  I've used "common mode" chokes at times.
Some of it is like throwing salt over your shoulder -- it can't hurt.
(This is a reasonable approach for "one-offs" but if you are designing
a product, more diligence is appropriate.)

If you want to try the ferrite bead idea, slip one or two over the +12 line.
If you have a toroid, wind the mic cable through it a few times.  If you have
a second toroid, wind the 12V line and the ground line through it a few
times.  I don't give this a big probability of curing the problem, but if all
else fails...

I'm not really well plugged in to this audio stuff, but do you really need
200x voltage gain here?  (I'm thumb estimating this based on the
ratio of collector resistance and bypassed emitter resistance, ignoring
the output loading.  What kind of ouput are you driving?  )

[BrianHG]

Worst case scenario, after performing the previous recommendations, you may need to encase the preamp in a shielded/metal container which is secured to the mic.  I would try tinfoil connected to the GND first just to see if it makes a difference.  If so, you may use a thin tin sheet which you can cut and bend into a solid container + you can solder the GND and mic case to the tin.

If the interference is making through your audio cabling or affecting your sound card audio input, maybe a USB mike might help you since the analog from those mics is connected directly to a sampler IC right in the mic, then sent digitally to your PC.

For example, have you tried a battery powered laptop or a tablet PC with your mic, the through the wall.  Your interference may be a result of a nasty spike coming through your home's AC power.

[eeFearless]

I had an electret preamp circuit that could pick up microwave tower interference (when I was nearby).  The preamp was an opamp configured as a miller integrator, so I was not expecting this.  But, semiconductors can do strange things in the presence of unwanted high frequency signals.  The entire circuit was shielded very well, as was the mic cable.  I fixed it with a RC low pass filter at the preamp input (1K and 100pF, 0402 SMT).   I was concerned about the resistor adding noise (as the preamp itself was very low noise).  But, in practice, it added no discernible noise.  The SNR was set by the microphone (which was something like 68dB using a Sanken COS-11D, an excellent mic).

This may work for you.  Give it a try, and good-luck!

[eeFearless]

Moreover the internal JFETs in those mike capsules tend to have a V_DS breakdown voltage of only around 6V

Although probably not related to the fence noise you are picking up, you should check the datasheet for the electret.  Most jellybean electrets that I've used have a max voltage spec of 10V.

[Buriedcode]

A common thing I've seen in small/cheap electret preamps is to add a filter on the bias resistor - R1 in your schematic.  That is replace R1 with two series resistors to make up the ~15K, and a large cap to ground where they are connected.  Say a 1k to V+, with a 22uF cap to ground, then a 15K resistor from this to the mic capsule.

Single transistor preamps can be surprisingly good for PC voice comms, plus they're simple.  But because of the bias requirement - the resistor form the V+ straight to the amplifiers input, any noise on the power supply is amplified as well.   A simple RC filter on this usually helps.