Electret 1st stage amp mod-DC coupled.

[Mosaic]

Hi all:
I was fooling around with a basic amplifier interface for a generic electret (from Newark) and I came up with this design that seems to exhibit some low noise floor properties with better low end frequency response. (I wanted it to feed a PIC comparator so the DC offset is good for me).
I note that once the LED isn't saturated (lit) the noise floor is better than the original circuit.
Original circuit:
**broken link removed**

Modified Circuit:


Perhaps those with experience in audio amps etc. can pitch in. This is purely experimental here.

 

[ronsimpson]

I have seen several "white noise" or pink noise generators. They can use a Zener diode as a noise source. A LED is the "bright" brother of the Zener. I would think a LED could be used as a noise source.

Try putting a capacitor across the LED. This might increase you gain a little.

**broken link removed**

 

[audioguru]

The original circuit is bad. I simulated it using a 2.7k resistor replacing the electret mic and the 10k resistor that powers it.
1) Its high frequency gain is only 30 times because the input of the transistor is a low impedance of about 2.8k ohms which loads down the signal from the mic and attenuates it. The mic should have a preamp with an input impedance of at least 27k ohms.
2) The input coupling capacitor value of only 0.1uF passes only frequencies above 500Hz so low sound frequencies are missing.

Here is my simulation:

 

[Mosaic]

HI AG:
Do you think my mod makes for an improvement then?

 

[audioguru]

Why is your schematic a negative image?

Your mod reduces noise because the mic and transistor do not work.
Get rid of the LED and use a coupling capacitor from the mic with a high enough value to pass low frequencies.
Because the low input resistance of the transistor loads down the signal from the mic then the output level from the transistor will be low.

The emitter of the transistor should be 0V with no LED. The collector should be about half the supply voltage at 2.5V
so it can swing equally up and down. Then the base will be about 0.63V.
The collector current will be about 2.5V/10k= 0.25mA.
The hFE averages about 200 so the base current is about 0.25mA/200=1.25uA.
The base resistor has 2.5V at one end and 0.63V at the other end and a current of 1.25uA so its value should be
(2.5V - 0.63V)/1.25uA= 1.5M ohms.
The electret mic needs 0.5mA and about 2V so its resistor should be (5V - 2V)/0.5mA= 6k, use 6.2k.

 

[Mosaic]

Thx AG!

 

[Mosaic]

Here's what I made with the amp....an Ironman Arc Reactor light engine with all discrete parts.

 

[audioguru]

Your Light Thingy works very well.
I also use those SuperFlux LEDs in the 4-pins package.

 

[Mosaic]

AG: for surface mount equivalent LEDs what would you suggest?

 

[audioguru]

AG: for surface mount equivalent LEDs what would you suggest?

I have never used surface-mount LEDs. Maybe Philips Lumileds Luxeon have some.