pramplifier circuit

[asrarbaig]

Hi everyone, well i m new to this forum an i need your help. i m building a voice scrambler but i am stuck in between. i would like to know which mic-preamplifier circuit is the best for producing an ouptut voltage of 0.7 Vpp or less than that so that i can give it to my scrambling circuit. thanking you all,

bye.

regards,

Mughal.

[audioguru]

Hi Mughal,
If you are using a two-wire electret microphone, this circuit works very well and you can adjust its pot for the amount of gain:

[asrarbaig]

Hi Mr. Guru,
                  thanks for the wonderful and easy to build circuit that you provded. I tested the circuit and it is perfect means that i can change the output as i like. I need your help again i am using this circuit for my project but i dont now how it works can you help me regarding this in detail if possible. well i have to write it in my projet report. Waiting for your reply,

Thanking you,

Yours Faithfully,

Mughal.

[audioguru]

Hi Mughal,
I'm glad that the preamp works perfectly. ;D
The opamp is low noise, wide bandwidth and very low distortion for an audio preamp. It operates as a non-inverting opamp circuit with the ratio of the resistance of the pot and R4 providing negative feedback determining its AC voltage gain. C2 is a dead short to audio frequencies.

R1 provides the FET transistor inside the electret mic with operating current and the electret material holds a high voltage charge for the "condenser" part of the microphone to work.
C1, C2 and the output capacitors pass the audio but block DC.
C4 provides a low impedance for the supply to keep the opamp from oscillating.

R1 and R2 are a voltage divider to provide a half-supply reference voltage for the input of the opamp (and its output since it is a DC-follower). Then its output can swing equally up and down for the most dynamic range.

Because the opamp has a wide bandwidth it can oscillate if a shielded cable is connected to its output due to the capacitance of the cable. Therefore a 100 ohm resistor should be connected from the output of the circuit to the cable to isolate its capacitance.

[asrarbaig]

Hi Mr. Guru,
                  Thank you for your kind reply. I have one more doubt regarding the circuit if we change the variable resisor with a higher value we can increase the gain to a more extent but is it safe to do that may be to use a 250K ohm instead of 100K ohm. what do u suggest.

well thank u again,

Yours Faithfully,

Mughal.

[audioguru]

Hi Mughal,
Anything over 100k is too high for a pot as a negative feedback resistance because it will pickup interference. To get more gain, reduce the value of R4. 470 ohms will give a max gain of 214 and since the TL071 is wideband then its audio high frequency response is still fine.
With 470 ohms as R4 then change C2 to 47uF for good low frequency response. ;D

[asrarbaig]

Hi dear Mr. Guru,
                            Thanks a lot for helping me out. actually i am using a low pass filter after the pre-amplifier circuit but when i am connecting both the circuits i am getting a voltage of 10-12mv at the low pass filter output where as as the input of the lowpass filter from the pre-amplifier is 100 mv. my cutoff frequency is 3.5 KHZ i dont know whether this preamplifier can use for this frequency. waiting for your reply,

thanking you,

Yours Faithfully,

Mughal.

[audioguru]

i am using a low pass filter after the pre-amplifier circuit but when i am connecting both the circuits i am getting a voltage of 10-12mv at the low pass filter output where as as the input of the lowpass filter from the pre-amplifier is 100 mv.

Is the microphone or opamp making hiss? Much more expensive microphones and audio opamps are available with a much lower noise level. A gain of 214 is very high.

my cutoff frequency is 3.5 KHZ i dont know whether this preamplifier can use for this frequency.

Are you using a lowpass filter for a telephone conferencing system? I made many of those systems and used a 10dB boost at 3.5kHz followed by a sharp cutoff.
Don't tell Bell. Telephone systems are supposed to cutoff frequencies above 3kHz. Bell told me that it is normal for telephone lines to have reduced response above only 1kHz which is what I measured. The boost made them sound crisp and clear.

With a gain of 214, a TL071 opamp has a flat frequency response to about 35kHz.
An old LM324 or LM358 is flat to only about 3kHz with such a high gain. ;D 

[asrarbaig]

Dear Mr. Guru,

I am sorry for being absent for a long time. I am not using a low passfilter and it is still ok. If i connect an audio power amplifier with a pre-amplifier i can hear a clear noise. Actually i m using a pre-amplifier circuit and would like to know what is the purpose of capacitor C2 in the circuit. You gave the explanation but i am sorry i didnt understand it. As you have said it block the dc but it is not at the input of the circuit so i was wondering. Well i will be grateful to you for the kind reply,

Thanking you,

Yours Faithfully,

Mughal.

[audioguru]

C2 is part of the opamp's inverting input circuit. It blocks DC and allows the opamp to be a DC follower with a gain of only 1 at DC. If C2 is shorted then the gain of the opamp would force its output to the positive supply. With C2 in the circuit, the DC output voltage is at half the supply voltage, allowing it to swing equally up and down.