Need Help Audio Preamp Circuit

[audioguru]

The LMV721 has a fairly high input bias current so its bias resistors R1 and R2 are 4.7k when the supply is only 3V. But with a 5.5V supply their values can be 10k each. My circuit uses 100k each.

The electret mic with the resistor that powers it has an output resistance of only 2.5k with a 9V supply and is about 2k with a 5.5V supply.
Then the ratio of the mic resistance to the preamp input resistance is only 2.5 times so the output level from the mic is reduced.
My circuit does not reduce the output level from the mic.

Here is a fairly good mic preamp using an LMV721 opamp:

 

[TecIE]

The LMV721 has a fairly high input bias current so its bias resistors R1 and R2 are 4.7k when the supply is only 3V. But with a 5.5V supply their values can be 10k each. My circuit uses 100k each.

Ok! But. The biasing resistors R1&R2 are in parallel so if i use 10k they will be at 5k bias
resistors so that will at about 3.7-4V.So i should use 20K resistors for R1&R2 which would
give me 10k in parallel with a voltage supply of 5.5V.Right.

Then the ratio of the mic resistance to the preamp input resistance is only 2.5 times so the output level from the mic is reduced.
My circuit does not reduce the output level from the mic.

How can i prevent the output from reducing from the mic.


Also can i use an variable trimmer pot to increase the gain on the feedback resistors.
And whats the highest gain can i produce from this amp.

I do not have 100uF capacitors.That's use in the circuit with the 330ohm resistor.
To prevent low frequency oscillation when the battery drains down.
The highest capacitors i have is 47uF or 10uF. Will they be ok to use. with my 5.5V supply.

 

[audioguru]

Ok! But. The biasing resistors R1&R2 are in parallel so if i use 10k they will be at 5k bias
resistors so that will at about 3.7-4V.So i should use 20K resistors for R1&R2 which would
give me 10k in parallel with a voltage supply of 5.5V.

No.
If you use 10k biasing resistors then they divide the 5.5V in half and produce a bias voltage of 2.75V that you want. Texas Instruments used 5k resistors with a 3V supply so you should use (5.5V/3V) x 5k= 9.2k but 10k resistors will work fine. Then the current in the biasing resistors is higher than the input bias current of the opamp.

How can I prevent the output from reducing from the mic level?

By using an opamp with high input resistance Fet inputs and using a non-inverting opamp circuit.

Also can i use an variable trimmer pot to increase the gain on the feedback resistors?
And what's the highest gain can I produce from this amp?

Yes you can use a trimpot to adjust the gain.
The gain of an opamp drops at high frequencies. For the LMV721 opamp the graph in its datasheet shows that 20kHz (the highest audio frequency) becomes reduced when the gain is higher than 55dB which is 600 times. But a mic does not need gain higher than about 200.
The highest gain is about 700 at DC and low frequencies which is very low for an opamp. Most opamps have maximum gain at DC and low frequencies of 200,000 to 2 million.

I do not have 100uF capacitors. The highest capacitors I have is 47uF or 10uF.

Buy some small and inexpensive 100uF electrolytic capacitors.

 

[TecIE]

I need to use the parts that i have on hand to build the amp.

Enclosed: Circuit thats in post #21.With the changed parts that i have on hand.

I need exactly a gain of 50.With out loosing any gain.at the highest FQ 20Khz to
the lowest fQ of 20Hz.

Tell me what will happen with those values with the changed circuit. And what parts would need to change.To get what i need.

 

[audioguru]

You need to learn when parts are in series and when they are in parallel.
You also need to learn the simple formula to calculate the value of a coupling capacitor for a certain cutoff freqyency.
I comment on and correct your schematic here:

 

[alec_t]

I need exactly a gain of 50

Why? For most audio applications the precise gain is irrelevant.

 

[audioguru]

The gain of the preamp is determined by the loudness of the band or singer, their distance from the mic, the sensitivity of the mic, the resistance of the mic (since this preamp is inverting) and the sensitivity of the amplifier.

The amplifier usually has an input volume control to be set for variations in all the previous things.

 

[TecIE]

You need to learn when parts are in series and when they are in parallel.
You also need to learn the simple formula to calculate the value of a coupling capacitor for a certain cutoff freqyency.

I mostly Been doing this electronics stuff on my own.

Yes! This is why im starting tech school this September...

These were the only parts i have on hand. Why buy them. When kit parts will be supplied in class,
to use in 2weeks.

The 1M trimmer pot can be use at 600K. The 1M is what i have.

My mistake.I Had a 4.7K and 10K. It was not 2.2k.I tested the resistor with my multimeter.Was
4.68K 5% gold band 1/4watt. Close enough.
So i will use this. As you suggested 4.7K.

What is the simple math for calculating these capacitors for the cutoff FQ.Please explain.If you May.

Also! I only have 1uF caps in my juck Box. So i wrote the wrong value it should of been 1uF not 0.1uF.Sorry!

For capacitor C2 should that be a 1uF cap also as C1... Is 0.33 to low.

Thank you very much for your help and your comments...

 

[audioguru]

The simple formula for calculating the cutoff frequency or calculating the capacitor value or resistor value if you know the other two is:

 

[TecIE]

If Capacitor C1 is at 1uF shouldn't C2 be the same value capacitor at the ouput.Correct!
Or should it be 3.4uF for a cutoff of 10hz.Not sure.

 

[audioguru]

If Capacitor C1 is at 1uF shouldn't C2 be the same value capacitor at the ouput?
Or should it be 3.4uF for a cutoff of 10hz.Not sure.

The value of C2 depends on the resistance it feeds. A power amplifier or a recorder?
If it feeds 12k ohms then the cutoff frequency of a 1uF output capacitor C2 is 13.3Hz.

The input 1uF input capacitor and the 1uf output capacitor each reduce the output at 13.3Hz by -3dB or 0.707 times the level. Then both add to produce an output that is -6dB which is 0.5 times the level of higher frequencies.
If the power amplifier or recorder input is 12k ohms then 26.6Hz will be -3dB which is 0.707 times the level at higher frequencies. -3dB is heard as a small drop of level.