Need more Vout from LM741

[PhillDubya]

**broken link removed**

Hey guys,

I am using a condenser microphone to pick-up sound pulses, and then input them in the form of very low voltage (micro-volt) signals into an LM741 op-amp to amplify them.

The problem is, the sound I am picking up (beep pulses) is not being amplified enough through the op-amp enough to saturate the transistor to .7 volts. What I am getting now, Vout is about MAX 100milli-volts.

As far as I know I should increase the Gain, but I am already using a 10M Ohm resistor as Rf, and a 1k Ohm resistor as Rin, therefore, technically the gain is around 10000 which doesn't make sense, but it is working better than the 200K (Rf) and 1k (Ri) which equaled a gain of 200.

Any ideas how I could boost the output of the condenser mic to much greater than .7 volts maybe at least .8 to 1 volt preferably.

BTW, the components are a little hard to see so here is a list:

*The box with the black X on it is the condenser mic.
*Rf = 10M Ohm, Ri = 1K Ohm
*The two resistors forming the voltage divider are 4.7K Ohm, which inputs 1/2 Vsource = about 5 volts input to non- inv input.
*The caps are .1uF, .01uF, and 1uF, all for de-coupling.
*2222a transistor, but I have available: 3904, 5088, and 4401
*1K Ohm limiting current through the LED in the collector.
*Output of condenser into the inv, input of LM741, with a de-coupling capacitor.

Thanks a lot for your time.

Phill

 

[Willbe]

Run the opamp open loop and attenuate the input to the amp, if needed?

 

[audioguru]

You saved your schematic as a very fuzzy JPG file type instead of as a very clear GIF or PNG file type.
Your schematic is too small to see and since it is a fuzzy JPG then zooming in on it is useless.
You posted your schematic over at PhotoBucket instead of attaching it here.

Your circuit has many problems:
1) The negative terminal of your battery is not connected to ground so the opamp is not powered.
2) A condenser mic needs a 48V power supply. Maybe you have an electret mic instead.
The electret mic has the 48V charge built-in and uses a few volts to power a FET transistor inside.
3) A 3-wires electret mic must be connected properly and some need a load resistor.
4) Your inverting opamp circuit has a very low input impedance (1k) that loads down the fairly high output impedance of the electret mic. If the opamp is re-arranged to be non-inverting then its input impedance would be high and the mic will have a higher output since it won't be loaded down.
5) If the opamp was powered then its output DC voltage would be trying to be at half the supply voltage, saturated and always turning on the transistor.
6) A lousy old 741 opamp is too noisy (hisssss) to be used as a mic preamp.
7) The frequency response of a lousy old 741 opamp drops above 80Hz when its voltage gain is 10,000.
8) The 0.1uF from the mic into the 1k input resistor of the opamp reduces frequencies below 1600Hz.

So you cut the highs and cut the lows and cut what is in between.

Use two low noise opamps (a dual TL072) with the total gain shared so the high frequency response is high enough.
The first opamp should be a high input impedance non-inverting amplifier.
The input coupling capacitor should be calculated to pass low enough frequencies.
The input to the transistor should be a voltage divider so that the transistor is turned off without signal and turned on with a signal.
Maybe the opamp should drive a peak detector circuit so that the transistor does not receive half-wave audio.

Please post the datasheet for your "condenser" mic.

 

[PhillDubya]

Thanks for the replies Willbe and Audioguru.

Run the opamp open loop and attenuate the input to the amp, if needed?

Not sure how.

@audioguru:

1) it is connected it just wasn't on the diagram that I posted, but the terminals for V+ = 10VDC, and the V- terminal is connected to ground.

2)Here is the one I am using: Mic link

3) It has an internal 1k Ohm resistor in the supply wire -->>> the Red wire shown at the link above.

6) Even if I am just using it to pick up audible pulses? Basically the same sound as a keytone on a cell phone makes, is what I am dealing with. I am actually using a cell phone from about 6 inches away and pressing the buttons to make a beep to test this circuit, as it is really close to the actuall pulses I am designing it for.

Is there a circuit you could suggest, using an lm386 that would amplify the really small voltages (microvolts probably) from this mic to voltages > .8 or so?
Reason is, the lm386 is the only other option I have right now.

Thanks a lot audioguru.

 

[PhillDubya]

Actually I just found a couple of op-amps that I have from TI here:

TLO64BCN, and TLO82BCP, LM339N

Would these work better, as I have read MANY posts in this forum and others to avoid the lm741 op-amp all together if you are limited to a single supply voltage, as I am.

Datasheets:

TL064

tl082bcp

lm339n

 

[audioguru]

RadioCrap never shows how to connect their parts because they don't know how.
They do not say if the supply wire has a resistor or needs a resistor. The supply wire might be red, the output might be white and the ground might be black.

The LM386 has a max voltage gain of 200 which might not be enough. Add a 10uF capacitor between pin 1 and pin 8 to make the gain 200.
So use the 741 opamp as a non-inverting high input impedance preamp with a gain of about 20. its output will be at half the supply voltage so use a coupling capacitor to feed the input of the LM386 which is at 0VDC. Calculate the coupling capacitors for a low frequency response down to 100Hz.
With a gain of 20 then the 741 opamp has a response up to 50kHz at low output levels. The LM386 has a response up to 30kHz when its gain is 200.

Make a voltage divider with two resistors to reduce the +4.5VDC output of the LM386 down to below 0.4V to feed the base of the transistor so it is not turned on without a signal but flickers the LED when there is a signal.

 

[audioguru]

Actually I just found a couple of op-amps that I have from TI here:

TLO64BCN, and TLO82BCP, LM339N

Would these work better, as I have read MANY posts in this forum and others to avoid the lm741 op-amp all together if you are limited to a single supply voltage, as I am.

Any opamp works fine from a single supply voltage if you bias its inputs properly like you did.
The TL062 is nery noisy and has low gain.
The TL082 is a noisy TL072. The TL072 has low noise.
The LM339 is a quad comparator. A comparator cannot be used as an opamp.