Thank you Irving, what a fast answer!

So, I lower R3 to lower the amplification? Or increase R2.

Can you also tell me what the 100k resistor does?

 

I mean the opposite with R2 and R3 of course ..... wops

 

I'd increase R3.

The 100k resistor sets the bias point for both inputs to 1/2 Vcc. Its not the best solution, but has the advantage of needing the least numbr of components. Its a poor option for opamps needing significant input bias current.

 

Thank you so much!

I'll try that right away.

 

I've tried to increase it to 47k, but I can only see a minimal decrease in decibel level.

Do you think I'll have too increase the resistor more? Or is there something else I can do?

 

What is the part number of your opamp?
Which resistor did you increase to 47k?
Look here:

 

The part number is OP27.
And it was R3 that I increased to 47k. Wouldn't that help?

 

I do not have the same circuit as you have though.
I still have the same circuit as I drew in the beginning of this thread, it's kind of hard to change. Values of resistors are alright be others might be hard since it's a PCB.

 

Well, I figured I'd to describe my new problem a bit more.

The thing now is that I have a microphone and an amplifier that works which I connect to a DSP, ADAU1467.
But when I connect it to the software included, Sigma studio, I measure the decibel level (dBFs I think, it goes from -96 dB to 0 dB at least) and at normal level there is no problem, but at high levels (I would like to be able to have at least 100 dB) it's clipping. So I have to somehow lower the amplification.

I would like to be able to calibrate my microphone so that I know what dB it is.

Do you guys have any good suggestions how I would do all this?

Thank you!

 

OK, my apologies I gave you a bum steer. My post #20 swapped R2 and R3. Gain = R3/R2 = 10k/100 = 100. So reducing R3 or increasing R2 reduces the gain.

Not sure what you're getting at, but 0dB for audio is, IIRC, 1v into 600ohm, so 100dB = 20log(1/g) => g = 10000
and your gain as set is 100 (40dB).

Your mic is spec'd as:

ie -40dB, so 40dB of amplification is going to get you to 0dB...

BUT that's only true if your mic is working into the recommended load impedance of 2.2k

SO
did you change the resistors feeding the mic as AGa recommended?
What supply voltage are you using?

 

Your original preamp had a serious problem that I discussed before:
The electret mic does not work or it barely works poorly when fed 5V through two 10k resistors. Then the mic is clipping, not the preamp circuit. I showed that you should use no more than 1k plus 5.1k.

Now you are using an OP27 opamp that needs a total power supply at least 12V but you have only 5V. All its specs in the datasheet show 30V. Its sales sheet says Not Recommended For New Designs so it is old.
An OP134 single audio opamp is low noise and works with a total supply that is a minimum of 5V but will have more output signal if its supply is 9V.

 

Hi @Audioguru again

Did you mean OPA134? I cant find datasheets for OP134 except an old Burr Brown design...

 

Yes, the OPA134 is a single low noise low operating voltage audio opamp made by Texas Instruments (they bought Burr Brown).

 

Ah that explains it. Sadly TI's Nov '21 Spice model wont run at 5v, it needs 5.5v though still manages a suitable output swing.

Here's simulation with the correct mic resistors... no clipping and about -0.5dB output (2.7vp-p, 0.95v rms)


and flat within 1.5dB from 230Hz to >50k


With the wrong mic resistors.... lots of clipping...

 

Out of curiosity: why did you change the original EK022 topology in such a radical manner?
https://www.electrokit.com/uploads/productfile/41017/EK022-datasheet-en.pdf

 

Hello, you have to filter out the noise, it is not enough to mute it, unfortunately I had the same, but I filtered it out and it works now as it should, there is no noise

 

Okay, so actually I have made some change to the circuit you posted AG and I now have this (without the resistors you call R6 and R7). Would this work for my microphone? Now it has +5V supply?

I might change C1 to what it was before 220nF. I'm so much of a beginner so I can't really say how changes in the circuit and different values will affect the result, that's why I'm really happy that you all are helping me out, I learn a lot.

 

There are several issues with the design. First one: why do you use the op-amp in a noninverting configuration. An electret microphone is a current source (because it contains a JFET transistor inside the capsule). Therefore the most natural choice is a transimpedance amplifier, converting current from the microphone (variable part) into voltage, not a high impedance (noninverting) voltage amplifier. Please have a look here for a detailed explanation:
Single-Supply, Electret Microphone Pre-Amplifier Reference Design (ti.com)

The mic resistor R1 = 10k is too large for the 5V supply and 0.5 mA electret microphone; the 0.5 mA current would cause a 5V voltage drop on R1, leaving the microphone without any voltage across its terminals. As a result, both the microphone voltage and current will be too low for proper operation in an actual circuit. C1=10u doesn't hurt, but it is unnecessarily large: your amplifier has an input resistance of 100k; therefore, the corresponding cutoff frequency is 0.16Hz, far below the hearing range.

 

The circuit in post #37 has problems:
1) R1 feeds all the power supply noise into the preamp input to be amplified. My fixed circuit added R6 and a 47uF capacitor to filter out the power supply noise from being fed to the preamp input.
2) The value of R1 is too high which causes the Jfet in the electret mic to clip the positive parts of the waveform.
The mic needs a minimum of 2V on it then the max resistor values are (5V - 2V)/0.5mA= 6k. That is why I used 1k plus 5.1k.
3) You shorted the opamp inverting input to the voltage divider which causes the opamp to amplify its DC input offset voltage which could cause severe distortion.
4) Your low value of C3 was not calculated to match the low value of R2 then all low frequencies are missing. That is why I used 100uF. The article from Texas Instruments shows the simple calculation.
5) Again, the opamp has no part number so it might not work when powered from only 5V, or it might be too noisy for a mic preamp. The opamp selected by Texas instruments is only available in a tiny package.

I forgot to add the important series output resistor to prevent the capacitance of the output cable from causing the opamp to oscillate at a high frequency. I use 100 ohms, Texas Instruments uses 49.9 ohms.

I do not use the Texas Instruments inverting circuit with its mic very close to the preamp because many of my mic preamps have a cable from a fairly distant electret mic. The inverting input of an opamp is extremely sensitive to interference. Some shielded audio cables are poor quality with poor shielding.

 

3) You shorted the opamp inverting input to the voltage divider which causes the opamp to amplify its DC input offset voltage which could cause severe distortion.

DC biasing is another issue with circuit #37. It relies on R2 for DC feedback. Any input offset is amplified 100x. The circuit would benefit from a more standard topology (with no DC amplification).