Why would the higher frequencies be reduced?

At the inverting input, the Thevenin equivalent of the two biasing resistors is 500K. This comes after a 10 nF capacitor, for a corner frequency of31.8 Hz. The cap is the series leg and the R is the shunt leg of a 2-element, frequency-dependent attenuator attenuator, a high-pass filter.

AG's statement might be a typo. Frequencies *below* 32 Hz are attenuated at 6 dB per octave.

ak

 

I figured it was probably a typo, was waiting for AG to correct.

 

Of course a comparator chip should replace the opamp.
An opamp is usually used with lots of negative feedback to reduce its distortion. But the small delay in the circuit causes negative feedback at high frequencies to become positive feedback and oscillation. So every opamp has a frequency compensation capacitor that cuts high frequencies at 6dB (half the voltage gain) per octave. A comparator almost always never uses negative feedback so it does not have the high frequency cutting capacitor.

Here is the gain vs frequency curve for the recommended MPC601 opamp. It has maximum gain from DC to 4Hz then the gain drops at 6dB per octave then the gain is 1 at 3MHz.
Negative feedback flattens the gain to a wide bandwidth. With a gain of 60dB which is 1000 time the gain is 1000 times for DC to almost 3kHz. With a gain of 40dB which is 100 times the bandwidth is flat from DC to almost 30kHz.

 

I said that the 10nF input coupling capacitor feeding the two 1M bias resistors is a highpass filter that cuts frequencies below 32Hz. A highpass filter cuts low frequencies. The openloop opamp cuts high frequencies.

 

Of course a comparator chip should replace the opamp.
An opamp is usually used with lots of negative feedback to reduce its distortion. But the small delay in the circuit causes negative feedback at high frequencies to become positive feedback and oscillation. So every opamp has a frequency compensation capacitor that cuts high frequencies at 6dB (half the voltage gain) per octave. A comparator almost always never uses negative feedback so it does not have the high frequency cutting capacitor.

Here is the gain vs frequency curve for the recommended MPC601 opamp. It has maximum gain from DC to 4Hz then the gain drops at 6dB per octave then the gain is 1 at 3MHz.
Negative feedback flattens the gain to a wide bandwidth. With a gain of 60dB which is 1000 time the gain is 1000 times for DC to almost 3kHz. With a gain of 40dB which is 100 times the bandwidth is flat from DC to almost 30kHz.

Thanks for the explanation. I understand the issue a little better now at least. I'll do some experimentation with negative feedback to see if it makes any difference here.

 

Yes, adding some negative feedback will reduce the extremely high gain and widen the range of frequencies.
My Sound Level Indicator project has 10 LEDs at 3dB steps and it uses an electret mic. The circuit has one opamp with a gain of 101 times (made with negative feedback) as the preamp with a bandwidth of 15kHz. Another circuit increases the gain 10 times at low levels.
Then it can be very sensitive to almost all sound frequencies. Your opamp with no negative feedback is too sensitive to only very low sound frequencies.

 

xox
Just for clarity what exactly are you trying to achieve with the microphone and LED?
If my understanding is correct you are just looking for the circuit to provide a visual indication via the LED when speaking into the mic. Is that correct?

 

xox
Just for clarity what exactly are you trying to achieve with the microphone and LED?
If my understanding is correct you are just looking for the circuit to provide a visual indication via the LED when speaking into the mic. Is that correct?

Precisely. It just needs to light up when the wave form is positive, otherwise it should produce an output voltage of zero.

 

It just needs to light up.

In that case the circuit you posted will do that just fine with a few modifications, C1 and Vr1.

 

Is this for speech only and not for music?

 

From the reply I got in post #28 I would say yes but it would work for music as well.
Do you mean music picked up by the microphone ?

 

It will also be activated by coughing, sneezing, dogs barking, a motorcycle driving by and many other sounds.

 

It will also be activated by coughing, sneezing, dogs barking, a motorcycle driving by and many other sounds.

Of course it would if those sounds are loud enough. The microphone is not that sensitive.

 

If the microphone can pickup your voice then the opamp's uncontrolled gain of almost one million times can pickup all kinds of noises, or you can turn down the threshold so that it misses detecting the noises and miss detecting some or all of your speech.

 

If the microphone can pickup your voice then the opamp's uncontrolled gain of almost one million times can pickup all kinds of noises,

Wrong!
Your theory is illogical.
Obviously you have not tried the circuit yourself.
I have and stand by my previous assessment of its operation.

 

You can turn the trimpot so that the LED lights all the time, turn it so the LED lights on extremely small sounds or the noise from the opamp itself, or turn it so that it detects only screaming and dogs barking.