# Microphone Filtering and Blocking

#### cl10Greg

Joined Jan 28, 2010
66
Hello Everyone,

I have a quick question about my audio section I am working on with a microphone. I have attached a screenshot of my setup. I am debugging the circuit in a couple of different areas and keep running across the same issue and I am not sure why....granted I am new at this stuff. In the section below, I am able to feed in some constant audio tones and observe the microphone signal. I see the input signal that looks clean. The pre-amp does its job and amplifies the output and does the DC offset so its all above 0. Then it comes to the inverting unity gain section and nothing! Their is no signal on the output of the op amp. I had a similar issue from another part of the analog section and it seems to be the resistors. Now I can do some basic ohms laws to see what the voltage might be. With my tone I am generating, I am seeing a sine wave of about 1V amplitude at 1.65kHz. It has a DC offset of right around 2.5V. With that being known, I can deduce that the current into the amplifier to be (2.5V/10000 = 25uA). I am guessing what is happening is that the resistor values are too high and essentially giving the op-amp nothing to use. If that is true, can I get some guidance on what size resistors I should be using for this type of application?

In the other section of analog signals, its a similar approach to this and again if I short out that resistor, the signal is back. So obviously the values are too high or I am missing something in the world of op-amps.

Before I did build this, I did some model simulation and I know what it "should be" with the caveat of this being the real world and not having all the parameters.

#### Ian0

Joined Aug 7, 2020
8,947
Non-inverting input of U21 should be biassed to half-supply, and the input may need to be capacitvely coupled.
With pin 3 at 0V, the op-amp would like inverting input also to be at 0V. The only way that can happen is if the output is at MINUS the voltage on the output of U20, and without a negative supply, it can't do MINUS!

#### cl10Greg

Joined Jan 28, 2010
66
Hello @Ian0 ,
Thanks for the response. Let me see if I can piece this together (probably not hah). The input into U20 pin 1 for my source (me whistling hah) is a DC offset of 1.25V with a amplitude of 72mv (1.22V to 1.29V), I get this from my scope. The output side after the preamp is a DC offset of 1.82V with am amplitude of 920mV (1.36V to 2.28V). That is the signal that is feeding into U21. With that being said....I think I am biased and it wouldn't have to be in negative land for that to work? Am I seeing this wrong?

#### cl10Greg

Joined Jan 28, 2010
66
I was more or less expecting something like the attached. The red being the signal from the preamplifier and the green signal being the inverting output. Now I did want a unity gain for noise cancelling aspect of everything and this appears to be clipped.

#### Ian0

Joined Aug 7, 2020
8,947
No. The op amp wants both of its inputs at the same voltage.
So, if +input is at 0V, then it will try to bring -input to 0V.
For -input to be at 0V, then 157uA has to flow in the 10k resistor (1.571V/10k), and that means that 157uA must also flow in the second 10k resistor. The only way that can happen is if the output were at -1.571V

Try it again with the +input at a different voltage (try half the power supply voltage)

#### AnalogKid

Joined Aug 1, 2013
10,790
I am guessing what is happening is that the resistor values are too high and essentially giving the op-amp nothing to use.
Nope.

The opamp input impedance is very high. The datasheet will list its bias current, the current that the inputs "use". The problem is that the two inputs are 2.5 V apart, and the output stage is saturating, The circuit is a unity-gain inverter, and the inverting input has +2.5 V on it. The output is trying to go to -2.5 V, which is the input times -1, which is 2.5 V below the part's negative rail,

1. AC couple U20 into U21 (R123).

2. DC couple U21 non-inverting input to Vcc/2.

Also, what is the part number for U20?

ak

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#### cl10Greg

Joined Jan 28, 2010
66
ooooo. I see. Yea I was looking at it wrong. U20 is MAX4466EXK+T. Thank you both for the responses.

#### Ian0

Joined Aug 7, 2020
8,947
AC coupling is always a good idea between stages, as otherwise the DC offsets build up.
Put a capacitor in series with the 10k resistor on the input, so that C>1/(2πfR) where f is the lowest frequency you want to amplify.

#### Audioguru again

Joined Oct 21, 2019
6,453
Your circuit will work properly without rectifying the signal if you bias the + input of the second opamp with the same +2.5V as the first opamp.

Why do you call cutting all low frequencies and cutting all high frequencies "noise reduction"? Is music and speech noise?

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#### cl10Greg

Joined Jan 28, 2010
66
@Audioguru again Thanks for the input. I agree with the biasing and that should fix the noise cancelling part of the circuit. I am confused about the second part about frequencies. This application is using this part of the circuit for active noise cancelling outside of the voice input. Basically doing the cheap method of taking the ambient sound and inverting it and injecting that into the signal. Maybe that is a bad way to do it? The environment will be pretty noisey so I am banking on the fact that the microphone is only picking up most of the ambient and not much of the users.

#### Audioguru again

Joined Oct 21, 2019
6,453
I am sorry that I mentioned cutting low and high audio frequencies without calculating them. The filters are fine and are shown on the datasheet of the preamp IC.
You can cancel background noise if you use two identical mics and preamp circuits with one signal inverted then the two signals are mixed together. One mic points towards the person talking close to a mic and the second mic very close to the first mic but pointing away from the person talking. The mics must be close together to avoid phase shifts caused by distance from preventing noise cancellation at some frequencies.
Most electret mics have a very wide pickup angle then the cancellation will not be much. The person talking must be very close to one mic so that the gain can be turned down.

#### LowQCab

Joined Nov 6, 2012
3,594
Electret Mics are also available in "Noise-Cancelling" versions as well,
eliminating the need to use 2-Mics, and without the Phase-Shift aggravations.
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