Microphone analog circuit  - Page 1
Forum section: electronics / beginners

author: nledinhthanh
author_rank: Newbie
timestamp: May 11, 2023, 01:54:49 pm
content: Hi, I am trying to record the signal from the microphone. I am using a 5V DC, connecting to a 2k Ohm resistor, then connecting it to the condenser microphone. The output is run through the low pass filter, cutting off around 480hz(33k ohm 10nF), and the high pass filter, cutting off at around 100hz(150k ohm, 10 nF). I don't intend to do any records; I just use the data for localization for my little project. After that, the signal is run through 100-times and 3.5-time op-amp amplifiers. Then run against another 100hz low pass filter to remove any DC offset left. However, I receive a signal as a straight line on my oscilloscope, which seems weird. I am trying various microphones but received the same result. Can anyone help me with it? Here is the list of microphones I have tried:https://uk.rs-online.com/web/p/condenser-microphone-components/7243153https://uk.rs-online.com/web/p/condenser-microphone-components/7542104https://uk.rs-online.com/web/p/condenser-microphone-components/7243134P/S: the circuit is base on the final project from Cornell University:http://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/f2013/agw39_mag338/agw39_mag338/agw39_mag338/agw39_mag338.html

author: Andy Watson
author_rank: Super Contributor
timestamp: May 11, 2023, 02:14:41 pm
content: Quote from: nledinhthanh on May 11, 2023, 01:54:49 pmIf this is correct, you've filtered-out all of your signal. You need to overlap the pass bands of your filters.

author: nledinhthanh
author_rank: Newbie
timestamp: May 11, 2023, 02:18:41 pm
content: Hi, I mean the other way around. Thank you for pointing that out. here is what I mean of the circuit look like

author: Andy Watson
author_rank: Super Contributor
timestamp: May 11, 2023, 02:31:24 pm
content: How are you powering the op-amps? If you are not using dual supplies you will have to bias the op-amp inputs to around 2-2.5V to give them "room" to work.

author: nledinhthanh
author_rank: Newbie
timestamp: May 11, 2023, 02:40:56 pm
content: Hi, I am using the same power source, but I checked the op-amp's power and ensured it has a stable 5V DC and no AC. However, when I checked by running the microphone output directly after the initial bias power supply, the output also stayed straight. That makes me wonder is normal for the condenser microphone. Because I have tried like 6 kinds of them, some from eBay and amazon brands

author: Andy Watson
author_rank: Super Contributor
timestamp: May 11, 2023, 02:48:21 pm
content: The signal from the mics will be the order of mV - probably too little to register on your oscilloscope.What have you done with pin 4 of the op-amps? Does it go to a negative supply, or have you connected it to ground (0V)?

author: nledinhthanh
author_rank: Newbie
timestamp: May 11, 2023, 02:50:03 pm
content: Hi, I have follow the lm358 pin out, pin 8 to VCC, pin 4 to GND

author: nledinhthanh
author_rank: Newbie
timestamp: May 11, 2023, 02:54:16 pm
content: I don't think the op-amp has anything to do with the current situation because the amplification process is fine. I have received an output of around 2-2.5V. However, the signal doesn't seem like respond to the sound. so it's kinda weird for me

author: mikerj
author_rank: Super Contributor
timestamp: May 11, 2023, 03:11:05 pm
content: Quote from: nledinhthanh on May 11, 2023, 02:40:56 pmAs drawn your circuit will not work with a single ended supply i.e. your 5v and 0v. You are trying to amplify an AC voltage which has no DC component (after C2), but without a negative supply rail the op-amp output cannot go negative. Either supply a positive and negative rail to the op-amps, or use a rail splitter to provide a mid-scale voltage to bias op-amp operating points.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 11, 2023, 03:19:08 pm
content: Quote from: nledinhthanh on May 11, 2023, 02:54:16 pmWas that before you added C2 to the circuit?Step away from the oscilloscope for a moment and consider the DC bias conditions of the circuit. Label op-amp inputs and outputs with the DC voltage that you expect to see (you should not need a calculator!) Then consider what happens when the input signal is negative.If the compnents were ideal you should see the positve portion of the mic signal, however, I suspect that the non-ideal offset voltage is swamping your signal and you are just seeing a striaght line.

author: nledinhthanh
author_rank: Newbie
timestamp: May 11, 2023, 03:20:55 pm
content: Just to clarify, is that the correct way to connect the op-amp? And I think I connected it to the positive power supply, as you said. Here is my actual design. The period photo was just the inspiration for my little project

author: Andy Watson
author_rank: Super Contributor
timestamp: May 11, 2023, 03:46:40 pm
content: Ah, I see! The bigger picture is that you're not interested in amplifying sound, but only in detecting the positive portions. In theory the circuit should work, however, I suspect you will fall-foul of the non-ideal chararacteristices of the op-amps.The resolution of the photo is obscuring the details, but the op-amp [? is it - can't tell] towards the lower right of the picture appears to have been placed such that its pins are shorted across.

author: nledinhthanh
author_rank: Newbie
timestamp: May 11, 2023, 03:55:48 pm
content: Hi, do you mean the circuit on schematic, because I am using the altium design. Therefore, tobe able to pass the schematic test, all of the pin need to be connect. And as I don’t use the last half of the last lm358 opamp, I was short it.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 11, 2023, 04:02:17 pm
content: Quote from: nledinhthanh on May 11, 2023, 03:55:48 pmNo, I mean on the bread-board - but it is difficult to tell due to the resolution (or lack thereof!)

author: nledinhthanh
author_rank: Newbie
timestamp: May 11, 2023, 04:19:45 pm
content: I am quite sure nothing have been shorted. However, I still have not received the expected results. It seem quite weird

author: Andy Watson
author_rank: Super Contributor
timestamp: May 11, 2023, 04:52:02 pm
content: My mistake - I was looking at the potentiometer. As far as I can tell, the bread board layout conforms to the circuit diagram. I assume the bias for the mic capsule (R1) is off screen?Do you have any other test equipment available - like an oscillator or function generator?

author: nledinhthanh
author_rank: Newbie
timestamp: May 11, 2023, 08:09:58 pm
content: I have a SFG-1003 function generator available for testing. However, their gate is too big, and it properly will not fit the breadboard

author: Zero999
author_rank: Super Contributor
timestamp: May 11, 2023, 09:20:29 pm
content: The op-amps needs be properly biased to work. The LM358's output can only swing to within 1.5V of the positive rail, which is 3.5V when run at 5V, so it should be biased at 1.75 for maximum voltage swing.The lower and upper cut-offs are 106Hz and 530Hz. Are you sure this is what you need?The LM358 produces nasty cross-over distortion, but presumably it doesn't matter. If it's an issue, add biasing resistors to the outputs, so they only source current.

author: MrAl
author_rank: Super Contributor
timestamp: May 11, 2023, 09:22:15 pm
content: Quote from: nledinhthanh on May 11, 2023, 01:54:49 pmHello there,The first thing you MUST do is make sure the microphone works correctly. You have to be able to measure the output some way, or else use a properly biased op amp stage with a gain of 10 or more and measure the output of the op amp. You should get a decent signal out of it if it is biased properly and AC coupled. AC coupling means the op amp can be biased to the right voltage yet still amplify the AC signal. Without that the DC bias gets amplified and that pins the output of the op amp to one power supply rail or the other.If the mic never works you will NEVER get this to work, so testing the mic in the right way is crucial.That circuit does not look like the op amps are biased correctly. There are a couple of problems that can come up if the op amps are not biased such that they stay in their linear operating mode.The first is the AC signal will not pass through the op amps as the original, and will get severely clipped. That may or may not be a problem.The second however is that the bandwidth of the op amps may be severely limited because the op amps are not always in their linear operating mode. What happens is the output gets clamped to one of the rails and when that happens the op amp takes much longer to respond to a new signal coming in to the inputs. That means that even a 100Hz signal may not be able to pass through because the op amp can not jump out of the non linear mode and back into the linear mode in a short enough time to start to reproduce the 100Hz signal properly or even not at all. This is due to the internal compensation.There is a chance that your signal is low enough to get through but if you really want to make sure the circuit works right you should bias it more like the circuit shown in the attachment. This is a more conventional way to bias the op amps for AC operation. Note that the DC bias becomes separate from the AC signal using capacitive coupling, and also the gain does not affect the DC operating point. The output of both op amps will be around 2.5v but you could check that and adjust if needed.The filter with the 150k resistor was changed to use two 300k resistors. The calculation is simple, just double the 150k.You might get away without doing this but you may find it works better if you bias it in a more conventional way.If you want to try this without adding components, simply get another 5v supply and use it as a negative 5v source to power the op amps.Good luck with it.