Microphone amplifier circuit help - Page 1
Forum section: electronics / projects

author: DaveHardy
author_rank: Regular Contributor
timestamp: November 17, 2013, 08:36:47 pm
content: I'm building a microphone circuit and was wondering if anybody could help me out. It is a high impedance capacitor based microphone.So far, the circuit works but I am getting a small 2mv 60 hertz buzz on the plus input that is roughly 20db higher than my noise floor and I can't figure out how to get rid of it. At first I thought that this 2 mv buzz was just my scope not seeing a load but I get the same wave at 4mv on the load of the op amp and my gain is set to two.Here is my question: What can I do to mitigate this? Should I change the 1 Meg resistor to a higher value so that the op amp input isn't loading my microphone capsule? Should I raise the voltage potential seen by the microphone cap? Should I look for a different op amp? Do I have an issue with ac line voltage contamination and improve shielding? I've read that I should employ a virtual ground ring and apply positive feedback to the cable shield but I'm not sure if I'm missing something obvious instead.-Dave

author: dannyf
author_rank: Super Contributor
timestamp: November 17, 2013, 09:01:10 pm
content: QuoteHow does it work?If you can figure out the answer to that question, you have figured out the solution to your problem.

author: DaveHardy
author_rank: Regular Contributor
timestamp: November 17, 2013, 10:52:20 pm
content: It works by superimposing an AC voltage generated by the capacitor microphone on top of the DC voltage generated by the power supply. The power supply is dead quiet right before the resistor and I mean dead quiet. Also, my parasitic AC wave is at 60 hertz. Yes, I know that 60 hertz is the power supply rectification frequency but the power supply looks OK right before the resistor. Also, if I disconenct the transformer and leave the circuit charged, I still get the same problem. I tried discharging the power supply caps too and still have the same problem.I tried putting an integrator capacitor in the feedback loop but it didn't do much other than roll off the high frequencies.

author: dannyf
author_rank: Super Contributor
timestamp: November 18, 2013, 12:13:21 am
content: QuoteGood 1st step.Where is the "AC voltage generated by the capacitor microphone" and where is the DC voltage generated by the power supply?

author: DaveHardy
author_rank: Regular Contributor
timestamp: November 18, 2013, 02:34:24 am
content: The DC voltage is generated by the power supply by rectifying the ac voltage through a voltage doubler, a half wave rectifier and a few low pass filters. Since the power supply is such a high impedance and operates with virtually no load, very little rectification is needed.The AC voltage is generated by the variable capacitor in the capsule.

author: dannyf
author_rank: Super Contributor
timestamp: November 18, 2013, 11:34:25 am
content: QuoteWhere are they in the schematic you posted?

author: andtfoot
author_rank: Unknown
timestamp: November 18, 2013, 11:43:57 am
content: Quote from: dannyf on November 18, 2013, 11:34:25 amI'd say here (in bodgo paint style):

author: andtfoot
author_rank: Unknown
timestamp: November 18, 2013, 11:50:00 am
content: I don't know enough to help sorry, but throwing ideas out there:- Have you tried with a known good DC power source and bypassing the current power supply? A battery maybe?- Are you touching any part of the circuit with your hands while measuring? You could adding noise to it that way.

author: dannyf
author_rank: Super Contributor
timestamp: November 18, 2013, 12:02:47 pm
content: That's progress.But where is that half wave rectifier? How much dc voltage is generated? What's the low-pass filter's frequency cornor? Or its output impedance? Why do you even need it?How much current does the macrophone consume?Why reinventing all the wheels? To make it more complicated than needed?