Hi,
I am trying to buildt a custom contact mic using https://www.richardmudhar.com/piezo-contact-microphone-hi-z-amplifier-low-noise-version/ (circuit picture below)
I want to record frequencies in range if 5-20 Hz. I simulated the circuit in ltspice and found better frequency response at the lower frequencies by replacing C1 by 100uF capacitor.
Frequency response:
I am looking for comments and suggestions for this change also can anyone suggest a suitable amplification circuit for amplifying mainly the lower frequencies (after the pre amplification).
My objective is to convert the microphone output into digital signal and process it further
I am trying to buildt a custom contact mic using https://www.richardmudhar.com/piezo-contact-microphone-hi-z-amplifier-low-noise-version/ (circuit picture below)
I want to record frequencies in range if 5-20 Hz. I simulated the circuit in ltspice and found better frequency response at the lower frequencies by replacing C1 by 100uF capacitor.
Frequency response:
I am looking for comments and suggestions for this change also can anyone suggest a suitable amplification circuit for amplifying mainly the lower frequencies (after the pre amplification).
My objective is to convert the microphone output into digital signal and process it further
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I had thots about this, but the approach i imagined was something that takes the output of a mechanic's tethescope to quntify its output. By far the best tool i have found for finding resonances.
dave
dave
C1 can certainly be larger, 220uF for instance, to avoid attenuating 5Hz appreciably. Adding a low-pass filter with cutoff around 50Hz might help reduce noise.
C4 is good for loads of 10k or greater as it stands. Adding a 100nF ceramic cap to the opamp's decoupling might be wise to ensure no high frequency shenanigans.
A useful comment might be that there's no way the piezo element is going to output much in 5 - 20 Hz. They use those as tweeters; ever see a piezo based woofer? Did you see the frequencies the fellow on that website was describing? I thought I saw kHz...
A mechanics steth, you could connect the hose to an ordinary, abundant electret mic which would have a far better chance of making signal at those frequencies. The compliance of the steth membrane would be more dominant; if it's not floppy, how's it going to move at 5? I'm pretty sure the hose length wouldnt be much of a factor at 20.
When I worked at Amzn, I thought I was going to be smart and design something that could listen to bearings in conveyors - but they had already purchased a professional setup for that.
A further suggestion for a direct contract pickup might be a "exciter" speaker (Dayton Audio) used in reverse. I'm pretty sure 5 - 20 would be below the resonant frequency of one of those. Still, I'd bet it would make signal in that range better than a piezo.
A mechanics steth, you could connect the hose to an ordinary, abundant electret mic which would have a far better chance of making signal at those frequencies. The compliance of the steth membrane would be more dominant; if it's not floppy, how's it going to move at 5? I'm pretty sure the hose length wouldnt be much of a factor at 20.
When I worked at Amzn, I thought I was going to be smart and design something that could listen to bearings in conveyors - but they had already purchased a professional setup for that.
A further suggestion for a direct contract pickup might be a "exciter" speaker (Dayton Audio) used in reverse. I'm pretty sure 5 - 20 would be below the resonant frequency of one of those. Still, I'd bet it would make signal in that range better than a piezo.
Piezo elements convert strain into charge movement (and vice versa), so you need to convert the sound to strain of the element efficiently at the frequencies of interest, which is a problem in acoustics. Piezo elements work down close to DC as witnessed by piezo gas lighters where a large DC strain generates many kV leading to a spark.
You may want to investigate FET opamp amplifiers. With a piezo source, you are forced to use large resistance in your design and so you need amps with low current noise.
I see. It is true that the piezeo element outputs very less signal. I will investigate further how woofers are made/designed. Although my objective is not to output but rather to input from a contact piezoeeletric microphone.
I see but the open loop frequency response of the FET OPAMP starts from higher frequencies such as 1Khz. I want to be able to use the op amp at frequencies as low as 5 Hz. I am a beginner so let me know if i am making a mistake.
Opamps work from DC upwards. Often they have a pole in that reponse somewhere around 5Hz to 100Hz, but that's of no issue, the open loop gain is massive at DC.