2 Mono mics (repost from Project Guidance Forum)

popjones

Member
No reply in the Project Guidance Forum, maybe Audio is where I should post this:

I'm trying to use 2 mono mics for a binaural signal. I have these going into a preamp and then into the line inputs on the teensy audio board. No signal is coming through. Am I mistaken in thinking the preamp will bring the mic level up to line level? If I avoid feeding the mics to the audio board and just go to the teensy, do I need the preamp? Ultimately I want be able to analyze the binaural signals so the device knows where it is relative to other sound making objects--a form of listening. Mics are on the sides of the gray cubes, ultrasonic sensor is on the front.

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Area doesn't usually have any effect - it just needs to be seen by someone understanding the issue to scan the postings.
Best to just keep one thread and perhaps add to it if you find more relevant info that might help get to an answer.
 
The linked electret microphone modules already have the preamps built in, based on MAX4466 as the name of the offer implies.
I see no output DC blocking capacitors, but the Teensy audio board have them installed on both Line inputs, so these modules can be connected directly to the Line inputs. If testing with other gear (ie. to hear if they are working at all), i'd add a 10uF-100uF DC blocking cap.
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Your best path is use of the audio shield, stereo line inputs. You'll definitely need preamps to get the mic signals up to the line level the audio shield expects.

Specific guidance is difficult when we have so little info about the microphones and preamps. I looked at that Amazon page, but the info is so limited. I couldn't find important tech specs, like the voltage gain or a schematic.

Many different types of microphones exist, with different analog properties. Virtually all with analog output give a tiny signal, so you will definitely need preamp hardware. How much the preamp needs to amplify the mic signal really depends on the mic sensitivity. Condenser type microphones (which need power to operate) usually give more voltage output than dynamic (just magnet and coil) types. Some mics are good at delivering the signal over a long wire if it's properly shielded, but others have higher impedance output which usually means you want to keep the wiring short. So even though that preamp board looks like it's designed for 2 signals, if your mics are physically far apart and they are the types that want short wires, you might need 2 separate preamp boards simply for the reason of sending the stronger signal over the distance for both to reach the audio shield.

To move forward, I'd recommend doing more testing with the mics and preamps. You will need a way to listen (with headphones) to a line level signal. You could program Teensy + Audio Shield with one of the passthrough examples. Before you start experimenting with the mics, first feel the line output signals from your PC into the audio shield line inputs. Make sure you can hear the sound the sound your PC sends on the headpones plugged into the Audio Shield. Or if you have a home stereo or similar audio gear with line level inputs and headphone outputs, you could use it for testing if more convenient. The key point is to get a test setup where you don't need to mess with the computer and loading new code simply to hear line level signals on headphones. The critically important step is to test it with known-good line level signals like the output of your PC or a stereo system or other audio gear with line level output.

Once you have a confirmed good way to listen, then start playing with the mics and preamps. Important to use headphones, so you don't get horriffic audio feedback from a speaker back into the mic. Placing the mic farther away and monitoring something that makes a distinctive sound can also help. If the mic is close and listening to you, it can be difficult to discern whether sound you hear really is from the headphones or just from the room. Over-ear headphones that physically attenuate room sounds can help for this sort of testing. You can also just tap the mic with your finder.

If the preamp is working, even if the gain is too low, you should be able to hear the faint sound. That Amazon page says "The on-board resistor R5 is adjusted by the magnification. If the magnification is not suitable, the resistance of R5 can be adjusted. It can be selected from 5k to 150k. The larger the resistance, the higher the magnification. If the plug-in resistor is used, the original chip resistor can be removed, and the plug-in resistor is soldered to the reserved pad on both ends of the original chip resistor."

If you get no sound in the headphones, that probably means the wiring isn't connected properly or the preamp just doesn't work. If you can't find the problem, your best path forward is to share photos of your wiring. Maybe we'll be able to see something you've missed. Better lighting and photos from multiple angles can really improve your odds of useful help.

If you get sound, but also unwanted noise, best to share photos and try to describe the noise. I could go on and on about common audio noise issues... but this is already long. Hopefully it at least helps you to move forward and get those mics + preamps working with the audio shield.
 
The linked electret microphone modules already have the preamps built in, based on MAX4466 as the name of the offer implies.
I see no output DC blocking capacitors, but the Teensy audio board have them installed on both Line inputs, so these modules can be connected directly to the Line inputs. If testing with other gear (ie. to hear if they are working at all), i'd add a 10uF-100uF DC blocking cap.
My problem with those modules is the MAX4466 is the noisiest opamp I've ever read the datasheet of (80nV/√Hz). Its a micropower opamp so its performance is compromized to get the ultra-low power consumption, which is of zero benefit here. Plenty of opamps have voltage noise down at the 3--6nV/√Hz range, over 20dB quieter.
So try them perhaps, but hiss may be a problem if you do.
 
Thank you all for your help. Standing on the shoulders of giants!

See my diagram and photos of wiring. I did some testing today running the mic directly into a headphone monitor (basically a preamp with a headphone jack) and it was insanely noisy. I'm not sure if it was my set-up, using unshielded wire, or what, but I couldn't really make any conclusions.

I'm wondering if I should just use digital mics? High sound quality is not imperative. I have the analog mics in a binaural configuration and want to be able to 1) compare the volume from left and right (to determine location relative to other robots), and 2) analyze pitch (so that the robots can improvise music with each other). So I need 2 mono mics--can I run those directly into the Teensy (and avoid using a preamp)?


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So it looks like I don't need the external preamp (red board in photo above) as one is built into the mic (purple board above). I am running one mic left, one mic right into the Line In on the audio shield. I get good volume from the mic board, but using the PassThroughStereo example from the HardwareTesting examples, I get a strange background noise, something like a distorted drone. The sound of the mics (tapping or scraping them) doesn't seem to be part of the drone, rather it sits on top of it, kinda. At first I thought it was feedback. It is not, I am wearing headphones. Maybe some severe crosstalk? Other ideas? I will need a cleaner signal than this I am certain.
 
Some progress. The drone tone mentioned above is related to my Neopixel code. As it cycles through it sounds like an LFO. I unplugged the LED ring and it went away. The sound quality is quite poor on this mic/board (Max 4466), very distorted regardless of the pot setting on the board. Weirdly, when plugged into the computer (Mac >USB-C hub>hacked data-only cable) it generates some noise/oscillation. I am going to try the MAX9814.
 
Also, I tried running the mic ground to the audio shield and alternately to the ground rail on the PCB--no apparent change in quality (a post on another thread suggested grounding to the shield was quieter). Does this make a difference? My power to the mic is 5v--does it matter that the audio shield is 3.3v?
 
The MAX4466 is billed as low-noise, yet it is by far the highest noise mic preamp I've even seen... Its _ultra_ low power, not low noise. It has 80nV/√Hz voltage noise, compared to 3 to 5 for common low noise standard opamps... Over 25dB noisier! Even a lowly TL072 would do miles better, and that's not exactly quiet.

Basically every chip ever advertized is "low-noise", because once there probably was something worse. No substitute for reading the datasheet!

You may be hitting the issue that the Audio adapter doesn't have separate analog and digital ground/supply, so picks up ground and supply noise from the rest of the setup. It can only help to have a good mic preamp first to boost signal level before it reachs the line-inputs, and avoid any ground-loops.
 
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