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Stereo (Mono) recording low noise low cost system

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DD4WH edited this page Dec 16, 2021 · 3 revisions

This is a DIY stereo microphone array with a self-built preamp and a self-built low noise power supply.

I use it for two purposes:

1.) Lightweight field recording with my Android phone

2.) Permanent ornithological monitoring with BirdNET-Pi HERE

MICROPHONES:

  • EM 272 with low self-noise: they have the lowest noise when the bias voltage is about 8 to 9 Volts
  • mic capsule EM272 [SNR 80dB !] https://micbooster.com/gb/primo-microphone-capsules/199-primo-em272.html
  • At the moment @nilspupils experiments with the AOM-5024 electret capsules (thanks Nils, for this important hint) and it seems they are equally suited, but this has to be thoroughly tested

PREAMP:

  • this is more a driver than a preamp, it has only 10dB of gain, but is ultra-low-noise

  • it was designed by ESP: https://sound-au.com/project93.htm

  • the additional supply voltage filter (10 Ohms & 470µF) is a MUST, if you use a power supply !

  • Transistors can be exchanged: 2N4401 == BC549, 2N4403 == BC559

  • for our purpose of feeding an unbalanced input of a USB audio sound card: Please use unbalanced/asymmetric output of the preamp: use GND, not OUT-

  • do not connect shielding of the cables, except for output end of preamp, where shield is connected to GND. Do not connect shield of the other end of the mic output cable. (Still experimenting with this...)

  • Do not use more than 10dB of gain, it's more than enough

  • please support ESP by buying their products, they also sell a PCB for this preamp! --> https://sound-au.com/

  • upper row of components --> preamp, its audio output goes to the upper waterproof connector. Please note that the GND connection is also connected to the shield of the audio output cable by a little solder bridge at the upper waterproof plug. This shield connection is NOT connected to anything at the other side of the cable (which has the 3.5mm audio plug for the mic input of the USB audio card.
  • the lower row of components is the power supply --> the input with 16 Volts AC goes to the lower waterproof connector, the output of the power supply (well regulated and filtered 9 Volts) feeds the preamp on the same Velcro board PCB

And this is what the whole setup looks like, when you design it for rough use in the field ("waterproof"). This is a MONO setup with the microphone electret capsule mounted in a rubber mic holder and waterproofed with kitchen cellophane foil and fitted into the aluminium housing.

This is a different version with TWO microphone capsules at the sides of the aluminium box, approaching some Stereo effect:

USB AUDIO INTERFACE WITH STEREO MIC INPUT:

  • most cheap USB audio interfaces are MONO input, so watch out for what you buy!

POWER SUPPLY:

  • best is a 9 Volt battery, it is superduper low noise
  • if using a power supply, it has to be a very low noise type
  • always use an additional lowpass filter at the Vcc input of the preamp: I used a 10 Ohm resistor and 470µF electrolytic cap --> very quiet power now
  • again: ESP has a perfect solution: https://sound-au.com/project05d.htm
  • please support ESP by buying their products, they also sell a PCB for this power supply!

The wall plug produces an AC output of about 15 Volts and was found in my juke box. Never mess with main current, it can kill you !!! Use / Buy a wall plug that is well isolated and already outputs AC low voltage (16 Volts).

Here are some hopefully self-explanatory photographs to illustrate the building process of the mic hardware:

  • the shield of the mic cable is NOT connected to the case of the electret mic
  • the shield of the mic cable is connected to the self-adhesive copper foil
  • the other end of the shield of the mic cable is connected to the GND pin of the preamp

I modified an existing cigar box to take the components (except for the power supply, which has its own shielded box). The two EM272 microphones are fixed on an aluminium tube and carry a plug to plug into the cigar box. The preamp is inside a shielded tin box. You can see the shielded cable from the mics to the preamp and from the preamp to the 3.5mm plug which plugs into the Stereo USB audio interface (UGREEN). The 9 Volt battery is replaced by the output of the low noise power supply for the longterm monitoring setup. In the latter setup (2) the USB-A plug of the soundcard goes into the BirdNET-Pi. For the field recording setup (1) this goes into an OTG-USB-C-adapter for my smartphone.

The distance between the microphones is 40cm and corresponds to a Stereo AB40 setup, which captures stereo soundscapes quite nicely (but not nearly as good as a SASS :-)).

If you only need a MONO setup, just use only one mic and one preamp. :-)

Have fun in building !

This is a recording with exactly this setup (powered by 9 V battery), recorded on an Android smartphone using USB digital input (featuring Tawny Owl, cricket Nemobius sylvestris and bush crickets Tettigonia viridissima and Pholidoptera griseoaptera): https://github.com/DD4WH/SASS/blob/main/Waldkauz_Dresdner%20Heide_2021_09_03.mp3

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