Help with mic preamp and AGC

Discussion in 'Analog & Mixed-Signal Design' started by allisonr, Feb 7, 2017.

  1. allisonr

    Thread Starter New Member

    Aug 16, 2016
    27
    0
    Hello,

    I'd like to build a few mic preamps for my field recordings which usually involve low background noise and low sound levels, I can solder by following circuit drawings however I am not able to design them.

    I thought I'd benefit from using a low noise high gain preamp with AGC and a switchable variable resistor to manually set gain those times I won't need AGC.

    Since I'd leave equipment out in the wild for a few days (everything would be weather/waterproofed and weather forecasts studied to avoid noisy days such as rainy and windy ones, but those are obviously the least problem) and come back just to retrieve recorded data.

    I thought that AGC, even with its obvious limitations, would be more useful to maximize sounds capture rather than setting a certain gain while on the field and leaving it at that level, potentially clipping sound sources coming closer and/or having many ones unrecorded due to too low of a gain set.
    The switchable manual gain would come handy when I would record in more forecastable situations, so that I don't have to build another similar circuit over just to include a single function.

    I'd couple each one of them (unless there is a fits-all circuit, which I'd gladly use) with a different input (a ceramic piezo transducer for water sounds and either an electret array or RF-condenser mic, due to its higher reliability in high ambient humidity, for air sounds).

    Here is a conceptual diagram I sketched (recorders would be portable battery powered ones, I currently have two, both with 3.5mm TRS jack mic input, 1 with 1.6 kohm impedance and another with 2 kohm impedance, unless I upgrade to something else):

    [​IMG]

    After looking for a good preamp I came across this one on a TI's datasheet which apparently has very good reputation (I did add the red line since I thought I won't need phantom power for my current project):

    [​IMG]

    I have been suggested this AGC circuit (from a THAT datasheet):

    [​IMG]

    However I am far from knowing how I should merge them.

    Could you please help me? Any suggestion is welcome.

    Thank you

    Allison
     
  2. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    8,078
    2,285
    I'm a big fan of logarithmic compression, like the THAT parts, as opposed to a simple peak limiter. They are more complex, but THAT has a wide range of parts for various applications. If you're not doing 20 Hz to 20 KHz recording of a Stradivarius, then the lesser parts are less complex. And THAT has excellent tech support. THAT compressors and expanders are *the* parts for high-end wireless microphones and receivers.

    Separate from that, know that a peak limiter and a true compressor are different, and the audio outputs have different characteristics. Decision time.

    Analog Devices bought Solid State Music and their SSM parts line. The INA217 is a Burr-Brown (now a part of TI) part strongly similar to an SSM part. It is an great mic preamp, but as the schematic indicates it really shines with a balanced input. For a single-ended input with a short wire run to the preamp, a single-ended preamp will work just as well with much less complexity. If you want to go to the source, look into the SSM2015 and SSM2017 from ADI.

    Overall, you are several steps down the right road. Check out the lower cost THAT compressors, and post more details about the piezo mic.

    ak
     
    Last edited: Feb 7, 2017
    allisonr and JohnInTX like this.
  3. allisonr

    Thread Starter New Member

    Aug 16, 2016
    27
    0
    Thank you for your help. I have no constrains, I simply like the idea to build a good low noise high gain preamp to serve me for the years to come in my "remote" (leaving out equipment and coming back a few days later) field recordings and research on animal calls without having to upgrade, hence what sold me the idea of that INA217 was an article about its performance, but I otherwise would gladly use any other IC if you say that it'd suit better.
    I've just checked the SSM2017 and SSM2019 (which apparently replaces it).
    By short wire, how short would it need to be? I know that it's probably the shorter the better particularly with low input levels (hence would probably embed it close to the piezo and later to the mic), but would there be a difference between 2 inches/5 cm and 12 inches/30 cm?

    As for the ceramic piezo I'm not yet set on anything specific since I'll probably have to get something custom cut, however a few ones I've been using until now have resonant frequency higher than 20 kHz, 3800 pF capacity and 3 ohm (if I'm not wrong) impedance.

    Thank you

    Allison
     
  4. allisonr

    Thread Starter New Member

    Aug 16, 2016
    27
    0
    I've been suggested this circuit for the piezo input:

    [​IMG]

    It is nice because it includes a high pass filter (yet no AGC).

    It uses two 12V batteries. Could I use just one yet be able to give both + and - voltages?
    Gain is set with R2 and R3. Could I replace both of them with a potentiometer?
    The high pass frequency is set with R3 and C1. Could I replace the R3 with a potentiometer and add a switch to disable the filter when it isn't needed?
    Since I would probably be building just one, I'd like to pick up the best IC for this use so that I can't later think about "I'd have implemented that other IC".
    Both AD743 and AD797 are suggested in the diagram. AD743 lists a 2.9 nV/√Hz at 10 kHz while AD797 lists 0.9 nV/√Hz at 1 kHz.
    Since I'd be using this in the 3-45 kHz range, does this mean that the AD743 has less signal distortion at upper frequencies (hence the reference to 10 kHz instead of 1 kHz), or are they equivalent?

    Thank you

    Allison
     
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