Microphone Preamp Question

Discussion in 'General Electronics Chat' started by tracecom, Aug 27, 2011.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    I record a podcast occasionally. I have been using a mic and a mixer/pre-amp with the output going to the line input on my sound card. I use Sony Sound Forge. With this setup, I was able to set the pot on the mixer channel with the mic at about mid range and get audio into Sound Forge that ran high but almost never exceeded 0 db.

    Then, my desktop computer was damaged and I replaced it with a laptop which has only a mic input. So of course the mixer/preamp completely overloads the mic input. I found that I could eliminate the mixer/pre-amp and run the mic directly into the mic input. However, I have to speak much more loudly than I want to in order to get peaks at more than -15 db. Of course, I can use Sound Forge to boost the levels after I finish recording, but somehow I would rather solve the problem instead of patching the results.

    I guess my two options are (1) reduce the gain substantially in the mixer/pre-amp or (2) build a low gain mic pre-amp. I can build either solution but I don't know where to even start a design. Can someone point me in the right direction?

    Thanks in advance.
     
    Last edited: Aug 27, 2011
  2. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    In thinking about this further, I guess what I would like to do is build a pad for the output of the mixer/pre-amp to reduce the output to a level that is suitable for input to the mic jack on the laptop.

    Here are the specifications of the mixer/pre-amp.

    Radio Shack 32-2056 4-Channel Stereo Microphone Mixer
    Input sensitivity:
    Mic: 1.5mV/600 Ohms
    Line: 150 mV/27k Ohms
    Output Level at 10k Ohms Load: 1.2V
    Input Overload:
    Mic: more than 250 mV
    Line: more than 6V
    Distortion: less than 0.2%
    Signal to Noise Ratio: better than 60 dB
    Frequency Response: 10 - 35,000 Hz +/- 3 db

    I think maybe a simple L-pad on the output of the mixer/pre-amp would be sufficient. Can anyone specify the R values and a schematic? Thanks.
     
    Last edited: Aug 28, 2011
  3. nucleargungus

    Member

    Apr 6, 2009
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    As much as I like designing and building electronic circuits I also believe in the law of diminishing returns... I also am a fanatic about sound quality. Please buy a usb soundcard (roughly $20) and continue to use your preamp setup ensure no amplifier stages are being overdriven (red lights indicate this) utilize your software to provide compression and de-essing and you will have great sound quality! The spoken word can be harder to make sound good than music sometimes. What's your podcast about? sounds cool...


    Robert
     
  4. #12

    Expert

    Nov 30, 2010
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    As a devout scrooge, I would build a pad. 20 cents worth of parts in a metal box (and $5 worth of connectors) should do it, but I have all that stuff laying around so it wouldn't cost me a penny. Just my 20 cents worth. Go with what you know.
     
  5. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    Thanks for the replies; I had given up on getting any responses.

    I did find a web-site that dealt with making pads, but in the process of trying to determine the data to use in designing the pad, found some references to USB sound cards. That led me to the Behringer UCA202, which I ordered from Amazon, and used this morning to record a new episode of the podcast.

    The neat things about the UCA202 are that it is made to use with a pre-amp/mixer (so I can continue to use my existing mic and mixer), it uses a USB interface to the laptop, it has very low latency (which allows me to hear what I am recording as I record it), and it was relatively inexpensive.

    The only downside is that the recorded result, when converted to MP3 at 22,050 Hz 16 bit stereo, sounds a little "tinnier" than it previously did. I think that may have to do with the 48 kHz sampling rate of the UCA202, but I am not sure.
     
  6. JDT

    Well-Known Member

    Feb 12, 2009
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    Does your laptop have a line-in jack? This should handle the output of your pre-amp OK. If it only has one input jack, go into the sound card application (or windows mixer) and check that "20dB boost" is switched off. Also check the gain control on this input.

    The 20dB boost is usually used for direct microphone input only.
     
  7. tracecom

    Thread Starter AAC Fanatic!

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    No line-in jack; only a mic jack. I didn't find a "20dB boost" setting, but I did adjust the gain on the mic circuit and was able to adjust that sufficiently to use the output from the mixer. The only problem is that the gain setting seems to readjust itself from time to time.
     
  8. #12

    Expert

    Nov 30, 2010
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    OMG! It has gremlins!
     
  9. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    More likely, it's something that the operating system resets automatically. I just haven't discovered the particular set of circumstances that precipitates the reset.
     
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