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Reducing noisefloor before going digital (technical)

Discussion in 'Recording' started by camsr, Dec 6, 2006.

  1. camsr

    camsr Active Member

    Hi I have a dynamic omni mic and a preamp. The mic is rated 500 ohm impedance and the preamp input is 10k ohm. I read on a website it is ideal for a mic/pre to have a ratio of 1/10 impedances but my setup is 1/20. Im just trying to figure out ways to lower the noise floor before recording as my current noise floor is workable (-51dB) but still a pain when tracking and compressing.

    If I increase the impedance the pre sees to 1k ohm is it possible to lower the noise? I tried a test on my preamp, by recording it's input into my wave editor, but I didn't plug a mic into the pre, I only completed its circuit with a variable resistor. I noticed that at 10k ohms the noise floor was about -48db, but turning the resistor all the way open to 0 ohm the noise completely disappeared, no signal at all. What can I do to lower the noise without having to use the digital FFT noise reductions?
     
  2. Kev

    Kev Well-Known Member

    get a real mic and a real mic-pre ?

    what are you using ?

    no matter where the noise floor is, there is still the issue of signal to noise ratio and it may be that your gain structure is all wrong or the combination you have for what you are recording could be better

    and the way you have described will also lower the signal level
     
  3. camsr

    camsr Active Member

    So are you saying I should have a lower impedance on the mic and a higher impedance on the pre? Frankly I didnt understand a word of your post except that last bit you quoted.

    Hmmm maybe you're right, these imaginary preamps and mics really suck
     
  4. Kev

    Kev Well-Known Member

    :roll:
    yeah funny
    and so do toy mics and toy mic-pres
    that some manufacturers try to pass of as pieces of real sound equipment

    perhaps you don't

    http://www.johnhardyco.com/JTS990Details.html

    read the specs
    and the terms of reference

    -51dB (see above) is a relative term but you haven't specified ... relative to what ?

    dBu ... the u specifies the reference
    dBm and dbV
    and many more ways to use the term dB(ref)

    or

    here they specify the reference ... ( ref 1kHz)

    The John Hardy M990 Mic Preamp is a real Microphone Preamp.


    let's move on

    :?
    so you shorted the input
    no signal and no noise to speak of
    :roll:
    why doesn't that surprise me ?

    :?
    so you recorded the input to the pre-amp ?
    no
    you recorded the output of the pre-amp with no source except the variable resistor
    8)
    almost a worthwhile test

    lets refer to the John Hardy specs
    IF you had of set the variable resistor to 150 ohms you could have compared your result with the John Hardy
    -128.7dBu

    you would need to know what load your computer audio interface presents to the Mic-pre output
    and that should be relatively high for it to be dBu
    HOWEVER
    the note at the bottom of the page
    confused yet ?
    8)
    this may help
    http://www.johnhardyco.com/pdf/MicPreampEval.pdf

    which microphone and which microphone pre-amp are you using ?
    ... then we can talk specifically to your problem
     
  5. camsr

    camsr Active Member

    Okay, the -51dB was actually dBFS, and the impedance of the soundcard input is 20k. Im pretty sure this soundcard is -10dBV, and the preamp is +4dBu so that is already a problem. I've tried correcting it with a T-pad but I got directions from some bozo on the internet and he gave me a schematic for a -6dB VU t-pad, when I actually need a -12dB VU. (http://www.tonmeister.ca/main/textbook/node94.html) So I still need to fix that.

    The mic is from radio shack (commence laughing), a generic dynamic omni with a 500ohm impedance. The preamp is built into my JVC cassette deck (TD-W318) and doesn't use phantom power. All lines are unbalanced.
     
  6. Kev

    Kev Well-Known Member

    yep
    very good info
    and does look like you have enough knowledge to move forward
    no offense meant

    so the JVC cassette deck (TD-W318) is balanced ? ... no unbalanced
    I've tried a quick google but not having much success

    even so
    the cassette deck will have an output control and you can manage some of your level into the sound card with that.

    :shock: :roll:

    where to start ?
    you are making it hard on yourself
    first try to optimise the connection from cassette deck to sound card

    perhaps with a good cassette and strong level music loaded, ( test tone cassette perhaps ? )
    try to get it set so that with the output knob in a normal position the levels into the sound card and the audio software are good and without clipping

    it could require the T pad but a simple 20 k pot and think resistive divider may get the result you want

    for general pad info start here
    http://www.uneeda-audio.com/pads/
    the unbalanced r1 with r2 shunt ( if the cassette deck is RCA output)
    :?

    either way 6dB here or there is not going to solve your problem

    more later

    :roll:

    :shock:
    Houston we do have a problem

    I just can't know the construction of that Dynamic Mic or the mic input circuit of the JVC and what it is optimised for.
    Generally speaking it probably isn't far from what it should be and to get the noise floor from -50dBFS down to -80dBFS+ is not going to happen without some major changes ... given there isn't a basic fault somewhere

    a good healthy sound source into the mic and with the normal knob positions on the cassette deck it should meter OK and therefore the levels into the audio software should be close

    it not
    :roll:

    don't tell me you are trying to get dialog from 20 feet away ...

    optimise the Cassette Deck to Audio Software first ... then start work on the Mic to Cassette Deck
     
  7. camsr

    camsr Active Member

    Kev thank you so much for your help. I've read that pad link and found out I need a new one. The one I got now is a 600 ohm and my cassette deck outputs RCA at 1000 ohm (this was tested with the deck shut off using an ohm meter, not absolutely sure if its right). Appearantly I have a 1/20 impedance on both the mic-to-pre and pre-to-soundcard, not sure how to interpret this but I guess it's pretty good?
     
  8. camsr

    camsr Active Member

    Still not sure how to interpret this, but I did the math and the the ratio of voltage out from the mic and voltage in to the soundcard is 1:0.907029478. That equates to about a 1 dB attentuation.

    But how do I get rid of the white noise!?
     
  9. Kev

    Kev Well-Known Member

    you can't measure input and output impedances this way

    a DMM (digital multi-meter) is a DC device (mostly)

    the impedance we are interested is over the audio range and often done at 1kHz

    Have you actually tried going from TapeDeck +4dBu unbalanced to SoundCard -10dBV unbalanced ??

    that will put the TapeDeck 14dB stronger than required but the output control of the TapeDeck may be enough to get a better result than what you have now.

    even so
    a simple 10 or 20 K ohm pot
    with the signal from the TapeDeck across the outer terminals and the wiper feeding the SoundCard.

    this will allow you to set for nominal levels on the TapeDeck and then trim to suit the SoundCard.

    then ... without moving the pot position ... measure the relative resistances of each end of the pot and then choose some static resistors to make lead.

    Personally I believe you should make a passive interface box for the SoundCard to allow for all sorts of sources ... I'd even include a transformer option
     
  10. camsr

    camsr Active Member

    The deck doesn't have an output control, if it did I wouldn't be having this problem

    :roll:

    lol yeah I know the impedance is resistance to AC and not DC but those values seem resonable. And yes I have gone directly from the tape deck to the soundcard and it clips badly. Adjusting any software settings is pointless because the DAC is getting too many volts.

    I will build another T-pad and let you know how it goes. Thanks for the help
     
  11. camsr

    camsr Active Member

    I got a tip from my electronics instructor today, he said connect the hot side of the pre input to the common ground and then adjust the pres volume and watch it for internal noise. I guess this will work for my unbalanced input. He also explained to me the value of a balanced connection when running super long cables.
     
  12. Kev

    Kev Well-Known Member

    also try 150 - 200 ohms between 2 and 3
    and let the input float against ground

    fully floating differential input

    look back at the John Hardy specs and think about the methods of testing

    cables don't have to be that long for it to be worth while
    you could be in a noisy environment

    balance lines and Common Mode Rejection is a good thing for small signals

    often people do like to use unbalanced were distance is short
    but the signal levels are much greater than that of Mic signals
    Mastering Suites are often single ended
    and HiFi is single ended ... even the most expensive
     
  13. camsr

    camsr Active Member

    What do you mean "between 2 and 3"? I dont know where I would connect to common ground from the tape deck cause the case is non-conductive, and the plug for the deck itself is only a two pronger. And I think the outlet it's plugged into is the only one in the room thats on a different circuit. Did he mean connect the input to the ground of an outlet?
     
  14. Kev

    Kev Well-Known Member

    ?

    oh

    you mean the Mic input to the Tape Deck is ALSO unbalanced ?
     
  15. camsr

    camsr Active Member

    Yah.

    Any ideas?
     
  16. Kev

    Kev Well-Known Member

    try the mic directly to the sound card

    without being there and trying every trick I know
    working with these low spec pieces of gear ... it is very hard to know which or where your weakest link is

    gut feeling tells me ... " get a real mic "
    that is where life starts and a Cardioid Dynamic like an SM58 or 57


    better still
    lets start again
    what is the end goal of the whole exercise
    and
    is there any budget to complete it ?
     
  17. camsr

    camsr Active Member

    I think its a combination of both the elements, not to mention how much this preamp colors the sound. Esses get blasted through!

    Budget huh? Well, I got like 20 bucks? It IS christmas time.

    I did do the mic direct to the soundcard. The input was very low as you could imagine so I boosted it in software and the noise floor, at an audible "lead quality" volume, was still -51dB. And thats before compressin. So I DO believe it is the mic. I just don't have alot to replace it so Im trying my options. The next mic I buy will have a balanced line and Im gonna get or build a preamp for it. One thats designed for its impedance rating.
     
  18. Kev

    Kev Well-Known Member

    yes the lack of linearity and poor frquency response can lead to those esses and scratchy sounds

    the EL Cheapo Mic can also do this but I don't expect the frequency response to be that great so it may be a dulling effect

    even at line levels with a real mic and pre-amp the SoundBlaster card is going to degrade the sounds you record

    without knowing what and how you want to record
    ... all I can recomend is SM58 or 57
    perhaps a Tandy PZM
    and do my balanced and phantom mod

    building a Mic to Line ... yes I have them too
    search for the Green-pre or KDMP ... JLM Audio for a more expensive Baby Animal

    but
    look out I'll get flammed here
    a secondhand M-Box or M-audio two channel interface ... obviously I'm a PT user

    A simple M-Box with PT-LE and Reason Adapted and an SM58 and a modd'd PZM
    and I can do things that impress most beginners
    ...
    oh ? ... we would need headphones and speakers or some sort
     
  19. camsr

    camsr Active Member

    Im getting a pair of headphones for xmas, Im looking forward to either the Grado SR 60 or maybe the Beyerdynamic DT 231. Fits my price range and according to headphone.com they both have a desireable response in the bass.

    About the Shures, are they balanced? Also are they condensers or dynamics?
     
  20. Kev

    Kev Well-Known Member

    SM57 & 58
    dynamic
     

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