parallel preamps for lower noise

Discussion in 'The Projects Forum' started by MattP, May 13, 2013.

  1. MattP

    Thread Starter Member

    May 21, 2012
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    I want to amplify a really low noise condenser microphone (5dBA self noise!), but I don't want to spend a load of money on a preamp for it.

    Why not just run it into a portable recorder's mic in, you ask? Well, portable audio recorders' pres are too noisy to take advantage of the mic's low noise, so the end result is just a noisy recording (by 'noisy' I mean that the noise is audible when recording quiet ambience).

    So, my question is, would it be possible to use several preamp circuits in parallel to have a low noise output? I'm not quite sure how it works, which is why I'm asking.


    Code ( (Unknown Language)):
    1.  
    2.  
    3.      --|  preamp 1 |
    4. mic >--|  preamp 2 |>---- line input
    5.      --|  preamp 3 |
    6.  
    7.  
    My thinking is that the signal coming out of the mic will get amplified by the preamps, with their own noise contaminating the recording, but because the preamps' output signals are combined it would mean that the microphone's signal will be INCREASED, but the noise of each preamp would stay the same because of the different phases. Is there ANY possibility that what I'm thinking might work, or is it a complete waste of time and not worth trying? :p
     
  2. crutschow

    Expert

    Mar 14, 2008
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    If the noise is uncorrelated noise from the preamps and not noise from the mic, then the noise increases by the sum of their RMS values (square-root of the sum of the squares). This is because some of the noise components are indeed out-of-phase but some are in-phase. Thus your signal to noise ratio will improve by the square-root of the number of amplifiers (e.g. 4 amplifiers would give a factor of 2 improvement). To properly combine then you will need to use a summing amplifier such as a summing op amp.

    It is usually preferable to use a low-noise mic preamp rather than trying to sum the signal from several higher noise amps.
     
  3. #12

    Expert

    Nov 30, 2010
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    Wrong idea. All noise sources add. Best you can do is use a low noise preamp. Either an op-amp or a single stage jfet amp.
     
  4. Ron H

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    tubeguy likes this.
  5. crutschow

    Expert

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    It may be a bad idea but it's not a wrong idea.;)
     
  6. MattP

    Thread Starter Member

    May 21, 2012
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    Ah, that's a shame. Thanks for the input though guys!

    Those links are interesting, esp. the first one (ultra low noise amp). I'm a bit baffled by the numbers, though. Can any one tell me what the EIN noise of it is in -dBu?
     
  7. crutschow

    Expert

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    EIN noise in -dBu is dB below .7746V of RMS input noise (1mW into 600Ω). So you just take 20*log(Vn / .7746) to get the value of EIN in dBu. The minus sign comes automatically from the calculation depending upon whether the noise is less than or greater than .7746V.
     
  8. #12

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    Nov 30, 2010
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    I'm puzzled. I say that expecting 2 or 3 completely separate pre-amplifiers connected in parallel to create less noise is the wrong idea and you say it's only a bad idea. Is my microscope out of adjustment that I can't see a nit that small or have I missed the idea that, under some circumstances, parallel pre-amps can be used to reduce the noise level?
     
  9. patricktoday

    Member

    Feb 12, 2013
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    Supposedly this is so. According to Douglas Self's Small Signal Audio Design:

    I probably shouldn't post the diagram but he goes on to show a schematic of two identical non-inverting op amp amplifiers with the inputs connected together and each output terminal going into a 47Ω resistor which are then connected together on the far side.

    But do not confuse it for this technique :p
    http://www.barberelectronics.com/parallelopamps.htm
     
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  10. crutschow

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    You missed the idea. ;) I guess my post #2 wasn't completely clear. The signal level is increased directly by the number of amplifier outputs added together (in a true analog adder), but the noise, being uncorrelated, goes up only by the square-root of the number of amplifiers.
     
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  11. #12

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    OK. I see the math. Have you ever seen a practical application?
    Is there an upper limit on the impedance of the source that limits this idea, say a piezo transducer vs a low impedance microphone?

    I'm thinking about the idea that resistor noise is insignificant in a 50 ohm application but 10's or hundreds of kohms can interfere.

    Edit: My bad. The answers are in the first paragraph of the PDF Ron posted.
     
    Last edited: May 14, 2013
  12. Ron H

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    I don't see how parallel voltage amplifiers can increase the signal gain, no matter how they are summed. Parallel transconductance amplifiers, I can see.

    I got a kick out of a quote in the link patricktoday posted:
    :D
     
  13. crutschow

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    If you sum two equal value voltage signals into an op amp inverting summer (say with each input having a gain of 1), then the output voltage will be the (negative) sum of the two input voltages. But I'm sure you know that. ;)
     
  14. studiot

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    Nov 9, 2007
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    You might like to look up the following articles:

    Noise cancelling preamp Elektor July 77 27-75

    Noise cancelling amp Elektor April 78 86/4/02

    Parallel amps yield low noise Elektor December 92 p48
     
  15. Ron H

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    I didn't express myself well. i was taking issue with patricktoday's post, which he only reported. he didn't endorse it.
    My point was, putting two identical preamp's inputs in parallel, and summing the outputs through 47Ω resistors, does not increase the signal by 6dB (indeed, not at all) above the gain provided by a single preamp. I did not question the resulting noise reduction.
     
  16. patricktoday

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    Feb 12, 2013
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    The left of U3 is the circuit from the book and I added U3 presuming this is how you would sum the results.
     
  17. THE_RB

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    Feb 11, 2008
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    The trouble with that is that the sources of noise must all be separate and different.

    In most real world cases the noise is from something external (RF/EM signal or PSU) which will likely affect all amps equally so it will still sum.

    On the other hand, if you run an even number of amps, and have half the amps circuitry inverted, then noise affecting all amps equally will cancel. You would still have to deal with inverting the signal from half the amps as an additionl step.
     
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