discrete electret balancer, low noise

Discussion in 'The Projects Forum' started by isthatme, Aug 1, 2010.

  1. isthatme

    Thread Starter New Member

    Dec 2, 2009
    3
    0
    Hello all,

    i'm trying a discrete electret balancer that works on +48V phantom.
    I found two almost identical schematics on the web, A and B.
    [​IMG][​IMG]
    I have some practical electronic skills but as a theoretic i'm a total newbie.

    My questions: why the 10K resistor between collector and base, and then why can the 6K8 be removed, why was it there?
    I tried experimenting but cant figure out how to get the lowest noise figure.
    btw i'm using BC560c and all metalfilm R's.

    Can anyone tell me what the best option is and why? thanks in advance!

    Jeroen
     
    Last edited: Aug 1, 2010
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    The 10k resistor is between the emitter and base. Hard to tell (without spending some time analysing the circuit) what the difference will be between the two circuits. I suspect the difference will be the standing operating current.

    The transistors are operating as emitter-followers and do not amplify the signal. In fact they have a gain of slightly less than 1. They are impedance changers and give a low output impedance for driving the long microphone cable.

    The collector current of both these transistors are added and used to drive the zener. A small part of this current (at a constant 12V) is used to energize the J-fet transistor input stage. This stage has a very high imput impedance to suit the electret microphone. It also has a gain of 1. Anti-phase outputs from this stage are fed to each transistor.
     
  3. isthatme

    Thread Starter New Member

    Dec 2, 2009
    3
    0
    thanks JDT for your insight.
    it raises the next question.
    i'm using http://jlielectronics.com/transsound/electrets/tsb-120a.htm, which needs nothing more than 2k2 impedance. but thats already been taken care of with the 2k2 resistors right, or is the impedance here higher?

    Can i now crank up the amplification in this circuit some way? (i think not). I need to use as less components as possible for balancing and amplification say 20dB or more.

    thanks again!
    jeroen
     
  4. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    This does not give out the anti-phase signals that your first example does. Normally you would not have much gain in a phantom powered microphone as the mixer expects a microphone-level signal.

    It is hard to tell what exactly is going on in the TSB-120A device but the diagram shows the 2k2 resistor placed before the DC blocking capacitor so the resistor is essential and probably carries a continouos DC current. The "FET impedance converter" is possibly just a single FET as in your original circuit.

    So if you put another 2k2 in series with the TSB-120A term 1 (Vs) you will probably get an anti-phase signal from there. Experiment.

    The data sheet says "standard operating voltage 1.5V at 0.5mA. So the supply from the zener diode needs to be 1.5V + (0.5 x 2k2) = 2.6V.

    Might work!
     
  5. isthatme

    Thread Starter New Member

    Dec 2, 2009
    3
    0
    Thanks JDT,

    I think there is a little misunderstanding.
    The drain and source in the 2 circuits are in fact the FET inside the tsb-120.
    I am now wondering after reading you if pin2 gives the 180 degrees reversed signal of pin3 (as it should).

    Both circuits work, but i'm trying to get a bigger distance between signal/noise.
    I have no specialized equipment, but when i compare it with a AKG condenser on my Edirol FA101 soundcart i get allmost the same output level. Only the AKG has 10dB less noise.

    So this concludes that the signal coming from the circuit is symetrical i think, otherwise there would be a 6dB drop. or the electret has twice as much output and this circuit is assym.

    I'll try lowering the zener to a 2.7V type.. but then again lower voltage zeners are noisier arent they?:)

    I'll experiment some more, thanks for your replies!

    jeroen
     
  6. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    That is why there is a R-C filter after the zener diode.
     
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