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weird(?) DC voltages at output of mic preamp

Discussion in 'Electronic Basics' started by tempus fugit, Nov 29, 2004.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I posted a message on this topic a few days ago (pops problems when preamp
    is turned on). I've done a little more experimenting, and found these

    1. When I switch the mic preamp on, I get a little (1 volt or so) pop. I
    assume this is pretty normal, and not much of a problem. Someone answered
    with some great ideas for a mute circuit, which I'm sure will solve the

    2. (this is the biggy) When I turn the phantom power switch on, I get a big
    (10 - 15v) DC blast at the output. If I probe the actual output pin (i.e.
    before the DC blocking cap) this DC voltage remains for several seconds.

    3. When I turn the phantom power switch off, I get another big (-15v) DC
    blast at the output. Again, it remains there for several seconds at the
    output pin of the IC I'm using, but drops away fairly quickly at the output
    jack (post DC blocking cap).

    4. Both 2 and 3 only happen with a mic connected - there is virtually no DC
    pop (<1v) with no mic connected. Also, there is a brief DC pulse at the
    inputs (with a mic connected) of similar size, but it disappears quickly.

    There are DC blocking caps at the inputs. Where is all this DC voltage
    coming from? Any suggestions as to how to get rid of it?

  2. You are seeing the amplified blocking capacitor charging and
    discharging currents. Any time the DC voltage across a coupling cap
    changes, that is a form of AC, so it passes through the capacitor with
    an RC time constant of the capacitance times the resistance that the
    current has to pass through. If this pop takes several seconds, a
    coupling cap is way larger than needed to pass frequencies no lower
    than about 20 Hz.
  3. tempus fugit

    tempus fugit Guest

    Thanks John.

    I would like to clarify a couple things: If this is an AC voltage, why does
    it show up as DC on my meter? Also, the pop isn't audible for several
    seconds, but my meter does show the voltage for several seconds (not sure if
    that was clear in my original post). Are your reponses still valid in this

  4. The only DC that capacitors block completely is steady, unvarying DC.
    AC comes in two forms, sinusoidal and exponential. They can combine
    as a decaying sinusoidal, also. AC frequency is naturally measured in
    radians per second (2*pi of those per cycle), and the natural measure
    of exponential frequency is its time constant.

    Capacitors pass current in proportion to the rate of change of voltage
    across them (I=C*(dV/dT)), regardless of whether it alternates
    direction or only varies on one direction. The amplifier is probably
    saturating at the beginning of the decaying pulse, so the output is
    steady till the decay goes low enough that the amplifier comes out of
    saturation. If the decaying pulse lasts long enough, your DC meter
    can measure it as it decays (especially the starting saturated part).
    Your solution is either to shunt the audio signal to ground while this
    decay occurs so it doesn't show up at the output, cancel it with an
    equal and opposite decaying voltage or shorten the duration by making
    the coupling cap much smaller (making a high pass filter) so that the
    pop is not long lasting enough to be a problem.
  5. tempus fugit

    tempus fugit Guest

    Thanks again John.

    I have 2 100v 100uF caps at the input (to block the 48v from the IC) - the
    schematic on the data sheet (it's an SSM2017) shows 2 47uF caps. Could the
    extra size be a factor here (I don't know how much smaller I could get away
    with without rolling off the bass too much)? Also, I have a 470uF 250v cap
    as a smoothing cap in the DC circuit before the regulator for the phantom
    power. Is the added voltage handling of the cap a factor also? I also have a
    1 uF cap as a blocking cap at the output (at 100v I think - I used a poly
    cap here).
    Also, if I reduce the cap values, will this reduce the amount of voltage
    hitting the IC as well as the duration?

  6. Do you have a link for this schematic?

    The closest thing I could find was this replacement chip:
    It certainly makes the problem worse. The application note on my data
    sheet (page 2) shows the phantom power connected to the mic side of
    the 47 uf caps through 6k8 resistors, and another 4R7 after them
    before the clamping diodes. So the caps charge with very nearly a
    6K8*47u=.3 second time constant, while the diodes conduct. But after
    the cap current falls to what will not drop 15 volts across the 10k to
    ground resistors (about 1 time constant), the time constant becomes
    (10k+6k8)*47u= .8 seconds.

    So the inputs settle toward ground at that rate. Since the amplifier
    has a differential input, it starts to work as soon as the common
    voltage on the two inputs reaches the input common mode limit, which
    is about 2 volts inside the supply rails. But the output will still
    stay saturated a while after that if the two input capacitors are not
    well matched, so that one input settles toward zero faster than the
    other. You can test for this problem by looking at the DC polarity
    out of the amplifier during settling, and then switch the two input
    capacitors and see if the DC switches directions.

    Regardless, the input caps have a long time constant with the input
    loading (grounding resistors) compared to 20 Hz. 1 uf should be
    enough. 2 uf plenty. And then you can use film caps with much better
    leakage and tolerances than electrolytics.
    I doubt that it is involved in the problem.
    If the amplifier is saturated, the output cap is not the problem. The
    huge over voltage takes place at the input, before all that gain.
    It won't affect the peak much, only the duration.
  7. tempus fugit

    tempus fugit Guest

    Thanks John.

    I don't think the chip is made anymore. I could email you the schematic or
    post it on ABSE if you don't mind.
  8. Either or both is fine.
  9. What is the reason for turning the phantom power off and on? I've used
    dynamic mics on phantom powered preamps with no ill effect. Obviously
    the condenser mic requires the power. I've also had no problems
    plugging the condenser mic into the preamp 'hot'. Just curious.
  10. tempus fugit

    tempus fugit Guest

    Well, the unit has to be switched on and off in the first place, so I'll get
    the DC problems whether I have a separate switch for phantom or not.
  11. True, but a simple power on mute system would take care of things
    without having to have a 'smart' mute that would mute, toggle phantom
    power, un-mute. The mute system would be more complicated than the
    actual preamps.

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