Input gain pad on QU16?

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This topic contains 19 replies, has 7 voices, and was last updated by Profile photo of GCumbee GCumbee 5 years, 5 months ago.

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  • #53085
    Profile photo of coffee_king
    coffee_king
    Participant

    Hi there
    Where is the input “Pad” button on the QU16 please?
    Is there even one? My drummers phantom powered mic for his hi-hats are coming in almost too hot, even with the gain all the way down.

    Thanks

    #53086
    Profile photo of Rob
    Rob
    Participant

    If you are using the local inputs (on back of mixer), there is no pad:
    Local (red Gain) – Rear panel Mic/Line sockets
    feeding the internal Qu mixer preamps. These are a
    pad-less design featuring wide gain range.

    If you are suing a dSNAKE, there is a pad on the main screen for each input.

    #53087
    Profile photo of Rob
    Rob
    Participant

    Some mics have a -10db switch on them, have you checked this?

    #53088
    Profile photo of [XAP]Bob
    [XAP]Bob
    Participant

    http://www.amazon.co.uk/In-Line-Balanced-Attenuator-Volume-Adapter/dp/B0060GDZTG if you’re still struggling…

    I’m sure better examples exist, but it was the first hit

    #53089
    Profile photo of MarkPAman
    MarkPAman
    Participant

    Do those attenuators pass phantom OK? Some do & some don’t & that one doesn’t seem to say which it is!

    Also, check impedance. 600Ω is probably a better bet than 200Ω.

    http://www.canford.co.uk/Search?q=line+attenuator

    #53091
    Profile photo of mervaka
    mervaka
    Participant

    Attenuators shouldn’t need to touch pin 1! Differential mode attenuation is all that should be needed.

    pad schematic

    #53093
    Profile photo of [XAP]Bob
    [XAP]Bob
    Participant

    Why is R1 halved?

    #53094
    Profile photo of mervaka
    mervaka
    Participant

    Because of the differential nature of the circuit. It’s a derivation of a simple potential divider.

    Here’s some light bedtime reading:
    http://www.ti.com/lit/an/slwa053b/slwa053b.pdf

    Using this method, the circuit I posted previously should in fact read R1 and 2*R2.

    #53099
    Profile photo of [XAP]Bob
    [XAP]Bob
    Participant

    Why? I presume R1 and R2 refer to different resistances, so there is no reason to arbitrarily halve or double one of them.

    Or is it that R1 is R2/2?

    #53100
    Profile photo of GCumbee
    GCumbee
    Participant

    I have never seen a pad where R1 and 2 were different. Basically they are impedance isolation providing for the shunt resistor to passively reduce the level. Basically shorting. Same with a H pad.

    #53103
    Profile photo of mervaka
    mervaka
    Participant

    It’s not arbitrary, see Figure 1 in the PDF I posted.

    R1 and R2 are independently respective of their single ended equivalents in a potential divider, where R1 is the resistor connected to the source, and R2 is the resistor connected to GND.

    #53104
    Profile photo of GCumbee
    GCumbee
    Participant

    The link is for filters. The one you show on your post is a balanced out. No ground. Single ended pads are usually T-pads.

    #53105
    Profile photo of GCumbee
    GCumbee
    Participant

    Here you go.

    Attachments:
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    #53107
    Profile photo of mervaka
    mervaka
    Participant

    In my intended example, it is inferred that ground would run 1:1 with no circuitry other than the screening of the cable. Same as what your schematic shows.

    Yes the link is for filters, but the same principles apply, just without reactive components.

    #53108
    Profile photo of Andreas
    Andreas
    Moderator

    Just some points from my side for clarification. The circuit expects a High-Z input to work correctly (reduce level by 6dB), otherwise R2 has to be seen in parallel to the input impedance. If R2 equals the input impedance, reduction will be 12dB.
    -6dB Pad isn’t very much for a pad and when choosing R2=R1 you’ll get -12dB.
    For symmetric signals R2 needs to be 2*R1 for the virtual ground in the “middle” of R2 to obtain -6dB, as Mervaka said.
    The non-trivial problem is to choose optimal values for R1 and R2, since you also need to take into account the source impedance, input impedance and capacitance.
    Anyway, I somehow doubt this is a pad issue, since level from a mic should be way below line level and the inputs allow signals up to +19dBU which I rarely saw from any device (particularly not a mic, and never from a phantom powered condenser).
    I’d probably check cabling first. A broken ground or signal line on a phantom powered mic can easily produce that high spikes when shaked.

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