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  1. #1
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    Do I need a line transformer?

    The specs on the Direct Out on my Soundcraft Live 4-2 mixer are "Input to Direct Output @ unity gain -87 dBu." The description of the Direct Out is "a post fade line level output from the input channel, bypassing the summing amplifiers, typically for sending to individual tape tracks during recording." I want to take the Direct Out output and plug it into a Mic input on a recording device. I think the line level output is too hot for the Mic input and will produce a rotten recording. Others I am working with say it should be fine. What do I need? Suggestions for proper devices (manuf. and model), if needed, are appreciated. Thanks for your help.

  2. #2
    Need more information. What's confusing here is that -87dBu is about 30 microvolts, which is very low even for mic level. Maybe -87dBu refers to the noise level at that output?

    If it's really a line output, you need to knock it down by at least 100:1 to feed it into a mic input. A typical matching transformer is around 20:1.

    1) Make and model of mixer and recording device?
    2) Balanced or unbalanced mixer output?
    3) Balanced or unbalanced recording device input?
    4) Is there phantom power on the input? Can you turn it off?

  3. #3
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    Good catch on the specs; yes, that is the noise spec.

    1) Make and model of mixer and recording device?
    Mixer is a Soundcraft Spirit Live 4-2
    http://www.soundcraft.com/product_sh...?product_id=14

    Recording device is a small handheld Sony recorder (ICD-P620)
    http://esupport.sony.com/CA/perl/mod...=ICDP620&LOC=3

    2) Balanced or unbalanced mixer output?
    The Direct Out for the channel I want to record is balanced. As an alternative, I could use the Rec Out RCA jacks, left and right unbalanced line out (-10 dBv) but I'm still thinking I need to knock the signal down to Mic level AND combine the L & R channels (??).

    3) Balanced or unbalanced recording device input?
    The Mic input on the recorder is unbalanced monaural.

    4) Is there phantom power on the input? Can you turn it off?
    No phantom power.

  4. #4
    You need a combining pad. Can you solder? Try this. Use the RCA outputs from the mixer. Values are fairly non-critical.
    Attached Images Attached Images  

  5. #5
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    Cool. OK, just 'cuz I'm like that, what is the math behind that and how many dB would that chop the signal?

  6. #6
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    The Direct Out on the channel l want to record from is a 1/4" balanced jack. I have a DI box (DOD 265 Stagehand) that has a 0, 20, or 40 dB pad switch on it. Can I use it to meet my needs by making a balanced XLR to 1/4" cable and an unbalanced 1/4" to 1/8" cable and switching the DI box to 40 dB pad?

  7. #7
    If we think of the inputs as the same (mono) it's essentially an L-pad with a 16.5K ohm series leg and a 100-ohm shunt. This gives you about a 165:1 attenuation, or in the neighborhood of 44db.

    In all such issues, keep in mind that it's very bad to short left and right outputs together, and it's very bad to short an output to ground. The latter is a tricky one because in modern boards, a balanced output is really two unbalanced, out-of-phase outputs. When you do a balanced to unbalanced cord, very often, depending on wiring, you'll short one side to ground.

    The direct box will work if you're going from mono to mono, but I was under the impression that the board's output is stereo.

    If you want to use the 1/4" outputs, get 3-circuit jacks. Take the signal from the tips. Leave the rings unconnected. Connect the sleeve to the shield, the low side of the 100-ohm resistor, and the shield of the cable to the recorder.

  8. #8
    Quote
    Quote: karl eilers
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    in modern boards, a balanced output is really two unbalanced, out-of-phase outputs.
    Wait, what?
    Eric Adler (tonsofpcs)
    http://www.videoproductionsupport.com/ Chat at: http://tinyurl.com/vpschat
    Follow me on twitter: @videosupport @eric_adler

  9. #9
    Eric, I gotta believe you know this, but for others who may run into this...

    Whenever you go from a balanced output to an unbalanced input, you have to figure out what to do with the side of the balanced line that won't be connected to the input.

    In the "olden days," when balanced outputs came from floating transformer secondaries, you'd hook pin 2 to the balanced input and pin 3 to the shield. That way the output, which appeared across the transformer secondary, would be connected between the unbalanced input and ground. That's the way you'd expect.

    Modern consoles, however, usually do not have transformer outputs. The outputs come off a pair of opamps, one of which feeds the signal straight out and the other which inverts the polarity. Since you have two outputs of opposite polarity, you have a balanced output.

    But the opamps don't see themselves as a balanced pair. Each amp puts out its respective signal against local ground, and neither amp knows or cares what the other is doing. They are really giving you two separate unbalanced outputs which, taken together, because they have opposite polarity, give you a balanced signal.

    (This is exactly what you're doing when you put a power amp in bridge mode and connect the speaker across the hot terminals. A stereo amp is two mono amps in the same box. In each channel, the signal comes out on one terminal and the other is connected to ground. However, since the two are arranged to be "out of phase," connecting between the hot outputs gives you a balanced output with a combined maximum voltage of twice the maximum voltage of each of the individual amps.)

    If you short one side of the signal to the shield, as you would with a transformer, you are shorting the output of one of the amps, plain and simple.

    Knowing this is going to happen, the designers of most modern consoles design so the outputs will survive shorting. However, if there's any common resistance in the ground part of the circuit - which there always is, and it may be highly nonlinear - the high current created by shorting one amp to ground will show up as distortion on the side which is not shorted. Better results can be had by leaving the "low" side of the balanced line open.

    One caveat - it is possible to design the two opamp circuits so they "look at" each other and work the way a transformer works. Mackie, I think, often did it this way, and maybe other people have. Most don't, though.

    I'm a big fan of the "impedance balanced" or "pseudo balanced" output where the "low" side opamp is replaced by a resistor to ground that's equal to the output impedance of the "high" side amp. This costs you 6dB in maximum output before clipping, but otherwise it acts exactly like a balance output and has the advantage that it doesn't matter whether you connect the low side to ground or leave it open.

    I bet this throughly confuses khnervick. Long and short: since you're going into an unbalanced input, you want an unbalanced output. As stated above, modern balanced outputs are really a pair of out-of-phase unbalanced outputs. So if you take the signal from one side of the balanced output, you've got what you want to match the unbalanced input of the recorder (witht he pad inserted, of course). That's why I said to connect to the tip of the 1/4" plug. You must use a 3-circuit plug; if you use a 2-circuit one, the long sleeve will short the low side of the balanced output like I warned against.
    Last edited by karl eilers; 7th Jan 2009 at 05:20.

  10. #10
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    Actually, you only somewhat lost me with the last one. I wasn't sure what you meant by a 3-circuit plug; I understand it to be what I call a TRS plug. So, long and short of it as I understand it, what I need to do is create 2 cables. First, an XLR-F to 1/4" TRS (3-circuit) where pin 3 on the XLR to R on the TRS is disconnected. Second, an unbalanced 1/4" plug to 1/8" plug. Then I set the pad switch on the DI box to 40 dB and connect the cables as follows: 1/4" TRS to Direct Out on the mixer, XLR-F to one side of the DI box, 1/4" TS (unbalanced) the the other side of the DI box, and 1/8" TS to the Mic input on the recorder. Good?

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