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mic splitter using resistors

Discussion in 'Pro Audio Equipment' started by birdsong, Nov 7, 2008.

  1. birdsong

    birdsong Guest

    In doing some research for a mic splitter I'm building it became obvious that I should have one isolated out and one pass through instead of just building essentially 16 y-cables in a box. There are a number of reasons and what I want to know is if there is a way to do it without buying 16 trannies. Transformers are the right way to go but the Jensens are cost prohibitive and so are Lundahl. I couldn't find a price on Cinemag but my guess is that they are too.

    Resistors are much cheaper and in my mind could provide a mic with the amount of resistance it wants to see. Is this true? So here are my questions:

    If I want to use resistors should I put a resistor each across the hot and neutral legs of the output?
    What Ohm rating should the resistors have?

    Also is there a way to isolate one split from phantom power? what if I were to permanently lift the ground, that would certainly not pass phantom but what are the reasons this should not be done?

    Any other input on the subject is welcome.
     
  2. RemyRAD

    RemyRAD Guest

    For many years I've had to use passive or active splinters from microphones. If you're going to use a pair of mixers, how far apart would these mixers be? Will you be pulling the power for both from the same source? Yes, for your secondary split, you do not need to connect pin 1. That can be lifted. The signal only appears differentially on pins 2 & 3. What you don't want to do is to load down the microphones. So your primary input is already terminating the microphone with 1500 ohms. So which you may want to try is to use 1000 ohm buildout resistors on pins 2 & 3. Of course this will knockdown level to your secondary mixer considerably. So you're going to need to crank extra gain. You may also have issues with phantom power. It doesn't matter if you listed the ground. Phantom appears on pins 2 & 3 as +48 volts DC. So you'll probably want to disable phantom on your secondary mixer, any way you can. Otherwise you might also want to use a pair of .1 microfarad ceramic capacitors to block the direct current on the input side to your secondary mixer.

    But in all honesty, you don't need any resistors. You can just parallel wire pins 2 & 3. I've done it many times. Perfectly fine for rock-and-roll not as desirable for orchestral. But watch out for those ground loops as the hum & buzz will kill you. And that's the biggest problem with direct wired splits.

    You're right, transformers, good splitter transformers will cost you dearly. But the benefits outweigh the expenditure & the problems associated with passive splitting. Some problem so bad, you may never want to go there again?

    You could consider providing a split from the consoles pre-fader Mike preamp direct outputs. In that respect, you'd be providing line level splits, which are great. And yes, with a pad engaged, you can go into a microphone preamp an extremely low gain settings. This keeps the noise in check. So you wouldn't have to worry. This is something I frequently do in my control room so as to be able to multitrack record while providing isolated sub-mixes & stereo broadcast feed. And that's actually a secondary split after I receive the first split from the stage.

    Good luck. You'll need it.
    Ms. Remy Ann David
     
  3. GentleG

    GentleG Guest

    yes, and

    for pop / rock
    the cheap transformers will do

    I use the direct signal to record and the cheap transformer split to send to PA etc.

    At the level I'm working at: even the cheapest transformer is better then: PA x operator ....

    cheers
     
  4. Boswell

    Boswell Moderator Distinguished Member

    There is no easy way to connect resistors to do what you want. Resistors are by definition lossy, so use of resistors is going to lose you signal in either the direct split, the side split or both, and that translates into needing more pre-amp gain and a higher noise floor. From a noise point of view, you would be better off paralleling the mics into both pre-amps using a Y-cable. It would help your cause if at least one of the pre-amps had a medium (5 - 10KOhm) input impedance rather than 1.5 - 2K. But see next section...

    This is where you may run into trouble. Connecting the mics directly to both pre-amps also parallels the phantom power, the return current flowing though the mains earth if you lift pin 1 on one of the pre-amp inputs. Now, depending on the design of the pre-amps, this paralleling may or may not be a problem. Any difference in PP source voltage between the two pre-amps will result in a d.c. current flowing between the pre-amps. However, this may actually be a better situation than turning off the PP in one of the pre-amps and having the PP voltage from the other loaded by the unpowered PP resistors (6K8s each) if their junction is grounded when PP is off. The other thing to consider is that PP blocking is normally done using polarised capacitors, and these won't take kindly to being reverse voltaged.

    So I think the answer is... it depends. If the PP voltage in your two pre-amps is sufficiently similar (say within a couple of hundred millivolts) that there will not be large currents flowing through 13.6KOhm, then I would try paralleling them up with the PP switched on in both pre-amps.

    The better solution is to have a single external conventional pre-amp per channel whose output is then split using a Y-cable to the line inputs of the two boards or interfaces. This pre-amp needs to be high-quality, but doesn't need to be anything fancy.

    If you are in a constructional frame of mind, you could make a row of fixed 20dB gain amplifiers using SSM2019s to do the job for a lot less per channel than quality transformers. This would give you unbalanced outputs but would feed the paralleled mic inputs of two conventional pre-amps (PP off!) without trouble. PM me if you need more details of this solution.
     

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