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isolating balanced and unbalanced equipment - help requested

Discussion in 'Hobby Electronics' started by Jeremy Taylor, Jul 30, 2003.

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  1. I have hum inserted into the signal path when I place a processor between mic and
    mixer, and I'm not sure exactly what to do about it, though I have a few ideas, I'm
    not sure what is the best practice.

    I have a couple of mics feeding into a simple mixer/amp (with no chan inserts). I
    cannot push the levels on the channels very far before the mics feedback. It gives
    adequate level, but more headroom is desirable. It is not practical to relocate
    either the speakers or the microphones.

    I have a Behringer DSP1100 Feedback Destroyer at my disposal. Unfortunately, while
    the rest of the equipment uses balanced lines and XLRs, this DSP1100 has 1/4" jacks.

    I mistakenly thought that they were TRS/balanced jacks, based on the PDF manual I
    read that I obtained from Behringer's website, and used a couple of balanced patch
    leads with XLRs at one end and 1/4" TRS jacks at the other. When connected up to the
    system, there was a distinct and unacceptable level of hum.

    At this point I double checked everything, and it was at this point that I realised
    that while the back of the DSP100 had screenprinted notes regarding the Tip (signal)
    and Sleeve (ground symbol) there was nothing about anything for Ring. I interpret
    that as meaning this unit has unbalanced inputs/outputs only.

    I suppose I have several options I could try.
    (i) I could use cables that have XLR at one end and TS 1/4" jacks at the other, with
    the cold signal line shorted to the shield/sleeve.
    (ii) Between the DSP1100 and the mixer, I could put a DI box. What about the other
    side, from the mic to the DSP1100? another DI box, passive, running backwards? Does
    that work/How well?
    (iii) Some other sort of line isolator(s) between the units (both sides?)
    (iv) Something like the balance line drivers/receivers at Rod Elliott's website
    http://sound.westhost.com/project51.htm and http://sound.westhost.com/project87.htm
    but using the receiver between the mic and DSP1100 and the driver between the
    DSP1100 and the mixer?

    Advice on best practice (for making do with this equipment that is) would be very
    much appreciated.

    (Yes, a viable alternative is to trade the DSP1100 in on a DSP1124P with XLRs. I'll
    go down that track if there is not a simple cheap solution with the equipment I have
    to hand, if the fix will cost more than upgrade).

    Thank you all in advance...
    Regards
    Jeremy Taylor
     
  2. Phil Allison

    Phil Allison Guest

    (snip)


    ** Domestic hi-fi systems are totally unbalanced - and do not hum - so
    can be done.

    Is the DSP1100 powered by a plug pack or a 3 core lead ?


    ................. Phil
     
  3. Russ

    Russ Guest

    I've saw a DSP1100 a while back, and I'm pretty sure it had balanced
    connections, but yours might be a different incarnation. One main thing -
    congenitally it is placed after the desk and before the amps - in the same
    way that you'd use a set of graphic equalisers to dip the offending
    frequencies, the DSP1100 is a bunch of parametrics with very tight Q's, with
    the ability to function in a fixed mode - used when the mics are static, or
    auto, where the mics are moving around.

    It sounds like you are putting it on specific microphones before they enter
    the desk, which it isn't designed for - particularly if there is phantom
    power coming from the desk. It is designed to work with -10 to +4dB levels
    (otherwise known respectively as domestic and pro "line" levels) and
    therefore needs to go on the output of the desk. If the unit is indeed
    unbalanced and the output of the desk is balanced, I'd be careful about
    tying either of the hot or cold lines to earth - if it is an old desk with
    output transformers, you can do this, but if it has electronic outputs,
    tying one line down could overload the output buffer. Either let one line
    float, use a matching transformer, or a balanced->unbalanced box. If you are
    having earth loop problems downstream, you may need to re-balance the signal
    before the amps so you can lift the earth.

    As Newsy mentioned, you should make sure you've done the basics first - you
    may have phase buttons on each channel of your desk - often this can reduce
    feedback on specific channels, as can using the parametrics on the channel,
    provided it has a narrow enough Q.

    Russ.
     
  4. Rob Judd

    Rob Judd Guest

    Jeremy,

    Modify the Behringer to add balanced inputs. That's what I'd do.

    Rob
     
  5. It is powered by a 3-core lead.

    I am unsure what you meant by your first statement. Perhaps I can clarify - I am not
    speaking of a domestic hi-fi system by a low-end public address system.

    Regards
    Jeremy
     
  6. I can get ok gain before feedback, but occasionally with the movement of a mic near
    a ceiling mounted speaker will cause some feedback, and an extra 3dB of headroom
    even would make all the different to getting occasional feedback and never getting
    any.
    In ceiling, flush mounted. Mics occasionally walked/moved under them.
    Cardiod, but sometimes the mic is pointing in the general direction of the speakers.
    Not practical in this situation, I'm afraid.
    Yeah, this is the sort of thing that I am thinking - any other recommendations of
    other such gear that I might source locally (NZ) that you would consider ok?
    church hall, ceiling mounted speakers overhead over auditorium, mobile mics. Mixer
    is a pretty simple PA mixer with no insert jacks (which is where I would like to
    stick the processor in) and integrated amplifier (unfortunately cannot fit the
    DSP1100 between the mixer and the amp.)
    Yes, you're right - the DSP1100 is basically not the best choice of equipment to go
    with the other gear there already. The rest of the gear is basically doing an
    adequate (not perfect) job; I guess the choices are to find some other appropriate
    equipment that will fit into the existing system ok; or talk about replacing the
    existing equipment - which would of course be more expense, and needs to be
    justified against the low severity of the feedback problem. And unfortunately that
    also counts out the remodelling that would be needed with changing the existing
    installation of speakers etc...

    Thank you for you help.
    Jeremy
     
  7. Probably not a bad idea either, Rob! Considering the DSP1100 units came in a variety of
    flavours (I've seen XLR versions myself), the board probably has the capacity for a
    variety of connectors, the selection of which would be probably a purely mechanical thing
    when the unit is constructed. I can't imagine they would produce seperate board designs
    for each variant...

    Wouldn't tackle the job myself, but I'd get a electronics tech to look at it. However,
    other advice I have received here has pointed out that the DSP1100 probably wants a
    stronger signal than mic level - will want either -10dB or +4dB (it is switch selectable).
    So I don't think I am going to solve the feedback problem with the current equipment to
    hand anyway. It was a reasonably minor problem anyway...

    Thanks
    Jeremy
     
  8. Yes, I expected some sort of isolation or ground lift would eliminate the hum.
    Right, I see what you mean now - I think you are absolutely correct.

    The main problem I have been experiencing with the hum has really been caused by
    trying to put a piece of equipment where it really didn't belong; elsewhere it has
    been pointed out to me that the unit will require signal levels higher than mic
    level anyway, so I've basically being trying to tackle the problem from the wrong
    point. One can't expect things to work right if you try to break the rules!

    Thank you for your assistance

    Jeremy
     
  9. Phil Allison

    Phil Allison Guest

    rules!



    ** If you search the ESP site again you will find a nice, low-noise
    balanced mic pre-amp by me.

    PCBs in pairs are now available for a modest price too.



    ............. Phil
     
  10. Ah, now that would bring the signal level up to what we want... I'll look into that,
    thanks!
     
  11. It is true - the Behringer Shark DSP110 has a mic preamp in it. It was just a stupid
    assumption on my part that the DSP1100 would also be able to fit between the mic and
    mixer.

    JT
     
  12. Newsy

    Newsy Guest

    You must have a preamp between the mics and the Behringer - consider a cheap
    microphone preamp. Then put the output of the Behringer into the
    mixer/amplifier. It then won't matter if the output from the Behringer is
    balanced or not. Just balancing the inputs is not going to help.


    Rod
     
  13. Newsy

    Newsy Guest

    Is it possible to disconnect the speakers that make feedback occur?
    Sometimes these are just the ones at the front of the auditorium.

    As I said in another post - get another mixer/preamp and connect thus:
    mics - new mixer/preamp - Behringer - amplifier - speakers. Look in Crap
    Converters for something reasonable but cheap.
    The other suggestion is to suggest to the mic users to not go underneath the
    speakers. Maybe map a path on the floor that is "between" the speakers.

    Rod
     
  14. Russ

    Russ Guest

    The integrated mixer/amp thing is a bit of a killer. Behringer do a bunch of
    really cheap but high quality mixers, but I imagine a new amp would put the
    price up a fair bit.

    What model mixer is it? There may be a work-around.

    Russ.
     
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