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Disabling or replacing piezo tweeter in MarkBass CMD121P combo?

Discussion in 'Amps [BG]' started by mcnach, Jan 7, 2014.


  1. mcnach

    mcnach

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    Jun 16, 2007
    Location:
    Edinburgh, Scotland
    I have a MarkBass CMD121P combo, which I love for many reasons and I've been using it a lot since I bought it last May.

    What I don't love is the harsh tweeter. Blergh.

    I am considering disconnecting the tweeter and see what it sounds like. I imagine that it would not be difficult to open the thing and disconnect a couple of wires.
    I am also considering replacing the tweeter with something else that sounds better.

    Would it be bad to simply disconnect the tweeter?
    I suspect the impedance of the internal speaker will change... The amp is designed to run at 4ohm minimum, and I often add an 8ohm cab to the combo... will disconnecting the tweeter be bad for the amp when using an extension cab?

    If I replace the tweeter, ideally for a non-piezo type... does anybody have any recommendations? And again, will the impedance difference between piezo/non-piezo types create problems?
    I have no idea what values I should look for. I came across a couple of threads where someone had replaced the piezo tweeter with this:

    http://www.bluearan.co.uk/index.php?id=MON100190

    which is an 8ohm tweeter...
    and reported good results... but I have no idea whether that was using the combo by itself or with an extension cabinet, and I have seen no reports after a reasonable period of time confirming that "it's all good after a couple of years of frequent use" etc...

    I'm a little lost here, so I'd appreciate any help/suggestions. I don't want to damage this combo that I love just to remove something that bothers me but it's not a deal breaker.

    Cheers!
     
  2. B-string

    B-string Gold Supporting Member

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    Just disconnect and tape one wire to the piezo horn. If you try to put a standard dynamic tweeter in there you will HAVE to install a crossover (LPF). Either way there is no change to impedance.
     
  3. bccbass

    bccbass

    Joined:
    Dec 31, 2010
    I use passive Fenders with mine. I keep the amp in the floor with the bass set on 7, mid bass on 6 med on 4 with the treble on around 2. The VLE on 3 and the VFE on 5. I find this the warmest sound and seems to move the frequency away from the tweeter. To me this is the closest sound to my B100R ampeg. There is a lot of discussion about disconnecting the tweeter, but this "work around" seems to work for me. Concerning the ohms, I would suppose you could disconnect the tweeter and then check with a meter. Hope you find your sound!
     
  4. mcnach

    mcnach

    Joined:
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    Thanks for the answers so far!

    So it's safe to simply disconnect the piezo tweeter, at least, eh?
     
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  6. B-string

    B-string Gold Supporting Member

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    Yes, piezo tweeters do not conduct (provide any load) until the frequency reaches the designed range so no crossover is needed. There will be no difference to impedance if disconnected.
     
  7. JimmyM

    JimmyM Supporting Member

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    It is safe to do it, but I did it when when I had one and discovered that there are absolutely no highs at all without it. Quite honestly, I think there's a response hole between the woofer and tweeter. Your best bet is to use the VLE to tame down the tweeter. You could possibly replace it with a better tweeter, but since it's only a little 1" tweeter, I wouldn't expect miracles.
     
  8. will33

    will33

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    Yes you can simply disconnect it, but leave it mounted just to plug the hole in the cabinet. Tape or cap off the wire ends so the can't short on anything.

    A nicer tweeter would be Eminences APT80 with their 3500hz highpass filter and adjustable L-pad....about a $100 mod.
     
  9. will33

    will33

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    Lots of other options out there too. Selenium has some nice ones.
     
  10. KJung

    KJung

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    The VLE control (it is a variable low pass filter) will take care of your issue. That is what it is designed for.

    The more you turn it up, the lower the frequency cut-off of the high end. So, you should be able to literally 'turn off' the tweeter.

    That is what that control is for. Of course, if you turn it way up, it will roll off the lower treble and eventually down to the mid mids.

    My guess is, set at 9 o'clock will make you quite happy!
     
  11. will33

    will33

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    A lot of peizo's "nasty" sound is in the lower end of their range, not so much the high treble. He'd probably be chopping his tone off at 2-3khz to get rid of it.

    BFM has some peizo filter schemes that get rid of the gank but let the treble play.
     
  12. JimmyM

    JimmyM Supporting Member

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    I'll be honest...the VLE is a great little control that tamed down the nasty of that tweeter, but will's right...it only tames it down a few degrees and doesn't stop it from sounding nasty IMHO.
     
  13. BassmanPaul

    BassmanPaul Gold Supporting Member

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    +1 with the caveat that the filter should be a high pass for the tweeter. the woofer can be run full range.
     
  14. will33

    will33

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    In your experience, the woofers in these lack some topend. I'm going off the assumption that the OP would like to get rid of the nasty sound but not make his entire tone "dark". Otherwise, a combination of rolling down the VLE and the tone knob on his bass would probably do it, but there goes the whole upper part of his tone along with it.
     
  15. KJung

    KJung

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    +1, but the driver wouldn't go that high anyway, so no different from removing it. If the OP decides to replace the tweeter with a better unit and true crossover, that would be a different thing, but I wouldn't recommend that.... just get a new cab if he wants a better, full range sound with extended top end.


    +1 though, the question is, does he want full range up top but just better tone (that is not how I interpreted it), versus just 'getting the piezo/upper frequencies out of his sound'. If it is the latter, he should definitely mess around with the VLE before he starts cutting wires, etc. IMO.
     
  16. mcnach

    mcnach

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    Thank you for all the responses, much appreciated! :)

    I'll probably just leave it as it is, 'though, as I really don't want to mess it up.

    I use the VLE control, which I see it as a great tone shaping control, but not the right tool to remove the piezo harshness without removing all top end as well. Same with the treble control... it can help remove the very high zing, but it does not remove the harshness on the top end.

    Having said that, it's not a terrible affliction: at gig volume in the band it's barely noticeable. I mean, I hear it because I'm close to it, and it puts me off a little... but I know people don't really hear that, and most of the times I use the combo alone as a monitor while the FOH gets my DI, and there's no harshness there.
    The hiss is only noticeable at low volumes in the house, but then it's not a deal breaker. I love the amp... I just know I could love it more. If there were a simple and safe solution, great... but I don't want to risk it for the sake of a small improvement.

    I was curious because I have found a number of people who said they replaced the tweeter and it solved the problem of the top end being harsh (and the hiss).
    Every single one of them used the tweeter I linked to earlier... But I'm concerned that it's rated at 8ohm, and I'm not sure how that will affect the impedance of the whole combo. It may be ok on its own, but what about adding an extension cabinet? That's a bit of info I could never obtain about the examples I found, and I use an extension cabinet a LOT.
    The combo has a crossover, set at 3.5KHz, if that matters. I don't know anything about these matters.

    Now, about simply disconnecting the tweeter... yes, I'd be interested in disconnecting the crossover, actually. It's not that I want to remove all top end, I want the main speaker to deal with it all. Again, I have found one example where someone did that and says the speaker has enough top end by itself that he does not feel he wants a tweeter at all, and bypassing the crossover+tweeter resulted in a smoother sound. So that sounds simple and promising... if safe as in not altering impedance in a way that adding an extension cabinet becomes bad for the amp.

    You can tell I don't know about these things, eh? :)
     
  17. mcnach

    mcnach

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    That's exactly what I want: full range but better tone.
    Or as full range as typical bass speakers get.
    VLE does tame the harshness in the top end, but it also changes the sound beyond it.
     
  18. mcnach

    mcnach

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    It's a combo.

    The specs state there is a crossover at 3.5KHz... but does not explain more beyond that.
     
  19. mcnach

    mcnach

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    would that be because the crossover was still in place and cutting all above 3.5KHz to the main speaker?

    I guess the crossover needs to be bypassed too?
     
  20. mcnach

    mcnach

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    There is a crossover already, apparently, set at 3.5KHz.

    I have no idea how crossovers work, 'though. That's why I worry about changing the impedance of the combo internal speaker (including tweeter), as I often use it with an extension cab.

    I guess that's my next stop: read about crossovers. There must be some basic layman-terms information out there.
     
  21. B-string

    B-string Gold Supporting Member

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    Piezo tweeters usually don't use crossovers, they are capacitive loads (natural crossover) so may include a resistive component. The horn you linked would fry pretty quick if you didn't install a crossover. There maybe better sounding piezo tweeter out there, or it may just be the way high frequencies are handled by the amp section. MarkBass cone speakers tend to be weak on the high frequency side. Don't worry about any change to impedance with piezos, if you go to a dynamic horn like the one linked you would want to match impedance of the cone speaker and choose the crossover frequency for that impedance.
     

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