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Acoustic Image Heads used with metal speaker cabinets

Discussion in 'Amps, Mics & Pickups [DB]' started by Bob McHenry, Jul 6, 2005.

  1. This note will outline a probably OBSCURE SCENARIO for most
    Talk Bass Members, but it is worth a "Heads Up" at least!

    I recently had occasion to play around with my Acoustic Image Clarus 2R head in combination with a G/K 112 MBX extension speaker. I had the Clarus 2R sitting ON TOP of the G/K speaker which happens to be in a METAL cabinet. When I turned the amp head ON there was a "clicking sound" heard through the speaker @ about a 60 Hz rate and there was NO amplifier output.

    What was happening was the switching power supply in the Clarus amp was TRYING to turn ON - but couldn't! The reason is because the speaker outputs on the amp are "floating" and the +/- speaker leads should never be tied to GROUND. It turns out that for whatever reason/s, it appears that at least one of the speaker terminals the G/K 112MBX speaker Cab is also tied to the metal Cabinet which is GROUND! Apparently the Clarus amp case was making contact with one of the carrying handle screws on the 112 MBX cab when I placed it on top of the cab. - thus causing this problem. Fortunately the built in safety features in the Clarus amps. precluded any damage to the switching power transistors in the amp . power supply.

    The bottom line: The Acoustic Image amps will work just fine driving one or two G/K 112 MBX speaker cabs. JUST DON'T SET THE AMP ON TOP OF THE METAL CASE OF THE SPEAKER CAB!

    It is not exactly clear to me why it appears that the speaker leads on the G/K 112 MBX cabs are not ISOLATED from the metal cabinet as I know of NO OTHER speaker system (regardless of the cabinet material) that does not have this isolation. Clearly this would NOT be a problem with speakers mounted in non-metal cabinets.

    My thanks to Rick Jones (President of Acoustic Image) for "shedding some light" on this potential problem area.

    Bob McHenry
  2. Thanks for the info! I've been wondering for some time wether the GK would work with the AI. I had allways thought that the low power rating on the GK would be a problem. I never thought about the metal cab though. How does it sound?

  3. Hi Bennet,

    The Acoustic Image amps should work just fine with the G/K 112 MBX metal speaker cab's as long as you take care to NOT place the amp on top of the speaker.

    I haven't experimented around much with that combo yet, but the little I did do (once I determined what my "problem" was) the cab sounded pretty good - using my 60+ year old Kay C-1 acoustic bass with a Fishman Full Circle pickup on it. Although G.K does not rate the frequency response of the 112 MBX and the MB 150S III combo unit, they have told me that the response is good from 50 to 19KHz. The 112 MBX is rated at 100 watts (not "barn burning" power handling, by any means,) but for small to medium size venues, it should be fine. Actually, I was considering using two of the 112 MBX's with my Clarus 2R which would represent a 4 ohm load to the amp. I've noticed that the G/K bass combo units are being used by a lot of keyboard player friends of mine - probably confirming that the unadvertised 50 to 19KHZ freq. response is probably accurate.
    I suspect, however that the preamp EQ section of the combo units are "voiced" for bass - even though they seem to sound O.K for keyboard use.

    I plan to do more experimenting around with the A/I amp and the G/K 112 MBX's as time permits. The "attraction" is the extremely light weight and diminutive size of the cab's. If they wind up NOT to my liking (soundwise) I'll scrap the idea. As the old addage goes "One Good Test Is Worth A Thousand Expert Opinions"!

    Bob McHenry
  4. Aleph5


    Feb 24, 2004
    19kHz from a 12" cone speaker? I don't think so, though it probably sounds quite decent for keyboards. Maybe they meant 9kHz?
  5. Ya Know??

    In thinking about it, I think you are probably right. It turns out that I still have the e-mail reply from Gene Lopes (G/K Customer Support) dated 10-11-2004. He did indeed say 50 - 19KHz, but in thinking about it I think one would be hard pressed to get even 9 KHz out of a single cone speaker. I would think that 5 KHz would be more "normal" and MAYBE out to 7 KHz. I suspect that G.K may be using propriatary drivers made by JBL (Paragon) or maybe Eminence. I have e-mailed Gene again to confirm what the response really is - and will re-post his reply when/if I get one..

    As a retired E.E. I should have questioned this myself. I think the only reason I passed it off, was because "way back when"- when I had one of my first stereo systems, I had a pair of JBL Cabs with 12" extended range single cone 12" speakers that DID get out pretty far in the high freq. range.

    Thanks for your thoughtful comment!
  6. O.K. !
    As I said in my previous post, I did re-contact G/K re. the 112 MBX Frequency Response. The latest input is from them is "from about 40 Hz to 5KHz" - which seems more likely. (Certainly more likely than the 50 to 19KHz originally quoted me!).

    Assuming that this same speaker is used in the G.K MS150S 112 III bass combo unit,that probably explains why there seem to be a number of keyboard players that are using the bass combo units "successfully" with their keyboards.
    According to the chart on Page 78 in Howard Sam's "Handbook Of Electronic Tables and Formulas", the frequency range of an 88 key piano goes from slightly below 30Hz to slightly below 8 KHZ. Therefore, the G/K's use with a keyboard MAY be marginal
    if one is interested in reproducing the full frequency spectrum of the keyboard/piano. Apparently, however, the unit must be "GOOD ENOUGH" for that use in the ears of the player.

    Interestingly enough, that same chart in the Handbook alluded to above lists the frequency range of a bass viol from 40 HZ to
    about 8 KHz so one could (I suppose) wonder if some of the
    frequencies between the 5KHz and 8 KHz range (and their harmonics) are being attenuated by the G/K units?

    Once again, what's the "bottom line" ? If the unit sounds GOOD to your ears and satisfies YOU then it not worth obsessing over
    technical jargon.

    Thanks for the opportunity to clarify (I HOPE) a few points.

    Bob McHenry
  7. This was an important post for me, since I have both an AI amp and the GK extension cabinet. I've never had a problem using them together, although I've done it only a few times (and I always have a wooden board in between 'em, since the amp just sits better that way!).

    Thanks for making me aware of this. Oh, I've also dealt with Gene at GK and I've found him quite knowledgeable and responsive!
  8. Ed Fuqua

    Ed Fuqua

    Dec 13, 1999
    Chuck Sher publishes my book, WALKING BASSICS:The Fundamentals of Jazz Bass Playing.
    I have a piece of foam (a largish rectangle) that I put in my bass case when I zip it up (so that it's in between the neck and my shoulder, being skinny, weak and aulden) as a cushion when I'm pushing the bass around on the wheel. I been using it between the Woods and my Polytone cab, just to keep **** from vibrating so much, I guess that'll work out for the GK. Whenever the hell it decides to get here...
  9. On my board, I cut a chunk out of the wood for the GK's handle, so the amp sits on it nice and flush. And now I know I was insulating it all the while!
  10. Christopher


    Apr 28, 2000
    New York, NY
    Someone tried an electronic drumset through my MB150E once - the cymbals were inaudible, though the other drums came through just fine. I'm inclined to believe the 5kHz cutoff.
  11. Freddels

    Freddels Musical Anarchist

    Apr 7, 2005
    Sutton, MA
    How does the GK 112 MBX sound? I just looked online and at $279 and 12lbs (that's a light cab) looks good. Paired with a WW 100 I wonder how that would sound.
  12. Aleph5


    Feb 24, 2004
    I would think a better sound for your dollar would be gotten from a cheaper amp paired with a better speaker. Make no mistake, I love my MB150S combo, which sounds very good to me (and is SO portable), and I would hope that using a WW head instead of the GK would be a big improvement. But from what I've read here (mostly), the GK speaker is far from the state of the art in tone compared w/ LDS, et. al. I should know the difference soon, since I took the (modest) plunge of ordering a 1x8 neo cabinet from Don at LDS. :hyper:
  13. Freddels

    Freddels Musical Anarchist

    Apr 7, 2005
    Sutton, MA
    Definitely let us know how the LDS 1x8 sounds with your bass.

    I'm leaning toward the wizzy cab right now. But you know . . . that effort to get the best sounding thimble sized rig possible ;)
  14. robgrow

    robgrow Supporting Member

    May 1, 2004
    Cincinnati, Ohio
    The sleeve contact of the 1/4-inch jack being connected to the aluminum case of the G-K MBX112 cabinet creates a situation that could potentially effect many different types of audio equipment adversely. One example would be when using a stereo power amp being run in bridged mono mode where neither of the leads from the amp to the speaker should be connected to ground. Even if an amp's speaker connections are not an issue, there is always the potential of introducing a ground loop if any other electronic equipment inadvertantly touches the case of the MBX112. Insulating the jack from the MBX112's case using non-conductive washers or, much better, switching to a Speakon connector would help prevent any of these problems.

    BTW the 12-inch speaker in the MBX112 is a high-compliance type with a foam surround and a relatively heavy cone, very much like speakers used as woofers in larger bookshelf-type speaker systems. It's very unlikely this speaker can effectively reproduce anything above a few KHz. (Of course this also acts as a very effective low-pass filter when using a very bright piezo pickup.) Finally, this type of speaker mounted in such a small sealed box is very inefficient. This is the trade off for relatively good low frequency response from a very small speaker cabinet.
  15. robgrow has posted some very interesting and very valid additionnal considerations re. the G/K 112MBX with one of the speaker leads grounded to the metal speaker cab. My particular sample of the 112 MBX DOES have the ring of the phone jack grounded to the metal case. In talking with the folks at G/K about this they first said that there should be an insulating washer in the the phone jack that isolates the speaker terminals from the metal cabinet. (However I noticed that the same scenario exists on my
    MB150S 112 combo unit) When I pointed this out to them the "tune" changed a bit and they said Oh well, the 112MBX was designed to be a companion extension speaker for the MB150's and they work O.K. together in that mode. My guess (and that's all that it is) is that they may not fully realize that their products are being used in other configurations and have basically ignored the issue. To me to have a manufacturer tell me that "I can put an isolating washer in the jack myself" is a fairly unacceptable alternative - EXPECIALLY when the units are under warranty. Sure I COULD do this myself, but in order to do so, I believe you have to take the metal (steel) grill and the speaker out of the cab. to access the Jack. (The Cab. is aluminum, by the way, but the grill and all the screws are steel). Wonderful things those magnets!

    Anyway, I don't plan to muss-futz around with this situation any longer. I bought the G/K units to experiment around with and to have here as backup units to my regular gear if needed. I personally don't like the idea of having bass electronics and speakers having "voiced" preamps, power amps and speakers having limited frequency response. Why let THAT be a limiting factor in a bass system when there are so many other considerations to address as well? (Input impedance of the preamp, for example?) MOST units (including the G/K) have a 1 megohm input inpedance. Many (but not all ) piezzo pickups
    sound BETTER with a 10 Megohm input impedance.(The David Gage Realist pickups - being one of them- in my opinion) Other popular pizzo pickups like the K& K Sound and Fishman pickups can "get along" with a 1 Meg Input Z and sound decent. The end product of an improperly loaded piezzo pickup CAN result in the low end response being attenuated (The LAST thing you would want!)

    The primary reason why I chose the Acoustic Image Clarus 2R amp was because of its small size, WIDE frequency response, and other features like a SWITCHABLE input impedance. Coupled with (in my case) a pair of Contra EX extension Cabs (NOT frequency response limited) seems to fulfill 98% of my needs. To be sure there is other FINE equipment available on the market place as well.

    Once again, it is clear that in our quest for small size and weight
    (important for my 75 year old back) we have to be careful about little "quirky" things like we've been discussing in this thread- that can crop up if we don't keep our heads on straight!

    Thanks and "keep on pluckin"!

    Bob McHenry
  16. Bob, if it helps, I'm now running my A.I. through a 1x12 Flite cabinet. It sounds great and keeps the weight off!
  17. Hi Rob,

    Thanks for that suggestion (ie. using a Flite Cab) with the A/I amp.
    Actually I still have a Flite 115 cab (weighs only 26 lbs) which I use occasionally and it sounds very nice. I used to also have a Flite 123 cab with a 12" Eminence and a attenuatable 5" midrange that sounded good too. I regretably sold that cab. to a guitar player friend of mine in a "weak moment". I bought my Flites directly from Kurt Meyers Jr. who was the founder of Flite Sound in Danielson CT. I don't know how the newer Flites sound as Kurt sold the business to some one else and I believe some of Kurts original designs have been altered. Those Flites are remarkably light with that propriatary light weight cab. material! Other manufacturer's would do well to start enginering light weight cabs that sound good!
    It CAN be done!!!!!

    Bob McHenry
  18. I bought a pair of 1x15 cabs from Kurt in the late '90s; the 1x12 cabinet was purchased recently from the new people (Guy LaPointe). Like a good bottle of scotch, both transactions were smooth and satisfying!

    (I found the AI into a 1x15 sounded "too sloppy" for me.)

    Sorry to hear about your "weak moment." I agree with you, there's a huge market for lightweight gear. I think a lot of product marketing guys are missing the boat here....
  19. robgrow

    robgrow Supporting Member

    May 1, 2004
    Cincinnati, Ohio

    I can only hope I'm fortunate enough to still be playing the bass and experimenting with gear when I'm 75. I think it's fantastic that you are doing so. Anyway, you've got over 20 years on me, and I'm also trying to save my back and still get "the sound".

    Being an EE, you may have already thought of this: since GK amps such as the MB150 and 400RB use an input stage with a JFET opamp which has an extremely high impedance, it is very easy to change to 1 meg resistor that is connected from the input to ground to any value such as 10 meg ohms. It's pretty subjective if this improves the sound or not however.

    BTW I'm a little dyslexic sometimes MBX112 = 112MBX, etc. :)
  20. Hi "robgrow',

    Yup, I COULD consider changing the input resistor on the preamp front-end but I'm always afraid that if the front end is not well shielded, then the "opportunity" for the noise floor of the preamp can increase significantly (perhaps even UNACCAPTABLY) if I were to do that. Fortunatly, I have enough gear made by folks who (in my view) really know what they're doing, so I don't have to resort to trying that.
    My A/I gear with preamps all have 10 Meg input impedances available (switchable) and my Fishman Platinum Pro and D-Tar Solstace preamps BOTH have 10 meg input impedences, so I'm not hurting for gear with front ends that can screw up the loading on SOME piezzo pickups.

    Thanks for your comments on my "ripe young age" of 75 !
    I'm still having a LOT of fun playing jazz (after over 59 years of so doing), and I don't intend to stop now. However, I must admit that my old fingers don't always do what my feeble brain tells them to do these days.

    Have fun, and don't STOP!

    Bob McHenry

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