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Shielding an Alembic preamp from a Power Amp

Discussion in 'Amps and Cabs [BG]' started by KPO2000, Feb 3, 2006.


  1. KPO2000

    KPO2000 Supporting Member Supporting Member

    Sep 5, 2001
    MA
    I just a got a used Crest CA9 power amp and it can't be within a foot (literally) of my old Alembic F2B preamp without emitting a subtle hum that gets louder as I turn up the power amp up. (I didn't have this problem with my QSC PLX.) If I turn off the F2B or move it to the side of the CA-9, the hum dissapears...and the tone is awesome. I guess they won't be sharing a rack together anytime soon, but it's a bit inconvenient. Not too big of a deal since the CA9 deserves it's own 2-space rack due to its weight. Maybe this is just a bedroom volume issue....I don't know.

    Anybody have any thoughts on how best to shield the F2B? Or, do I just get longer cables and put the CA9 on it's side next to the cab or something? Is this an issue with other tube pre+"lead sled" power amps?

    Thanks!:bassist:
     
  2. Fuzzhead

    Fuzzhead

    Sep 26, 2005
    Australia
    I'd seriously leave it on it's side next to the cab if it has it's own rack, and control the volume from your preamp on top of the cab. Sounds like a grounding issue, but why lift it up there if you don't have to, it weighs a ton right?
     
  3. That's exactly what I do. Who wants to lift that big beotch to the top of the stack anyway. :rollno:
     
  4. KPO2000

    KPO2000 Supporting Member Supporting Member

    Sep 5, 2001
    MA
    Is it OK to have these power amps operating on their sides for extended periods? Thanks! Ken
     
  5. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    If it's a grounding issue, lifting the ground on the signal cable might do the trick. Also, does the power amp have balanced inputs?
     
  6. KPO2000

    KPO2000 Supporting Member Supporting Member

    Sep 5, 2001
    MA
    The humming problem dissipates when moving the F2B away from the Crest about a foot or so, so I'd expect that this is happening because the F2B is emitting something. (The same low hum is audible when the power amp volume is turned all the way down as well.) Any ideas?

    Ken
     
  7. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Yes. There is a ground loop inside the preamp, and it is picking up the stray field from the power amp transformer. A power amp with a toroidal tranny would eliminate the problem.
     
  8. zombywoof5050

    zombywoof5050

    Dec 20, 2001
    That's weird, my F-2B is 'dead quiet'.
    But I've never tried it with a CA9. I've only used it with an old Crown DC300A II and my current Carver PM1400.
     
  9. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Now I am confused. The CA9 has a toroidal power transformer.
     
  10. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Even a toroid can throw off a goodly amount of EMI, and short of a quarter-inch thick mu-metal case there's not a lot you can do to shield the pre. Moving the pre is likely the easiest solution.
     
  11. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Hey, maybe quarter-inch mu-metal is going to be the next thing in audiophile bass amps. Hydrogen annealed, of course.
     
  12. zombywoof5050

    zombywoof5050

    Dec 20, 2001
    My F-2B has an old style 2-prong unkeyed power plug. As I stated earlier, I haven't had any hum/noise problems. But if I did run into it somewhere, would unplugging it and turning the plug around in the wall outlet help? I think I remember we used to do this with other equipment a long time ago before the invention of the grounded/keyed power plug.
     
  13. KPO2000

    KPO2000 Supporting Member Supporting Member

    Sep 5, 2001
    MA
    One thing I hadn't thought about, but no, it didn't have any effect. Separate case on its side seems like the way to go. Thanks!
     
  14. Fuzzbass

    Fuzzbass P5 with overdrive Supporting Member

    Too bad about the inconvenience. The F2B is a killer preamp, though... it's worth the additional effort!
     
  15. svtb15

    svtb15 Commercial User

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig
    My F2B is dead quiet too.. How old is it? when was the last time you changed the 7025/12ax7 tube?
    There is a chance that the power amps power transformer is emmitting RF that is being picked up by a bad pre tube.. possibly! someone wrote that the F2B may be emiiting noise and the poweramp is picking it up.. BUt I think it is more likely that the poweramp is emmiting the RF and being amplified by the more sensitive in high gain sensitivity of the preamp.
    Are the preamps tubes metal sheilds in tact?
     
  16. KPO2000

    KPO2000 Supporting Member Supporting Member

    Sep 5, 2001
    MA
    It's one of the old ones with the red knobs and blue sides. You might be right about the tubes, but they're not that old...or at least I didn't get them that many years ago. For kicks I changed the setting on the trim pot inside the F2B tonight, but it made no difference. Once again, not a big deal, because the problem doesn't exist when the power amp is separated from the preamp. Crest says it doesn't matter how you position (sideways, upside down, etc) the power amp provided it has enough ventilation.

    Thanks for all of the ideas.
     
  17. theshadow2001

    theshadow2001

    Jun 17, 2004
    Ireland
    What about a farradys cage? Put some chicken wire around the pre amp or around the power amp. This might inhibit some of the induced noise.
     
  18. KPO2000

    KPO2000 Supporting Member Supporting Member

    Sep 5, 2001
    MA
    Update: I put the CA9 in its own 2 space rack and stacked it above the 2 space rack containing the Alembic F2B and power conditioner and the hum is gone. I'll still probably put the CA9 on its side, but at least I know I can stack them if I need to.