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B2R buzz when used with SVPCL

Discussion in 'Amps and Cabs [BG]' started by mgess, Mar 24, 2009.


  1. mgess

    mgess

    Apr 8, 2008
    Virginia
    Hey,

    I am new to using separate preamps and poweramps. I have always used integrated heads. However, I recently purchased an used ampeg svpcl. I still need to save the funds to get my Crest CA9 so, in the meantime, I thought I could use my old Ampeg B2R with a poweramp in on the back panel.

    I tried connecting the two using a 1/4 cable. As soon as I plug into the poweramp in on the B2R, it hums like crazy. It does this while the svpcl is not plugged in at all. So, I don't believe it is the preamp.

    Is there something else that I could/should be doing? OR is the B2R the problem?

    Thanks
     
  2. Did you try a different cable. You did use a screened cable yes?

    Paul
     
  3. mgess

    mgess

    Apr 8, 2008
    Virginia
    I did. I tried different cables and used speaker cables which I believe are shielded.
     
  4. it is VERY unusual to find shielded speaker cables, I've never seen one. You should use an INSTRUMENT cable. That's where your hum is coming from.

    Paul
     
  5. toobalicious

    toobalicious

    May 6, 2008
    triad, nc
    ground loop.

    try lifting the ground on the preamp. you can use a two-prong cheater sort of deal if you dont have a fancier way.
     
  6. Very unsafe way to do things. However it's happening when the pre-amp is not plugged in!

    Paul
     
  7. JustRob

    JustRob

    Dec 15, 2008
    Central USA
    Possibly a bad ground on the B2R Power amp in jack?
    Plug your cable in to the jack, at the other end of the cable short the tip to the sleeve with a piece of wire, the hum should stop. If it does not, the B2R is at fault. If it does stop, there's something else going on. (duh)
     
  8. <Sigh> The hum is coming from the fact that he is using an UNSHIELDED cable. That is picking up on all the hum and induced AC noise in the area.

    Speaker cables are aptly named. They connect an amplifier to its SPEAKER system and are used nowhere else. That's why they are called SPEAKER cables!

    Paul
     
  9. toobalicious

    toobalicious

    May 6, 2008
    triad, nc
    yeah maybe not a long term solution, but if grounding *were* the issue, that would trace it down. missed the part about it not being plugged in, lol.
     
  10. I thought y' might have! :)

    Paul
     
  11. mgess

    mgess

    Apr 8, 2008
    Virginia
    I tried using an instrument cable and the buzz was still there. I did use a "cheater" to go from 3 to 2 and it cleared up the buzz.

    Is this safe to do? If not, would a power conditioner solve the problem?
     
  12. No it won't help. You need to plug both units into the same duplex socket. If the pre-amp has a ground lift switch use it. If not see if the amp has and try that.

    Paul
     
  13. JustRob

    JustRob

    Dec 15, 2008
    Central USA
    Something doesn't add up there.:confused:


    Safe? Not really. Done frequently? Yes.
    Power conditioner help? No

    I'd try using the xlr balanced out, it is transformer isolated and has a ground lift switch. Problem here is that it bypasses the master volume on the preamp.
    Using the preamp out (1/4"), another way I've been able to stop ground buzzes is to attach a ground bonding wire between the two units. There should be a zinc plated (silver) phillips head bolt on the back of each unit near the AC receptacle or power cord entrance. Loosen the bolt, wrap a wire (14-16 gauge) under the head, tighten it down, repeat on the other end, buzz should be gone. Safety level high, but not very convenient if you gig with the amp. Spade lugs on the wire would be more convenient.
     
  14. toobalicious

    toobalicious

    May 6, 2008
    triad, nc
    that pretty much sums it up....

    and as far as safety.... well, you probably wont get UL to approve it. but, the two units (which, as rackable units, *are* actually in a rack, right?) chassis' are very likely to be electrically tied together by the rails (even nix the nylon washers or add star-type lockwashers between the rail and the unit if you need to, which will dig through any isolation offered by the finish), as well as by the shield on the signal cable, which is virtually always tied to chassis ground anyway. you can check it with a voltmeter, if you like (and you should). it HAS to be, though, as otherwise there would be no ground loop. ground loops are very very very common in rack gear, and transformer isolation is often not available. if you are truly terrified of the small chance of electrocution, use a wireless, and electrically separate yourself 100%. also, the physical ground wire between the units as mentioned above would be even more assurance, and is exactly why said lugs exist. if none of this makes sense, see here:

    http://en.wikipedia.org/wiki/Ground_loop_(electricity)

    or
    someone will read that as bad advice, but i read it as the truth.
     
  15. Try an instrument cable and disconnect the ground and insulate at one end. The cable ground will not be continuous but it will be still grounded. If this cuts the hum try reversing the cable and use whatever orientation provides the least hum.

    Paul
     
  16. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    If the hum exists with the system plugged-in and all attached, but the preamp NOT ON.... then you have a "pin 1 problem".................unless the wire is really badly unshielded. Since lifting ground fixed it, shields were not the major issue, although they are good to have (!).

    Most likely, there is a power transformer whch is inducing hum from its "leakage field" into the ground of the cable to the B2R. The B2R is amplifying that hum. I suspect the B2R also has some "pin 1 problem" coming in that input. It is hard to avoid with complex signal paths in amps.

    Lifting ground, so long as there is no other ground, will generally fix it.

    Often, moving the mains cords and signal cable around will also fix it, with no need to lift grounds.

    The use of one outlet or extension cord instead of two separate ones also reduces this sort of hum.
     

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