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PLX2402 & Preamp question

Discussion in 'Amps and Cabs [BG]' started by caesarbass, Mar 23, 2005.


  1. I just got a PLX2402 and man do I love POWER!!!! I've been running it with my Sadowsky Pre/DI pedal, and I have to use the XLR out from my pedal to the PLX just so I get a strong enough signal. My question is, what should I look for when choosing a pre amp? I'm looking at the Fender TBP-1, an older Alembic F2B, BBE SS or T, SansAmp RBI. when I go with a pre should I use a balanced TRS from the preamp Out to the balanced In of the PLX? I'm going to be using the Alembic this weekend and hopefully soon I'll have my hands on the Fender TBP. Thanks for your wisdom!!!!!!
     
  2. Big D

    Big D

    May 26, 2004
    Orlando, FL
    Congrats on your purchase. I just bought a PLX3402. I also purchased an Ampeg SVP-Pro preamp, which has a very hot signal that seems to be a good match with the PLX. I have not tried any of the other preamps you mention. I went with the SVP-Pro vs. the SVP-CL since the Pro has an effects loop (the CL does not). I am using the ¼ inch output from the pre to the amp. It sounds great.
     
  3. zombywoof5050

    zombywoof5050

    Dec 20, 2001
    The Alembic F-2B only has unbalanced outs, so I'd just use a regular 1/4" TS cable. If you've got a good quality direct box, I bet it would work great to run thru that into the XLR of the 2402.

    When I play at home, I mostly run thru a chorus (set so low you can't really tell it's a chorus) into both channels of the F-2B thru a Radial JDI Duplex into two channels of a Mackie mixer panned at about 9:30/2:30....sounds awesome, much better than just running 1/4" cables from the F-2B to the line in on the Mackie!

    So I imagine it would work great to go thru a DI into the XLR's of a poweramp. Come to think of it, I've never even tried that with my rig, I ought to give it a try. Thanks for helping me think of that!

    Let us know how it turns out, however you hook it up.
     
  4. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    The PLX2402 works just fine with an unbalanced input too...

    Make sure that you engage the limiter and low filter to protect your speakers...the diagrams on the back of the amp tell you how to set the little dip switches.
     
  5. Great info guys, I think I just have to get the F-2B and start dialing it in. Please feel free to suggest other preamps. BigD, I'll have to check out the Ampeg pre's, I've never tried them but you've peaked my interest, thanks.
     
  6. For sure, I have the limiter on and the rolloff set at 50hz. Is the unbalanced signal as good as the balenced??
     
  7. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    For the short run of cable that you'll use between preamp and power amp, there should be no difference between going balanced and unbalanced.

    With a long run of cable (i.e. for PA use), I'd run balanced...
     
  8. Passinwind

    Passinwind I Know Nothing Supporting Member

    Either generally works fine for short runs, but there's a 6dB voltage gain using balanced lines as compared to unbalancing the same output. That could make a meaningful difference with some preamps. I don't sweat it with most of my bass gear, but I still always use balanced if the cables to so are handy.
     
  9. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    Wow, I didn't know that... I believe I've read the PLX' manual and don't recall any mention of that phenomenon either.
     
  10. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    That's because he was talking about preamp outputs, not power amp inputs.
     
  11. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    Fair enough... however, if I was writing the manual for a power amp, I might be inclined to add a little "Did you know?... statement or something...

    I'll have to look at the manuals for my two mixers to see if it is mentioned there...
     
  12. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    What would you add to a power amp manual?
     
  13. Joe Beets

    Joe Beets Guest

    Nov 21, 2004
    I've had a few preamps over the years and the line out jack usually put out twice the signal of the XLR out jack. The cable from the preamp to the power amp was always about a foot long and the unbalanced line out (TRS) worked best for me. Maybe the new preamps have a super pumped up output on the XLR?? :eyebrow:
     
  14. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    It all depends on the preamp designer and manufacturer. There is no standard for what does what.

    In pro audio, line level products can generally be counted on to be able to deliver nominal levels of about +4 dBu (1.23 volt) and peaks of +18 dBu (6.2 v) or higher. Those that don't won't be widely accepted unless they are meant to fill some niche need. There is no such criterion in MI; some instrument preamp models can't even put out 1 volt, while some can put out over +20 dBu (10 volts).
     
  15. Joe Beets

    Joe Beets Guest

    Nov 21, 2004
    Bob, has anyone told you today how much we love having you on this forum? If not, thanks for being here for us. And by the way, my PLX3002 has made more than one guitar player soil their drawers when they failed to control their stage volume. Thanks again. ;)
     
  16. Passinwind

    Passinwind I Know Nothing Supporting Member

    Joe, notice I said: as compared to unbalancing the same output. You comparison amounts to apples and oranges, not that there's anything wrong with that. :cool: BTW, if your 1/4" out is really TRS, that should be ablanced output, in most or many cases.

    The XLR out jacks can be at nominal mike level (DI level, IOW), or some in between level from line to mike, or ?? As Bob says, well defined standards don't hold with instrument preamps.

    BillyB: I think the old Mackie 1202VLZ manuals might mention this. I remember a big song and dance in there about how the various output configurations correlate to what the mixer meter says.
     
  17. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    A cute little cartoon character saying "Did you know that a balanced source has 6dB more voltage gain than the same source connected in the unbalanced mode?".

    Or in the troubleshooting section...where people hopefully look when they don't think that they're not getting enough power out of their power amp (it's never the source's fault ;) )

    I'm trying to understand the 6dB part though... Is the voltage to ground half of the voltage from + to -? If so then 20 log 2 equals 6 dB, which I guess makes sense...

    :help: Bob Lee :help:
     
  18. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Well, that's not really power amp stuff, though; it's just way too general. It would have to say that some balanced sources lose 6 dB when connected unbalanced, but some don't, and some cause ground loops, and none of this is specific to power amps, BTW, and wait, there's more ... ;)
     
  19. Joe Beets

    Joe Beets Guest

    Nov 21, 2004
    Let's see if I've got this figured out. On the line output jack on the back of my preamp. If I plug in a 1/4 stereo plug (balanced) instead of a 1/4 mono plug (unbalanced), the output from the preamp will be different. I wish they would have said something about it in the owners manual. :meh:
     
  20. Passinwind

    Passinwind I Know Nothing Supporting Member

    Is your preamp the Rolls one in your profile? That is a different case, because the 1/4 inch output doubles as a headphone jack. It's maybe dual mono instead of balanced, but still uses a TRS, from what I can infer? It does say unbalanced output in the specs...and if it's a headphone level output, no wonder it's louder than the line level XLR.

    It's easy to see why Bob wants to avoid this can 'o worms in an amp manual. ;)