Kern into QSC PLX issues

Discussion in 'Amps and Cabs [BG]' started by Hollow Man, Nov 21, 2005.

  1. Hollow Man

    Hollow Man Supporting Member

    Apr 28, 2003
    Springfield, VA
    Hi all, this is my last bastion of hope before I call Les at Kern. I recently picked up a used Kern, but I can't seem to get useable volume out of it without damn near diming the preamp gain and output volume. With all other settings/cables/etc on my basses and PLX 2402 power amp being equal, my Read Purity only requires the preamp and master volumes to be at about 9:00-10:00. I've tried both the Main Out (has unbalanced 1/4" output) and Line Out (XLR out), and neither seems to give me anything more than the other. I've also tried swapping tubes, but replacing any one of the two 12AX7's or single 12AU7 did nothing for me.

    I figure if something's wrong with the Kern, I'll have to send it back to Les. But the user before me reported no problems running it into his PLX 3002 from the 1/4" out, and so I was thinking that if there's a flaw with my setup, you guys could find it, or at least give me suggestions of things to try before I trust my preamp to UPS or Fedex.

    Random thought: I know that some preamps don't work well with the QSC PLX's because of their relatively low sensitivity, and there are some people who have had their PLX's modded to accomodate lower input voltages. Those of you who have used Kerns into PLX's, is this for me? Have any of you Kern veterans noticed low output signal coming from them?
  2. The PLX does not have relatively low sensitivity. It is designed to work at pro sound input levels.

    Evidently the Kern is the same is my SWR IOD: gutless output. My IOD is 0.7v maximum output (full cranked).

    My solution is using a Rane ME30B equalizer, or other signal processing between the pre and the power amp. The Rane boxes boost the signal sufficiently to fully drive my PLX.
  3. e.maynard


    Nov 11, 2004
    Afton, Virginia
    That seems like a real mickey mouse solution. I'd call Kern pronto. I've used Demeter, Alembic and a few mic pre's with a PLX 1202, and never experienced mismatched gain like you describe.
  4. Hollow Man

    Hollow Man Supporting Member

    Apr 28, 2003
    Springfield, VA
    I agree with your first point; I should have rephrased that in my original post. The QSC does not have low sensitivity; however, there are several preamps that send outputs that are too low to fully drive the QSC. Many people have had mods done to their PLX's to accomodate these low input signals.

    I highly doubt that the Kern suffers from 'gutless output'. There are enough rave reviews on this site and others to lead me to believe that a properly functioning Kern into any reasonable power amp is a perfectly formidable setup. I'm just trying to figure out where my weak link is.

    Like maynard, I believe that inserting a separate EQ into my signal chain isn't a true solution, it's a sidestep. While I believe that you are right that it would help, I would rather fix the problem at its source (be it my setup, or the Kern) than work around it. Having said that, I really appreciate your suggestion, as it was only designed to help me. If you have any other thoughts, I would very much like to hear them.

    Keep 'em comin', guys. I know a lot of you have experience with Kerns. David Wilson, Jae, Boogie, I'm lookin' your way. :)
  5. msquared


    Sep 19, 2004
    Kansas City
    I had the same problem with a PLX 1602 that you're describing. No matter what preamp I used I had issues driving the PLX (although some were worse than others) Putting gear into the signal chain to boost the signal helped, but eventually I dumped the PLX/preamp method altogether.

    Lord Valve will mod the PLX so that it matches preamp outputs (and iirc can do it without voiding the warranty). I read on a newsgroup somewhere that there's a mod that can be done allowing it to switch between pro sound and preamp output levels, but I can't remember any details.
  6. Hollow Man

    Hollow Man Supporting Member

    Apr 28, 2003
    Springfield, VA
    Yeah, I bought my QSC from him, and I've read about that mod as well. He's an authorized QSC retailer/repairman, so any work he does should keep the warranty intact. There shouldn't be anything fundamentally wrong with my PLX, so I'd rather not modify it until I'm sure that everything else is as it should be. I'm still hoping to hear from some of the Kern gurus around here... I'd like to get a feel for their experiences with it (hopefully pertaining to a PLX, but if not, just their setup in general will do) to contrast and compare against my own.
  7. Fuzzbass

    Fuzzbass P5 with overdrive Supporting Member

    I hope you get the Kern issue squared away quickly. I'm very interested to hear your thoughts on Kern vs. Read. I'm keeping my Read, but might add a Kern in the future!

    FWIW, I've been using my SWR IOD a lot lately. It doesn't sound gutless through my Stewart World 2.1. :bag:
  8. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    QSC builds its amps to professional standards, which is a +4dB input sensitivity. Many manufacturers build their gear to the -10dB consumer elctronics standard. In most cases they do so because it's cheaper.
  9. Dear HollowMan 227,

    I just got off the phone with Les and read him your e-mail. He has asked me to respond as follows. The only way the Kern could have a low output signal is if there is a power supply fault in the preamp. This has happened in one other unit. The only way to determine if this is the problem is to try the unit with another power amp. If this is not feasible, Les will take a look at it for you and determine what the problem is. You can get ahold of Les via his website at Just go to the contact page for his e-mail address. He wanted me to assure you that he answers every e-mail he receives promptly. If I can be of any more help feel free to call me at home at (219) 326-5819.

    Mark Bruso
  10. jokerjkny


    Jan 19, 2002
    NY / NJ / PA

    i'm pretty sure the Kern doesnt fall into the latter catagory.

    and yea, i'd try the preamp w/ another power amp. and be sure all cord connections are tight.

    btw, Fuzz,

    we had a LONG thread about the both of them going head to head.

    while both were of the most tubiest and fattest preamps i have ever owned, i tip my hat to the Kern's smoother, glossier sound. the Read while awesome in its own right has a more furry glow kinda tube sound that's like a Demeter on steriods.
  11. 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
    If this is an IP-777 pre, Kern publishes some info here:

    It says it has a balanced transformer output "capable of +20 output level". I assume they mean +20 dBu, which would mean a maximum RMS output voltage of nearly 8 volts. That would be more than enough to drive a PLX amp to full power even with its gain turned down a fair amount. If that's the case, I think you may have something set up wrong.
  12. Hollow Man

    Hollow Man Supporting Member

    Apr 28, 2003
    Springfield, VA
    Thank you so much man, that was really great of you! As you, Jae, and Bob Lee have suggested, I'm pretty sure that the fault is not with the Kern's design. I'm also reasonably sure that my setup is not to blame, unless there's a particular cable or output that I should be using. But, like I said, I've tested with both the 1/4" unbalanced Main Out and the XLR Line Out.

    I played around with it some more today, and I noticed that there seems to be a huge volume swell at about the 1:30 position. What I mean by that is, if you leave either the gain or master volume high (this is true of either master volume, the XLR out when using it as well as the Main 1/4" out) and slowly crank the other control, it seems like the volume goes from minimal to HUGE somewhere between 1:00 and 2:00. With both the input gain and correct master volume knob high (about 3:00), the preamp sounds enormous, just like I would expect it to with everything cranked. It's just that when the volume knobs are at "normal" settings (anywhere between 9:00 and 12:00, since I'm just playing at home right now), there's just very little appreciable sound. After having performed this test about 30 minutes ago, my new suspect is that some, if not all, of the pots are of incorrect value. Thoughts everyone?
  13. Sorry you feel that way. The solution is both valid, and fully balanced.

    I don't own the Kern, so cannot comment on its design. I do own the IOD, and am passing along what I have in writing from SWR tech support.

    The IOD puts out 0.7v maximum, by design. If that is Mickey Mouse (and I agree it is), it is by SWR design. The IOD balanced output is obviously not intended to drive pro sound level gear running at +4 dBu.
  14. getz76


    Apr 3, 2005
    Hoboken, NJ
    I have not experienced a problem with my Kern IP-777 or SWR Interstellar Overdrive into either a Stewart World 2.1 or a Peavey DPC 1400X.
  15. jokerjkny


    Jan 19, 2002
    NY / NJ / PA


    mine does not do that.
  16. jerry

    jerry Doesn't know BDO Gold Supporting Member

    Dec 13, 1999
    Hmmm...a curious bump. :cool:
  17. Hollow Man

    Hollow Man Supporting Member

    Apr 28, 2003
    Springfield, VA
    Just the laugh I needed this morning, Jae... thanks! :)

    What do some of you other folks think of this volume-swelling-type dilemma here? Think it could be the pots, or another culprit that I've not considered?
  18. Fuzzbass

    Fuzzbass P5 with overdrive Supporting Member

    Thanks Joker... excellent thread, I remember it now! I look forward to HollowMan's comments too.
  19. Hollow Man

    Hollow Man Supporting Member

    Apr 28, 2003
    Springfield, VA
    And I'll be happy to provide them, just as soon as I get the Kern up and runnin'. The entire purpose in me buying the Kern was to A/B them, and see which one I prefer. I will sell the loser. I simply don't need two preamps, but sometimes I get that grass-is-always-greener feeling, so I like to try the competition whenever possible. This will help me decide which preamp is right for me.

    I emailed Les (or whoever is on the other end of the email link on Kern's website), so hopefully I'll get a reasonably prompt response. In the meantime, additional thoughts about my predicament are more than welcome... :)
  20. The person on the other end of the email link on Kern's website is......Les Plopa.