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Preamps that don't Drive QSC's to Full Power

Discussion in 'Amps and Cabs [BG]' started by rickbass, Jan 13, 2003.

  1. rickbass

    rickbass Supporting Member

    As a result of my "Punch in the Gut Power Amps" thread, I've been giving QSC PLX's a hard look to go with the Aguilar preamp that's on its way to me. BUT..........FYI - Today, I received an interesting email from Lord Valve about makes/models of preamps that don't perform with QSC PLX's as one might hope -

    "It has come to my attention that many of you are trying to use the QSC PLX-series amps with preamps which operate at less than standard +4 line level; Nearly all of these devices operate at a level somewhere between -10 and +4; problems occur because (especially with the higher-powered amps in the
    PLX series) the preamp can't drive the amp to full power....I now offer a mod to the input stage of any PLX amp which *doubles* the input sensitivity, ensuring full power output with preamps or other devices which operate at less than industry-standard output levels. This mod is reversible,
    and does not affect your warranty...."

    So, another $49 (cost of the mod) would have to be figured into the final cost if I paired my Aguilar with the PLX configuration.

    Also, I'm curious about which other brands/models are included in his mention of "among" and "etc."???? ("...among these are the
    Trace Elliot models, some SansAmp DI-type products, Aguilars, Line 6 Bass PODS, etc.....").

    So, I guess the moral of the story is - Find out about line level before buying a preamp to go with a PLX.

    I think this info is germane to the thread Petebass recently posted where he tried the same Yamaha preamp with various power amps and was disappointed in the volume. He said,

    -"No matter what I tried, none of the power amps would go as loud as a standard 400w head, not even the 1100w so-called-monster. I'm talking brands such as QSC, Mackie and Yorkville, all reputable. It wasn't even close. I even tried bridging to double the power and still no cigar."
  2. Very interesting. I'm chiming in because I use a SVP-Pro and want a PLX or RMX in the near near future. I'd rather not have to have a device to drive greater than unity gain.
  3. rickbass

    rickbass Supporting Member

    Thanks for raising a question I hadn't considered yet, Dave - Whether the RMX's CAN be driven to full power - OR - if they are one of Lord Valve's "etc's"???

    It would seem that the RMX's are okay since he chose to single out only the PLX's. Then again, I have no idea of what others are included in his statement. :confused:
  4. geshel


    Oct 2, 2001
    I've been able to clip my PLX with my F-2B, FYI.
  5. My SWR IOD will not drive a PLX to full output. The IOD is 0.700 volts maximum output at 100 Hz from a 140mV input signal.

    Everything is greater than unity gain in the preamp stages. The preamp is not a unity gain device. I use a Rane crossover to boost the IOD signal to a true LINE level output. In essence, the Rane is just the final stage of preamplification.

    I'd ask LV exactly what the input sensitivity voltage is after the modification. Doubling the sensitivity means a 3dB change to me. If so, the 1.7v PLX 3002 sensitivity is 6.82 dBu. Doubling that would be 3.82 dBu, or 1.2v sensitivity... still not enough for an IOD or anything else that puts out 0.7 volts (0 dBu).
  6. geshel


    Oct 2, 2001
    Doubling a voltage sensitivity would be 6dB, changing it from 1.7v to 0.85v - you know, half. :)

    Also, don't forget that your 0.7v spec for the IOD was based on the output of one of your basses. That might be typical, but it's not a real limit on the output voltage the IOD can produce given a hotter signal.
  7. LowRanger


    Dec 24, 2000
    I briefly owned a Warwick Quadruplet that would not push my PLX 3002 anywhere near the limit.
  8. My error.

    Power is +3dB for a doubling.
    Voltage is +6dB for a doubling, or -6dB for a halving.

    I have two passive basses, and an active G&L L1500. The passive J is the weakest, and the L1500 is the highest output. I also have multiple gain devices between the IOD and the PLX. I suspect the IOD was designed for active basses with higher outputs.
  9. jokerjkny


    Jan 19, 2002
    NY / NJ / PHL
    my Demeter VTBP-201s has a variable internal volume trimmer that according the website:

    "Internal Volume Trimmer: This control is designed to match the output volume of the Tube Bass Preamplifier with the input sensitivity of any power amplifier.
  10. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    I use an old Pearce BC-1 and it's output is kinda low. When I ordered my PLX2402 from Life Effects Pro Audio, they had QSC set the sensitivity for my preamp at no additional charge.

    Works great!!!!

    In the long run, it's worth the cost to have them match up properly...
  11. That's what I meant, but did not say lol :D. I'd rather not have to have an EQ or similar device between the preamp and the poweramp. I've finally got my basses I play to have very similar outputs, so input voltages into the poweramps are comparable. I should prolly check everything with the scope at work, just to make sure.
  12. Tapp


    Aug 29, 2001
    USA, Mississippi
    Since the SWR IOD was mentioned by Bgavin I thought I would chime in since I'm an IOD owner too. I recently did some testing for the heck of it and connected my function generator, scope, and my Fluke 87 true RMS meter to the IOD.

    The IOD is a very interesting preamp since it uses the signal of a true push-pull EL84 tube amp to derive a "tube-like" distortion quality to the tone. I injected a 100hz signal at around 120mV into my IOD. I monitored the output on my scope to see when I was starting to clip the signal and I stayed in the "clean" zone.

    Now, here's what interesting. When I monitored the VAC on my Fluke with the "Blend control" set to "Direct" I did get around .70VAC (as Bgavin said)with the Master volume cranked. Move the Blend control to "LINE" and set the Drive control to around 10:00 and I still get a clean sinewave (or at least my IOD did). With it set like this I was getting somewhat over 1VRMS and if memory serves I believe it was even 1.6VacRMS which would indeed drive a QSC to full power.

    This is just what I found by playing around with the preamp.

  13. Jerry J

    Jerry J Supporting Member

    Mar 27, 2000
    P-town, OR
    All three of my pre's, F-1X, older Demeter 201 and the Eden Navigator, will drive my PLX's to clipping.

    These are the only ones that I've used into the PLX's.
  14. bben


    Feb 28, 2002
    Santa Fe, NM
    I had a PLX 1602 for a while, and if I got another one, I would definitely go for the modification.
  15. Eric Moesle

    Eric Moesle

    Sep 21, 2001
    Columbus OH
    The Eden Navigator pre is fine without the upgrade. It has an internal output selection switch so you can choose either 2.8v or 1.4v. The switch is on the top front of the PC board, and the preamp must be disassebled to access it.
  16. Bonafide


    Oct 15, 2002
    My 'Big Rig' consists of a QSC PLX 1602 (Bridged) with the Sansamp RBI. The RBI is actually TOO hot at unity gain and will easily clip the PLX with the master level at 60%. It has a -20 pad on both the input stages which when activated is still plenty hot and capable of clipping if I max the master level on the pre.
    I still use the 0db setting on the pre so I have the option of getting the most from the QSC.
    Bridged at 1600w with the Sansamp RBI and my volume levels can be ear shattering while staying spanky and clean. Truly no problems at all.
  17. rickbass

    rickbass Supporting Member

    Rodney - What speakers are taking that beating??? Your Eden (XLT?) 10's??? Also, does that include a B string???

  18. geshel


    Oct 2, 2001
    What bass? I thought the RBI had the same (not-quite adequate) output as the BDDI.
  19. Bonafide


    Oct 15, 2002
    Rick Bass,
    The cabinet is my Ampeg 6X10 HLF. Front ported with a tweeter. 600w rms 1200w peak. Best Ampeg cab IMO.
    No B string with this setup though I have pounded a B through it with my SVT-3Pro as a Preamp and had great results.
    Obviously I don't even have to turn the QSC up to 50% for outdoor volumes. I have headroom galore with this setup.

    Passive basses mostly. Usually A Jazz or Precision, both equipped with Antiquity's. My Stingray has a hotter output level due to the pre and requires me to re-adjust my pre-amp gain. Levels stay primarily the same.
    As for the Pre itself, I can't say for the BDDI but the RBI has more gain that neccessary, truly. When I first got it I was a little dissapointed because it had TOO much output at unity. I wanted to run the QSC at a healthy level and run the Pre's master at a similiar level or hotter depending on my application. As is I only really get the results I want if I am cranking that rig. At lower volumes it still sounds good (The RBI is really responsive at lower levels) but at stage volumes the rig just cooks.
    The RBI, being designed for both live and studio accepts both high and low impedances with enough gain cut/boost to compensate for either. I Think the BDDI and RBI have less in common than many people realize.
  20. Bonafide


    Oct 15, 2002
    I just noticed that the RBI has a a pad for both inputs AND output. That is 40db total. I have them both set to 0db, perhaps that is the reason that unit is so hot but -20db is a little extreme from 0db. Design engineers usually have the most practical experiance in real-life situations..:rolleyes: