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QSC power amp mods

Discussion in 'Amps and Cabs [BG]' started by Matthew West, Jan 16, 2002.


  1. This may be of value to some of you. It's from Lord Valve's latest power amp catalog.


    OF SPECIAL INTEREST TO BASS PLAYERS:It has come to my attention that many of you are tryingto use the QSC PLX-series amps with preamps which operateat less than standard +4 line
    level; among these are theTrace Elliot models, some SansAmp DI-type products, Aguilars, Line 6 Bass PODS, etc. Nearly all of these devices
    operate at a level somewhere between -10 and +4; problems occur because (especially with the higher-powered amps in thePLX 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, as I am a QSC-authorizedwarranty technician.
     
  2. ganttbos

    ganttbos The Professor Supporting Member

    Dec 18, 2001
    New Orleans area
    I just ordered a PLX 2402 and I use an Aguilar DB659 preamp. I've seen some older threads where this issue was addressed and I don't remember exactly what I read but I thought my worries had been laid to rest. Maybe Bob Lee could comment on this product and whether there is an issue with my particular preamp / power amp combination. Bob I apologize if you've already addressed this in previous threads. I just can't remember exactly. Thanks.
    Gantt
     
  3. 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
    I've tried with mixed success to get maximum output voltage information from various preamp manufacturers. Unfortunately, it's not readily available data like it is with mixers, equalizers, crossovers, and other pro audio gear in general.

    The input sensitivity of a PLX amp depends on the model and the load, and the spec itself is only applicable at full gain. Each model has a maximum gain of 40×, or 32 dB. That means if the amp channel is turned all the way up and you put a 1-volt signal into it, you'll get 40 volts out. If you turn the amp channel down, you'll get correspondingly less output voltage, and it'll take correspondingly more input voltage to reach full power.

    So a PLX 2402's input sensitivity spec is 1.5 V rms when loaded at 8 ohms/channel and 1.3 V rms when loaded at 4.

    A level of -10 dBV is 0.316 volt rms, which really is not much (in comparison: a AAA alkaline cell is only 1.5 V), so if the preamp can do -10 dBV as a nominal output level, it probably can do much higher on peaks.

    This is important because you only want to hit maximum power on your absolute highest peaks, if at all. Otherwise, you'll get clipping and distortion.

    1.5 V rms is +3.5 dBV, so it's 13.5 dB higher than 0.316 volt. And 1.3 V rms is +2.3 dBV, putting it 12.3 dB higher than 0.316 volt. So if the preamp is rate to put out a nominal -10 dBV level and still have at least 12 to 14 dB of headroom, which is more than reasonable (I think Joris had posted some findings on peak-to-average measurements), you'll still be able to drive the PLX 2402 to full power if you need to. If the preamp has an output gain (or "volume") control, it probably has a good-sized output voltage capability and is designed to drive a wide variety of power amps.

    I would encourage anyone to try their preamp with the power amp (whatever the brand or model) before deciding to have the gain modified. Do this test: turn the power amp down, about halfway or so. Turn your preamp down a little lower than usual, and gradually turn it up while you play your bass as hard as you normally would. If you get to a point where you clip the preamp, back off a little bit, then switch over to gradually turning the amp up. If you can get to the sound level you want without turning the power amp gain all the way up and without clipping the power amp, you've got more than enough gain and it doesn't need modification. If the power amp clips before you get the sound level you need, you need a more powerful amp and/or more speakers or more efficient speakers.
     
  4. ganttbos

    ganttbos The Professor Supporting Member

    Dec 18, 2001
    New Orleans area
    Thanks, Bob. I'll try the preamp / amp combination including performing the test you described when the amp arrives next week. I'll post the results in this thread.
     
  5. Bob,

    I ran into this problem with my MX-3000a and Alembic F-1X preamp.

    Using the XLR/DI on the F-1X, I was going balanced through a Rane signal chain into the MX power amp, and just not getting anywhere close to the loudness I should be getting with that much amp.

    I built an unbalanced to XLR cable and started using the Unbalanced TS out jack on the F-1X and wound up cutting the preamp volume control down from 7+ to 2~3 to achieve the same stage volume. The downside was a slight increase in hum resulting from unbalancing the system.

    I bought an SWR Interstellar Overdrive preamp, and will be using it at my Thursday gig. SWR claims the XLR Out has a +24dB capability.

    Question: is it possible to inject pink noise into a preamp, and put a DVOM across the LINE OUT to measure the output voltage?
     
  6. 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
    Hi Bruce,

    You could, but it would only tell you the voltage of the pink noise and not the maximum output voltage of the pre. (For best results, use an AC voltmeter with RMS detection.) Unless you view the waveform on an oscilloscope, you won't know where clipping begins.

    A sine wave will give you a more accurate measure of the preamp's maximum output voltage. But +24 dBu (is it +24 dBu or dBV?) is a pretty healthy output—comparable to what a quality mixing console can deliver. +24 dBu is 12.3 V rms, and +24 dBV is 15.8 V rms. The MX 3000a has a max gain of 80×, or +38 dB, so its input sensitivity is 1.0 V rms (@ 8 ohms per channel).

    You're going to have to keep your levels down with that setup. ;)
     
  7. What is the typical input levels for a mixing console?

    I suspect my F-1X XLR/DI Out is putting out a MUCH lower voltage than does its LINE OUT jack.
     
  8. 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
    Most mixing console inputs have to be capable of handling dynamic or ribbon mic levels, which could be a few millivolts, up to condenser mic levels, which could be tens of millivolts, up to line-level audio signals, which would be nominally around a volt.
     
  9. Bob,
    I'd generally agree with that, especially for higher end consoles, but many lower to midrange desks have serious trouble dealing with true line level (ie +4 dBu nominal) signals at their XLR inputs. I guess the manufacturers are trying to save money and don't put proper pad circuitry in. Many of them don't have any pad, just a line/mic switch which switches between the XLR and TRS inputs. The Allen Heath GL 2000's and some of the lower end Yamahas and Soundcrafts have this problem. Perhaps the bass amp manufacturers have adjusted the level of their XLR balanced outputs in light of this, because having lower than line level output would make the signal universally acceptable to mixer inputs. Most bass players don't play gigs where they have a Midas or a Series 5 (which can take line level at their XLR inputs) out front
     
  10. unlined4string

    unlined4string Supporting Member

    Mar 18, 2000
    Baltimore County, Md.
    I solved this problem with an EbTech line level shifter. It made a huge difference and doesn't seem to degrade the signal at all. It's also useful as a diagnostic tool in case you're not sure whether the preamp is driving the power amp input at +4.
     
  11. ganttbos

    ganttbos The Professor Supporting Member

    Dec 18, 2001
    New Orleans area
    Spaceboat is right about less expensive desks. We use a Mackie 24-4 and the line output from my Avalon U5 pegs the input indicator when using the board XLR. I have to use the mic out on the Avalon if I want to go balanced to the board.
     
  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
    My idea of a perfect solution: :p

    A pre with about +20 to +24 dBu output capability, with a gain knob providing at least 30 dB of decent-resolution adjustability, plus selectable 20 and 40 dB pads.
     
  13. lo-end

    lo-end

    Jun 15, 2001
    PA
    ...yet another reason why this whole preamp + poweramp setup scares me. This is wayyyyyyy too confusing and involved for me.

    Learning that I have to wire my Neutrik Speakon to +1 +2 was hard enough! :D
     
  14. So wait, should the IOD run at full capacity through a PLX series QSC poweramp?
     
  15. 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
    I don't know what you mean by "full capacity," but an IOD can put out +24 dBu on its balanced line output. Thus, it's got more than enough juice to drive a PLX amp to fulll rated output power. You'd want to turn both the amp itself and the preamp down a bit.
     
  16. 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
    Hey, it's not all that complex. That's why our gear has knobs. ;)
     
  17. I'm going to fire up the IOD on my QSC MX-3000a power amp at tonight's gig. I'll post a report tomorrow.
     
  18. ganttbos

    ganttbos The Professor Supporting Member

    Dec 18, 2001
    New Orleans area
    I used my new PLX 2402 on a gig this past weekend. Using the Aguilar DB659 I had no problem with the output level of the preamp. The two 1/4" outs on the Ag are unbalanced. I ran an unbalanced patch cable to the channel 1 balanced 1/4" input on the QSC (I was running the amp bridged). There was no audible hum and plenty of preamp output signal. I didn't try to make the amp clip but just adjusted the preamp master output gain until the green input LEDs on the QSC (there are three green lights before the red clip indicator) were all lighting (the third one only on the dug-in low notes). I had the pre-gain on the Ag at about 10:00 and the master at 12:00 and got massive output volume with the QSC level on full - perfectly quiet and no hum when not playing.
    So it seems that the whole issue of inadequate output from an Aguilar 659 when coupled with a QSC PLX amp is not valid, at least with the signal path configured as above. From reading the other responses to this issue, it seems that if you used a balanced preamp out, you'd have even more signal. The balanced out on the Ag is intended as a board send (is not affected by the master output gain). I guess you could use it to feed a power amp. But I had plenty of (dead-quiet) gain even when I unbalanced the chain so what's the point?