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QSC's PLX amps

Discussion in 'Amps and Cabs [BG]' started by Jerry J, Aug 2, 2001.


  1. Jerry J

    Jerry J Supporting Member

    Mar 27, 2000
    P-town, OR
    I have a chance to grab a QSC PLX 2402 at a fairly reasonable price but I want to see if anyone has any positive or negative comments on the PLX amps. I'm not going to get a chance to give it a test other than just as a PA amp. I also will not be able to return it if it isn't my cup of tea.

    I went throught the archives and found limited comments on the QSC line. Most the comments were favorable. I did find several that stated that the switching power supply contributed to a mushy bottom. I'm not real certain what a mushy bottom is when referring to a power amp.

    If this amp isn't what it's cracked up to be then I'll probably be looking at a Crest CA9 at twice the weight and 18" deep. I do know the Crest is a solid performer but it's almost 50 LBS.

    I've been told that the PLX's are very solid so I'm not too worried that the amp won't hold up. My concern is TONE.

    Jerry J
     
  2. Phat Ham

    Phat Ham

    Feb 13, 2000
    DC
    Most of what I've heard about the QSC PLX series is positive. If the price is good I'd say go for it.
     
  3. The big three are Crest, Crown, and QSC. Crown has a noticeable "coloration" that is apparent in A/B tests. This is neither good nor bad, simply there.

    QSC amps are built like tanks. I own the RMX-850 and the MX-3000a and have been inside both of them. Very well made, generous heat sinks, industrial grade quaility. I spoke with a QSC engineer at length on the phone about reliability of the PLX. He stated the reliability factor is the same for the PLX as it is for the MX.

    I was very squeamish about buying a PLX because of the switching power supply technology. After a lifetime as an IBM engineer, I had a belly full of those switching pieces of sh*t. As it turned out, the amp people have made significant strides in reliability. This thing switches 260,000 a second (not a typo), so I did have concerns...

    The PLX runs in a variety of different modes. If you are considering bridging, read ALL the fine print in the manual and understand it. Bridging involves MUCH higher voltage swings, and you CAN kill yourself if you get across it. It also involves impedance awareness, as bridging effectively cuts the load seen by the amp in half. An 8 ohm cab bridged is seen by the amp as a 4 ohm load. RTFM and understand it.

    The 2402 is a fine amp, and you will NEVER get 2400 watts from it, not because of the amp, but because a 120 VAC 15-amp wall outlet only provides 1800w maximum. The power put out by the PLX will be less than 1800w. Don't let this scare you off. I cannot get 3,000 watts from my MX-3000a either, but it sticks kicks *ss and takes names.

    Don't over pay. The average eBay used price is $680 for the 2402.
     
  4. LowRanger

    LowRanger

    Dec 24, 2000
    I've had a PLX 3002 for a little over a year, and am entirely happy with it. Sounds great and weight is NOT an issue!

    Of the few problems I've read about or experienced: the unit can produce an annoying rattle when placed on a speaker cab. I've experienced this, and read where others have also. It must be inherent in the chassis design; I subdued mine to barely noticeable by tightening hardware, others have "shimmed" chassis gaps.

    There is also an input sensitivity issue. I've seen posts that you can increase input sensitivity by ordering them modded direct from the factory, have the two resistors changed by a factory authorized tech, or (if you're brave or good with a soldering iron) change them yourself. I can't say I've experienced the problem, but it has been documented with Trace Elliot and (I think) Demeter preamps.

    I've also got a Mackie 1400. Maybe I have lead ears, but I don't hear any mushy bottom, and at half the weight, the QSC gets the gig calls every time!

    Best of luck...
     
  5. Jerry J

    Jerry J Supporting Member

    Mar 27, 2000
    P-town, OR
    This sounds good to me. Great comments regarding this amp. I'm most likely going to nab it. The price is $550 but I'm pretty certain that I can pick it for $50-100 less. It was used by a church for their PA system so not a scratch on it.

    I'm sure that I'd never expect the amp to even get anywhere near an 1800 watt output. I'm really looking for a strong output into 8 ohms.

    I am quite aware of the leathal potential of a bridged power amp but thank you for the warning. I used to work on old Fender tube amps as a hobby. Scary stuff when shunting a transformer to bias power tubes. I will look into the mod for the amp if I find any troubles with it.
     
  6. Norre

    Norre

    Jan 5, 2001
    Antwerp, Belgium
    Hey Lowranger,
    Could you give me a little more info about the input sensitivity issue? Does this mean that certain pre-amps won't work with PLX power amps? If so, how do I know which pre-amp can be used and which can't be used? I'm just asking because I'm going to get a PLX soon (probably this weekend), but I want to make sure it will work with my pre-amp (= Aguilar DB659).
    Thx,

    KN
     
  7. Bob C

    Bob C

    Mar 26, 2000
    Duluth, MN
    My new PLX 3002 produced plenty of deep, clean lows, but it didn't seem as "loud" as I imagined it would. Everyone I told this to thought I was nuts. The dealer bench tested it for output and it checked out fine. But he did add that it has a different input sensitivity than a Crest or Crown.

    I'm considering a Demeter preamp right now, and intend to look into the sensitivity issue, as well. Most high-end preamps list different sensitivity figures for balanced and unbalanced operation.

    Anybody else out there have experience with this subject?
     
  8. The PLX line has decreasing input sensitivity with increasing power ratings:

    PLX-1202 1.00v
    PLX-1602 1.20v
    PLX-2402 1.50v
    PLX-3002 1.70v
    PLX-3402 1.90v

    I'd send an email to QSC and get their input, then I'd find out exactly what output is provided by my preamp. Matching the QSC to the preamp would be a good idea, I think. Ask QSC for sure.

    My MX-3000a has an input sensitivity of 1.0v as a reference point. I can't comment about sensitivity issues directly, as I have Rane crossovers and EQ between the preamp and my QSC, and both have variable gain associated with them. When both are set at Unity gain, the system seems pretty wimpy. Then again, I run massively ineffecient subs.
     
  9. Norre

    Norre

    Jan 5, 2001
    Antwerp, Belgium
    Hey Bob C,

    I just e-mailed QSC support to ask about the input sensitivity issue. When (if?) I get an answer I'll post it here.

    KN
     
  10. Bob C

    Bob C

    Mar 26, 2000
    Duluth, MN
    Thanks Norre. I just e-mailed Demeter to get preamp side of the story. The VTBP-201S preamp has an internal volume trimmer; don't know if the HBP-1 model does.

    Last time I e-mailed them, I never got an answer. We'll see...
     
  11. Norre

    Norre

    Jan 5, 2001
    Antwerp, Belgium
    Hey Bob C,
    I e-mailed QSC yesterday and I already got an answer from them. That's what I call a good customer's support !!!

    My mail:

    Hello,
    I'm about to buy a QSC PLX power amp (1602 or 2402 - I haven't decided yet).
    I was looking at the specs and noticed their input sensitivities are 1.2 and 1.5 Vrms (8 ohm).
    I'm not a technician and I was wondering what this means.
    I ask this because there's a discussion going on at the talkbass forum (www.talkbass.com --> bass --> amps) about QSC PLX-series power amps. Someone said that there were input sensitivity issues when using certain pre-amps.
    I already ordered my pre-amp (Aguilar DB659) and I want to make sure there are no *surprises* when I buy the power amp. What specs from the pre-amp should I check to make sure everything will be OK?
    Thanks in advance,
    Nordin


    Answer from QSC:

    Hello Nordin,
    I believe the issue is of how hot an input signal needs to be in order to max out a PLX series amplifier. As the amps increase in model numbers, the hotter the input signal needs to be in order to drive an amplifier to its maximum wattage.
    In regards to the PLX1602, the Aguilar DB659 needs to at least be able to output +3.9 dB, hopefully more in order to supply some headroom.
    In regards to the PLX2402, the Aguilar DB659 needs to at least be able to output +5.5 dB, hopefully more in order to supply some headroom as well.

    Brad Sambrano
    Chief Service Engineer 
    QSC Audio Products, Inc.
     
  12. I have a request into Alembic for output information on my F-1X. Will post here when I get a reply. Alembic is always slow, so don't hold yer breath..
     
  13. Bob C

    Bob C

    Mar 26, 2000
    Duluth, MN
    Demeter's preamp spec sheet lists "max output +28dBv".

    QSC's reply to Norre says "+5.5db".

    What formula do I use to match these terms?
     
  14. Norre

    Norre

    Jan 5, 2001
    Antwerp, Belgium
    Bob C,
    I can't answer your question, but I'm sure bgavin can. I also e-mailed Aguilar today to ask about the output of the DB659 pre-amp.

    KN
     
  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
    Actually, you can get 2400 watts of short-term power from the amp without tripping a 15A breaker. If you want to do continuous sine wave full power (like in a lab), though, you'd need to use a beefier AC feed.

    Your advice to RTFM is right on the mark. I wrote, illustrated, proofed, and laid that thing out, so I want people to read it! :oops:

    -Bob
     
  16. Yeah, I suppose short term is better than none. I could claim I get that performance out of my kid, but her room is still a mess :D

    Seriously, can you speak to the increasing input sensitivity of the PLX line as the power rating goes up? Many of us here are contemplating the PLX, but I notice the 3402 has a sensitivity of 1.9v, where my MX-3000a has 1.0v.

    We are concerned if we should make the modification and increase the sensivity of the input stage. Your thoughts?
     
  17. 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
    Brad should've said "dBu" instead of "dB." I'll walk over and slap him upside the head for that. :)

    What the specs mean is that the preamp has more than enough output to drive the amp.

    -Bob
     
  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
    The maximum voltage gain of each PLX amp is the same: 40x, or 32 dB. Thus, if you have the amp turned up full, 1 volt in = 40 volts out = 200 watts at 8 ohms = 400 watts at 4 ohms, etc., regardless of model.

    Only a very wimpy preamp wouldn't be able to put out more than 1.9 volts RMS, so you probably don't need to spend extra for a gain mod.

    -Bob
     
  19. Thanks Bob.

    Problem is, many of the preamp people won't spec the outputs. I've got an email in to Alembic asking about my F-1X, and heard nothing. Another player has an email in to Aguilar, and has heard nothing (or not posted).

    Is there any technical reason you can divulge about why the 3402 is a 1.9v input, vs the MX-3000a which has a 1.0v input?

    Just curious.
     
  20. 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
    Yes, that's something a lot of the preamp manufacturers have dropped the ball on, IMHO. If you make something that's meant to drive another device, you need to describe its output, plain and simple.

    As I mentioned, the PLX amps all have a maximum gain of 40x, so 1.9 volts into a PLX 3402 will produce the rated power into an 8-ohm load, if the gain controls are turned up all the way.

    The MX 3000a has a maximum gain of 80x (6 dB more than the PLX), meaning 1 volt in = 80 V out = 800W @ 8 ohms. The problem with having so much gain in an amp is that any amp boosts not just the signal, but also all the hum and noise that gets picked up upstream. So more gain equals more noise. If you've got the amp turned all the way up, you've probably got whatever's in front of it turned way down, so you've got full gain on the noise, and a reduced signal level to boot. That's not a good combination. You're much better off, signal-to-noise-wise, reducing the amp gain and hitting it with a hotter signal, as long as you don't clip the preamp or anything else upstream from the amp.

    IMO, people concern themselves with sensitivity way too much, when they should pay attention to gain.

    -Bob