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Increased Cooling for QSC PLX Series

Discussion in 'Amps and Cabs [BG]' started by bgavin, Aug 12, 2002.

  1. Bob Lee,

    What can I do to replace the fan in my PLX 3002 with a model that provides more air flow?

    I'm curious about the power requirements for the stock fan, and what to replace it with. I play outdoors every week in the hot Sacramento summer, and my PLX is working very hard.
  2. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Hi Bruce,

    Here are the specs of the fan that's in the PLX amp:

    24 VDC brushless
    Max voltage: 27.6 volts
    Max air flow: 1.3 m^3/min
    Max air pressure: 5 mm H2O
    Noise: 32 dBA (@ 100 cm from air intake side)
    Size: 80 × 80 × 25.5 mm
    Mounting hole spacing: 71.5 mm
    Mounting hole dia.: 4.5 mm

    So you'd want one that fits but has a higher air flow and pressure.
  3. Bummer on the 24VDC. There are a zillion Delta in 80mm (at 12 VDC) in various flavors of Screaming Loud. If there is room, perhaps a 90mm or 120mm could be arranged. Computer cooling fans are VERY available, albeit all in 12VDC, but they come in all sizes and displacements.

    I suppose it would be simple to build a voltage divider to drop 24 VDC to 12.

    Thanks for the info, and I'll see how it shakes out. My outdoor gigs are running my 3002 too high for my taste (that and always bumping the clipping LED in the subwoofer channel).
  4. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Usually cooling is not a problem in the PLX 3002, even outdoors in summer. One thing you could try is mounting a fan panel on the rear of your rack that will force air into the rear slots of the amp.
  5. seamus


    Feb 8, 2001
    Hi Bob,

    Would you happen to have the specs for the RMX series fans as well? In particular, the 1850HD?

    If not, no biggie, just curious.

  6. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Sorry, I don't. It also is a 24V fan, but a different make from the Panasonic and Sanyo fans we've qualified for the PLX amps.
  7. I play in 100-degree heat, and worry about the amp getting too hot. My background is in computer hardware, and heat is the enemy.

    The force-fed solution is an idea I've toyed with for awhile.
  8. fast slapper

    fast slapper

    Dec 11, 2001
    Fresno, CA
    I think the fan panel idea sounds the most effective. My brother had the same problem so I clipped on a 4" fan to the back of the rack. Works pretty good.
  9. I suppose the right way to do it is install a temperature probe in the amp and take proper measurements on the job.

    My current rack is 10RU, so I have miles of open space and ambient ventilation from the rear is not the cause of the problem. Force feed the rear of the rack will do nothing for my heat problem. I have to force feed the rear input of the PLX, or use a higher capacity internal fan.

    Or wait for winter.


    We are currently in a 105-degree hot streak right now, so waiting for winter is a problem. I'm just really worried about smoking a very fine amp from the combination of high temps plus driving the hell out of the subwoofer channel.
  10. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Hi Bruce,

    The amp won't thermal until the heat sink temps reach 90° C (194 ° F). Anything below that is still within the design range of the amp.

    There are a lot of sound guys who use PLX amps in the Caribbean in 100°+ climates, for all-day-long festivals and such, at 2 ohms per channel, and the amps hold up fine.
  11. Wow...

    I've spent plenty of time in the Caribbean, and know that heat (and humidity) well.

    Add to that environment a 2-ohm load... all day... yikes!

    I have all my client's computer systems set to thermal off at 70C, which to me is just plain hot as hell. But 90C... damn near boiling.
  12. looks like a PLX is next on my list. Can you tell me a little of your experiance with the power amp and why you chose it over others? Will the XLR output on my SWR IOD be loud enough to drive it? sounds as though our set-ups will end up rather similar eh? I will be running my IOD w/ my DBX 120XP and maybe my Alembic SF-2 into a QSC PLX 2402 or 3002.
    How has all this worked for yourself?
  13. The IOD will not drive any PLX to full power without assistance.


    The IOD puts out a maximum of 0.7 volts at 100 Hz from a 140mV (passive bass) input signal, both balanced and unbalanced jacks. It is my understanding the dbx 120xp / 120a will put out greater than unity gain and the specs say +20dBU into 600 ohms. That is sufficient to drive a PLX all the way, and then some.

    I have several pieces of Rane gear, plus the dbx, in my signal chain. I have no trouble hitting the red LED for clipping all the time. All the Rane equipment provides greater than unity gain. My IOD is running at the 9 o'clock position on the volume control... this is just barely cracked open.

    I have a pair of PLX 3002, and a PLX 1202 because Bob Lee told me to... ;) Actually, I like the 21 pounds a whole bunch. My previous QSC was an MX-3000a at 69 pounds. And yes, it cooled real well, also. Bob says the PLX is rated up to 90C which is smokin' hot... even in Sacramento.

    I chose the PLX after having a very long phone call with a QSC product engineer. I asked pointed questions, he gave good answers, I tried QSC and am 100% pleased. When I was an engineer at IBM, switching power supplies were just coming on the market and they were pure, dogsh*t. They failed constantly and gave me a real bad taste for this technology. Many years later, switching supplies have the bugs worked out, and all they do is work. Oh yeah, they also weigh a WHOLE lot less.

    I'd be squirmy about running a PLX at a true 2-ohm load, especially as hard as I push my subs. The MX series is probably better suited for 2-ohms, plus the MX appears to cool better. I run 4 to 8 ohm loads on mine, and they get warm but not enough to break a sweat.
  14. Customer service is a VERY big factor for me whenever I look to purchase gear. Nice to hear they were so supportive for you.
    Is there not some mod that will make the XLR I/O on the QSC and IOD compatible? I will just have to run the IOD into the DBX into the QSC is what you are saying. Hmmm...
  15. I'd try one of those double window fans vertically against the back of the rack. Keeping the air moving around the amp will cool it down significantly. Even though cool air is being forced into the amp keeping the air moving around the amp will will eliminate heat build up from the amp case. I've been using a single rack EV Dynacord power amp Bridged into 8 ohms for the past 6 years. When I first started using it, it would get very hot, so hot it would burn your hand. Even though I had a rack space open above it. I mounted a Radio Shack computer fan on the side of the SKB case. The fan didn't really cross vent the rack, it just kept the air moving. Like magic, the amp stayed cool, at least 20 degrees or more. (Just My Experience)
  16. If you buy a new PLX from Lord Valve, he will do the factory authorized modification to increase the sensitivity. The PLX 3002 requires 1.7v to reach full amplification, where the smallest PLX 1202 requires 1.0v. The modification is a couple of resistors, and not a big deal to anybody competent with a soldering iron.

    Bob Lee (QSC) can tell you how low the sensitivity can go, as I have no idea. Frankly, I'd like to see it at 0.775v / (0dBu). I don't know if there is a down side to this or not.
  17. Downside being possible compatibilty issues when using other preamps perhaps...? Seems like if my main focus was bass preamp amplification then raising the sensitivity could be helpful in dealing with the variety of signal levels found in musical instrument amplification. Did you do the mod yourself? I know of the crazy deal you got on E-bait for your PLX 3002...less than $500 was it? Any tips on finding a similar deal or how much you think I should expect to pay.
    I can only imagine how terrific that DBX 120a must sound with your QSC and subs. Have you tried driving them just with the "subwoofer out" on your DBX? Don't need no ELF with that setup...
  18. The Crest amps are 0.775v.
    Crowns switch from 0.775 to 1.44v (from memory)
    Carvins are 1.0v
    SoundTechs are 1.23

    Only the PLX are way up high in the 1.7v range. Maybe Bob Lee can offer a good explanation.

    As for compatibility with other amps, the 0dBu (0.775) and +4dBu (1.23) are both standard pro audio line levels. If your PLX is modified to the same sensivity as a Crest or Crown, it will be in good company. The attenuator controls on the PLX will reduce a hot LINE IN to prevent over driving the PLX, but they cannot boost it.

    I have no need for using Subwoofer Out (only) with the DBX, so I don't use it.

    I have cataloged 51 PLX 3002 sales on eBay, and my 2nd one is the cheapest in the range of $545 to $1,000. Average and Median prices are both $745, which is far too expensive, considering LV sells them for $839 shipped.

    My "cheap" one was still within warranty, and my $710 unit was not. But.. the $710 had just been repaired by QSC (verified) so I don't worry about it. I've been driving hell out of that one since I bought it. The PLX 1202 I got cheap also, but it is for a PA system. I don't need a 2nd PLX 3002 in my rack to drive a pair of JBL E110, but I use it anyway because it is a spare power amp. If I lose the subs' PLX 3002, I can reconfigure at the gig and use the other 3002 to drive the whole rig. The weight is the same as the 1202, so I keep both 3002 in the rack.
  19. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    The #1 downside of high gain in the power amp stage is increased noise, buzz, and hum from upstream cables and gear. PLX amps use the same gain structure as many pro amps, such as the Crest Professional Series and the highest-powered QSC PowerLight models: a maximum gain of 40×, or 32 dB (Crown's constant-gain figure is even lower: 20×, or 26 dB). Even 1.7 volts rms is, by pro audio standards, a very low signal level to max out at. Another downside of high gain is that it only takes a very low signal level to drive the amp into clipping. Since most power amps are used in sound reinforcement and playback systems, having a lot of amp gain makes operator error more likely to blow speakers.

    As with everything else in audio, it's a tradeoff.
  20. So what is happening is, bass players are mixing musical instrument preamps with pro audio power amps.

    I have to fault SWR for providing only a miniscule 0.7 volts at maximum output for their preamp. +4dBu (1.23v) is a standard pro audio line level, so you'd think SWR would at least provide that much. After all, SWR took the time to design an attenuation pad into the IOD for active basses... why not provide the IOD with some more output?

    Demeter, EBS, Peavey all provide preamps that put out much more than the IOD. I don't about Aguilar or Ampegs. My old Alembic F-1X was pretty anemic for output also.

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