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Read Purity into a QSC PLX

Discussion in 'Amps and Cabs [BG]' started by Hollow Man, Mar 17, 2005.


  1. Hollow Man

    Hollow Man Supporting Member

    Apr 28, 2003
    Springfield, VA
    I've read several times about people having problems with their preamps having a sufficiently low output voltage such that their QSC PLX power amps can't be driven to the full output. Has anyone ever noticed this problem with the Read Purity preamp? I just bought a Purity, and I'd like to know if people have issues with the Purity/PLX combination before I go drop more money on a QSC.
     
  2. msquared

    msquared

    Sep 19, 2004
    Kansas City
    I had issues with my PLX with at least one preamp (the Trace GP12SPX) but they weren't insurmountable. I've also read of issues with the various Aguilar preamps going into PLXes but I can't vouch for that. In general you should have no problems, but there is a recommended aftermarket modification that you can have done to PLX amps which increases the input gain to be more in line with what the preamp is putting out. It's a very easy mod and is sufficiently simple that the input section of the PLX can be set up with a switch to go between "PA input gain" and "bass amp input gain".

    If a licensed QSC repair place does the mod, it's still warrantied. A guy who goes by "Lord Valve" on Usenet is a licensed QSC dealer/repairer and will sell you one new with the modification done. His website with contact info is here: http://www.nebsnow.com/LordValve

    EDIT: I was just reading the Demeter vs Eden preamp thread, and apparently the Crest amps have this switch built in. Maybe you should consider a Crest instead, despite the extra weight.
     
  3. Hollow Man

    Hollow Man Supporting Member

    Apr 28, 2003
    Springfield, VA
    Thanks for the replies. I'm actually thinking about buying my QSC from Lord Valve, and I know about his mod, which is one of the reasons I asked this question now. If I found out that I needed the mod done, I could have him do it before he ships it out. Any further commentary would be appreciated.

    Also, at least one person has indicated to me via PM that they feel the Read would be better served if it were paired with a Stewart or a Crest power amp rather than a QSC. I know this is an oft-debated topic, and I've read countless posts on the matter. I'm curious as to whether anyone has AB'ed the Read with different power amps, and what their results were.
     
  4. Rod B.

    Rod B.

    Jun 11, 2002
    Montana
    I run a Read into a PLX.

    Originally I ran with all switches to the right; the PLX amplifier is in parallel and bridge mono mode with the clip limiter on and a low-frequency filter engaged at 30 Hz. I got an OK signal but I tried this; switches 4, 5, and 6 off and all other switches on/right, the amplifier is in bridge mono mode with the clip limiter on and a low-frequency filter still engaged at 30 Hz. I got a hotter signal this way.

    I believe it was Bob Lee that said this changed the input level. Whoever said it, I tried it and it did.
     
  5. msquared

    msquared

    Sep 19, 2004
    Kansas City
    I'm curious about this too. All I've seen on the topic so far is a handful of "it feels better" type stories. I'd love to see something with a little science in it, if anyone has a sine wave tone generator and a few PA amps and an oscilloscope. :)
     
  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
    If the preamp can put out a maximum signal voltage of at least about 1.5 volts (that's not a difficult level, considering that nominal pro line levels are about 1.2 volts ), it can drive a PLX amp to full power without any modification needed.

    Some preamps may require that you flip a switch or simply adjust your gain to do this.
     
  7. 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 advocate paralleling the inputs of a PLX or RMX amp in bridged mono because it simply adds another load onto the preamp output unnecessarily. If the preamp has a significantly high output impedance, that extra input impedance will decrease the signal strength. In bridged mono, all you need to do is feed signal into channel 1's input, and the bridge circuitry inside the amp takes care of the feed into channel 2's output section.
     
  8. Rod B.

    Rod B.

    Jun 11, 2002
    Montana
    BTW, Jack Read told me he designed the amp with the QSC in mind.
     
  9. mgmadian

    mgmadian

    Feb 4, 2002
    Austin, TX
    FWIW, I've spoke with Jack Read a couple times regarding the Purity, and he recommends Stewart amps. Perhaps most importantly, the Stewart is the amp that he runs the Purity thru when he's building it...and he uses a Stewart in his playing rig (World 2.1, with a switch to defeat the fan).

    You may wanna ask him about the fan-defeat mod...the fan noise I had on my PLX-2402 was unfortunately one of the things I really didn't care for. Jack mentioned that the Stewart 2.1 often/typically doesn't get very warm even with the fan switched off. Can't vouch for this myself, but if I go back to using a separate solid-state power amp, this would be something I'd look into.
     
  10. Hollow Man

    Hollow Man Supporting Member

    Apr 28, 2003
    Springfield, VA
    Thanks, it's good to know that if nothing else, the QSC will work. Now all I have to do is decide between a PLX 2402 and a Stewart 2.1. Anyone got any recommendations? Theory (and Bob Lee) suggests that well-made power amps shouldn't color or detract from the sound, but people around here seem to have varying opinions.
     
  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
    It's not just theory. ;)

    Many times I've seen people absolutely certain they hear a difference between two good power amps when they know which one they're hearing become unable to tell them apart when they have to rely on the sound alone. I was one of them once.
     
  12. Fran Diaz

    Fran Diaz

    Mar 28, 2002
    Santander, Spain
    Bassist
    I run my Reap Purity into a PLX1602 and the Purity has plenty of gain to run the QSC to full power. (I usually bypass the eq, which means that no extra boost coming from that)

    keep groooovin'
    Fran
     
  13. gregbackstrom

    gregbackstrom Supporting Member

    Feb 3, 2005
    Tacoma, Washington
    Hey, Hollow!

    I've had both amps. Although I regret selling my Demeter VTBP-201 / Stewart World 2.1 combination, my Read Purity / PLX 2402 combo sounds better. The Read was designed with enough output voltage to drive the QSC. It's also nice having the filters on the PLX (the Stewart doesn't have any filtering).

    I originally had two 8 ohm Acme Low B-2's that I drove with the Stewart. No matter what I did with the gain from the Demeter, something about the bridged mode seemed to make the Acme's fart. When I substituted a Mackie 1400i for the Stewart, the farting went away. I even sent the amp back to Stewart, who checked the bridging circuitry and gave the amp a clean bill of health. They recommended that I avoid turning up the input attenuator past two o'clock (which I had already figured out on my own).

    I no longer have the Acme's, so I can't comment on how the PLX would handle them, but it sure sounds great with my Eden D-410 XLT! I suspect Bob Lee is right about the similarity between power amps (although I can sure hear a difference between the transistor amps I use and the power amp stage of my Mesa Bass 400+! I don't like the Mesa's preamp very much, but I love the power amp!

    - Greg Backstrom
     
  14. Just a thought...I recently purchased a QSC PLX2402 and I'm looking for a preamp, I'm running my Sadowsky pre/di pedal as a pre at the moment. I do have to use the XLR out of the pedal to get a hot enough signal, the 1/4" out is not that hot. But my point is that when I purchased my PLX I was told to use a Speakon cable and that "any old" speakon cable would work for bridging the PLX. I called numerous stores, even the store that I purchased the amp, and they all said "use any speakon cable." I finally put a post on TB and was told that I had to rewire the cable to 1+ 2+, I had no idea what this was but with the help of the geniuses at TB, I was able to fix the cable and now I'm in POWERAMP heaven. Could the problem be that when you're using a speakon cable that is not wired correctly for bridged mode it affects the output? Just a thought.