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QSC PLX Input Sensitivity Issue

PolkaHero

PolkaHero

Supporting Member
Jan 5, 2002
2,642
3,296
Michigan
#1
I just bought a PLX 3002 to replace my Carvin DMC1000 power amp. I've noticed posts in this forum about guys not being able to drive their PLX's to full volume with preamps that don't put out a certain voltage (usually Tech 21 Sansamps, which I also use).

Why is it that the input sensitivity of the PLX series goes up as you move to the higher models whereas the Carvin DCM series all have the same input sensitivity (1.000 volts at 4 ohms)? It seems like a law of diminishing returns: as you get an amplifier with more power available, the harder it is to drive it with your preamp!

I guess there's a modification available to bring the sensitivity down to .775 or so, is this something you can do yourself or any repair shop can handle? Other than this potential issue, I think this amp should be quite a step up from the Carvin. :cool:
 
#5
Ericman197 said:
Unless the QSC PLX 3002 has a much lower sensitivity than any other, you should be fine. I've never had any problems causing my QSC PLX 1202 to clip with my Sansamp RBI, and I don't have to max it either.
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<TABLE style="WIDTH: 649px; HEIGHT: 37px" borderColor=#dddddd height=37 cellSpacing=3 cellPadding=0 width=649 bgColor=white border=1><TBODY><TR height=14><TD width=217 bgColor=#dddddd height=14>
[font=Verdana,Arial,Helvetica]PLX SPECIFICATIONS[/font]​
</TD><TD bgColor=#dddddd height=14><CENTER>[font=Verdana,Arial,Helvetica]PLX1202[/font]</CENTER></TD><TD bgColor=#dddddd height=14><CENTER>[font=Verdana,Arial,Helvetica]PLX1602[/font]</CENTER></TD><TD bgColor=#dddddd height=14><CENTER>[font=Verdana,Arial,Helvetica]PLX2402[/font]</CENTER></TD><TD bgColor=#dddddd height=14><CENTER>[font=Verdana,Arial,Helvetica]PLX3002[/font]</CENTER></TD><TD bgColor=#dddddd height=14><CENTER>[font=Verdana,Arial,Helvetica]PLX3402[/font]</CENTER></TD></TR></TBODY></TABLE>
<TABLE style="WIDTH: 649px; HEIGHT: 86px" borderColor=#dddddd height=86 cellSpacing=3 cellPadding=0 width=649 bgColor=white border=1><TBODY><TR height=13><TD width=217 height=13>[font=Verdana,Arial,Helvetica]Input Sensitivity @ 8 ohms[/font]</TD><TD height=13><CENTER>[font=Verdana,Arial,Helvetica]1.0Vrms[/font]</CENTER></TD><TD height=13><CENTER>[font=Verdana,Arial,Helvetica]1.2Vrms[/font]</CENTER></TD><TD height=13><CENTER>[font=Verdana,Arial,Helvetica]1.5Vrms[/font]</CENTER></TD><TD height=13><CENTER>[font=Verdana,Arial,Helvetica]1.7Vrms[/font]</CENTER></TD><TD height=13><CENTER>[font=Verdana,Arial,Helvetica]1.9Vrms[/font]</CENTER></TD></TR><TR height=13><TD width=217 height=13>[font=Verdana,Arial,Helvetica]Input Sensitivity @ 4 ohms[/font]</TD><TD height=13><CENTER>[font=Verdana,Arial,Helvetica]0.9Vrms[/font]</CENTER></TD><TD height=13><CENTER>[font=Verdana,Arial,Helvetica]1.1Vrms[/font]</CENTER></TD><TD height=13><CENTER>[font=Verdana,Arial,Helvetica]1.3Vrms[/font]</CENTER></TD><TD height=13><CENTER>[font=Verdana,Arial,Helvetica]1.5Vrms[/font]</CENTER></TD><TD height=13><CENTER>[font=Verdana,Arial,Helvetica]1.7Vrms[/font]</CENTER></TD></TR></TBODY></TABLE>
 
#6
i had a similar problem/issue with my eden navigator and qsc poweramp. turns out, there's a switch inside the navigator that you can turn to the correct VRms to match the amp at whatever ohmage you're running. you might check with the preamp people. ever since i fixed that switch, no more clipping!
 
#7
It's because all five models have the same maximum gain. The higher-power models have higher maximum output voltages than the lower-power ones do, so they have correspondingly higher input sensitivity voltages

However, all line-level pro audio equipment and most preamps should be able to produce a higher output voltage than the 8-ohm input sensitivity (1.9 volt) of even the PLX 3402.

If your preamp can't produce that much signal and can't be easily made to do so, your options are to either boost the signal between the preamp and power amp or have the amp modified to boost the gain.
 
#8
I have a PLX 2402 that I've requisitioned from our PA system to use in my new bass rig. I need to mod it (input sensitivity of 1.5v @8 ohms) to work with my Phil Jones Bass Buddy preamp, whose manual reads: "Line Output: 1.2v".

Now does that mean that the preamp's O/P is variable (depending on playing dynamics & pickup O/P) with a MAXIMUM of 1.2 volts?

My choices are to have the PLX 2402's input sensitivity changed to either 1.0v or .775v. Which one should I opt for? With the intent of making sure I get the maximum O/P from the PLX, should I go for .775 volts with the worst case scenario being that I start to clip the PLX, which I should be able to fix by dialing back the preamp O/P knob?

Also, are there downsides to increasing the I/P sensitivity this much? If I choose to use the PLX as a PA amp again, will this present a problem?

Anyone with experience with this situation? ...Thanks...
 
#9
Yes, the Bass Buddy's maximum output is 1.2V clean, although it can probably be pushed beyond that with some unpleasant distortion.

There are enough cool preamps out there with low output- the old AMP 320, the Ampeg SVP's, etc. that if you felt you were more likely to keep the QSC as an instrument amp then I would go for the .7V mod, and dial back the output of stronger preamps as needed to avoid clipping. Manage the output of the pre's and there should be no negative side effects.

If you choose to use the QSC for PA again, it would be a problem if anyone but you is setting up or controlling the PA connections, because they might assume from prior knowledge that the sensitivity rating was higher, and send an extremely high input into the amp, causing much distortion.
 
#10
Thanks Bongo! From what you say, I think I will go for the .775v mod. If I end up using the PLX as a PA amp again, I'll just have to lower the mixing board's O/P send to it & check for no clipping on the PLX. The sound wouldn't be any different, would it, as the gain staging would now be less preamp, more power amp? Am I over analyzing this?
 
#11
alyerpal said:
I have a PLX 2402 that I've requisitioned from our PA system to use in my new bass rig. I need to mod it (input sensitivity of 1.5v @8 ohms) to work with my Phil Jones Bass Buddy preamp, whose manual reads: "Line Output: 1.2v".

...Anyone with experience with this situation? ...Thanks...
Click to expand...

I bought my PLX-2402 to use with a low output preamp (Pearce BC-1). I ordered it with lower input sensitivity. At the time (2001?) it was a no cost option as I was told that most pro sound companies have their amps configured to their needs and QSC does it all the time. It worked great!

I shifted the PLX-2402 over to PA duty since I started using an integrated bass amp (first a Hartke, now my beloved Walkabout) and haven't had any problems with overload.

I wouldn't hesitate to do the same thing again. Assuming that you're using good quality, short length cable (or cables) between your preamp and the power amp, you shouldn't have any problems.

IIRC, the modification involves changing just four resistors so it's not a big deal at all. I'm sure that there are limits to how sensitive you can make the amp, but I'll let Bob Lee comment on that if he so chooses.
 
#12
BillyB_from_LZ said:
I bought my PLX-2402 to use with a low output preamp (Pearce BC-1). I ordered it with lower input sensitivity. At the time (2001?) it was a no cost option as I was told that most pro sound companies have their amps configured to their needs and QSC does it all the time. It worked great!
Click to expand...

Do you know if the input sensitivity on your 2402 is now 1.0v or .775v?

I shifted the PLX-2402 over to PA duty since I started using an integrated bass amp (first a Hartke, now my beloved Walkabout) and haven't had any problems with overload.
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Nice. I was concerned. Good to know.

I wouldn't hesitate to do the same thing again. Assuming that you're using good quality, short length cable (or cables) between your preamp and the power amp, you shouldn't have any problems.
Click to expand...

I think I'm using short Pro Co cables right now. Do you have any suggestions on brand? Is the concern that we want cables that specifically are known to reproduce low freqs well?

IIRC, the modification involves changing just four resistors so it's not a big deal at all. I'm sure that there are limits to how sensitive you can make the amp, but I'll let Bob Lee comment on that if he so chooses.
Click to expand...

Yes, two SMT resistors per side. I have a QSC approved service center in L.A. that'll do it for me.
 
#13
Any "good quality" cable is sufficient. The ones that claim to be "better for bass" are bogus, they just increase the capacitance of the cable, which attenuates the highs, causing the lows to sound more prominent. ProCo is fine. I like Canare, Belden, and Mogami, but I also use ProCo.
 
#14
My preamp is the SWR IOD. It only produces 0.7v maximum output.

My solution is simple: a Rane ME30B equalizer between preamp and my PLX-3002. The ME30B produces much higher than unity gain output, so you can drive hell out of the PLX without cranking the IOD up very high. This also lets you EQ your rig to Flat (or taste) so you can use the preamp tone controls as you see fit.

I did not want my PLX modified, so the Rane box is an easy fix. Cheap too... you can find the ME30B on eBay readily, and for reasonable pricing. Be careful, there are multiple models, and ME30 (obsolete) is not the same as ME30B.
 
#15
Nice solution with the Rane EQ. Spurred me on to work something out to be able to use the PLX 2402 on a gig tonight. I have 2/3rds of a rack space left to work with - currently a 3 space rack with the PLX and a 1/3rd space PJ Bass Buddy on top, so I've put an FMR RNC (Really Nice Compressor) between the two that I had lying around. It's 1/3rd space also. Just using the make-up gain to drive the PLX, but I suppose I could use the compressor a touch if I wanted to.
 
#17
Rick Auricchio said:
PolkaHero, are you running a balanced cable between preamp and the power amp?

If you don't, you'll automatically lose 6dB of gain.
Click to expand...

You might or might not, depending on the pre's output topology.

In general, you don't need to have the power amp gain modded unless the preamp is incapable of putting out signal voltages as high as a volt or so. Most preamps can. Many preamp outputs have ratings based on average or nominal levels instead of maximum, which may run 10–20 dB higher (~3–10×). My advice: check with the preamp manufacturer to be sure of the maximum output level; it should equal or exceed the power amp's input sensitivity voltage.
 

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