Bridging a QSC PLX1602

[andertone]

Howdy Ed! You don't know me, but I'm a fellow Tucsonian and

good friend of our beloved Bob Mick (my band used to practice

in his studio last year in the cold locker!) I've seen you play

at Boondockers and Reid Park at various festival so I thought

I'd introduce myself. I play in two bands, one original rock

and the other very corportate (finances GAS attacks)

 

Question: I have an Epifani T212 which I drive with a

QSC PLX1602/Alembic F2b using only one channel (500 watts at

4 ohms). The QSC folks warn me about bridging as it limits

amp life (they really don't like to see 2 ohms per channel).

But I noticed you had the same problem I did before you went

to bridge mode to increase your volume.

 

What's up? They rate the amp safe in bridge mode, but

warn quietly "don't do this for extended periods of time".

 

Thanks!

 

Phil Anderson

Tucson, AZ

[Rick Turner]

Probably a current problem....the amp might try to draw too much in bridged mode, thus leading to thermal problems or perhaps power supply transformer problems....

Just talked to an audio engineer about bridging (a generic conversation, not about QSC), and he stated that two advantages of bridged mode (all other things being up to the task)are double the slew rate and half the noise of non-bridged mode. He used these effects in designing a mic preamp at one time!

[rumpelstiltskin.]

to get 500W into 4 ohms, you need 11.2A (amps). all QSC PLX-series amps come with a 15A IEC power cord (like the one on your computer), meaning the power supply + amp has to be 75% efficient in order to actually deliver that kind of power and not fry the power cord. it isn't that efficient, so you're not actually getting 500W. in fact, you won't actually ever get 500W from that unit if you use it consistently with UL/CE/CSA safety guidelines. you can get 500W from it, though, and that is why it is rated at 500W.

 

also, this is a continuous rating, not a program rating. you can get 500W, just not for an extended time. so you may well be getting 500W from it if you're able to drive it sufficiently and get it adequate power.

 

that may be another reason why QSC quietly discourages such a practice.

[Ed Friedland]

Phil,

Well, I use the QSC bridged at 4 ohms all the time. I find that more than enough power for me, I have cranked it up a few times, but mostly I play low to medium volume. I think pushing high volume at 2 ohms for extended periods of time is pushing the amp too hard, even if they say you can do it. Would you want to drive a Ford Fiesta at 95 miles an hour to Lake Havasu during July? Sure, you can do it, but it's not the best for the car, or it's passengers.

Come out and see The Friends OF Shooby Taylor sometime, we play at Plush periodically, next one is July 25th. I'm also there on Fridays 6-8 with Dickie Thompson. Come on down!

 

 

------------------

www.edfriedland.com

[Ben]

Just thought I'd dredge this old thread back up with some further info about this since last June.

 

According to this recent thread at TalkBass, according to Bob Lee of QSC, it's OK to run PLX amps bridged.

 

http://www.talkbass.com/forum/showthread.php?s=&threadid=41261&highlight=qsc+plx

[Ben]

Or at least the 2402. I'm assuming Mr. Lee's advice applies to the 1602, too...

[Bob Gollihur]

I've only ever run my 2402 bridged, and have run it loud enough to hit the red lights on occasion. Only time I ever had a "failure" was an outdoor Summer gig where the line voltage sagged to 100 volts, according to my power bar, while I was pushing it very hard - and all it did was drop out due to the thermal overload. Came back in a few seconds; did it maybe three times before I reduced my volume (still quite near the top) and it was fine for four sets being pushed very hard.

 

Don't know that this experience applies to the 1602- there are differences between the two besides power output.

[doctordale]

to get 500W into 4 ohms, you need 11.2A (amps). all QSC PLX-series amps come with a 15A IEC power cord (like the one on your computer), meaning the power supply + amp has to be 75% efficient in order to actually deliver that kind of power and not fry the power cord."

 

The flaw in this argument is the 15 amps in is at 115VAC, which is 1,725 watts max input power.

[Ben]

If you look at the schematics on the QSC website, it shows that the 1602 and 2402 have the same basic class AB amp, but the 2402 has some class H output circuitry, too. The circuit board in the 1602 has a bunch of spaces for the extra components but they are not installed. I assume there is additional power supply circuitry to supply the additional output stuff, which runs at a higher voltage, but I didn't check that.

 

Since I am only driving a pair of Epifani 112s, I couldn't feature paying another couple hundred dollars for additional power I don't really need.

 

With regard to how much AC current these amps draw, there is a very interesting chart showing just that on page 29 of the PLX owner's manual (which is also on the website). Under what QSC calls a "typical" scenario, pink noise at 1/8 power "with occasional clipping", bridged into 4 ohms, the 1602 draws 14 amps and the 2402 only 11.8, so the benefit of class H is to allow the 2402 to produce another couple of dB of output while using less line current.