ProSoundWeb Community
Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Pro AV Forum => Topic started by: Brad Gibson on August 18, 2021, 06:28:31 PM
-
I have several QSC PLX2 3602 amps and I am considering a new speaker system that will require me to run them at 2 ohms per channel. What are your thoughts or experience running at 2 ohms?
-
My general advice is not to do this for normal operation, but it is OK to do this in case of emergency when say an amp fails and the show must go on.
JR
-
I have several QSC PLX2 3602 amps and I am considering a new speaker system that will require me to run them at 2 ohms per channel. What are your thoughts or experience running at 2 ohms?
PLX sucks at 2ohms.
I'd say "2 ohms" is relative. My subs (TH118) are 4ohms 'nominal' they have a dip at 3ohms.
I run 2 in parallel on a DNA20K4 or PLD4.5 Technically the amp should see 1.5ohms or 2ohms...
I rarely see less than 4 ohms (at any freq) even with both paralleled. Why? Power compression/voice coil heating.
---
You said 'speakers' nominal impedance means very little. It's just so that the MFG don't have to answer questions like "what amp do I use" (Ivan speaks lots about this issue when DSL used more granular impedance).
-
I have several QSC PLX2 3602 amps and I am considering a new speaker system that will require me to run them at 2 ohms per channel. What are your thoughts or experience running at 2 ohms?
Used PLX amps, and for that matter, pretty much any class H amp design, are pretty cheap these days. Why not just get more amps?
-
Brad - refer to post's #2, #3 & #4. Never setup the baseline in any part of any system to the extreme limits of performance.
There are probably amps out there that can survive driving 2 ohm loads - there are more amps out there that will not.
Experience says don't do it.
-
I have several QSC PLX2 3602 amps and I am considering a new speaker system that will require me to run them at 2 ohms per channel. What are your thoughts or experience running at 2 ohms?
Require 2 ohms? That is a red flag.
As others have said -
Don't do it.
Doubly don't do it with subwoofers.
Triply don't do it with PLX or any other lower tier amps.
10AWG cable that's really short, as a temp band-aid, depending on the amp, is OK. Linea amps, as long as you look closely at the crest factor and de-rate the power, and account for line losses through the speaker cable, are OK (but not great).
-
Require 2 ohms? That is a red flag.
As others have said -
Don't do it.
Doubly don't do it with subwoofers.
Triply don't do it with PLX or any other lower tier amps.
10AWG cable that's really short, as a temp band-aid, depending on the amp, is OK. Linea amps, as long as you look closely at the crest factor and de-rate the power, and account for line losses through the speaker cable, are OK (but not great).
Does 4 Ohm Bridge count the same as 2 Ohm dual channel in your opinion Caleb? I have one rig with six PL236's running 4 ohms briged.
-
Does 4 Ohm Bridge count the same as 2 Ohm dual channel in your opinion Caleb? I have one rig with six PL236's running 4 ohms briged.
4 ohms bridged IS 2 ohms per channel
JR
-
4 ohms bridged IS 2 ohms per channel
JR
I have been running JBL SRX 728 bridged at 4 ohms for years and not 1 failure of amps or drivers. That is why I wanted to start this discussion. Why can I run the bridged at 4 ohms but others are saying stereo at 2 ohms is a no, no?
-
I have been running JBL SRX 728 bridged at 4 ohms for years and not 1 failure of amps or drivers. That is why I wanted to start this discussion. Why can I run the bridged at 4 ohms but others are saying stereo at 2 ohms is a no, no?
Because you have no fallback. Two is one, one is none.
-
I have been running JBL SRX 728 bridged at 4 ohms for years and not 1 failure of amps or drivers. That is why I wanted to start this discussion. Why can I run the bridged at 4 ohms but others are saying stereo at 2 ohms is a no, no?
You can run them at 1 ohm (2 ohms bridged) if you like. You asked for advisement and got it. I will guarantee you if you push the amp at 2 ohms it won't live long and prosper. 2 ohm loads, as has already been stated, is something I would never do unless in a real bind, as has also already been stated, in which case the drive to the amp would be reduced and temperatures monitored closely.
Why have you been able to pull off a 4 ohm load on a bridged amp? An educated guess is that you haven't been pushing it very hard.
-
I will reiterate what has been said several times: As a design goal, running your system down to 2ohms is NOT prudent or in the best of interest. Running down to 2 ohm's is something to be done when there is nothing else left that you can do. It is very much a caveat emptor sort of thing when you do it purposefully. For a small, easy-going gig where there will be little stress on any part of the equipment, sure, but if you need any amount of real performance, I would highly suggest NOT going that way with your system design.
-
I have been running JBL SRX 728 bridged at 4 ohms for years and not 1 failure of amps or drivers.
What amps are you using this way? Some amps cannot handle this use at all, others can tolerate it but don't really excell at it, then there are amps that are designed for it. This mode of operation puts the heaviest electrical load and thermal stress on the amplifier and often performance at 2R is compromised in the name of keeping the amp functional. In numerous threads over the years it has been suggested and confirmed that using a bigger amp that can generate roughly the same power into a box at 4ohms/ch results in noticably tighter sound than using smaller bridged mono amps, and a side benefit is that the total AC electrical draw is reduced... significantly reduced (1/2 as much or less) if going from an old school class AB or class H amp to a class D.
-
I have been running JBL SRX 728 bridged at 4 ohms for years and not 1 failure of amps or drivers. That is why I wanted to start this discussion. Why can I run the bridged at 4 ohms but others are saying stereo at 2 ohms is a no, no?
MTBF
JR
-
When you look at the specs on the PLX amps, what you will notice is that as you get lower in ohms, to get to the rated power, your trade off is a MASSIVE increase in distortion. The PLX 3602 is 1250 watts into 4 ohms at .1% distortion, but at 2 ohms, it's 1800 watts at a whopping 1% distortion. So less power per speaker, way more distortion. Also notice that their spec ONLY shows 1khz, not full band range. That's telling that the full range has even less impressive numbers.
Running an amp at it's max load rating just means you're driving it harder. Well designed amps should be able to operate just fine, but like anything else, if you run it harder, it may not last quite as long. But, with current equipment, the life of the gear is long enough even at high load that it may not really matter.
BUT, what does matter is if your amps are all running to their max load, that means there's no option to re-patch to account for the possibility that an amp fails.
1 amp running at 2 ohms dies, show over. 2 amps running at 4 ohms each and one dies, and you can patch together and get back to running with very little loss.
-
Have used rigs running PLX at 2ohm. When I convinced them to buy more amps jaws were dropping because the difference is not at all subtle.
All things being equal, if you insist on running amps at 2ohms the PLX is one of the worst possible choices to do it with. It just doesn't have the grunt to do it well.
iTech's and Macrotech-i's do 2ohm loads reasonably well. As do a number of other "D" amps out there that I don't have a lot of experience with. Unfortunately all the amps that do it well are also expensive.
So you have the choice. Buy the expensive amps and run at 2ohms per channel and have no backup plan, Or buy double the amount of PLX's and run at a happy 4ohms per channel, and if you have a failure you have the choice to run one or more amp at 2 ohm to get through the gig.
-
Have used rigs running PLX at 2ohm. When I convinced them to buy more amps jaws were dropping because the difference is not at all subtle.
All things being equal, if you insist on running amps at 2ohms the PLX is one of the worst possible choices to do it with. It just doesn't have the grunt to do it well.
iTech's and Macrotech-i's do 2ohm loads reasonably well. As do a number of other "D" amps out there that I don't have a lot of experience with. Unfortunately all the amps that do it well are also expensive.
So you have the choice. Buy the expensive amps and run at 2ohms per channel and have no backup plan, Or buy double the amount of PLX's and run at a happy 4ohms per channel, and if you have a failure you have the choice to run one or more amp at 2 ohm to get through the gig.
PL2's do OK though. I haven't tried PL3's. I have PL236's running 4 ohms bridged (effectively 2 ohms) but I have 4 of them if I lose one the show goes on. I am using them on STX828S's
-
PL2's do OK though. I haven't tried PL3's. I have PL236's running 4 ohms bridged (effectively 2 ohms) but I have 4 of them if I lose one the show goes on. I am using them on STX828S's
It's a Pro amp compared to a Pro-sumer amp though. Much better power supply and the cost new was quite a bit more. Maybe double? I don't know what either of them cost when new.....
-
I have several QSC PLX2 3602 amps and I am considering a new speaker system that will require me to run them at 2 ohms per channel. What are your thoughts or experience running at 2 ohms?
I have been running Subs at 2 ohms for many years with no issues . I had plx3402 for a while and now I have the Pl380 . They were powering EV MTL 4 .