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Author Topic: To Bridge/Parrell or Stereo my power amps.  (Read 3334 times)

Bill Hayden

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To Bridge/Parrell or Stereo my power amps.
« on: September 16, 2014, 02:13:39 pm »

Using new speakers and power amps for benefit this weekend. using two Crown X4000 power amps FOH, one for both subs (mono) JBL JRX118's (4 Ohms) with crossover (left mono) and pair of JBL JRX225's (4 Ohms) (right mono). Speakers are 4 ohm and I have been having discussions with others about bridging/ parellell or stereo mode for power amps. If I bridge Im at 8 ohms (should I ad another pair of 225's paired or should I run parrellell or stereo mode. venue is outdoors about 200-300 people. Im also using two other 18" cabs using another power amp ground level center stage. testing all weekend in bridge mode, sounded great" but if I need to push them Saturday with a band am I gonna feel comfortable about it. I have two pairs of the JBL JRX 225's but was only planning on taking one set, and Im only using four power amps S
I've included the specs for the Crowns FOH, any help would be welcomed. Thanks

Crown X4000 Stereo 2x1350W Power Amp Specifications:
Guaranteed Minimum Power: 1350W (per channel) @ 4 ohms / 1000W (per channel) @ 8 ohms / 2700W bridge mono @ 8 ohms
Frequency Response (at 1 Watt): 20Hz - 20kHz, +0/-1dB
Total Harmonic Distortion (THD): < 0.5%, 20Hz - 20kHz
Intermodulation Distortion (IMD) 60Hz and 7kHz at 4:1, from full-rated output
to 30dB: 0.35%
Slew Rate: >10V/us
Voltage Gain: 29dB 31dB 33dB 36dB
Damping Factor (8 Ohm), 10Hz - 400Hz > 200
Signal-to-Noise Ratio: (below rated power, 20Hz to 20kHz, A-weighted): > 100dB
Crosstalk (below rated power)
At 1kHz: 75dB
At 20kHz: 59dB
Input Sensitivity for full rated power at 8 Ohm 0.775V or 1.4V
Input Impedance (nominal)
Balanced: 20 kOhm
Unbalanced: 10 kOhm


Hayden J. Nebus

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Re: To Bridge/Parrell or Stereo my power amps.
« Reply #1 on: September 16, 2014, 02:39:01 pm »

You need to change the display name in your profile to your real full name before anyone can help you, or the mods will lock your thread.

Mac Kerr

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« Reply #2 on: September 16, 2014, 03:00:27 pm »

Using new speakers and power amps for benefit this weekend.

Please go to your profile and change the "Name" field to your real first and last name as required by the posting rules displayed in the header at the top of the section, and in the Site Rules and Suggestions in the Forum Announcements section, and on the registration page when you registered.


Luke Geis

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Re: To Bridge/Parrell or Stereo my power amps.
« Reply #3 on: September 17, 2014, 11:46:04 pm »

Watts are watts and speaker efficiency determines what those watts will do. I think your mistaken about ohm's ratings though.

If you parallel the subs together you will be pushing a 2 ohm load and same with the tops. The subs and tops are 4 ohms each as you stated. That being the case if you double them up you reduce the impedance by half making a 2 ohm load. The amps in bridge mode are not designed to run a 2 ohm load. I get the feeling they will thermal out and shut down in short order if you start to ask much of them. It also looks as if your amp is not rated for a 2 ohm load at all? This again means that it will shut down in short order if you ask it to push a 2 ohm load even in stereo/parallel mode.

From a set up standpoint I would try to get all the subs in one spot. Center stage is perfect. If you can run all four that's great, but be aware that if they are different models, you may have reduced performance. As for the tops, being that they are 90 deg. boxes, running all four will not net you much except a larger coverage area. Pointing them at the same general spot will only make it louder at frequencies that don't comb out. In other words don't bother doing it for added volume. You will have to splay them in order to get 180 deg. of coverage at basically the same volume as 2 boxes would get you. If you cannot achieve the volume needed in the listening area with what you have, then you don't have enough rig for the gig. Plain and simple......

If I were you I would have one speaker for each amp channel. That is: run both amps in stereo mode and connect only 1 speaker to each channel. If your speakers are in fact 4 ohms, you will be getting the most out of that amp your going to get safely. If the amp has a built in crossover, be sure to use it and connect the speakers to the appropriate amp channels to utilize that feature.

You can mathematically calculate the potential volume of your system, but I feel I would be shooting over your head to explain it. I touched on it in my opening sentence. From what I know of what you have I would guess about an at best db level of about 107db at 50'. This is only hypothetical though and it's rather likely that you won't break 100db. The weighting is non specific and depending on which weight you go with will alter the results as well. For 200-300 people you should have enough rig to get rock level in front of the stage area though. At about 50' your going to be at a more comfortable, less rock and roll level. That level assumes full rated power, so that would be the level you could get as the amp begins to clip. However it doesn't factor power compression and cable losses. So that number doesn't mean much except that in a perfect world you could get there. I factor low leaving at least 3db for speaker safety and another 3db for the unknown. If I can't get the level I need after that, I either need more PA, or more efficient speakers. I'm fortunate that most of my local gigs are volume limited to 80db at listening position ( per city laws ), which is usually between 50-75' from the stage. Don't need a super duper PA to get that!

My biggest concern is getting the tops and subs working together. In general you should shoot for at least 2:1 ratio of subs to tops power. That means 4 subs at the 1350 watts power you mention. You have to properly crossover the tops and the subs to work together. Putting all the subs in one area ( in a line in front of the stage ) will help get you some free level gain. Putting two subs together nets you 3db and placing them on the ground nets you another 3db. That is 6db of free gain! So 6db is great right? It's not quite enough in your case. Your subs are less efficient than your tops and will only get about the same level at the same distance even after factoring the free gain. That means that there is no room for extra boom. Most engineers shoot to have at least 3-6db of extra room in the subs to have for added impact. I shoot for 6db over the mains as a minimum. This makes it easier to get the chest thumping low end without worry of clipping. You would need all four of your subs in line to properly meet the 2:1 ratio and you still would only just clear your mains by around 5db at best. It takes twice the power to create an extra 3db in level. So adding the other 2 subs gains you another 3db for a total of 9db's of possible extra level. Even with that extra 9db, you still only clear your mains by about 5db. This assumes full rated power too. So if your sub amps are clipping and you don't have the level you need, then you either need more subs, or subs that are more efficient.

I find that when the subs are fed the same send as the mains and simply crossed over, the extra content that goes through the crossover still eats up headroom and can muddy up the mix. The only real fix is to put the subs on an auxillary mix. This makes it so that only the things you want in the sub, go into the sub, freeing up headroom and allowing for higher sub impact and less muddy mix.

Another thing I think I see you trying to do is separate your subs. It sounds as if you plan on using a set to put the mains up on and then another set in the middle of the stage. This is a bad idea. This will create bad comb filtering that will totally cancel out sub frequencies in several places and you would also no longer be getting all 9db of that free volume I was talking of. If you have to use the subs to put the mains on, your best bet is to run the subs in pairs on each side with the tops. This will get you the classic power alley and you will still have lots of comb filtering , but it minimizes it to a known area. If you put all 4 subs in line in the center you keep the 9db of added level on the subs. This would clear you of the mains by 5db and allow for a lot of highly predictable sub power. With little to no comb filtering with the subs the entire listening area can have powerful sub content. If you separate the subs you start to loose the coupling and the extra gain is lost and then comb filtering further reduces level at several spots in the listening area. The potential volume level and outcomes are of course hypothetical. It is not hard fact, but generalizes the potential in order to make educated decisions of the likely outcome. 
I don't understand how you can't hear yourself

Thomas Le

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Re: To Bridge/Parrell or Stereo my power amps.
« Reply #4 on: September 18, 2014, 09:22:38 am »

Since you have your ohms terminology wrong, I suggest you not bridge.

 8 + 8 = 4
 4 + 4 = 2

JRX dual 15's and a single 18 sub? this should be in the lounge...

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Re: To Bridge/Parrell or Stereo my power amps.
« Reply #4 on: September 18, 2014, 09:22:38 am »

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