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post #1 of 58 Old 11-05-2013, 05:57 AM - Thread Starter
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I'm in the market for a new amp, and like the power it puts out at 4ohm bridged. I'm hearing it's not a good idea because that's basically running it at 2ohm. It's running the amp at it limits?

How do you all feel about this? Is it good practice?

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post #2 of 58 Old 11-05-2013, 06:09 AM
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Originally Posted by SeekingNirvana View Post

I'm in the market for a new amp, and like the power it puts out at 4ohm bridged.
Bridging isn't about power, it's about voltage swing. You bridge when the voltage swing of one channel isn't adequate to drive the speaker to full output. That means you have to know what the voltage swing is when bridged, and what the required voltage swing is to drive your speaker to full output. When you have these facts at hand then you can buy, so do your homework before placing an order.
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post #3 of 58 Old 11-05-2013, 07:32 AM - Thread Starter
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When I look at amp specs I see watts. The driver I'm going to use can take 2000 rms, but I'm putting it in a small sealed so I'll give it more, like 4000. The amp I'm looking at is 1250 WPC at 8 ohms or 4000 bridged at 4 ohm. What am I missing?

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post #4 of 58 Old 11-05-2013, 08:32 AM
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Power ratings by a manufacture are more of ballpark figures. For example, feeding most any driver 4kw continuous will cook the driver (exceeding thermal limits). However, for very short periods, you could give a lot of drivers 4kw without cooking them, but they may bottom out and become damaged (mechanical limits). So long as you exceed neither them thermal nor mechanical limits, you'll be good.

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post #5 of 58 Old 11-05-2013, 08:36 AM - Thread Starter
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I'm going to feed my LMS ultra 4000 watts for peaks. It will be in a all sealed so that will help it not bottom out.

The question I'm trying to ask is, Is it good practice to bridge a amp at 4ohm?

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post #6 of 58 Old 11-05-2013, 08:44 AM
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My personal preference is no, but there's no reason why you can't.

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post #7 of 58 Old 11-05-2013, 10:04 AM
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You are not running the amplifier "harder" by bridging it. As long as you're within the rated specifications of the amp, then there is no need to be concerned about the longevity of the amplifier. In other words, bridging an amplifier is standard practice.

In the pro sound arena with limited budgets, it is fairly standard to run subwoofer amps bridged with two subwoofer cabinets in parallel....this usually maximizes the amplifier power per dollar invested. In 21 years of running live sound, I've never seen a bridged amplifier fail.

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post #8 of 58 Old 11-05-2013, 10:17 AM - Thread Starter
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Quote:
Originally Posted by MBentz View Post

You are not running the amplifier "harder" by bridging it. As long as you're within the rated specifications of the amp, then there is no need to be concerned about the longevity of the amplifier. In other words, bridging an amplifier is standard practice.

In the pro sound arena with limited budgets, it is fairly standard to run subwoofer amps bridged with two subwoofer cabinets in parallel....this usually maximizes the amplifier power per dollar invested. In 21 years of running live sound, I've never seen a bridged amplifier fail.

Now that's the type of answer I was looking for. Thank you!

I was hearing people say that bridging at 4 ohms was running it at its limits, but I've also heard of people doing it all the time.

I think I will make my purchase.....2 QSC pl340's, one per LMS ultra.

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post #9 of 58 Old 11-05-2013, 12:14 PM
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I contend bridging the amp can be harder on it because it feels a heavier load and (can) put out more voltage swing/power if you command it to.

Example, the PL340 is 1250wpc @ 4 ohms stereo, 4000w bridged @ 4. If you push it, it will be working harder. That said, it is rated for 2 ohms stereo / 4 ohms bridged (these two are the same load to the amp) so it should be able to take it.

Just because there is a knob doesn't mean you should turn it.
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post #10 of 58 Old 11-05-2013, 12:25 PM - Thread Starter
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I pulled the trigger on 2 QSC 4050hd's. They are also 4000 watts @ 4 ohm bridged. Something about the large toroidal transformer says high current to me.

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post #11 of 58 Old 11-05-2013, 02:56 PM
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Quote:
Originally Posted by whoaru99 View Post

I contend bridging the amp can be harder on it because it feels a heavier load and (can) put out more voltage swing/power if you command it to.

Example, the PL340 is 1250wpc @ 4 ohms stereo, 4000w bridged @ 4. If you push it, it will be working harder. That said, it is rated for 2 ohms stereo / 4 ohms bridged (these two are the same load to the amp) so it should be able to take it.
Voltage swings will remain the same. It's the current that increases as the resistance goes down.

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post #12 of 58 Old 11-05-2013, 03:19 PM
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I believe I covered it properly with the "heavier load" and "more voltage swing" comments, relative to the OP's concern about 4 ohms stereo vs. 4 ohms bridged.

Just because there is a knob doesn't mean you should turn it.
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post #13 of 58 Old 11-05-2013, 03:35 PM
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Quote:
Originally Posted by whoaru99 View Post

I believe I covered it properly with the "heavier load" and "more voltage swing" comments, relative to the OP's concern about 4 ohms stereo vs. 4 ohms bridged.
+1. What most users don't consider is what actually happens when you bridge. Yes, power delivery goes up. Why? Because voltage swing doubles. That's all well and good if you need the extra voltage swing, but if you don't all you accomplish is to increase the risk of blowing drivers. What most users are actually looking for when bridging isn't increased voltage swing into a high impedance load, but rather increased current capacity into a low impedance load, and that's not what happens.

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post #14 of 58 Old 11-05-2013, 10:08 PM
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Quote:
Originally Posted by MBentz View Post

You are not running the amplifier "harder" by bridging it. As long as you're within the rated specifications of the amp, then there is no need to be concerned about the longevity of the amplifier. In other words, bridging an amplifier is standard practice.

In the pro sound arena with limited budgets, it is fairly standard to run subwoofer amps bridged with two subwoofer cabinets in parallel....this usually maximizes the amplifier power per dollar invested. In 21 years of running live sound, I've never seen a bridged amplifier fail.

Bridged or not makes no difference, if it's gonna fail, it's gonna fail no matter what config you run it at.

Blasting brown notes for 10 years and counting!

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post #15 of 58 Old 11-05-2013, 10:57 PM
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corrected.

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post #16 of 58 Old 11-06-2013, 05:26 AM
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Originally Posted by LTD02 View Post

???
both channel's capacitor banks serve as current reserve in the case of bridging. all output devices are in use, thus more current capability. this is why bridging an old skool amp gives about twice the power.
Look at the specs, these for instance:
http://qsc.com/products/Power_Amplifiers/PLX2_Series/PLX1802/
Note how the bridged minimum impedance load is twice that of the per channel minimum impedance load.
What you're saying would be true if the two channels were parallel bridged, an option that used to be seen on some amps. But the vast majority of amps today use series bridging, and just like placing a pair of batteries in series voltage doubles, current delivery remains the same.

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post #17 of 58 Old 11-06-2013, 09:27 AM
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corrected.

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post #18 of 58 Old 11-06-2013, 10:49 AM
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Quote:
Originally Posted by Bill Fitzmaurice View Post

Look at the specs, these for instance:
http://qsc.com/products/Power_Amplifiers/PLX2_Series/PLX1802/
Note how the bridged minimum impedance load is twice that of the per channel minimum impedance load.
What you're saying would be true if the two channels were parallel bridged, an option that used to be seen on some amps. But the vast majority of amps today use series bridging, and just like placing a pair of batteries in series voltage doubles, current delivery remains the same.

+1



All the capacitors are used the same way regardless if the amp is bridged or not.

When you bridge the amp one channel signal is inverted and the voltage swing of the two channels sum. It's as simple as that. Voltage swing capability is doubled, current capability is the same as a single channel.

Ohms Law bears it out quite clearly. When the amp is run at 4 ohms bridged it's like running 2 ohms in stereo.

In the amp being discussed...

2 ch operation @ 2 ohms is 2000WPC (4000W total output) per the spec, which is the result of (approx.) 63V and 32A current (per channel).

When the amp is bridged with 4 ohm load, the spec is 4000W. This is the result of (approx.) 126V and 32A current.

As you see, current isn't increased at all per channel in this bridged mode situation but you get doubling of the voltage swing resulting in double the power.

Just because there is a knob doesn't mean you should turn it.
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post #19 of 58 Old 11-06-2013, 11:31 AM
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Quote:
Originally Posted by whoaru99 View Post

2 ch operation @ 2 ohms is 2000WPC (4000W total output) per the spec, which is the result of (approx.) 63V and 32A current (per channel).
When the amp is bridged with 4 ohm load, the spec is 4000W. This is the result of (approx.) 126V and 32A current.
As you see, current isn't increased at all per channel in this bridged mode situation but you get doubling of the voltage swing resulting in double the power.
Double the power but only into twice the impedance. If the voltage swing is doubled into the same impedance then power is quadrupled, but that would cook the power supply, so to keep that from happening the impedance must be doubled. You never get something for nothing, and in the case of bridging the doubling of the minimum load is the proverbial fine print. That's what gets bridgers into trouble, as they typically parallel drivers for higher power handling and then bridge to get more power from the amp without reading said fine print. Where bridging works best is when multiple drivers are series wired, and bridging supplies the required higher voltage swing to drive them to full excursion.
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post #20 of 58 Old 11-06-2013, 11:38 AM
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corrected.

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post #21 of 58 Old 11-06-2013, 11:52 AM
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That's not true in most cases because in most amps the power supply is common to both/all channels.

Besides, it's an irrelevant point in the discussion of what happens in bridged mode vs. two channel operation. It should be obvious if you're not using one channel of an amp that unused channel isn't contributing to the output. That said, its capacitors are being charged just the same using that channel or not.

Just because there is a knob doesn't mean you should turn it.
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post #22 of 58 Old 11-06-2013, 11:54 AM
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"That's not true in most cases because in most amps the power supply is common to both/all channels."

ah. then i'm mistaken.

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post #23 of 58 Old 11-06-2013, 11:58 AM
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Quote:
Originally Posted by Bill Fitzmaurice View Post

Double the power but only into twice the impedance. If the voltage swing is doubled into the same impedance then power is quadrupled, but that would cook the power supply, so to keep that from happening the impedance must be doubled. You never get something for nothing, and in the case of bridging the doubling of the minimum load is the proverbial fine print. That's what gets bridgers into trouble, as they typically parallel drivers for higher power handling and then bridge to get more power from the amp without reading said fine print. Where bridging works best is when multiple drivers are series wired, and bridging supplies the required higher voltage swing to drive them to full excursion.

Guess I'm not sure if you're clarifying something to me or building off what I said.

In either case, the specs of the amp in topic were used so there isn't any abstract/mystery power coming from anywhere re. 2 ohm stereo vs. 4 ohm bridged.

Just because there is a knob doesn't mean you should turn it.
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post #24 of 58 Old 11-06-2013, 12:16 PM
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Originally Posted by whoaru99 View Post

Guess I'm not sure if you're clarifying something to me...
Not to you, as you obviously get it. wink.gif

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post #25 of 58 Old 11-06-2013, 01:34 PM
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Quote:
Originally Posted by LTD02 View Post

"That's not true in most cases because in most amps the power supply is common to both/all channels."

ah. then i'm mistaken.

The reason they have to share the same rails is if you didn't they could float apart generating a possible DC bias. Beside being bad for speaker, the common mode voltage generated might exceed that of the voltage rating of the silicon and fail it. There are ways to manage this, but just simply linking the rails makes the problem a joke.

I must be guilty because people say I am guilty because they chose to call me guilty because they refuse to see the truth. Much easier to be part of the mob..
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post #26 of 58 Old 11-06-2013, 02:47 PM
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so I've got a curiosity question:

if a single channel output device can swing voltage from +60/-60v when operated in stereo mode and it can swing 1/2 of a cycle up to +120/-0v in bridged mode, why can't it swing +120/-120v in stereo mode?

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post #27 of 58 Old 11-06-2013, 02:55 PM
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Hi LTD,

It can swing +/- 60 volts because the power rails are just over +/- 60 volts, but the minus speaker terminal is tied to zero volts. ie: The plus terminal swings +/-60 volts while the minus terminal stays at zero volts.

In bridged mode, the minus terminal is replaced with the plus terminal of the second amp, and is driven by the compliment of the input to the first amp. So while the first amp's plus terminal swings toward +60 volts, the plus terminal of the other amp swings toward -60 volts, giving you a total swing of 120 volts across the two plus terminals.

A single amp can't swing +/-120 volts because the power rails are only +/-60 volts.

We used to call the former "unipolar drive", while the bridged mode would be called "bipolar drive" (not to be confused with Class A and Class B).
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post #28 of 58 Old 11-06-2013, 03:16 PM
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still confused. if the single channel can swing +/-60v on the positive side with the negative side held at zero, isn't that still 120v total swing?

i'm not seeing how having each channel produce have the wave increases the amplitude of the wave.


ah! i went back an read it again for a fourth time and see how it works. thanks!

so in stereo mode, channel a is 0v on negative and +/-60v on the positive. this creates up to 60v potential difference between the positive and negative terminals.

in bridge mode, as the positive swings up +60v, the negative swings down -60v. this creates up to 120v potential difference between the positive and negative terminals (then -60v on the positive and +60v on the negative for the second half the cycle).

so, it's not that each channel is producing "half" the wave, but that the channels are push/pulling together to double their potential difference.

correct?

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post #29 of 58 Old 11-06-2013, 05:24 PM
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Quote:
correct?
correct!
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post #30 of 58 Old 11-07-2013, 08:53 AM
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I have a Behringer EP2500 ( same as EP4000 but marketed for RMS not peak watts ). I had been using it in bridge mode to power a pair of dual voice drivers wired to present a 4 ohm load. I ran it that way for 5 years. Recently, the fan began kicking into turbo when things were idle. I had to clean out some dust in it. I am reluctant to do a fan mode in the configuration because I suspect it runs a little warmer.

I have since built two separate boxes for the woofers and now have 2 2ohm loads. Same thing to the amp really.
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