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swoo View Drop Down
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    Posted: 05 June 2019 at 7:54pm
I am about to buy some very powerful 21" drivers. 
They come in the option of 4 or 8 ohms. 
I will be wiring 2 in parallel. 
It seems to me that going for the 8ohm (giving a 4ohm overall load) version would be silly as would just require a more powerful amplifier to drive them, as opposed to the 4ohm (2ohm overall load) version which could receive more watts from the same amplifier, assuming its stable at 2ohms. 
Is there a disadvantage to opting for the lower impedence driver?
In the past I have hear people say they would only run their bass at 8 ohms. I understand that less resistance makes the amp run more "loosely - so to speak", but is this really am issue if the manufacturer has stated that it is stable at 2 ohms by putting a power rating at 2 ohms on the specification sheet?
For the record I am allowing around 50% headroom on amp power capabilities to driver RMS.
In my ideal scenario I would get 2x B&C 21DS115 (1700w RMS) at 4 ohm, wire them in parallel to give an overall 3400w RMS load at 2 ohm, then power them with an amp that is capable of around 5000w @ 2 ohm.
Can anyone shed light on issues here or the physics behind why this may be bad/good? 
Thanks
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Elliot Thompson View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 05 June 2019 at 10:37pm
Originally posted by swoo swoo wrote:

I am about to buy some very powerful 21" drivers. 
They come in the option of 4 or 8 ohms. 
I will be wiring 2 in parallel. 
It seems to me that going for the 8ohm (giving a 4ohm overall load) version would be silly as would just require a more powerful amplifier to drive them, as opposed to the 4ohm (2ohm overall load) version which could receive more watts from the same amplifier, assuming its stable at 2ohms. 
Is there a disadvantage to opting for the lower impedence driver?
In the past I have hear people say they would only run their bass at 8 ohms. I understand that less resistance makes the amp run more "loosely - so to speak", but is this really am issue if the manufacturer has stated that it is stable at 2 ohms by putting a power rating at 2 ohms on the specification sheet?
For the record I am allowing around 50% headroom on amp power capabilities to driver RMS.
In my ideal scenario I would get 2x B&C 21DS115 (1700w RMS) at 4 ohm, wire them in parallel to give an overall 3400w RMS load at 2 ohm, then power them with an amp that is capable of around 5000w @ 2 ohm.
Can anyone shed light on issues here or the physics behind why this may be bad/good? 
Thanks


Operating an amplifier at 2 ohms per channel will not give you any leeway in terms of headroom. You are operating the amplifier at its limits in which, the performance can/will suffer at times. When a manufacture states an amplifier is 2 ohms stable, it does not mean long-term.

 

This is why, cabinets housing two drivers will equate to a 4 ohm nominal load (8 ohm nominal per driver). B&C states a continuous power handling of 3400 on the 21DS115. It would be best to find an amplifier that delivers 4000 watts per channel @ 4 ohms if, you want to drive the loudspeaker(s) to its/their full capacity without stressing your amplifier with a low impedance load during the process.

 

Best Regards,




Edited by Elliot Thompson - 05 June 2019 at 10:41pm
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Risc_Terilia View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Risc_Terilia Quote  Post ReplyReply Direct Link To This Post Posted: 06 June 2019 at 7:16am
What's the short term application for 2 ohm loading?

Many manufacturers state 2 ohm stable unless you're driving bass btw


Edited by Risc_Terilia - 06 June 2019 at 7:17am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pfly Quote  Post ReplyReply Direct Link To This Post Posted: 06 June 2019 at 10:27am
As 2ohm will be the nominal impedance, depending on your loading it might get even lower. Less impedance = closer to short circuit. Imagine connecting amp's + and - together with thick cable, that's what you're getting closer to. With lower impedance cable cross area and length start to matter much more.


Edited by pfly - 06 June 2019 at 10:28am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 06 June 2019 at 11:26am
You pays your money and takes your chances. If you know the behaviour of your enclosures with regard to power angle and you know what the typical frequency content of the content you typically feed them then you can make an informed judgment on whether 2ohm loading is possible given your amp choice. If all of that makes no sense to you then you will have to make a choice and take a chance. All I would say is that if you want to use boxes hard at low loads then you'd better not skimp on the price of the amplifier. I'd be looking at a decent powersoft or similar. Good internal protection/limiting is a must.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 06 June 2019 at 12:13pm
Amps are relatively cheap these days - unless you start asking for 1000´s of watts @ 2 ohms. Then things start getting (very) expensive. And really, best avoided.  Ermm


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tonskulus Quote  Post ReplyReply Direct Link To This Post Posted: 06 June 2019 at 1:19pm
In general, its best to use 4ohm/channel minimum to avoid reaching amplifiers current limits = happy amplifier and better SQ (amplifier can feed full power for all frequencies). 
It should be noted that even when using 4ohm speakers, amplifier might see it as 2ohms or even less at 
certain situations because speaker impedance is never stable, it has dips and peaks. 


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Elliot Thompson View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Elliot Thompson Quote  Post ReplyReply Direct Link To This Post Posted: 06 June 2019 at 2:29pm
Originally posted by Risc_Terilia Risc_Terilia wrote:

What's the short term application for 2 ohm loading?

Many manufacturers state 2 ohm stable unless you're driving bass btw


Whether you use such a configuration for Bass, Midrange or, Treble does not matter. What matters is the load.

It depends on what the manufacture feels is appropriate. It is usually a matter of seconds. The power supply (Transformer) of the amplifier is the deciding factor. Amplifiers designed today that are classed 2 ohms per channel minimum would be looked upon as not recommended below 4 ohms per channel 30 years ago.

An amplifier brings forth less weight in addition to, consuming less current has its pitfalls when you must operate at 2 ohms per channel when, an amplifier falls within that category.


Best Regards,


Elliot Thompson
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 06 June 2019 at 2:59pm
And a lot of the systems today come with either built-in amps, or amps with DSP, so all processed to avoid those excessive current draws, ie you actually get less than you expect.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote logsquared1 Quote  Post ReplyReply Direct Link To This Post Posted: 06 June 2019 at 3:03pm
With these new super woofers,  21DS115, Ipal, etc you need lots of voltage swing to get the most out of them.  The "idea" of spec sheet watts and ohms is an outdated way of determining the amp to speaker match.  Its really about volts delivered to the driver and long term power handling of the driver.  This needs to match the voltage swing of the amp and the long term power capabilities of the amps supply.  From my experience, if you are not using a high tech amp to drive these new breed of drivers its a waste of money.   You will either not get the most out of the driver (waste of money) or you will blow the drivers (waste of money).  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Chris Grimshaw Quote  Post ReplyReply Direct Link To This Post Posted: 06 June 2019 at 3:32pm
For modern high-power sub drivers, I'd use 4ohm drivers, and one per amp channel. The big touring amps usually work best at 4ohm anyway.

That way, if an amp goes down, you can move the cabinets across, load the other amp down to 2ohm, and get through the night. If the amp is already running at 2ohm, you have nowhere left to go, and must leave cabinets disconnected.

There's also the increased distortion, loss of damping factor, and increased heat in the amplifier. It'll have a short unhappy life.

Chris
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Post Options Post Options   Thanks (1) Thanks(1)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 06 June 2019 at 4:04pm
Originally posted by Elliot Thompson Elliot Thompson wrote:

Originally posted by Risc_Terilia Risc_Terilia wrote:

What's the short term application for 2 ohm loading?

Many manufacturers state 2 ohm stable unless you're driving bass btw


Whether you use such a configuration for Bass, Midrange or, Treble does not matter. What matters is the load.

<p ="western" style="margin-bottom: 0cm">It depends on what the
manufacture feels is appropriate. It is usually a matter of seconds.
The power supply (Transformer) of the amplifier is the deciding
factor. Amplifiers designed today that are classed 2 ohms per channel
minimum would be looked upon as not recommended below 4 ohms per
channel 30 years ago.


<p ="western" style="margin-bottom: 0cm">An amplifier brings forth less weight in addition
to, consuming less current has its pitfalls when you must operate at
2 ohms per channel when, an amplifier falls within that category.

<p ="western" style="margin-bottom: 0cm">


<p ="western" style="margin-bottom: 0cm">Best Regards,


<p ="western" style="margin-bottom: 0cm">




I’m sorry Elliot, with all due respect there’s been a few comments from you of late that are flat wrong. This is one such comment.

It is relatively trivial to measure the impedance of a speaker load, and many modern amplifiers are out there running 2 Ohm nominal on all channels week in week out, and have been for years.
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