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Why 2 ohms is not a good idea..

#21
greenboy said:
There's some truth to what you say. But I think your figures might be off a little, just by looking at this chart several cabling companies use for load/gauge/length recommendations:

Code: 
Speaker Impedance   8 Ohm Load       4 Ohm Load
   Wire Gauge      Distance (ft)    Distance (ft)
     18 AWG             10               5
     16 AWG             20              10
     14 AWG             35              18
     12 AWG             60              30
     10 AWG            100              50

Deriving a 2-ohm column from that I think it's easy enough to deal with 2-ohm cabling losses.
Click to expand...

I don't see any reference to power levels on the above chart. Do we need to worry about power level and impedance in order to determine the correct wire gauge to use over a given distance?
 
#22
ehque said:
Blah. Any physics student worth his salt will be able to calculate that a speaker cable does not have nearly as much resistance as you think it does.



And any physics student worth his salt knows that a 2ohm load made of multiple resistors will never become a dead short, unless one of the speakers shorts out, and then you are fried anyway.

2ohm load in parallel - one speaker fries, you're left with a 4ohm load
2ohm load in series (unlikely) - one speaker fries, you're left with a open circuit, which is technically infinity ohms.
Click to expand...

Please provide some specific numbers that'll make it easy for the rest of us to understand this better.
 
#23
Just grabbed the meter and a nearby cable.

6' of 14 gauge speaker cable measures up at less than 0.05 ohms (i.e. below the resolution of the meter I'm using)

... so that means that the voltage drop across the cable is less than 2.5% of that across a 2 ohm load ...

... as it's in series, the current is the same ...

... so at 1200W there's less than 30W being dissipated in the cable. Probably a lot less, but I'd need to grab a better meter to be sure

... so you can expect the cable to warm up a bit, but unless you're using an instrument cable or a very long speaker cable it's not going to be anything to worry about

If anyone's got 100' of speaker cable lying around, I'd be interested in the resistance measurement for that.
 
#24
Taking the worst-case numbers from Invalid Link Removed gives a rough estimate of 0.3 ohms for 100' of speaker cable.

That would translate into 150W into the cable, which I would regard as something to worry about, especially if the cable were coiled.
 
#25
Jazzdogg said:
I don't see any reference to power levels on the above chart. Do we need to worry about power level and impedance in order to determine the correct wire gauge to use over a given distance?
Click to expand...

That was the question I was wondering. Is 500w watts at 4ohms somehow different from 500 watts at 8ohms.

Again, I always run at 4, so it's more of an academic interest for me.
 
#27
82Daion said:
A "dead short" is a connection with no resistance.

2 ohms is a very low impedance, but as long as the equipment in use is designed to handle it, there's no cause for alarm, as greenboy noted. Your amp setup isn't going to decide to go into a dead-short situation just because the impedance is low.
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+1 to you and Greenboy (and also Agedhorse, who I'm sure will chime in).

I'm more just interesed in learning about this stuff, and I realize most of my 'uneasy feeling' at running stuff at 2ohms is 'emotional/irrational'.
 
#29
UncleFluffy said:
Taking the worst-case numbers from Invalid Link Removed gives a rough estimate of 0.3 ohms for 100' of speaker cable.

That would translate into 150W into the cable, which I would regard as something to worry about, especially if the cable were coiled.
Click to expand...

Better to think of a loss in dB than think about watts. A 1dB loss where the most energy is in the signal is not even audible anywhere but in the midrange, and then, only in controlled environments without loud punters masking it ; }
 
#30
KJung said:
That was the question I was wondering. Is 500w watts at 4ohms somehow different from 500 watts at 8ohms.

Again, I always run at 4, so it's more of an academic interest for me.
Click to expand...

There's some difference - the power gets divided up according to the voltage drop across each stage, so the lower the speaker resistance the higher the proportion of the power that gets soaked up by the cable.

A quick concrete example:

1200W of power going into an 8 ohm speaker with 0.1 ohms of cable means:

speaker power 1185W
cable power 15W

with a 4 ohm speaker

speaker 1170W
cable 30W

with a 2 ohm speaker

speaker 1142W
cable 58W

(Yes, I know I'm simplifying a lot here and should be dealing in terms of impedance rather than just resistance, but the overall message is the same).
 
#32
UncleFluffy said:
There's some difference - the power gets divided up according to the voltage drop across each stage, so the lower the speaker resistance the higher the proportion of the power that gets soaked up by the cable.

A quick concrete example:

1200W of power going into an 8 ohm speaker with 0.1 ohms of cable means:

speaker power 1185W
cable power 15W

with a 4 ohm speaker

speaker 1170W
cable 30W

with a 2 ohm speaker

speaker 1142W
cable 58W

(Yes, I know I'm simplifying a lot here and should be dealing in terms of impedance rather than just resistance, but the overall message is the same).
Click to expand...

OK, I get it at a very basic layman's level. So, it seems the OP was right in general, but greatly overstated the impact of a high quality, low gauge, cable (the typical 10 or 12 gauge 3 foot cable with speakons that many of us seem to use).

Thanks.
 
#33
I generally run at 2 ohms with no problems. But I don't have a 1,200W amp ;) I have run for 6 hours playing time with no noticeable heat generated from the cable.

The acoustic 370 was designed to run at 1.7 ohms, so this isn't new. Yes, you need heavier cables at higher power with lower impedance.

Here is another table for 5% loss that includes 2 ohms:

http://www.roger-russell.com/wire/wire.htm
 
#34
KJung said:
OK, I get it at a very basic layman's level. So, it seems the OP was right in general, but greatly overstated the impact of a high quality, low gauge, cable (the typical 10 or 12 guage 3 foot cable with speakons that many of us seem to use).
Click to expand...

Pretty much. If you're not using long+thin cables you've got nothing to worry about. If you are, don't coil them up when you're cranking it.

KJung said:
Thanks.
Click to expand...

You're welcome.
 
#35
The resistance of stranded copper wire is so low it's measured per 1000 ft.

10 gage wire is a litte more that 1 ohm per thousand.

If you want to know exactly how much power loss there is across a given distance you need to know length load ohms AND voltage applied.

Whats your assumption, that your 1,200 watt amp is running flat out warp factor 10 (it ain't) and hitting a constant 2 ohm (it ain't) load?

Because if it is, your cable will carry 24.49 amps adding a whopping .301 ohms to the circuit if your speaker cord is 30' long.

There are no shortage of ohms law calculators on the 'net. just plug your favorite numbers in....

I'm just a master electrician, I'm sure a real EE will show up and as specific as you want soon enough.

I'm not saying it's hunky dory to drive your amp 100 miles per hour in first gear all day long but, the situation isn't quite as grave as it's been made out to be either.

At least not as far as your power loss through cabling goes.
 
#37
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#38
Steve said:
The resistance of stranded copper wire is so low it's measured per 1000 ft.

10 gage wire is a litte more that 1 ohm per thousand.

If you want to know exactly how much power loss there is across a given distance you need to know length load ohms AND voltage applied.

Whats your assumption, that your 1,200 watt amp is running flat out warp factor 10 (it ain't) and hitting a constant 2 ohm (it ain't) load?
Click to expand...
Agreed. These are peak figures we're dealing with here. To know the actual dissipation we'd need to know both V and R, but the % dissipated by each component is just down to R.

Steve said:
Because if it is, your cable will carry 24.49 amps adding a whopping .301 ohms to the circuit if your speaker cord is 30' long.

There are no shortage of ohms law calculators on the 'net. just plug your favorite numbers in....

I'm just a master electrician, I'm sure a real EE will show up and as specific as you want soon enough.
Click to expand...

In theory, I am one, but I've spent so long working with pico-amps and below in my current day job that I've forgotten what it's like to work with the meatier stuff. A rough Ohm's Law calc is enough to put us on the right planet though.
 
#40
Poor uneducated me. I foolishly ran my old Peavey Mark IV head for years at 2 ohms getting it's seemingly very realistic 300 watt output rating, now only to find that the perceived wattage boost was stuck in the speaker cables coupled to the two 4 ohm cabs I ran back then. Is this like the electronic version of a gunk lined colon?;)
 

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