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Author Topic: Converting amps needed based on ohm change  (Read 824 times)

David Allred

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Converting amps needed based on ohm change
« on: August 03, 2017, 10:18:07 am »

If a power amp draws 9.5A @ 1/8th power @ 4 ohms, is there a formula to convert the draw @ 8 ohms.  Is it basically 1/2 or 8 and x2 for 2?  Seems a little too easy.

thanks.
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John Roberts {JR}

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Re: Converting amps needed based on ohm change
« Reply #1 on: August 03, 2017, 11:37:02 am »

I like easy....  take it when you can.

JR
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Tim McCulloch

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Re: Converting amps needed based on ohm change
« Reply #2 on: August 03, 2017, 12:20:31 pm »

Georg Ohm laid it out for you.
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Bill Koonce

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Re: Converting amps needed based on ohm change
« Reply #3 on: August 03, 2017, 12:22:40 pm »

I'd think that a lot of that would depend on the amplifier class and power supply. A Class A amp will draw nearly full power regardless of output power, for example. If your amp draws 9.5A @ 1/8 power and that draw was linear, then it would draw 76A @ full power @ 4 Ohms, and over 150A @2. That's one mighty big AC circuit that your amp would need...if input power was linear to output power.
« Last Edit: August 03, 2017, 01:30:10 pm by Bill Koonce »
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David Allred

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Re: Converting amps needed based on ohm change
« Reply #4 on: August 03, 2017, 12:55:51 pm »

I like easy....  take it when you can.

JR

Thanks, JR, for confirming that I was correct.  (Or at least being the least cryptic.) ;)
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John Roberts {JR}

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Re: Converting amps needed based on ohm change
« Reply #5 on: August 03, 2017, 01:22:27 pm »

I'd think that a lot of that would depend on the amplifier class and power supply. A Class A amp will draw nearly full power regardless of output power, for example.
What do you think the odds are that the OP has a class A amp?  ::)
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If your amp draws 9.5A @ 1/8 power and that draw was linear, then it would draw 76A @ full power @ 8 Ohms, and over 300A @2. That's one mighty big AC circuit that your amp would need...if input power was linear to output power.
That was not the OP's question.

The 9.5A was for 4 ohm 1/8th power.   8 ohm 1/8th power would be half that current, all else equal. (not exactly 1/2 because power supply sag would be different but sag at 1/8th power should not be significant for either load..

A current draw @ 1/8th (speaker?) power for a class A amp is not a meaningful spec.  Try not to confuse the issue with superfluous information.

JR
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Jeff Bankston

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Re: Converting amps needed based on ohm change
« Reply #6 on: August 03, 2017, 02:43:26 pm »

How would you figure a multi step amp ? I would use an amp meter. But what would be the point of this ?
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John Roberts {JR}

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Re: Converting amps needed based on ohm change
« Reply #7 on: August 03, 2017, 02:59:51 pm »

How would you figure a multi step amp ? I would use an amp meter. But what would be the point of this ?
You are over thinking this... if 1/8 power for 4 ohm is X...  1/8 power for 8 ohm is half X...  finis.

The point of 1/8th power is for sizing mains ampacity since that is closer to typical consumption than full power.

JR
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David Allred

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Re: Converting amps needed based on ohm change
« Reply #8 on: August 03, 2017, 03:58:32 pm »

You are over thinking this... if 1/8 power for 4 ohm is X...  1/8 power for 8 ohm is half X...  finis.

The point of 1/8th power is for sizing mains ampacity since that is closer to typical consumption than full power.

JR

I found another document on the QSC site.  The PLX3102 is listed 8.8A (not 9.5) @ 4 ohms @1/8th power and 5.7A @ 8 ohms @ 1/8th power.  So not 1/2, but 35% less.
Per QSC... 1/8th power is light clipping, 1/3rd power is very heavy clipping, all using pink noise to simulate heavily compressed program material.
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John Roberts {JR}

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Re: Converting amps needed based on ohm change
« Reply #9 on: August 03, 2017, 05:49:55 pm »

I found another document on the QSC site.  The PLX3102 is listed 8.8A (not 9.5) @ 4 ohms @1/8th power and 5.7A @ 8 ohms @ 1/8th power.  So not 1/2, but 35% less.
Per QSC... 1/8th power is light clipping, 1/3rd power is very heavy clipping, all using pink noise to simulate heavily compressed program material.
OK sorry not that simple...

for probably more info than you want the 3102 is a  "2 tier class H technology" amp. What class H means is that they use different voltage power supply rails inside the amp, so for small voltages they can pull from a low voltage rail and not waste all the heat dissipation from a single higher voltage rail. The 3102 is 2 tier so has 3 different power supply voltages inside.

1/8 power @ 4 ohm is a smaller voltage than 1/8 power at 8 ohm. The higher voltage 8 ohm 1/8 power is probably drawing from the bottom and middle rail voltage so less efficient than the 4 ohm just pulling from the smallest bottom rail. So not exactly 1/2 the current.

A class D amp without the multiple PS rails would be more like the linear 2:1 ratio, but the 3102 is not class D or class A.  ::) Sorry.

JR

PS: Since 1/8th power is an important metric for install amps it is likely that the internal power supply voltages were selected to be most efficient at 1/8 power for typical load impedance. Note: these internal voltages are probably different for most efficient 1/8 sinewave power and 1/8 pink noise power. I'll bet they are tweaked for best noise power.
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Tim McCulloch

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Re: Converting amps needed based on ohm change
« Reply #10 on: August 03, 2017, 09:20:29 pm »

^^^

This from the "I'm not the amplifier guy."  8)

JR, you just illuminated a lightbulb over my head.  Thanks.  I think... :D
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John Roberts {JR}

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Re: Converting amps needed based on ohm change
« Reply #11 on: August 04, 2017, 12:13:27 am »

^^^

This from the "I'm not the amplifier guy."  8)

JR, you just illuminated a lightbulb over my head.  Thanks.  I think... :D
sorry it's probably too late, class D makes multi-tier class H obsolete....    8)

JR

PS: I am not the "speaker" guy, I can find my way around power amps.
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Chris Hindle

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Re: Converting amps needed based on ohm change
« Reply #12 on: August 04, 2017, 08:18:26 am »


PS: I am not the "speaker" guy, I can find my way around power amps.

Nicely worded, for the understatement of the year......  ;)
Chris.
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Tim McCulloch

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Re: Converting amps needed based on ohm change
« Reply #13 on: August 04, 2017, 10:23:37 am »

sorry it's probably too late, class D makes multi-tier class H obsolete....    8)

JR

PS: I am not the "speaker" guy, I can find my way around power amps.

I'm not getting design ideas, John.  It's the way you sussed out the approximate points where the power supply rails switch that helped me with an unrelated trouble shooting problem.  Kind of  a liberal arts education thing - learning allows interpolation.
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David Allred

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Re: Converting amps needed based on ohm change
« Reply #14 on: August 04, 2017, 11:46:57 am »

sorry it's probably too late, class D makes multi-tier class H obsolete....    8)

JR


Are you putting down my amps, like they are obsolete and that I might not should have made a recent purchase of them?   :P :'(
I have had them for many years.   Relatively new product at the time.  Just in the middle of a major reconfig (digital mixer) and seeing if 3 amps on a 15A circuit is doable.
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John Roberts {JR}

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Re: Converting amps needed based on ohm change
« Reply #15 on: August 04, 2017, 02:11:39 pm »

Are you putting down my amps, like they are obsolete and that I might not should have made a recent purchase of them?   :P :'(
I have had them for many years.   Relatively new product at the time.  Just in the middle of a major reconfig (digital mixer) and seeing if 3 amps on a 15A circuit is doable.

Back last century when class D was expensive "gee whiz" technology, it was mostly a premium expensive solution.  Now that the technology has caught up and become cost effective it does IMO obsolete the old ways. From a purely engineering perspective a multiple tier class H or class G amplifier (similar but different) involves more complexity with additional points of failure than modern class D, so arguably more expensive to sell and less reliable.

That said do not throw away your amps, but if somebody offers you top dollar for them, don't chase them away.  8)

Back when I was product manager over all Peavey power amps I did some bench comparisons between class D and multi-rail designs. And the efficiency benefits to class D was not a much as touted. The class D technology from decades ago had idle energy dissipation that diminished the efficiency benefit for modest power output.

Now I see cost effective class D being a win-win-win... simpler, cheaper, and more efficient. I am mainly angry that I didn't have today's class D technology to sell back last century.  >:(

JR
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