How much current is high current?!

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Eastern
Eastern Posts: 43
edited August 2003 in Electronics
Hi,

I just received a response from Rotel Tech Support regarding Rotel RMB1075 current drive capability.

The Rotel RMB1075 has 4 output device per channel rated at 130W 15A (kind of ambiguous so I asked the question)
Their answer was that each channel has a peak current drive of 24A. Since the 4 devices are Positve negative pairs. So each pair PN can supply a max of 15A. But to be safe, 12A is the limit. Therefore 24A for 2 PN pairs.

Deriving from this, the Rotel RMB1095 is 36A (6 output devices)

I am surprised that this is lower than some denons/HKs rated at 45A

Has anyone done any Math to derive how much current is actually needed? I have seen specs of AMPs out there delivery over 100A.

My thinking is this, for the most demanding 4 ohm speakers out there, the impedance may dip to 1 ohm, And typically the amps output voltage rails at 70V this equals an instantaneous current of 70A and would require 4900 W (70A x 70A x 1ohm) output.

Even at a modest 2 ohms, 35 A is required and would require 1225W peak power is needed.

But realistically, peak power output by real life affordable receivers/amps are like 500W per channel, which means that the current is then limited to 22 Amps per channel.

That means that most of the affordable amplifiers/receivers out there in the market are very far from capable of driving the Speakers to it's full potential ... :eek:
Of course listening at these levels would make you deaf ...
but am I smoking some bad weed or what?

V3Max, maybe this is the reason you are blowing your tweets. And if you like cranking the volume up on your amps, you may need to check your amps current drive/power capabilty out.
Post edited by Eastern on
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  • HBombToo
    HBombToo Posts: 5,256
    edited August 2003
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    Here is the math:

    P=V*I and V=I*R

    Inserting I*R into V of the power equation yields P=I^2R then solve for I yields I=SQRT(P/R).

    I used my outlaw specs at 200W/channel and inserted (2, 6 and 8 Ohms) into the above and calculated (10, 5.77 and 5 Amps) respectively.
    *Outlaw has 1800 Watts max power consumption which is 30 Amps. Looks like 3 times over the requirement at 2 Ohms. Thats a good deal!:)

    Notice @ 8 Ohms the current requirements are at the minimum. Each channel in the respective amp adds together for the max/min power supply requirements.

    1/2 Twin
    ***WAREMTAE***
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  • Eastern
    Eastern Posts: 43
    edited August 2003
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    I am not talking about RMS power, rather peak power, because that's what you get with THD etc. For 4 ohm nominal speakers, the impedance can vary between sub 1 ohm to over 10 ohms dynamically, but with THD, you get a spectrum of Frequency sent from the AMp to the Speaker.

    WHen you have THD, the amp will rail it's output voltage, typically 70V (in some designs may go higher). So if you rail the voltage is 70V over a load of 1 ohm, you get 70A instantaneous current into your speakers. Typically, mid/bass drivers have heavier gauge winding in coils, tweeters have very light windings (less current capability) so if your speaker is rated at 250W nominal, than it can nominally withstand SQRT(250W/4Ohm) = 8 A, so 70A would definitely turn it into a light bulb :D when THD hits.
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  • pjdami
    pjdami Posts: 1,894
    edited August 2003
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    Yes Hbomb is on the money with his calculations. Most house circuits are 15 amp I believe then you trip the breaker. So Bomb's calcs look very reasonable. I always did wonder about the 45 amp instantaneous claims some manufacturers make. Maybe just instantaneous and not enough to trip the breaker??

    PJ
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  • HBombToo
    HBombToo Posts: 5,256
    edited August 2003
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    Eastern, pjdami of course we are talking about peak instanteneous and not average. At those levels the power supply would be about the size of a truck. I worked on a power supply bigger than the above mentioned and when the breakers blew ya **** your pants on the spot! The first time it happens that is:D

    1/2 Twin
    ***WAREMTAE***
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  • Eastern
    Eastern Posts: 43
    edited August 2003
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    That's where the Caps come into play!! To deliver those instantaneous AMPS.

    I accidentally touched one of those 15000 uF / 63V caps and luckily lived. But I tell you, no amount of money can ever get me to touch them again. I can taste bile as I am writing this.
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  • pjdami
    pjdami Posts: 1,894
    edited August 2003
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    Cool. I have seen the lightbulb go off in my head now! :cool:

    PJ
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  • HBombToo
    HBombToo Posts: 5,256
    edited August 2003
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    Originally posted by Eastern
    That's where the Caps come into play!! To deliver those instantaneous AMPS.


    we exploded caps often and decided 0 pcb types were in order.:D

    Pulsed Power and Tubes go hand in hand.

    twin
    ***WAREMTAE***
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  • Eastern
    Eastern Posts: 43
    edited August 2003
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    No Kidding, I was conscious the whole time feeling the current pulsing through my arms. I couldn't move or breathe and thinking to myself what a terrible way to die! Fortunately, I was sitting on a bench and as I lost balance and fall over, my fingers lost contact. I landed on my face, but better that than meeting the maker. And all this occurred with the unit unplugged.

    So to all you folks out there, discharge the caps first! Even unplugging is not enough!
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  • HBombToo
    HBombToo Posts: 5,256
    edited August 2003
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    Originally posted by Eastern
    Even unplugging is not enough!

    short +/- too ground.

    twin
    ***WAREMTAE***
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  • mantis
    mantis Posts: 17,061
    edited August 2003
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    I am surprised that this is lower than some denons/HKs rated at 45A


    Ummmmmmmm no.The mighty Denon avr5803 is rated at 15 amps of current.I believe thats total across all channels.

    Now if you want to talk about current lets talk about B&K's avr307 having 42 amps of driving current............;)

    B&K's ref 200.7 has 75 amps of current........

    The mighty B&K ref 200.1 has 150 amps of bad **** current.

    Even the nice little B&K sr55.2 has 20 amps of current.....nice numbers......but there is something to be said about real world performance.
    Dan
    My personal quest is to save to world of bad audio, one thread at a time.
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  • Eastern
    Eastern Posts: 43
    edited August 2003
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    Actually I advise against shorting to ground ... if the cap is charged up, you are gonna get a fire ball of an arc and the contact point would probably melt. Just unplug the unit and power it up .. provided you disconnect the Speakers. THan watch the LED light up and dim off. And to be safe, check with a multimeter again. If that doesn't do it, hook up to a source and play some music (amp still unplugged resistor in place of speakers)
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  • Tour2ma
    Tour2ma Posts: 10,177
    edited August 2003
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    Originally posted by Eastern
    Just unplug the unit and power it up ...
    :confused:
    That sounds difficult. ;)

    Actually killing the AC with the source on and the speakers still connected will slowly discharge the caps. It's cool, and reassuring, to hear the music spin down...

    Dan,
    Since we're slamming our amperes on the table, I'll throw down my Sunfire's 120 amps (peak to peak) per channel... of course, I only own the 5 ch, not the new 7's... :D
    More later,
    Tour...
    Vox Copuli
    Better to remain silent and be thought a fool, than to open your mouth and remove all doubt. - Old English Proverb

    "Death doesn't come with a Uhaul." - Dennis Gardner
    "It's easy to get lost in price vs performance vs ego vs illusion." - doro
    "There is a certain entertainment value in ripping the occaisonal (sic) buttmunch..." - TroyD
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  • gidrah
    gidrah Posts: 3,049
    edited August 2003
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    I've powered down my amp but the music keeps playing for 15-20 seconds some times. Gotta love those big blue bad **** caps.
    Make it Funky! :)
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