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Is this true of all switching amps?

Discussion in 'Amps and Cabs [BG]' started by Zoom, Aug 21, 2002.

  1. Zoom


    Mar 31, 2002
    "Bridging involves MUCH higher voltage swings, and you CAN kill yourself if you get across it. It also involves impedance awareness, as bridging effectively cuts the load seen by the amp in half. An 8 ohm cab bridged is seen by the amp as a 4 ohm load. RTFM and understand it".

    I was looking at some past posts, and came upon this one. I know it's regarding QSC's PLX series, but I was wondering if it's true for other (switching) amps like the Stewart World series?

    Is this what I think it is?
    >Hook up a 4 ohm cab to an amp running bridged/mono, and that amp "sees" that cab as being 2 ohms??
    >I was going to order my cab in 4 ohms, now I'm not sure.

    Anyone know for sure?
  2. All partially true.

    It's higher power that causes more danger, not bridging.

    By bridging an amplifier you can only load it with twice the impedance of each channel separately. Their powers are added together, as are their load capabilities. A 2 channel amp with 4 ohms load capability per channel, can drive down to 4+4=8 ohms when bridged. The impedance "seen" by the amplifier doesn't change, just the minimum load capability changes to twice the value for each separate channel.

    I hope this helps
  3. Also, this has nothing to do with whether the power supply is switching or conventional.
  4. Zoom


    Mar 31, 2002
    Thanks for the replies.
    Yeah, I was only concerned with the impedence info from that past post. I didn't realize that fact.

    I'm thinking, my amp, running bridged/mono @ 4 ohms = 1200 watts. I figured I would need a 4 ohm speaker cab to take advantage of that spec.

    So just to clarify >
    I want to use "X brand" (switching or non-switching) amp and intend to run that amp, bridged/mono.
    Wattage spec for that amp is, 1200 into 4 ohms, bridged/mono.
    I need a speaker cab wired @ 8 ohms?
    Cab will receive (the FULL) 1200 watts?

    Excuse my confusion on this one.

    Thanks again.
  5. Ian Hall

    Ian Hall

    May 31, 2002
    Nearly any voltage and current can be dangerous. My AC theory teacher brought in an article that showed 2 or 3 kids a year get killed from licking 9 volt batteries. It depends basically on how good of a connection the juice has to you. The higher voltages are more dangerous because it is easier for the electrons to "jump" to you, but on an amplifier, the voltage level is no more dangerous than plugging a lamp cord in to an outlet.

    Several times at work I've been shocked because some dip*@$& decided to come by and turn on the juice while I was working on an electrical box.(yup, I almost got in a couple of fights over those incidents) Consider this- I've been shocked by 220 several times, and if I was a good enough conductor(which luckily I'm not) the circuit could've run 30 amps through me before it blew. The only thing that has saved me is that I have very dry skin, and when I have been shocked, I was not grounded with any other part of my body.

    The bottom line after my ramble is that ANY amount of voltage and current can be deadly- if the connection is good enough to you. A watch battery could kill you if it had a good enough connection to your heart, and you happen to be unlucky enough to be that sensitive to juice. The only surefire way to keep from getting hurt is to use your brain; don't use cords that have wire showing through the insulation, don't use cracked connectors, etc.

    If the amplifier says "1200 watts bridged at 4 ohms", then use a 4 ohm cab, and your amplifier should output 1200 watts. If you use an 8 ohm cab, you'll probably get something around 600-800 watts, depending on the amp.
  6. Zoom


    Mar 31, 2002
    "If the amplifier says "1200 watts bridged at 4 ohms", then use a 4 ohm cab, and your amplifier should output 1200 watts. If you use an 8 ohm cab, you'll probably get something around 600-800 watts, depending on the amp".

    Yeah inwbcstm,
    That's what I originally thought!


  7. It should be pointed out the voltage swing is 2x higher than non-bridged mode. The higher voltage makes the amp more dangerous from electrical shock.

    This is a direct lift from the QSC PLX manual.
  8. Ian Hall

    Ian Hall

    May 31, 2002
    Very true, bgavin. I was just pointing out that any voltage can be dangerous, to prevent non-techies from being scared away from bridging because of the higher voltage. Instead of three paragraphs I probably could have just said "be careful".

    The statement is fully correct, though, in that doubling the voltage definitely doubles the amps ability to deliver a nasty shock:)
  9. Fuzzbass

    Fuzzbass P5 with overdrive Gold Supporting Member

    OMG! Guess I'd better stop testing my batteries that way... :eek:
  10. Ian Hall

    Ian Hall

    May 31, 2002
    2 or 3 out of the world population, so it is a pretty rare occurence. I thought that the teacher was pulling our legs until he showed us the article- I can't remember which one, but it was from a major publication; if I ever find a copy of it I'll post it.
  11. camoe


    Sep 7, 2001
    Lafayette, CO
    It's not the voltage that hurts you...it's the current. Anything over 50 milli amps and above can kill if you are in the right circumstance (standing in a pool of water, current across the heart, etc.) Iv'e been shocked across the body (literally thrown off a chair) by 2KV and I'm just fine...other than this involuntary twitch that won't go away ;) Actually, I was in a daze for hours afterward. The good thing is that getting shocked by wall outlets really doesn't bother me anymore. And I still play (work really) on high voltage tanks that generate as much as 1 million volts. Electricity doesn't kill people, people getting in the path of electricity kills people :D

  12. geshel


    Oct 2, 2001
    Only because you're not very well grounded at the time :rolleyes: I know that you alluded to this in your post but it to me you weren't clear enough. Wall outlets can very easily kill you.
  13. camoe


    Sep 7, 2001
    Lafayette, CO
    I thougt the humor and sarcasm was self evident. It's just a joke. Next time I'll be more illustrative of my intent.
  14. Ian Hall

    Ian Hall

    May 31, 2002
    Ummmm, the reason voltage is dangerous is because the higher it is, the easier it is for a connection to happen between you and the source. We have some very high current transformers at work that we use for resistance evaporation in our coating sytems. They put out 600 amps at 3 volts. You can lick your fingers and touch the secondary side of the transformer and not be hurt at all.

    If you flip those numbers around, 3 amps at 600 volts, you could be one very crispy critter.
  15. geshel


    Oct 2, 2001
    OK Camoe - thanks. :)

    Well. . . .the transformers don't put out 600 amps unless you "ask" them to by being a good conductor. People are pretty high-impedance (at DC and low AC frequencies, anyway - so frequency plays a part to), so when you get across those 3-volt terminals you're probably drawing about 3uA of current through you. Which means you're dissipating 10 micro-watts of power.

    If, on the other hand, you managed to get some cells in your body to string together and form a conductor that ran from one finger to the other :D, and dropped your resistance to, say, 0.1 Ohm, you'd pull 30A, and burn about 90w of power, which would cook a nice little path through you.

    This is why you can touch a car battery terminals with your hands no problem, but if you get a wrench across them (500A at 12v -> 6kW), watch out (even though you're not the conductor the sparks and heat and physical shock are pretty dangerous).

    With the case of 3A at 600 volts (DC?), you'd (normal person) draw about 0.6mA of current, and dissipate .36W. Not a lot of power (so you probably won't get burned), but from what I understand (I'm basing this on the fact that 600v can kill you - I don't know what exactly affects the heart and how) enough current to disrupt the heart and possibly kill you.

    voltage: ability to move through higher resistance (you). The higher the voltage at a given resistance (people, dry, are around 1 Meg Ohm), the more resulting current flows (I = V/R)

    current: amount of electricity moving through you, doing what electricity does (which can mean disrupting normal electrical signals in the body, nerves mainly (but nerves control muscles))

    power: ability of the total shock (voltage & current, P = I*V) to do things like cook you or knock you across the room.
  16. My son was playing with a spent disposable camera mom got from work. He asked me if it had film. I opened it to check and pow. the cap for the flash got me damm I have to be carfull. it didn't even have a battery in it. And I like to fix tube amps and should have known better. BTW the 67 Fender Pro Reverb I recapped sounded great at the gig last night.
  17. Ian Hall

    Ian Hall

    May 31, 2002
    Geshel, you are further supporting(well) the point that I was trying to make- current doesn't mean diddly without voltage to give it the potential to make a connection.

    My whole point was that people are poor conductors, and low voltages are not dangerous to us for that very reason.

    We control the power on the primary side of the transformers with an SCR, and on average they run about 400 amps through 2/0 cable on the secondary to the resistance source inside the coating systems, they max out at 600 amps- I just was using that as an example. Saying that they put out 600 amps was assuming that they are provided with power to do so. No biggie.:)
  18. All this talk of transformers and shocks! hehe. I often use transformers to step down 347/600 to 208/240 for shows. Let me tell you, a 400A service at 600V could do you some damage! 600V is downright scary anyway. I see some of the old school electricians working around this stuff like it's nothing at all. I worked with one guy who would test 600V by putting his fingers very close to the hot lugs. He said that if the hair on the back of his neck stood up, it was live!

    To reinforce what's been said: you probably would not get hurt by touching a service with a 400A capacity at 3V, but 400A at 600V would almost certainly do you some serious damage! Higher potential enables more current to flow through a given resistance.

    50mA across the heart muscle is all it takes to induce arrythmia (spelling?).
  19. redjeep!

    redjeep! Guest

    Jan 19, 2002
    I work in a similar environment. But we use a procedure called Lock Out Tag Out, whenever anybody is doing anything like this .

    YOU NEED TO INSIST ON THE SAME THING - It will probably save your life.

    This procedure padlocks the power OFF to any device that you are working on so that nobody but you can switch it back on. Anybody not following this can be dismissed on the spot.

    PM me if you'd like a copy of the procedure.

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