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What is bridging an Amp?

Discussion in 'Amps and Cabs [BG]' started by reedo35, Jul 30, 2000.


  1. I hate to sound like an ignoramus, but how exactly do you bridge an Amp? And what does it do for your sound?
     
  2. embellisher

    embellisher Holy Ghost filled Bass Player Staff Member Supporting Member

    Bridging is using both channels of a 2 channel power amp or bass head to drive a cabinet(or cabinets) full range..

    The main reasons you would want to do this is say, you have a 2x400 watt(at 4 ohms) head and only 1 8 ohm cabinet. You can either drive the cab with 1 of the 200 watt channels, but it will only put out around 100 - 140 watts at 8 ohms.. But, if you bridge the amp, it will put out 400 watts at 8 ohms(the total output of the amp at a higher impedance)..

    Thus, if you bridge in this situation, you get almost 4 times as much power in your cab as you would get running it from 1 channel..

    For a better explanation of the ohms thing, ask PsychoBassGuy or Joris, but, put simply, a lower impedance, ie, 4 ohm cabinet will allow the amp to put out more power than a higher impedance, ie, 8 ohm cabinet, and more power does equal more volume..

    BTW, with the exception of SWR, most popular bass heads cannot be bridged at 2 ohms, so to bridge you would need 2x8 ohm or 1x4 ohm cabinet..

    ------------------
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  3. :Thanks for the info. I have what I consider to be two pretty good bass rigs,a Walter woods with a Carvin 2x10 for Acoustic and
    an SWR head with a Goliath lll 4x10 for Electric. I never tried the bridge mode on either of them, and I was just wondering if I wasn't utilizing the full potential of the amps.


    [This message has been edited by reedo35 (edited July 30, 2000).]
     
  4. reel big bassist

    reel big bassist

    Mar 27, 2000
    Maryland
    So bridging is using both channels of a 2 channel power amp or head to drive cabinet(s) at full range. I'm confused on how you would drive only one cabinet.

    Most cabinets I've seen have only one place for the 1/4' plug that goes from the cabinet to the poweramp/head.

    On a 2 channel head/poweramp their would be a place on each channel to plug speakers in,correct. So how would you plug both channels into 1 cabinet? Would you use a y cable?

    Thanks,
    Greg
     
  5. Mo' bass

    Mo' bass

    May 4, 2000
    Netherlands
    <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by reel big bassist:
    So how would you plug both channels into 1 cabinet? Would you use a y cable?
    <HR></BLOCKQUOTE>

    NEVER USE A Y-CABLE TO CONNECT TWO AMP CHANNELS TO ONE CABINET, NEVER!!!!!!!!!!!!!!!

    This can (and WILL) damage you amp! If the amp can be bridged there will be a switch on it to select between stereo or bridged mode. For stereo mode you use both speaker outputs, for bridged mode only ONE output. Some amps have a seperate bridge output, then you have to use that.

     
  6. Thank you for asking that question. I have often wondered about that...


    [​IMG]
     
  7. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    Additional questions for our tech-experts please:

    1. If the specs of a stereo amp say 2x350W at 4 Ohms, does this mean I have 4Ohms on each side, so that I can use two 4Ohm cabs?

    2. What will be the minimum load and max. power output in bridged mode? 700W@2Ohms, @4Ohms or @8Ohms? And what's the reason for this?

    Thanks,
    Matthias
     
  8. Mo' bass

    Mo' bass

    May 4, 2000
    Netherlands
    <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Matthias:
    1. If the specs of a stereo amp say 2x350W at 4 Ohms, does this mean I have 4Ohms on each side, so that I can use two 4Ohm cabs?

    2. What will be the minimum load and max. power output in bridged mode? 700W@2Ohms, @4Ohms or @8Ohms? And what's the reason for this?
    <HR></BLOCKQUOTE>

    1. Yes, you can use two 4Ohm cabs, one on each side of the amp.

    2. 850W @ 4Ohms. I'm not sure why, but it is. Check out the specs of the SWR stereo 800 power amp. (2*350W @ 4Ohm, 700W @ 8Ohm, 850W @ 4Ohm)

    [This message has been edited by Mo' bass (edited July 31, 2000).]
     
  9. Rockinjc

    Rockinjc

    Dec 17, 1999
    Michigan
    Mo,

    that looks backwards!

    jc
     
  10. Here's some of the science and math behind bridging an amp:

    A standard solid-state amp usually has a power supply that is +/- with respect to ground. For this example, we'll use plus and minus 40V, a total swing of 80V possible. During normal operation, one leg of the speaker is connected to ground (common), and the other is connected to the output transistors. The output transistors put out a voltage that is between the "top" voltage (+40 V) and the "bottom voltage" (-40V), depending on the instantaneous value of the input signal, they pull the output voltage up or down. So the most voltage the speaker can see across it is magnitude 40 volts, either positive or negative.

    When you bridge a stereo (2-channel) amp, you hook one side of the speaker voice-coil to the transistor output of one channel, and the other side of the speaker voice coil to the transistor output of the other channel. Special circuitry then drives one channel 180 degrees out of phase with the other, so when one output is going up towards +40V, the other output is going down towards -40V. This allows all 80 volts of the power supply to be used as the voltage swing for the speaker.

    But the output transistors have a current limit, and this current limit still holds true even in bridging. So even though the voltage has doubled, the current remains the same (Wasn't that a Led Zeppelin movie?) So Power, which is voltage times current, is doubled, since the voltage across the speaker is doubled. But to have the same current at 2x the voltage, the impedance has to double as well.

    On the amp above,

    40volts peak is equal to 28.28 volts RMS, so If the amp is rated for 4 Ohms per side, then
    Vsquared / R = power &gt;&gt;&gt; 800/4=200Watts and Current=7.07Amps RMS

    So now doubling the voltage to the speaker, 28.28 * 2 = 56.56 V RMS, times the same 7.07 amps, gives 400 Watts, as expected. V=I*R, so if V=56.56, and I=7.07, then R has to equal 8 Ohms. If you have a single 4 Ohm speaker bridged from ch1 positive to ch2 positive, the amp may fry, since it will try to put out too much current. Over-current is one of the main things that kills transistors.

    It makes sense, since you are electrically "stacking" the two channels of the amp, then you need to elecrically "stack" the 2 speakers they each want to see. 4 ohms + 4 ohms = 8 ohms. 200 Watts + 200 Watts = 400 Watts.

    This is just a general explanation, specific cases are slightly different, depending on the amp in question.

    Chris

     
  11. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    Wow!
    One CAN learn a lot here!!
    Thank you very much! [​IMG]

    Matthias
     
  12. Mo' bass

    Mo' bass

    May 4, 2000
    Netherlands
    [​IMG] [​IMG]

    Thanx for the input guys. Sorry if I gave misleading information. I did not know that SWR amps can be bridged in a special way. [​IMG] Now I do....

    It's a good thing Psycho is back [​IMG]