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Voltage gain + Bridging

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  • Voltage gain + Bridging

    Does voltage gain differ by 6 dB when bridged vs single channel or does input sensitivity change to compensate? Reason I ask, I have a 3way system with the amps having 38dB voltage gain but the lows are bridged and wondered about setting the correct processor settings.

  • #2
    Does voltage gain differ by 6 dB
    Yep

    Comment


    • #3
      Everything I know of would require to back down the input sensitivity knobs by 6 dB to compensate for the additional 6 dB gain you pick up when you bridge.
      Don Boomer

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      • #4
        Thats what i was thinking.... Im not an EE but know enough of the math to strike a balance between useful and dangerous. Another question... if you lower your input 6db, would the amplifiers still track the same? It would seem that youd lose that 6 dB of headroom (which would seem to defeat the purpose of bridging in the first place) if the processors setting stay the same.

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        • #5
          That's not the "advantage" of bridging. That's also not how it works.

          The voltage gain is 6dB higher so yes you would wantt o reduce the input sensitivity by 6dB. The halves of the amp will automatically track.

          Be sure you understand REAL power handling before you assume that bridging is the best solution for your rig. I would say the 90% of the time it's not a good choice.
          -------------------------------------------------------------------------------------------------------------------------------------------------------------------
          Former product development engineer: Genz Benz, a KMC Music/Fender Musical Instruments Company, continuing factory level product support and service for Genz Benz

          Currently product development engineer: Mesa Boogie

          Comment


          • #6
            Old post I know butan old QSC amp got me wondering about this again... The amp is 1600W @4ohms and 4000W bridged @4ohms. That's 80V and 126.5V respectively. Thats a difference of about 4dB. I was wondering, how we all arrived at the 6dB of additional gain from bridging? Maybe I'm missing something or confused myself. Another question I have is, does load impedance change your input sensitivity? The spec sheet says "Input Sensitivity @ 8 ohms 1.06Vrms". I dont see how it would matter to the sensitivity if it was 4, 8, or 16ohm loads.

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            • #7
              You have failed to take into account the power supply loss at the increased load. The voltage gain increase is 6dB, but the MOL increases by 4dB because of power supply limitations. Gain and MOL are two different and unrelated variables.

              The input sensitivity (for rated output) will be a little bit different at different impedances, again because of power supply losses as well as slight nonlinear voltage gain transfer function versus load current the global feedback can not make up for 100%.
              -------------------------------------------------------------------------------------------------------------------------------------------------------------------
              Former product development engineer: Genz Benz, a KMC Music/Fender Musical Instruments Company, continuing factory level product support and service for Genz Benz

              Currently product development engineer: Mesa Boogie

              Comment


              • #8
                I figured supply losses had something to do with it but I was wondering what the math was. I forgot how I arrived at 6dB, to begin with, that inspired the original question

                Comment


                • #9
                  Most amps that I have seen require less input voltage to reach full output as you go from 8 to 4 to 2 ohms.

                  Power amps in general don't give twice the wattage every time you 1/2 your load.
                  If the amp is giving 100 watts at 8 ohms it won't in general give 200 at 4 ohms or 400 at 2 ohms. That is why you only see a 4 db gain on the bridged output.

                  Just know it takes less voltage to drive an amp to full rated power as you go from 8 to 4 to 2 ohms.

                  As an example. If you look at the below link on page 17.
                  The amp takes 1.54 volts for 4 ohm rated power, 1.27 volts for 2 ohm rated power.

                  http://www.peavey.com/assets/literature/manuals/80304679.pdf

                  Only guessing but you may find the amp gives full 4 ohm bridged output at the 2 ohm input voltage. I had these amps for some time and I found this to be very , very close.

                  Dookietwo

                  Comment


                  • #10
                    You have failed to take into account the power supply loss at the increased load. The voltage gain increase is 6dB, but the MOL increases by 4dB because of power supply limitations. Gain and MOL are two different and unrelated variables.

                    The input sensitivity (for rated output) will be a little bit different at different impedances, again because of power supply losses as well as slight nonlinear voltage gain transfer function versus load current the global feedback can not make up for 100%.


                    Sorry. You beat me too it!

                    Doug

                    Comment


                    • #11
                      Im not worried about the power. I'm trying to make sense of the numbers involving the actual voltage gain. How does one arrive at the value of input sensitivity at a given impedance? I know the formulas for gain, 20log(Vout/Vin), and 10^x(dB/20)*Vin, for voltage out

                      Comment


                      • #12
                        20 Log is for voltage, 10 Log is for power. The difference accounts for the squared factor in the power formula which is represented by a "2" in the log.

                        If you have an amp that swings 100V RMS output with a 1V RMS input, the voltage gain is 20Log(100/1) and the power gain is 10Log(Pout/Pin) but power gain in this fashon has little bearing. Generally we talk about RATIOS of output power. The power is calculated from voltage and impedance, you need to know the INPUT impedance of the amp to come up with a real number for power gain. which is not used much (any more) in audio. This is essentially a voltage to power conversion these days with "bridging" (NOT the same a s bridging an amp) inputs.
                        -------------------------------------------------------------------------------------------------------------------------------------------------------------------
                        Former product development engineer: Genz Benz, a KMC Music/Fender Musical Instruments Company, continuing factory level product support and service for Genz Benz

                        Currently product development engineer: Mesa Boogie

                        Comment


                        • #13
                          so this amp should swing 160V and 6400W if the supply could actually provide the power?

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                          • #14
                            Yes, under no load (or 16 ohms if your initial measurement was based on 8 ohms per channel) that's what you should expect.
                            -------------------------------------------------------------------------------------------------------------------------------------------------------------------
                            Former product development engineer: Genz Benz, a KMC Music/Fender Musical Instruments Company, continuing factory level product support and service for Genz Benz

                            Currently product development engineer: Mesa Boogie

                            Comment


                            • #15
                              thanks! So to calculate the sensitivity for a bridged 4 ohm load, i'd take 126.49V/158X voltage gain (44dB) for .8V? However, by attenuating the processor output by 6dB to match the gain of the tops, Id calculate with 80X voltage gain for a sensitivity of 1.58V right?

                              Reason I ask is for setting the proper limiter setting as that would put the low amps into clip at 0dBu or 6dBu and wanted to make sure which was was correct.

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