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Matching preamp with amp for 2 channel setup

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  • Matching preamp with amp for 2 channel setup

    As the title states, looking for criteria on how best to match amp/amps(monos) with a preamp. Have researched this a bit in the past, and read some saying matching output voltage of a preamp to the input sensitivity of the power amp is the way to do it. Also saw where it was mentioned that if an amp has high enough voltage gain, then matching the input voltage to the output voltage of a preamp is not all that important. There may have been other factors brought up, but can't remember them now.

    To sum up the research never provided me with a 'warm/fuzzy" answer. Guess I am looking for some comments on whether I understand the concepts used to match a preamp with a power amp. Maybe the best way to get meaningful feedback is to just explain the way I understand it, then have others comment:

    My understanding of output voltage and input sensitivity:

    Output voltage of a preamp is the voltage at which the signal leaves the line outs of a preamp. Some preamps will increase the voltage on the signal given to them (active), and others will not (passive). Input sensitivity for a power amp is the input voltage that will drive it to its maximum rated power (measured at a given speaker impedance).

    Since I have a passive preamp, lets use that as an example, the dac feeding it is listed as having a 2v output. Using this value as the input voltage on the signal to the passive preamp, the max output of the preamp will be 2v, and each step down from max of the gain/volume knobs on the premp will reduce the output voltage. So the preamp should be able to output 0v -2v to the input of the power amps.

    Lets say the power amps have a voltage gain of 34, and an input sensitivity of 1.44 at an 8 ohm resistance. Then the passive preamp at about 72% of the max gain, will provide the 1.44v needed to allow the amps to reach their max power rating at 8 ohm resistance.

    Would also like to verify my understanding on how exactly to measure input sensitivity of a power amp...but will wait until seeing some feedback on the statements above. Also, if there are other important factors to consider when matching a preamp to a power amp, please feel free to chime in on them. Links to helpful information is also greatly appreciated.



  • #2
    Hope this reply is still relevant after 7 months John.

    Most matching isn't needed between components unless you are going from unbalanced to balanced or pro audio +4dbm standard to consumer -10dbm standard.

    In striving for perfection ... small improvements can still be had in s/n ratio, or drive for maximum output between same impedance/level class devices though.

    You might look here for some examples

    Good luck
    :group: Long live draft beer and good old rock 'n roll :grinning:


    • #3
      The input sensitivity of the amp is usually stated as the input voltage needed for full power output into a specific load resistance. So in your example it takes 1.44Vrms to drive the amp to full rated power into 8 ohms.

      If your amp has 34dB of voltage gain with a 1.44Vrms input, the output voltage is 72.2Vrms into 8 ohms. Using Power = V^2/R, this would be 651 watts of output into the 8 ohm load.

      One thing about passive preamps is that the output attenuation changes with output load impedance. So that has to be taken into account as part of the gain losses from input to output into the power amp.

      Hope this helps.
      Better living through Audio Nirvana!


      • #4
        Taking HAL's equation, you can figure out the input sensitivity of an amp by going from its rated power output and voltage gain back to the input voltage.

        You usually don't need to try and match the input sensitivity of your amp to your pre-amp. In pretty much every single case, your amp's gain is going to be such that feeding it a full 2Vrms input will be past its "low distortion" power rating. In other words, if you do feed it 2Vrms and it is able to push out that power, it is probably doing so at much higher distortion levels than compared to its operation at lower voltages.

        It tends to be more important to match your amp with your speakers, since some speakers are a harder load to drive.
        Wes Miaw, Neko Audio LLC


        • #5
          There are some cases in which a preamp's output can be a bit too high. I know the Melody P1688 Signature has both high and low gain outputs to combat this. I believe it's not the only one, but I have no examples off the top of my head.
          Angel City Audio
          East Street Audio

          ACA, Melody, Onix, NuForce, KR Audio


          • #6
            Thanks for the replies guys. Even though the thread is 7 months old, the information in each of your posts helped clear things up for me.