Pre amplifier

Discussion in 'General Electronics Chat' started by justin77, Apr 10, 2010.

  1. justin77

    Thread Starter New Member

    Apr 3, 2010
    19
    0
    How can i use a 50 watt amplifier as a pre amplifier for a another amplifier?

    Can i install a series resistor to reduce the output so that it would act as a pre amp?
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Why would you want to?

    If you want the processing of the 50w amp, check to see if there is a LINE out. This will be a PRE-AMPED output that you can run to another amp for amplification. Start at a low gain on the pre-amp. You dont want to blow the input on the 2nd amp.
     
  3. justin77

    Thread Starter New Member

    Apr 3, 2010
    19
    0
    Nope there isn't a line out. And i forgot to mention that the other amplifier does not have a pre amp. So if i put a 20K resistor in series with the 50Watt output and connect will it harm the other main amp?
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Most likely, yes. You probably shouldnt put more than .25w into the input of the other amp. There are devices that do this for you.. Im guessing thats what you want.. I know for cars there is a device that takes the high watt speaker signal and trims it down to a line signal for amp input.
     
    justin77 likes this.
  5. Bychon

    Member

    Mar 12, 2010
    469
    41
    You need to use at least 2 resistors per channel. One in series and one to ground. You calculate the voltage the amp can put out and calculate resistor values to cut the voltage down to the right level to drive the input of the pre-amp. Also calculate the power the resistors will be required to handle so they don't smoke.

    A 50 watt amp driving 8 ohms will have available more than 28 volts peak. You want to keep the 50 watt amp turned down to a tenth of its maximum power so music peaks don't get distorted. It will still hit 28 volts peak, but 90% of music information is in the bottom 10% of the power. If your pre-amp can handle 1 volt peak input, you calculate to chop 28 volts down to 1 volt. 27k ohms in series, 1 k ohm to ground, and presto...28 volts just became one volt. With these values, common quarter watt resistors will not smoke.

    In addition, you have to consider if the 50 watt amp output doesn't use the normal ground as one side of its output. Connecting a 2 channel amp to a 2 channel pre-amp could blow the whole deal if the ground system isn't right. Now we're talking about transformers to isolate the big amp's 2 channels from hurting each other. That still requires math, but the job changes to finding audio quality transformers with the right ratio and power handling abilities.

    I hope this gets you on the right track. I actually did this job to get an electric piano to drive other amplifiers in a band.
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    I would follow Bychons advice with one added note. Follow what he said in your calculations for the MAX power the 50w amp can provide, and when you THINK your done, check your output on a meter well before pluging it into your amp. Also, Start at the lowest gain level and find a good setting to use as MAX when using this as a pre-amp device.

    If you already have an keyboard or instrument with a line out, measure it with a meter while playing it at a fairly high level and use that as your base line to match for your 50w amp.
     
    justin77 likes this.
Loading...