Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mosfet amplifier upgrade

  1. Aug 16, 2015 #1
    I have an old mosfet amplifier which I plan to upgrade, I have never done this before but I am very experienced in electronics and have built many amplifiers before, just not mosfet amps. I am not 100% sure how the n and p channel mosfets are biased or set up but one upgrade I want to do with this amp is increase the power supply from +-24v to +-36v. So given that all components can handle this voltage and extra current which may come as an affect, will increasing the voltage screw up anything within the capacitor resistor and transistor setup circuit that comes before the mosfets themselves?(which I am not Familiar with). Or will it be an easy upgrade?
  2. jcsd
  3. Aug 16, 2015 #2


    User Avatar
    Science Advisor

    Why increase the voltage of the earlier stages? Only the output stage needs greater voltage to handle greater power. Duplicating the output stage and running the amps in a bridge configuration will double the effective output voltage without changing the supply voltage.

    Your question has no answer without a circuit diagram of the existing amplifier.
  4. Aug 16, 2015 #3
    I know it may be a general question and i understand that to increase voltage i can simply bridge but my objective is to increase the power transformer to a 36v output, and add parralel/higher current mosfets to allow for 2ohm stability with an increased voltage output, thus turning a 200 watt amplifier into around a 1000 watt.

    From what i can see, because i do not have an exact circuit diagram, it seems to be a pretty basic 100watt mosfet amplifier such as the ones you can find on circuitstoday.com. although there is a second channel which is why i say 200watts.

    So with minimal information lets ask, if i were to take a regular 100 watt mosfet amplifier circuit you can find online, useing a +-24v supply, if i know for a fact that all components can accept +-36v, then will making this increase cause the circuit (for whatever reason) to not work?
  5. Aug 16, 2015 #4


    User Avatar
    Science Advisor
    Gold Member

    I wouldn't do this. You will need extra heatsink, etc. Too much trouble, but this is just my opinion and of course you know what you paid for this advice. :) Also, plus and minus 36 volt supply rails won't get you to 1000 watts into 2 ohms unless you have a bridgeable 2 channel amp that you are doing this to.
  6. Aug 16, 2015 #5


    User Avatar
    Science Advisor

    Just build a new 1kW booster amplifier, then drive it with the old one.
  7. Aug 16, 2015 #6
    Dont worry :) i got that covered!
    I am hand assembling longer and taller heatsinks in order to overcome the extra heat, plus every component will be mounted to the chassy which has fins, and i am adding in 2 3cm fans, and in my experience even small fans like that can make a crazy difference. And yes it is a 2 channel bridgable amp but to keep things simple i have been talking about one channel.

    So my question is about the circuitry, there are bipolar transistors and resistors which come before the power mosfets in order to "set them up for the audio"
    Will increasing the voltage screw any of this up?
    Or will it work juat fine but on a higher rail voltage

    Attached Files:

  8. Aug 16, 2015 #7
    Ok i have discovered that the op amps are running off of a split supply composed of half transfomer output, so on a +-24 v system the preamp chips are getting +- 12 and they can only go to 18.
    So i am switching out those with the lm883 which can get to 30v.
    Also their coupling cap is only 16v so gotto bump that up too.

    What are the large resistors places on the n channel mosfets drain pin for?
    They are only like .1 to .3 ohm so what is their use?
  9. Aug 16, 2015 #8


    User Avatar
    Science Advisor

    Without seeing the exact circuit I can only guess but it's likely those are current equalization resistors. With several devices in parallel the device with the lowest ON resistance will carry the highest current so these resistors are selected to balance that current by making each leg close to equal without too much power dissipation (~1-2W) with a voltage drop of around 50 to 100mV on each leg at close to full power.
  10. Aug 16, 2015 #9
    Thats ingenious! I feel bad for not realizing myself lol.

    So unning a 36v supply, by the time i start clipping it should be no more than maybe 32v, but lets jave headroom here....

    36v into 2 ohms plus the .2 from the resistors is 16.36 amps, now divide that by two because i am adding in parralel mosfets to share the high current and we get 8.18amps at 36v.

    8.18 times the .2ohms and we see a voltage drop of 1.62v and a heat disapation that 10watt resistors should hold up to Nicely.
    Does that seem right?
    How neccesary are these?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook