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

Need help with a 12 watt Audio amplifier

  1. May 18, 2009 #1
    I have recently built a 12 watt amplifier spec'd by G. Randy Sloan.
    The problem I am having is many fold.

    1.) I am not an EE and learning as I am going

    2.) The amp WILL carry a signal but so far I have only been able to use it as a passive circuit.

    3.)The output leads don't provide an output but if I banana clip my speaker leads to the transistor heat sinks then it carries a signal!?!?

    4.) The power supply I have built for it may be built incorrectly. It is a 24 volt 2 amp transformer going to a 2 amp bridge rectifier which then goes to a 1000uF electrolytic capacitor. It is meant to be a mono power supply applying approx 38 volts to the amp.
    The transformer has two leands on one side which is going to the ac outlet and three on the other, The far left and far right are then attached directly to the ac leads on the recifier. What do I do with the center tap? Is this a ground?

    4.) Im not 100% sure how to test the amplifier circuit but I have checked all the resistances with an ohm meter and it seems to work out.

    Below are two pics, on rather crappy one of the schematic and one of the actual amp.
    Thank you in advance for any help


  2. jcsd
  3. May 18, 2009 #2
    It could be a nightmare troubleshooting this over the forums, so many things could go wrong. You would have to systematically check every part of the circuit to be sure, but maybe we can narrow the search down.

    I'd suggest checking the voltage across the base-emitter of the two output transistors, just to see if there is a 0.7 voltage drop or not. It's also possible the leads on the transistors are mixed up.

    It's hard to tell from the schematics, there looks to be a PNP and NPN for the output transistors
  4. May 18, 2009 #3
    The transformer is fine. The center tap is used with a full-wave center tap rectifier. With your voltmeter, make sure the positive dc voltage goes to the top of your schematic, and the negative to the bottom. Your schematic is too small to read and analyze, but I see 4 PNP's and 3 NPN's. Make sure you have the base, emitter, and collector properly selected on all 7 transistors.
  5. May 18, 2009 #4


    User Avatar
    Science Advisor

    I'd have to agree with Waht on this one.

    There shouldn't be any audio on the heatsinks because there should be insulating washers between the transistor bodies and the heatsinks. They might be little rectangles of mica or some other insulator.

    If you have a multimeter, make a chart of voltages (relative to the negative rail), redo the picture of the schematic so it can be read and put the list and the new photo on this Forum.

    That schematic is pretty bad. Get some more light on it and focus it and hold the camera steady on a tripod. And don't shrink it so much.

    Measure the resistance from the junction of the two output transistors (on the heatsinks) to the large capacitor that leads to the speaker and then from the capacitor to the speaker. There seems to be a fault just there, somewhere.

    Check that you have the right transistors in the right positions on the board, particularly the transistors on the heatsinks.
  6. Nov 6, 2011 #5
    I am sorry to bring such an old tread back from the dead. I am building the same amp and also having trouble. I have a schematic but cant post it now, as im writting this on a tablet. I have finished this amp, and the only output I get from the 4 ohm speaker is a constant clipping with no music. When there is no source pluged in it sounds lime static with a pulsing noise in the background. I have swaped out all the transistors for new ones, in case there was a bad part, with no results. I am using the exact same power supply as the op. This amp is based off a tip32c and tip31c power transistor. The rest of the transistors are small signal amplifers for the first stages. Any advice would be greatly appreciated.
  7. Nov 6, 2011 #6
    Well, the unreadable schematic attached to this thread is useless, so, without seeing your circuit, we can't tell you much of anything. Is the schematic you're using available online anywhere?
  8. Nov 6, 2011 #7
    Here is the schematic scanned out of the same book that the first picture was taken of. It's a simple circuit, well it is supposed to be... haha. However it has been a while since my electronics engineering courses. I have found that a few of the resistors I am using have a loose tolerance (ie. 22k, is 21). I'm not sure how tight these need to be.

    Attached Files:

  9. Nov 7, 2011 #8


    User Avatar
    Science Advisor

    The obvious thing missing from that circuit is decoupling. This would be why you are getting that pulsing effect. It is sometimes called "motor-boating".

    If you feel experimental, you could open the wire between the tops of R5 and R8 and put in a 470 ohm resistor between them. Then put a 22 uF capacitor between the top of R5 and ground.
    I'm just guessing the values, so be prepared to alter them. If it improves, but doesn't fix the effect, make the capacitor bigger.
  10. Nov 7, 2011 #9


    User Avatar
    Science Advisor
    Homework Helper

    I suppose the original was meant to be used with a VERY low impedance power supply, not something knocked together from a full wave bridge rectifier and a 1000uF smoothing capacitor. If your reference has a section on power supplies, try doing what it says. Otherwise, try what vk6ro said.

    Or your could find an amp design by somebody who doesn't have such strong opinions on why everybody else is doing it wrong...
  11. Nov 7, 2011 #10
    Hello folks, sorry that I haven't updated anybody on the situation. I had a little bit of a melt down when I shorted out the circuit while testing voltages. I have had to start from scratch and test all my components to see which ones survived. I am working with several forums, most posters have mentioned the "decoupling" situation. However somebody with access to good software was able to build and simulate this circuit exactly as it is in the schematic; and it worked. It is an unorthodox design from what I've gathered. The book calls for a raw 24VAC 2A power supply with a 1000uF @ 50V cap for smoothing. However I have a 4700uF @ 50V cap and will try that after I'm done picking up all the pieces.
  12. Nov 7, 2011 #11


    User Avatar
    Science Advisor

    Decoupling is like this:

    [PLAIN]http://dl.dropbox.com/u/4222062/amplifier.PNG [Broken]

    I enclosed the proposed components in a red box.

    The idea is that with an unregulated supply, there will be some variation in supply voltage due to the supply, the fuse and the assorted supply wires.

    Without decoupling, the variation can be applied to the sensitive preamp stages and produce feedback effects.

    Because the preamp draws relatively little current, it is easy to filter out this variation in the preamp's supply line rather than try to filter it out for the whole amplifier.
    Last edited by a moderator: May 5, 2017
  13. Nov 7, 2011 #12
    Success!! I have finally been able to get this working. I used the 4700uF cap directly on the breadboard, and cleaned up the circuit after testing all the components. This amp doesn't sound all that great for all the trouble it's caused me, but I am happy to say it works haha.

    Now I have some MJ15003 and MJ15004 power transistors waiting for my next project :biggrin: However I don't feel like frying those, so I'll save that for another day.

    I've attached a picture of the circuit below, I'm aware that I'll have to put the heat sinks onto the power transistors; but just lost my patience. Also the diodes should be attached to the power transistors for thermal stability, according to the book.

    Thank you for everybody's help and suggestions. @vk6kro thanks for the mod, I will try that tomorrow and see how it sounds, for now I think I'm putting this one to rest though. haha

    Attached Files:

    Last edited: Nov 7, 2011
  14. Nov 8, 2011 #13
    You can save yourself a lot of headaches (and destroyed components) by investing in a bench-top power supply. There are also a bunch of online instructions for building one from an old computer power supply. http://hackaday.com/?s=adjustable+power+supply
  15. Nov 8, 2011 #14
    I've got a good power supply, with current limiting but it only goes to 30 v @ 3 amps. I am building one that will goto 50 v and will be adjustable, and is a dual voltage supply.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook