Need Help with this Circuit Please

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  • tfs2015
    Senior Member
    • Feb 2015
    • 135
    • USA

    #1

    Need Help with this Circuit Please

    Dear Forum members,

    I hope that you can help me out with this amp circuit, I cannot get it to work correctly even on +- 12 vdc supply. I end up with 10vdc between the speaker output and ground.

    Thanks
    Tom
    Attached Files
    Computer and Small electronics troubleshooting and repair
  • stj
    Great Sage 齊天大聖
    • Dec 2009
    • 31009
    • Albion

    #2
    Re: Need Help with this Circuit Please

    i'm not surprised, it has no bias ajustment or anything.
    it looks like a classroom example of what not to try to build!

    Comment

    • Khron
      Badcaps Legend
      • Sep 2006
      • 1350
      • Finland

      #3
      Re: Need Help with this Circuit Please

      Might i suggest taking a look at some of Rod Elliott's projects? As a starting point, i'd like to point you towards his Project 3A - http://sound.westhost.com/project3a.htm

      He has strived to make his designs as simple and forgiving (if not even bulletproof), using mostly "jellybean" parts, so they should make a decent starting point
      Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

      Comment

      • SteveNielsen
        Retired Tech
        • Jun 2012
        • 2327
        • USA

        #4
        Re: Need Help with this Circuit Please

        Originally posted by tfs2015
        Dear Forum members,

        I hope that you can help me out with this amp circuit, I cannot get it to work correctly even on +- 12 vdc supply. I end up with 10vdc between the speaker output and ground.

        Thanks
        Tom
        It's no wonder, there is no output DC decoupling. At least put a cap in series with the speaker before you burn out the voice coil.

        Comment

        • Khron
          Badcaps Legend
          • Sep 2006
          • 1350
          • Finland

          #5
          Re: Need Help with this Circuit Please

          At least in theory, it shouldn't need any, since at least on the schematic, it has +/-45v supplies, so...
          Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

          Comment

          • SteveNielsen
            Retired Tech
            • Jun 2012
            • 2327
            • USA

            #6
            Re: Need Help with this Circuit Please

            Yes, if the circuit was really balanced but it doesn't look like it at a glance and the circuit is not drawn very well. It's not possible to tell how the transistors are biased without knowing what they are. One could look them up and redraw the diagram but I'm not gonna do it.

            Comment

            • budm
              Badcaps Legend
              • Feb 2010
              • 40746
              • USA

              #7
              Re: Need Help with this Circuit Please

              Originally posted by Khron
              Might i suggest taking a look at some of Rod Elliott's projects? As a starting point, i'd like to point you towards his Project 3A - http://sound.westhost.com/project3a.htm

              He has strived to make his designs as simple and forgiving (if not even bulletproof), using mostly "jellybean" parts, so they should make a decent starting point
              +1, great site for learning also.
              Never stop learning
              Basic LCD TV and Monitor troubleshooting guides.
              http://www.badcaps.net/forum/showthr...956#post305956

              Voltage Regulator (LDO) testing:
              http://www.badcaps.net/forum/showthr...999#post300999

              Inverter testing using old CFL:
              http://www.badcaps.net/forum/showthr...er+testing+cfl

              Tear down pictures : Hit the ">" Show Albums and stories" on the left side
              http://s807.photobucket.com/user/budm/library/

              TV Factory reset codes listing:
              http://www.badcaps.net/forum/showthread.php?t=24809

              Comment

              • budm
                Badcaps Legend
                • Feb 2010
                • 40746
                • USA

                #8
                Re: Need Help with this Circuit Please

                Originally posted by tfs2015
                Dear Forum members,

                I hope that you can help me out with this amp circuit, I cannot get it to work correctly even on +- 12 vdc supply. I end up with 10vdc between the speaker output and ground.

                Thanks
                Tom
                Link to the site where you get this diagram from?
                Never stop learning
                Basic LCD TV and Monitor troubleshooting guides.
                http://www.badcaps.net/forum/showthr...956#post305956

                Voltage Regulator (LDO) testing:
                http://www.badcaps.net/forum/showthr...999#post300999

                Inverter testing using old CFL:
                http://www.badcaps.net/forum/showthr...er+testing+cfl

                Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                http://s807.photobucket.com/user/budm/library/

                TV Factory reset codes listing:
                http://www.badcaps.net/forum/showthread.php?t=24809

                Comment

                • SteveNielsen
                  Retired Tech
                  • Jun 2012
                  • 2327
                  • USA

                  #9
                  Re: Need Help with this Circuit Please

                  Here are a couple I found:
                  http://circuitdiagram.net/150w-power-amplifier.html
                  http://www.circuitdiagramworld.com/a...ram_16729.html

                  Comment

                  • budm
                    Badcaps Legend
                    • Feb 2010
                    • 40746
                    • USA

                    #10
                    Re: Need Help with this Circuit Please

                    The reason I want the link is to fiond out how many people or if the Author actually build the circuit.
                    I see the power supply circuit (http://circuitdiagram.net/150w-power-amplifier.html) suggested to use 35V-0-35V and the filter caps are rated at 50VDC (4500uF 50V), @35V, the DCV on the cap will be about 49VDC if the line Voltage stays still.
                    I found this:
                    http://pcb.jspayne.com/150wattamp.html

                    I would also implement speaker protection relay.
                    I.E. http://sound.westhost.com/project33.htm
                    Last edited by budm; 11-17-2015, 11:48 AM.
                    Never stop learning
                    Basic LCD TV and Monitor troubleshooting guides.
                    http://www.badcaps.net/forum/showthr...956#post305956

                    Voltage Regulator (LDO) testing:
                    http://www.badcaps.net/forum/showthr...999#post300999

                    Inverter testing using old CFL:
                    http://www.badcaps.net/forum/showthr...er+testing+cfl

                    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                    http://s807.photobucket.com/user/budm/library/

                    TV Factory reset codes listing:
                    http://www.badcaps.net/forum/showthread.php?t=24809

                    Comment

                    • SteveNielsen
                      Retired Tech
                      • Jun 2012
                      • 2327
                      • USA

                      #11
                      Re: Need Help with this Circuit Please

                      The alternate schem is easier to read:
                      Attached Files

                      Comment

                      • tfs2015
                        Senior Member
                        • Feb 2015
                        • 135
                        • USA

                        #12
                        Re: Need Help with this Circuit Please

                        Originally posted by budm
                        Link to the site where you get this diagram from?
                        I got the schematic from http://electronics-diy.com/electroni...tic.php?id=477

                        I simulated it MultSim and found out that with +- 12vdc power I would get about 10vdc on the + speaker output to ground. If I raised the supply voltage to +- 45vdv the vdc would drop to 950 mv which should not burn out the speaker. When I ordered the components I forgot about the TIP 141 and ended up using a TIP 142 in its place. According to the specs it has 100 watt handling capability where as the TIP 141 has 80 watt. I have a couple of 1200 uF 63v caps laying around, can I use one of those as a decoupling cap on the speaker output? I don't have a lot of money to spend on this project right now so I am hoping I can use what I have on hand.
                        Computer and Small electronics troubleshooting and repair

                        Comment

                        • stj
                          Great Sage 齊天大聖
                          • Dec 2009
                          • 31009
                          • Albion

                          #13
                          Re: Need Help with this Circuit Please

                          you will need a lot of aluminium - and probably a fan.
                          that is going to get real hot.

                          Comment

                          • SteveNielsen
                            Retired Tech
                            • Jun 2012
                            • 2327
                            • USA

                            #14
                            Re: Need Help with this Circuit Please

                            1200uF is a bit much at 8 ohms, that puts the resonant freq center at below 20Hz. Around 20uF at 8 ohms would center close to 1KHz.

                            http://www.calculatoredge.com/enggca...apacitance.htm

                            Comment

                            • tfs2015
                              Senior Member
                              • Feb 2015
                              • 135
                              • USA

                              #15
                              Re: Need Help with this Circuit Please

                              Which is going to get hot, the cap or the transistors? I have a heat sink to mount them on.

                              I need to clarify something: I am trying to rebuild the power amp on my Fender PD250 PA system since the original board is not able to be repaired.

                              The SMPS outputs +/- 52 vdc for powering the amplifier section. I need a circuit that has low component count, can use most of what I have on hand and is cheap and easy to build. The SMPS is already mounted to a very big heat sink and the power amp board also mounts to this heat sink as well. The orginal board had a lot of transistors and diodes on it plus trimmer pots for setting all of the volategs and currents correctly. I want to get away from that so I can just plug the circuit in and go.

                              Any help or recommendations would be greatly apprecaited.

                              Thanks
                              Tom
                              Computer and Small electronics troubleshooting and repair

                              Comment

                              • stj
                                Great Sage 齊天大聖
                                • Dec 2009
                                • 31009
                                • Albion

                                #16
                                Re: Need Help with this Circuit Please

                                the transistors will generate heat, your psu will need to be pretty capable too.

                                Comment

                                • SteveNielsen
                                  Retired Tech
                                  • Jun 2012
                                  • 2327
                                  • USA

                                  #17
                                  Re: Need Help with this Circuit Please

                                  I don't think you're going to have a good time of it going that route. Please explain why the original board cannot be repaired. It might be the best and easiest solution and there are people here willing to help with that. I found a svc manual for the unit.
                                  Attached Files

                                  Comment

                                  • stj
                                    Great Sage 齊天大聖
                                    • Dec 2009
                                    • 31009
                                    • Albion

                                    #18
                                    Re: Need Help with this Circuit Please

                                    the chinese probably sell a pre-built pcb for peanuts.

                                    Comment

                                    • keeney123
                                      Lauren
                                      • Sep 2014
                                      • 2536
                                      • United States

                                      #19
                                      Re: Need Help with this Circuit Please

                                      Here is a link to a course on understanding amplifiers. http://www.learnabout-electronics.or...plifiers50.php
                                      It even has tests to verify your understanding.
                                      Last edited by keeney123; 11-17-2015, 04:34 PM.

                                      Comment

                                      • tfs2015
                                        Senior Member
                                        • Feb 2015
                                        • 135
                                        • USA

                                        #20
                                        Re: Need Help with this Circuit Please

                                        Originally posted by SteveNielsen
                                        I don't think you're going to have a good time of it going that route. Please explain why the original board cannot be repaired. It might be the best and easiest solution and there are people here willing to help with that. I found a svc manual for the unit.
                                        The circuit board traces are badly damaged due to repeated repairs to the board. I called Fender about obtaining a replacement board. They told me that the board is no longer available and that the new systems they make use a digital technology instead of analog. Therefore I am kind of stuck on what to do that is why I reached out for some help. Any help and recommendations would be helpful.
                                        Computer and Small electronics troubleshooting and repair

                                        Comment

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