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  • 2.5ch preamp build

    As I promised casey in his thread here is my preamp build.

    Brief:
    Its a 2.5ch affair where there is a common boost stage for each channel. The clean is pretty much taken from a VH100R as that has a great clean IMO, dirt channel is fairly generic three stage cold clipping and I'll no doubt play around with the values in there quite a lot when I get the thing working. Boost stage has a mid/full boost switch to select the bypass capacitance for lead/rhythm work. The channel and boost are all footswitchable using relays.

    A shout out goes to PaoloJM for helping me with the switching and to olaf (VH4) for donating an output stage. Much appreciated guys. I may be cutting it a little fine with the heater current to run three relays so I may have to get inventive...

    And now, the schematic:



    If thats too small a hi-res version can be found here.

    A few other piccies:


    PT (from ampmaker.com) and the tag board I'm building on


    Chassis is it grey primer glory. This will be sprayed white after its been drilled.

    More stuff to follow. Going to put the order for the last few part in tomorrow and they'll hopefully come within a week or so which should be enough time to motivate me to redo the layout and drill plan!
    DCC Engineering. Custom Cabs and Effects.






    Originally Posted by -Assy-


    Okay I summed up this entire thread in three posts. Let's go home and beat our children.

  • #2
    That is a nice looking schematic and design. Did you design this yourself? It looks like a really cool preamp with the switching.

    Keep us posted as you move along with this.

    Casey4sDIY Links:102 Amphttp://www.guitarkitbuilder.com/cont...-amplifier-kitChassis Layout:http://www.guitarkitbuilder.com/cont...lifier-chassisTurret Board:http://www.guitarkitbuilder.com/cont...d-construction Tolex Tutorial:http://www.guitarkitbuilder.com/cont...lifier-cabinet

    Comment


    • #3
      Yeah, well apart from a little plagiarism and advice
      I bought merlin blencowes book earlier in the year and I learned a hell of a lot from it. I dont think I've seen anyone say a bad word about that book. I'm certainly interested to try the LED biasing out after I've got it working using the resistors/caps. I know its popular with hi-hi builders as you get full gain without the sluggishness of the bypass cap.

      Still disputing what voltage to put in the plates. I'll get 320V or so after rectification and I want 300-310 for the clean channel but havent given the rest much consideration yet.
      DCC Engineering. Custom Cabs and Effects.






      Originally Posted by -Assy-


      Okay I summed up this entire thread in three posts. Let's go home and beat our children.

      Comment


      • #4
        Here's the Duncan power section calculator.

        http://www.duncanamps.com/psud2/index.html

        I used this for the first time today and it is pretty slick. It saved me hours of calculating time.
        Casey4sDIY Links:102 Amphttp://www.guitarkitbuilder.com/cont...-amplifier-kitChassis Layout:http://www.guitarkitbuilder.com/cont...lifier-chassisTurret Board:http://www.guitarkitbuilder.com/cont...d-construction Tolex Tutorial:http://www.guitarkitbuilder.com/cont...lifier-cabinet

        Comment


        • #5
          The switching is odd. The boost switching is normal, but shunting in the channel select is gonna bring some frequency response issues because of the change in load. Have you factored those in to your analysis?
          Originally Posted by Anderton


          FYI God posts here, but under an alt.









          Originally Posted by BernardAlbrecht


          Yeah, it's SomeoneYouKnew.



          Apparently I'm God.

          Comment


          • #6
            The switching is something I'm still learning about. I may have to lose a relay due to current draw in which case I'll use a more standard line select method.
            DCC Engineering. Custom Cabs and Effects.






            Originally Posted by -Assy-


            Okay I summed up this entire thread in three posts. Let's go home and beat our children.

            Comment


            • #7
              The switching is something I'm still learning about. I may have to lose a relay due to current draw in which case I'll use a more standard line select method.
              Take a look at what happens with the "Cn Sw rly A" in the position, as drawn.

              The signal to the bottom section is shunted to ground. So that avoids driving it with a signal that isn't gonna be used at the time anyway. All well and good.

              But look at what happens to the signal that continues through the top part of the circuit. The 10k resistor and 22nF coupling capacitor form a high-pass filter. Everything below the corner frequency of about 725Hz drops off @ 6dB per octave. This can't be good.
              Originally Posted by Anderton


              FYI God posts here, but under an alt.









              Originally Posted by BernardAlbrecht


              Yeah, it's SomeoneYouKnew.



              Apparently I'm God.

              Comment


              • #8
                Thanks for spotting that. I could have been scratching my head for a while at that one!
                DCC Engineering. Custom Cabs and Effects.






                Originally Posted by -Assy-


                Okay I summed up this entire thread in three posts. Let's go home and beat our children.

                Comment


                • #9
                  Thanks for spotting that. I could have been scratching my head for a while at that one!


                  There are several really sharp guys here in DIY now, him and SnorkelMonkey
                  have helped me quite a bit with my new build.
                  Casey4sDIY Links:102 Amphttp://www.guitarkitbuilder.com/cont...-amplifier-kitChassis Layout:http://www.guitarkitbuilder.com/cont...lifier-chassisTurret Board:http://www.guitarkitbuilder.com/cont...d-construction Tolex Tutorial:http://www.guitarkitbuilder.com/cont...lifier-cabinet

                  Comment


                  • #10
                    Thanks for spotting that. I could have been scratching my head for a while at that one!
                    No worries, mate. You also have some weirdness happening at rly B. The position of VR9 is gonna affect the tone of the signal on the upper channel. More cut of the highs when VR9 is at minimum or maximum. Not sure if the effect will be very noticeable, but it will be there.

                    Is that a typo on the value of R22? Should that be 33K, or did you miss the decimal point for 3.3K?
                    Originally Posted by Anderton


                    FYI God posts here, but under an alt.









                    Originally Posted by BernardAlbrecht


                    Yeah, it's SomeoneYouKnew.



                    Apparently I'm God.

                    Comment


                    • #11
                      Cool thanks again. I'll swap the channel switching to a one relay input select later. Hopefully I wont make too much noise on channel switching. I need to add a large resistor to ground from the coupling cap on the boost stage too for this purpose. I think the channel.

                      Yup 33K, going to try a 39K too for some real cold clipping. Might end up using a more subtle value in the end though.
                      DCC Engineering. Custom Cabs and Effects.






                      Originally Posted by -Assy-


                      Okay I summed up this entire thread in three posts. Let's go home and beat our children.

                      Comment


                      • #12
                        Yup 33K, going to try a 39K too for some real cold clipping. Might end up using a more subtle value in the end though.
                        Ah, so that's what you're after. Well, the gain of the circuit should be pretty low, with all that degen happening at the cathode. Maybe like 2 or something. And the idle current should be very low, 100~150uA. Cold indeed. And close to cutoff. So yeah, it probably won't take much to drive it into clipping.

                        I've never played in this region. Dunno what it's gonna sound like. I'll be interested to hear from you after this had been built.
                        Originally Posted by Anderton


                        FYI God posts here, but under an alt.









                        Originally Posted by BernardAlbrecht


                        Yeah, it's SomeoneYouKnew.



                        Apparently I'm God.

                        Comment


                        • #13
                          Yeah its pretty low. Did the calculation with merlins spreadsheet and its certainly under 10 and probably even under 5 as you say. Its pretty much what mike soldano did with the SLO to give it that sound and quite a lot of high gainers since have copied that design feature. I like that sound but I dont really want a silly 5+ stage design as it robs too much of the subtlety from the tone. I like to get a good bit of dirt going on the amp and then slam the front with a TS or fuzz, sometimes a TS and a fuzz
                          DCC Engineering. Custom Cabs and Effects.






                          Originally Posted by -Assy-


                          Okay I summed up this entire thread in three posts. Let's go home and beat our children.

                          Comment


                          • #14
                            Updated the schematic to change the switching and go a bit colder on the V2A clipping. Removed a few redundant/ineffectual parts.



                            Big print edition.
                            DCC Engineering. Custom Cabs and Effects.






                            Originally Posted by -Assy-


                            Okay I summed up this entire thread in three posts. Let's go home and beat our children.

                            Comment


                            • #15
                              Sweet how much did the pt cost you?
                              Jca 50h/ 100h/ 22h Mod thread and Owners Club
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