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Help with circuit: Acoustic guitar preamp based on Fender

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  • Help with circuit: Acoustic guitar preamp based on Fender

    I would like to build acoustic guitar preamp based on Fender Acoustasonic circuit.
    I got 15V power supply and some TL072, I need one-channel preamp with line-level output of 600-700mV.
    Could you help me to understand, following the input and EQ (U1) which part (op-amps) of the circuit to use and how ?

    Click image for larger version

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    mtsitsishvili.strikingly.com

  • #2
    Hi Mikheil.
    Not recommended .

    This is a quite complex Acoustic Guitar preamp, lots of features which are not big deal if assembled in some robotic factory but deal killers in a homemade one.
    To boot it has two very different channels, yet you donīt indicate *which* one do you want.
    What is your intended purpose? to drive an external power amp or powered cabinet or send line out to a mixer?
    You intend it as a floor unit or built as, say, some kind of rack preamp?
    Will you drive it straight from a raw Piezo pickup or passive Dynamic capsule or you already have some kind of preamp or active pickup built into your guitar?

    To boot, youīll have to *design*, print, etch and drill your own PCB, not an easy task and which takes some time to learn.
    I suggest you build the *excellent* sounding yet simple preamp found in the Peavey Solo acoustic guitar amplifier.

    The reason why I am trying to avoid the straight Fender design is that although itīs excellent, is way too complex but much worse: uses very hard to get (not only in Georgia but most anywhere else, including most of Europe) C/reverse Log taper potentiometers which are a staple in Fender designs but which, of course, they custom order by the thousands; not easily available on standard Electronic parts vendors.
    Juan Manuel Fahey

    Comment


    • #3
      Log taper pots: what they do such an exclusive to the circuit?

      I would like to build a circuit with just two TL 072 op-amps and get a line-level output of 600-700mV. The Acoustasonic schematic is much larger and complex and little bit hard to realize which 'spot to cut' after input and EQ sections.

      Here is the question:
      Could you please share your thoughts on the issue and indicate those particular op-amps with R-C periphery that would suit the application with standard 10kOhm input impedance amplifier?

      Attached is the option...

      Click image for larger version

Name:	Acoustasonic_04.JPG
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      mtsitsishvili.strikingly.com

      Comment


      • #4
        If the volume pot is a 30% log reverse taper, I'd source a linear taper to replace it.
        If it still won't get loud enough, it's probably broken. - Steve Conner
        If the thing works, stop fixing it. - Enzo
        We need more chaos in music, in art... I'm here to make it. - Justin Thomas
        MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

        Comment


        • #5
          Concern about the EQ section: I have never seen such a small EQ circuit of just 2 caps, does anyone have? should it be switched with other TL072 of the circuit?
          Thank you!
          mtsitsishvili.strikingly.com

          Comment


          • #6
            Originally posted by Mikheil View Post
            Concern about the EQ section: I have never seen such a small EQ circuit of just 2 caps, does anyone have? should it be switched with other TL072 of the circuit?
            Thank you!
            I've seen it thousand times. This is typical cost-saving design. I thought that you already decided to build preamp based on this schematic. And now you suggest that the schematic is incorrect . I can assure you that the schematic is correct (they build many amps using this schematic). Such a design (three tone pots and one opamp) is very popular on the internet.
            If you want other schematic, use other schematic.

            EDIT: more info at: http://www.diyaudio.com/forums/instr...ed-fender.html
            Last edited by MarkusBass; 01-27-2017, 12:34 PM.

            Comment


            • #7
              Originally posted by MarkusBass View Post
              ... And now you suggest that the schematic is incorrect .......
              Hi!
              I do not say the circuit is incorrect. As you see, I have cut U2 op-amp from the next section and switched it to U1 myself, so, the question was ob this issue on whether I did it right way ???
              Sorry for confusing ...... and thanks for comments!
              mtsitsishvili.strikingly.com

              Comment


              • #8
                You missed the:
                C/reverse Log taper potentiometers which are a staple in Fender designs
                part

                That said, you *may* replace it with, what else? a Linear pot but be warned that you will have a lot of gain variation on a small part of the pot range.
                Say, not much effect from 0>7 and a big jump from 7>10
                I suggest you use a larger diameter Volume knob which will make handling easier.

                The block you cropped is fine, I thought you needed other features included in that amp.

                The phase inversion section and switch is not needed, since you are building a single channel preamp, but wonīt hurt either.

                To get 600 mV out with everything else being the same, increase R10 to 33k and you are done.

                Those Op Amps require +/- 15V on pins 8 and 4 respectively; IF itīs the front end for an amplifier, no big deal; if itīs intended as a belt pack (or "pedal" case) preamp it complicates things a little, you might need a +9V to +/-15V converter.

                If power amp input impedance is 10k, then replace R23 with a 100 ohm resistor.

                Good luck.
                Juan Manuel Fahey

                Comment


                • #9
                  [QUOTE=J
                  To get 600 mV out with everything else being the same, increase R10 to 33k and you are done.

                  Those Op Amps require +/- 15V on pins 8 and 4 respectively; IF itīs the front end for an amplifier, no big deal; if itīs intended as a belt pack (or "pedal" case) preamp it complicates things a little, you might need a +9V to +/-15V converter.

                  If power amp input impedance is 10k, then replace R23 with a 100 ohm resistor.

                  Good luck.[/QUOTE]


                  Thanks!
                  Yes, the negative feedback resistors do regulate the output volume of op-amps but the other rule is that the final section op-amps are used as filters requiring both input and feedback resistors to be of the same value, isn't it ?
                  mtsitsishvili.strikingly.com

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