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Intro - Background; project, the OverCompensator, etc.

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  • Intro - Background; project, the OverCompensator, etc.

    Hello world! (Sorry about the old programming joke

    Every winter, I set out and pass the time with a project; usually something unique and fun, just to push (sometimes aggravate) myself, and see what happens. I'm a shift troubleshooter with Michelin by day, freelance programmer by night; and in between the jigs and reels, family life, work, etc. I get to play.

    Close to 20 years ago, trained to be an electronics engineering tech; have been into guitar previous to that (lately v-drums, but that's another story). During one jam session, a local guy came in with a Fender Twin Reverb and sat to play with us. I remember being memorized with the amp and how it sounded, and have wanted one ever since. The price, availability, politics (wife!), life, the universe, everything, haven't made it viable in the past. C'est la vie.

    I've recently taken up guitar again after a long hiatus, but have sold off my old 75W Peavey long ago. I've scrounged and saved umpteen magazines, articles, plans over the years thinking I'd get back into it; seems that the time has finally come.

    This time around, it's not so much the cost really. I *could* afford a Twin, but there's no fun in just buying one. I've decided on prototyping a 2x12 Fender Twin clone. Dual channel, (one low Z input for guitar, one high Z input for ipod, mp3, v-drum, etc.), vacuum tube preamp and 100W class-D power amp stage, feeding into 4ohm Eminence Texas Heat 12" speakers. My wife has called it the "OverCompensator", which is actually not a bad name for it!

    It's actually been an interesting project thus far. From building the PCBs, acquiring parts, the cabinetry, working through various issues has been quite rewarding and fun.

    I'm hoping to go with a yellow tolex and black trim (if there's any car guys on board, they will tell you yellow adds +10hp just on principle alone!); one potential issue I foresee is if the chassis straps and tilt legs can be sand blasted to remove the chrome and then painted black. We'll know more next week. If it doesn't turn out, I'll go with the original Fender colour scheme.

    Various pics of assembly below, comments welcome.

    Chris












  • #2
    Very nice!

    But how can it be a Fender Twin clone when it has a Class-D power amp? What Class-D circuit did you use? I've experimented with Class-D, but it all ended in smoke and shattered MOSFETs. I hope those are powdered iron or MPP cores.

    Your comment about yellow adding hp is all well and good, but this is the guitar world. Red ones are louder.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

    Comment


    • #3
      It will be more of a Twin look-alike, perhaps 'clone' is too far removed from what I'm doing; though I've kept with the tube spirit at least

      The circuit is a design from Barry Ward; he authored a couple of articles back in '87 (Modern Electronics iirc) on class D's amps. The cores are indeed MPP; one of the more challenging things I had to acquire thus far.

      Duly noted about the red!

      Comment


      • #4
        I googled "Barry Ward Modern Electronics Class D" and came up with the schematics, at least:

        legalities of posting schematics from a defunct magazine - diyAudio

        Class-D has come a LONG way since 1987. But then, a guitar amp doesn't exactly need awesome sound quality either. If (when? ) it blows up, you can get modules based on the Tripath chips quite cheaply on Ebay now.

        TDA8920 BTH Class D Digital Amplifier Board 2X100W on eBay (end time 10-Mar-11 17:51:15 GMT)
        "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

        Comment


        • #5
          Yes sir, that is the schematic. If it blows up, (which reminds me of a question I have for you actually...) I might give the Tripath a go. Personally, I like to build it all up by scratch; PCB right straight through. I also have a few class A articles & schematics in my tickle trunk yet. And again, I'm not ruling out getting a Twin kit if all is lost on the power amp stage; chassis, turret board, et all. Loads of paths I can go depending on what happens here

          My question: out of mild curiosity, I checked your web pages and quickly glanced at your 130W class D. You mention that you installed a fan to cool down the MOSFETs. I'm not incredible happy with my heatsinking and may redesign it for more surface area. In addition, I'm still worried about heat dissipation and have thought of a fan running inside the chassis. One design question lingering in my mind was that of electrical noise introduced into the circuit by adding a fan. Did you encounter any problems with this? I'm looking at grounding techniques, and haven't decided between a grounding bar or star topology. Would you mind sharing your experiences with the fan?

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          • #6
            I've never actually built a Class-D. The smoke and MOSFET shrapnel came from a 400W Class-D module that I bought.

            The amp you see on my site is a regular Class-AB type, hence the extra cooling requirements. The theoretical best efficiency is 78% or something, as opposed to 100% for a Class-D. Also being a MOSFET amp it liked to dissipate about 50W at idle for minimum crossover distortion.

            There was no trouble whatsoever with electrical noise from the fan. They don't leak noise to ground or anything. Anyway, the fan in that thing is a 220V AC one with an induction motor, but I've also used the DC brushless type. I would not run one of those off the same regulated supply node as my preamp. While they may not radiate noise or leak it to ground, the current they draw is large and somewhat noisy.
            "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

            Comment


            • #7
              Ah, I apologize. I read MOSFET and immediately went into class D train of thought. Appreciate the advice on the fans... for me, it's just a game of hurry up and wait.

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