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Thread: Interesting G-120 amp design...

  1. #11
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    Quote Originally Posted by buffo View Post
    Glad you mentioned something about it, Bill. Was actually thinking about picking up a set of G-120's from Aron Bacs at FLEM just to play with this. But if you don't think it's a viable option, then I'll hold off.
    I just don't think that this thing will really work "as advertised".

    It may be possible to tune it up to 24K for very small angle... BUT, then what will happen when you put some content in with large angles? Experience tells me that it will massively overshoot and possibly (probably) break a torsion rod.

    Then, it may be possible to tune this amp up to work at larger angles (which is what the G-120 is supposed to do), but then small signal response will be crap (like 15K or below, as he shows in some of his other pictures).

    This amp has no "angle intelligence", so it can either be tuned up for small signals, or large signals, but not both at the same time. This amp will also "sing", depending on the mirror/mount you put on it.

    So, in the end, I think (gulp) Chinese scanners and amps would work better than G-120s and this amp, and probably be less expensive too.

    But in any event, if someone has some money burning a hole in their pocket, sure, go ahead and get it. It never hurts to experiment and sometimes that's the best way you learn.

    Best regards,

    William Benner

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    Maybe I will heed your warning for now Bill. Besides, I may be getting some Catweazle II's soon. Anyone have any opinions on those? These would be the 30K catweazles. Based on the tests I saw on the Pangolin site, they looked somewhat comparable to the g-120s running 30K on a Turbo Track.
    Those who fail to grasp art are the ones who criticize it.

  3. #13
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    Quote Originally Posted by Pangolin View Post
    This amp has no "angle intelligence", so it can either be tuned up for small signals, or large signals, but not both at the same time.
    This is interesting. How does one implement "angle intelligence" in a scanner amp? Is it some sort of amplitude filter that kicks in at a given level and changes the tuning for larger steps? Or maybe an integrator that continually adjusts some portion of the amp in response to the magnitude of the signal change? I'm really curious about this...
    This amp will also "sing", depending on the mirror/mount you put on it.
    I'm not sure what you mean by that, but it sounds bad! Is this some kind of resonance that will develop in the rotor of the scanner?
    So, in the end, I think (gulp) Chinese scanners and amps would work better than G-120s and this amp, and probably be less expensive too.
    Wow... *That* is really saying something! I've already decided not to pursure this, but I am still interested in learning more about scanner amps in general.

    Adam

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    I think one question, for me, still remains. What is it about the Turbo Track that makes it so fast?

    Could one build an amp that employs the same acceleration feedback that the 124 open loop amp has and combine it with an efficient feedback circuit?
    Those who fail to grasp art are the ones who criticize it.

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    Quote Originally Posted by absolom7691 View Post
    I think one question, for me, still remains. What is it about the Turbo Track that makes it so fast?
    "Benner Magic"

    Seriously though, Bill put a lot of thought into that design, and it's just one hell of a well-built amp. But that's why they were expensive when they came out, and why people are reluctant to sell them now. They still perform really well, so people hang on to them. (Think Mercedes vs Yugo)

    As for the specific technical details behind the amp's continued success, you'd have to ask Bill. Scanner amp design theory is *way* over my head...

    Adam

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    Very interesting amp. Hmm, schematic diagram is very close to Chan's amateur amps (the first link on that page), even components values are the same.
    I'm also about making new amps for my galvos. I saw on many drivers two power amp ICs. I suppose that is bridged current feedback drivers. Does anyone know how to do that?

  7. #17
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    Sergey, LM3886 is just a big inverting opamp and with split supplies, can be treated as such.
    Thats why many drivers have dual lm3886

    A little knowledge is dangerous, test with a speaker or wire wround resistor before you go galvo:

    but here:

    http://apex.cirrus.com/en/pubs/appNote/Apex_AN20U_D.pdf

    if you google " lm3886 bridged with dc servo", you might find your goal.


    Also national semi conductor ap note bpa200

    Steve

    sTEVE

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    Still I love OPA549 series, except the price
    Piotr.K

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    Okay, let me add a few words to the main subject; to my opinion the best way to go with fast & universal galvo drive is DSP. There's no fixed parameter schema for a galvo driver to be both fast and accurate in small & large step area. It takes non-linear functions, so preferably parameters should be fetched from previously filled (self-learned, if implemented) table of parameters for each step required.
    And it's commercially available ! - x-drive by MediaLas, perhaps others. Too complicated for DIY.
    Piotr.K

  10. #20
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    Quote Originally Posted by buffo View Post
    This is interesting. How does one implement "angle intelligence" in a scanner amp? Is it some sort of amplitude filter that kicks in at a given level and changes the tuning for larger steps? Or maybe an integrator that continually adjusts some portion of the amp in response to the magnitude of the signal change? I'm really curious about this...
    the amp in question has the second thing you mention.. if anything the angle intelligence probably amounts to a properly placed capacitor or op-amp-capacitor pair in the feed-forward section.

    I'm not sure what you mean by that, but it sounds bad! Is this some kind of resonance that will develop in the rotor of the scanner?
    I am reminded of the notch filter that had to be added to some earlier galvo amps to compensate for a resonance in the feedback pickup of the galvo. If the G120s have a strong 'Q' self-resonant frequency then a similar notch filter might be appropriate.


    Wow... *That* is really saying something! I've already decided not to pursure this, but I am still interested in learning more about scanner amps in general.

    Adam
    Given that the author has scope plots proving operation at 15pps at what he claims as 40 degrees (no reason to assume he's lying) then there's reason to expect he would have noticed if his software wasn't actually drawing at 24kpps.

    The amp documented on his page may not be perfect, but it's still pretty good, considering the design is released free. Definitely worth considering building and tweaking if there were a glut of amp-less galvos available.

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