Volt/Amp dependent RLC band pass -> Loudspeaker expander using lightbulbs

Discussion in 'Analog & Mixed-Signal Design' started by maxbauer, Jul 6, 2018.

  1. crutschow

    Expert

    Mar 14, 2008
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    You do know that LTspice is a free download from Analog Devices?
     
  2. danadak

    Well-Known Member

    Mar 10, 2018
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    Last correction (I Hope) -

    5 Ohms -

    upload_2018-7-9_12-31-37.png

    2 Ohms

    upload_2018-7-9_12-33-5.png
     
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  3. Max Bauer

    New Member

    Jul 6, 2018
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    hey Alec, thanks for chiming in over here, I hope its not a big problem that I posted on electro tech too!?
    I will add the graphs down below new post
     
  4. Max Bauer

    New Member

    Jul 6, 2018
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    hey crutschow, yes now I am aware but the circuitlab was too tempting and I opted for the cheap 1 month option but Spice is on my radar too!
     
  5. Max Bauer

    New Member

    Jul 6, 2018
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    ok here is what circuitlab put out,
    they look quite a bit different than in Spice it seems hmmm .. o_O

    I added zener diodes in series with the midrange filter to the spaker, they should put more pressure on the bulb and then open up when the bulbs resistance increases, zeners are actually quite smooth in a certain range and not a sharp cutoff at all, but its hard to simulate that stuff, or rather I am not sure how or if all at circuitlab simulates this or just goes from sharp off to on ?!

    is this the correct way the zeners should be implemented, back to back right?

    the top one is with the bulbs at 5, the bottom with bulbs at 25ohm.
    also the graphs change quite a bit depending on whether its a voltage or a current source.

    filters1.png


    new.png


     
  6. crutschow

    Expert

    Mar 14, 2008
    20,265
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    The most accurate analog simulations you can do is with a Spice based simulator.
    I have my doubts about others, like Circuitlab. :rolleyes:
     
  7. danadak

    Well-Known Member

    Mar 10, 2018
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    Here is 25 ohms from LT Spice -

    upload_2018-7-9_20-0-53.png
     
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  8. Max Bauer

    New Member

    Jul 6, 2018
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    ok to keep you guys updated, thanks for pointing me to Spice, now about to go over the numbers and try to refine the circuit, and see how spice calculates compared to circuit lab.
    for the bulbs I think it be good to choose lower wattage ones in parallel, they have low resistance cold but shoot high faster than higher wattage bulbs of course, which should help to make the thing responsive. also the z-diodes seem to make a good difference,..
    will get back soon

     
  9. Max Bauer

    New Member

    Jul 6, 2018
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    ok here is the latest draft,
    I eliminated the 3mH inductor in the midrange bypass to the speaker, this gives
    a more favourible response,
    and got rid of the Z-diodes for now, they are a pain in the ass to get working in the simulation,
    will be left for trial in real life situation.

    I also realized that putting lightbulbs in parallel to find a resistance sweet spot, will potentially flatten the resistance slope too much,
    which preferably is as steep as possible,
    so looking now to get just one high wattage 110v bulb of some sort with a low enough cold resistance, maybe a several hundred
    watt halogen bulb or something.
    Retro addison bulbs would look nice but they come only in 60w max rating and at least 20ohm cold i suppose,
    but will get one and measure. I would prefer a cold resistance of 10ohm or less.

    The speaker is now an inductor with a 0.5mH and 16ohm which is average for a guitar speaker, changes the response quite a bit.

    Okay, for now I cant find figure out anything simple to improve the design, so its up to real life testing I guess :)

    Additional feature will be a switchable distortion circuit but this will be another story, trying to get this to work first!

    the plots are 5ohm and 30ohm for the bulbs. frequency response of a typical guitar speaker drops off rapidly from 5k up.
    lowest fundamental of a regular tuned guitar is 80Hz.

    around 300Hz to 400Hz is the fatness and mud region of an electric guitar, taking it down a notch there should clean up the sound when played clean, and give the thump and punch when turning up the volume and digging in for distortion.

    new.png

    5ohm.png

    30ohm.png

    edison.jpg
     
  10. danadak

    Well-Known Member

    Mar 10, 2018
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    Here is what I got in LTC -

    upload_2018-7-12_7-36-0.png
     
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  11. Max Bauer

    New Member

    Jul 6, 2018
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    Circuit lab shows the same unfavourable response with a voltage source, it should work with a current source in spice tho

    I trust the notion that guitar tube amps indeed function more like current sources than voltage sources, or else I wouldnt even
    need to try this ;)

    here is what some gentlemen wrote on the fractalaudio forum:

    "...a power tube is nearly a current source. Nearly! No power tube has an infinite plate impedance and that's why power tubes sound different.
    A current source has infinite output impedance, an actual power tube has a finite output impedance.

    The output impedance of a power tube (or any active device for that matter) is defined as delta V / delta I which is the change in voltage vs. the change in current.

    Let's take a 6L6 for example. Let's assume that the tube has a quiescent operating point of 300V and let's assume we swing +/- 100V around that point. If we look at the plate graphs for a 6L6 at a bias of -10V we see that the plate current at 200V is 95 mA and at 400V it's 105 mA (roughly).
    Using our formula for impedance we get 200/0.01 = 20 Kohms."

    6L6 tubes are indeed among the most common output tubes for higher wattage tube guitar amps (30+ Watts),
    some use little feedback, some none at all such as some VOX amps which the beatles and some crunchy brit rock bands are known for.

    In the end this thing has to prove working in real life conditions with different amps and speakers, and we will find out as soon as I
    get around and assemble a prototype :D:cool:

     
  12. mtripoli3

    Member

    Mar 1, 2016
    32
    15
    Hi,

    I appreciate what you're going for here, and you've got lot's of good input. The one thing I would like to add is this; don't do it.

    I design (and work) on guitar amps and effects and have repaired more than one amp that someone hung something off the output transformer (and by relation, the output tubes). You talk about the tubes, but this is just a small part of the total output section. As robust as these things are, they are very sensitive and it only takes one "flash back", where the OT has had something cause it to act like an inductor, charges up, arcs and causes the insulation to break down in a spot; next thing you know, total meltdown of the output of the amp. This happens with almost ALL the "power soaks, attenuators" out there. Everything is "fine" one minute, next thing you know there's smoke pouring out the back of the amp.

    If you have an amp to sacrifice for experiments, knock yourself out. However, I wouldn't do anything to my "main player"... ;)

    Good luck!

    Mike Tripoli
     
    Last edited by a moderator: Jul 16, 2018
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  13. Max Bauer

    New Member

    Jul 6, 2018
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    Hey Mike!! very cool that you chime in here!

    I was not aware of the trouble that some of these load boxes / power soaks cause, google brings up a couple reports from
    failing amps with the use of such devices, speaking for myself and some friends I dont know anyone personally who had this kind of problem.

    I have used 2 popular models, the weber mass now for years, on a really low setting on a dimed fender amp and no problem so far.

    So yes It will be mainly for myself and run on a mediocre cheap 90s tube amp and will see how it goes, before I even attempt to give it to a friend to tryout.

    Will see how it goes, now getting the parts together,...

    thanks for giving me a heads up appreciated!

    chris
     
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