Attenuators / L-pad values?

Discussion in 'Shock Brother's DIY Amps' started by Bitsleftover, Jul 13, 2020.

  1. Bitsleftover

    Bitsleftover Tele-Meister

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    Hi, I’ve built Robs 10% 16 ohm attenuator into an enclosure.
    (32 ohm in series, 24 ohm in parallel)
    I’d like more attenuation though.
    What values should I use to cut this output in half? Or even 75%? (While maintaining the 16 ohm load)
    I’ve tried and tried and tried to do the maths but it’s made my head hurt and I’ve started dribbling out of the corner of my mouth while slowly rocking back n’ forth in the corner of a dark room with my underwear on my head.
    Please help.


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  2. Andy B

    Andy B Friend of Leo's Silver Supporter

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    Go to Ted Weber's site. There is a calculator there which lets you calculate resistance values for attenuators. Works very well. I've used it to do a number of them recently.
     
  3. johnDH

    johnDH Tele-Meister

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    I posted on your other thread, and you might be able to adapt my design there. But I can also run the maths. Are you looking to add further switched stages? or just increase attenation on on the current stages? Adding an lpad could spoil the tone unless its done right, and it needs care with both input and output impedance.

    Your build looks very neat! how do you do the lettering?
     
  4. Bitsleftover

    Bitsleftover Tele-Meister

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    Thanks Andy, I’ll take a look.


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  5. Bitsleftover

    Bitsleftover Tele-Meister

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    Thanks John. I think you answered my other post while I was typing this one.
    All I WAS looking to do, was keep the 10% setting, ditch the 25% setting and replace it with something around 5% or even 1%
    I’m intrigued by your attenuator now though and I’m in desperate need for a project to occupy my thoughts and time.
    I have two questions.

    1/ Will your design get me down to where I want to be? Ie around 1 to 5%

    2/ I have an adjustable l-pad which gets the level right down but loses a bit of top end. Would there be any advantage to adding your inductive stage before the adjustable L-pad?

    I’ll get plowing through the 60 page mega thread this morning and start getting my head around it.

    Thank you for your help.

    By the way, I use some free software called ‘Inkscape’ to do the graphics and lettering.
    It’s really easy to use.
    Once happy I save as PDF as “no scaling” or “100%” then print the PDF onto inkjet self adhesive vinyl.
    Once I cut the decal out I touch up the white edge of the vinyl with a suitable coloured sharpie then stick it on. I should probably lacquer over the top but haven’t bothered up until now.
    You can also buy clear inject vinyl which I’m going to try next once I think of something to build.
    Here’s the label of the L-pad I have built into my 5f2A-esque. [​IMG]


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  6. johnDH

    johnDH Tele-Meister

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    neat idea with the labels

    Yes those attenuators, if you build any of the ones on the first post, will get you down to -31.5db (-7, -7 -14 -3.5) in small equal steps. At that point, a 50W amp is running at about 35mW and it is still sounding clear. I use that setting often.

    I can work out a reduced stage, but if you actually want 1%, which is -20db, then you can just do two of Rob Rs stages end to end (near enough), , so 4 resistors, and you get -19db, with the same impedances in and out which is key to consistent tone. If you have the parts, how about just lash that up quickly for a test to see if its right or if you need a bit more or less

    I think I've followed the -10% RR circuit, its a 24 Ohm and a 32 series to the speaker? How does it get the 25%?, which values?
     
  7. voskarp

    voskarp Tele-Holic

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  8. Bitsleftover

    Bitsleftover Tele-Meister

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    I might do just that. (Add a second 10% in line). I’ll raid the leftover bits box and see what’s going spare.

    I think my mind is made up though, I’m going to build one of the versions of your attenuator. It sounds perfect for my needs.

    Yes the RR 10% version uses 32r in series with 24r to ground.
    Then the 25% version uses 16r in series with 32r to ground.
    That’s the bollock I dropped!
    I incorrectly had it in my head that the series resistor was 32r in both cases and I just switched the ground resistor.


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  9. Bitsleftover

    Bitsleftover Tele-Meister

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