Robs 1/4 Power / 10% Switch Attenuator

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

  1. Bitsleftover

    Bitsleftover Tele-Meister

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    Quick mini project to fight the boredom.
    I knocked up a 16 ohm attenuator based on Robs 1/4 power and 10% switches.
    Sounds great!
    My Plexi micro is too loud even at 10% in my flat though. I might turn it into a 10% 1% attenuator!
    I must be the most considerate neighbour in the whole guitar community! I deserve an award!
    Here’s a few picks-


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  2. Bitsleftover

    Bitsleftover Tele-Meister

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    [​IMG]


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  3. Bitsleftover

    Bitsleftover Tele-Meister

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    [​IMG]


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  4. Bitsleftover

    Bitsleftover Tele-Meister

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  5. rangercaster

    rangercaster Poster Extraordinaire

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    That is cool !!!

    Very nice work !!!
     
  6. archetype

    archetype Fiend of Leo's

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    I'll need to make one, too. Did yours deviate from Robs prescription, other than treating it as a separate circuit?
     
  7. Bitsleftover

    Bitsleftover Tele-Meister

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    Thanks!
    The first switch is similar to Robs switch which bypasses the 32r series resistor. .
    The second switch just selects which shunt resistor is used for 25% or 10%.


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  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I've built the 1/4 and 1/10 power boxes and was 100% happy as well. Also great for diagnosing/tuning an amp to hear what's going on without the SPL killing your ears.

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  9. James Knox

    James Knox Tele-Meister Ad Free Member

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    Are those SPST switches?
     
  10. Bitsleftover

    Bitsleftover Tele-Meister

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    SPDT, but I’m starting to doubt myself now.
    I think I’ve messed up the design.
    The 10% side is right, but I think I’ve messed up the 25% side of things!

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  11. James Knox

    James Knox Tele-Meister Ad Free Member

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    does it not sound right? I’m trying to figure one out as well. I’ve read all the posts and looked at all the pics I can find on “1/4 or 1/10 Power Mod in a Box” I’m stumped. I guess I’m not understanding it. Your box and design looks great though. I will stay tuned!

    Here is mine (not sure how to wire it yet,lol):

    2AB2C6F8-4A35-4C67-8CDA-5FA508EE5CB1.jpeg

    1/10th Power for 8 ohm Speaker
     
  12. Bitsleftover

    Bitsleftover Tele-Meister

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    Yeah it sounds good. My meter is in my work bag at the minute but I’m pretty sure if I checked the resistance on the 1/4 setting, it’ll be nowhere near 16Ohms.
    I mistakenly left the 32r resistor in series for both 1/4 and 1/10 modes. It’s not as simple as that.
    I need to redesign that second switch.
    1/10th isnt quite enough cut for me so Ill just rethink the whole thing.


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  13. Bitsleftover

    Bitsleftover Tele-Meister

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    How does that switch work?
    Is the single contact on the right the common?


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  14. James Knox

    James Knox Tele-Meister Ad Free Member

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    That switch was a leftover “standby switch” from my 5E3 build.

    I finally figured it out from RobRob’s picture:

    6303B9EF-329B-4FE8-95B6-81F68F2758A7.png

    Changed my switch to a SPDT mini:

    08A835E3-1797-4168-8A19-2B9DE75FCF9A.jpeg

    60DEBCCA-E4A1-45CF-AFAF-65831F986F9F.jpeg

    It’s rough looking, but it works. Works great as a matter of fact. Still pretty loud. Now I want to build one for my 5F1 Champ bit it has a 4ohm speaker, so I need different resistors.

    Hope you get yours sorted out!

    Peace,

    James
     
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  15. jtcnj

    jtcnj Tele-Holic

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    @James Knox
    Idk which OT you used but on my 5F1 I used a classictone that has 4Ω and 8Ω impedance taps, originally hardwired at 4Ω for the combo speaker.

    I later added a SPDT on-on switch to add 8Ω, and I just plug into either the 8 ohm cabs or the 12" Weber Sig. in my 5E3 combo. Very useful.

    I built the 1/4 power first, in a pedal box, with speaker leads and plugs. Works ok but still very loud.

    I then built the 10% , pedal box with jacks. I just set up that way to use it. Didnt really see the need for any switching.
    This one really works well. I need to drill some vent holes. It gets warm with a cranked 5e3 or 18w, and hot on a 50w amp. I use it a LOT.

    It is not bedroom quiet, but manageable volume at sweet spot settings.
    Well, it can be pretty loud still with boost or OD pedals pushing the front end, which is my usual scenario.
     
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  16. johnDH

    johnDH Tele-Meister

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    Given a simple two resistor circuit, this is the eight way to do it, ie with series resistor on the speaker side. Nominally, given an 8 Ohm load Its reducing power to 1/9 th = 9.5db.

    Most simple ones would put the series resistor on the amp side, and those are the ones that sound dull due to too much speaker damping.

    You can stack these modules end to end to get whatever attenuation you like. Two of these ones and you have -19db, which can be just right for late nights.

    I arrived at a similar basis a couple of years ago and have pursued it ever since. My designs work with a series of switched steps using the same basic module, and I use -14, -7 and -3.5 db stages. Combining them gets you any combination in the range. I've also added reactive circuits to the front end which lift the performance to the next level. Its all here, (nearly 60 pages but post 1 has a summary update):

    https://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/

    This is the latest simple reactive version:
    https://www.marshallforum.com/threa...design-and-testing.98285/page-56#post-2000746

    Here it is stripped back to just a resistive design:
    https://www.marshallforum.com/media/attenuatora-180624.9965/

    Its worth doing the reactive ones though. If you like Rob R's basic circuit, then check out mine if you'd like to take it further, there are lots of successful build reports.
     
  17. James Knox

    James Knox Tele-Meister Ad Free Member

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    My 5f1 build was 4ohm, but I am getting ready to build another 5F1 with an 8ohm speaker tap. Seems more versatile. Can’t wait to try the 1/10 box on the new 5F1 when it’s done.

    It works pretty good on the 5E3. Like you said, not bedroom quiet, but very useful. I can use my 5E3 for small gigs, if/when we ever get to gig again, lol.
     
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