Attenuators; Volume trimming; L-Pads, and ?

Discussion in 'Amp Input - Normal or Bright' started by StratUp, Jan 14, 2022 at 1:55 PM.

  1. StratUp

    StratUp Senior Stratmaster

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    (Two part post: This is part 1; another angle in the followup comment )

    So... on the side of another project, I ran into this "attenuator" in an amp. That sent me into a little research on the side. I have some questions I need to answer to broaden my understanding:

    This Chinese 5W tube amp was sold with a slider switch for .1 watt, 1 watt, and 5 watt. Here's what the output looks like:

    upload_2022-1-14_13-47-21.png


    --

    It looks to me like it's basically volume reduction, not attenuation. Just bleeding of output to ground.

    I'm thinking that the transformer & tubes will not be driven to the level of realistic high output as a "real" attenuator (or speaker) would do. So it's really just cutting output to the speaker jacks.

    But I'm a lightweight in electronics and particularly tube amps. Tell me what this does in terms of sounds.
     
    Last edited: Jan 14, 2022 at 2:10 PM
  2. StratUp

    StratUp Senior Stratmaster

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    This is part 2 of my attenuation questions.

    There are some attenuators selling for $50, like Carl's Speaker Soak. And some upscale attenuators selling for $250+.

    Inside a Carl's is just an L-Pad and a capacitor. It's like the ones you can DIY with a youtube video. (To be fair, I don't think you can even buy the materials for what Carl charges for his complete unit).

    What do they do inside the upscale versions? Some sort of circuit for emulating the oscillations in the signal that occur in an actual moving speaker coil? What makes them worth $250, other than the name and fancy graphics on the box?
     
  3. rolandson

    rolandson Dr. Stratster

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    Damn, that looks like a 'booger' schematic (bugera t5?)?

    My understanding is that, yes... the 'upscale', read that as "expensive", attenuators are employing a means of reactive resistance as opposed to static...Weber calles it a "speaker motor". Yes... the ability to follow the rise and fall of impedance relative to frequency.
    "Worth", versus cost? I don't know about worth, but it's what they charge...

    [I'll admit to having acquired a Weber Mass that employs the reactive circuit... I'm using it as a dummy load. It's very handy. It has variable amp load capability. I can select the amplifier load I want, from 2 thru 16 ohms. Rather than wiring static resistors to meet the need. (Super Reverbs are 2 ohms) But as far as being the perfect means of achieving the dynamics of a driven amp at conversational volumes, no.]

    I think you are correct, the booger simply sends output to ground. It is/was I think a cheap attempt to steal Andy Marshall's PicoValve design. The Pico actually employs an attenuator...And you can use pretty much whatever octal power tube you want...
    Too bad they're discontinued.
     
  4. StratUp

    StratUp Senior Stratmaster

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    Thanks.

    As for what this amp is that I stole the circuit snapshot from... I'll check into the Bugera and compare. There's a good chance I'll end up owning one very similar to the schematic. May or may not have the resistor based attenuator wiring (might have been an model option).

    I should either own it, or not, by tomorrow afternoon. If I end up with it, I am already thinking of modding it a bit to tame it. It has a reputation of getting dirty fast (EL84 based). But I'll have to do a lot of reading, then ask my dumb questions. I can elevate the level of my dumbness up to at least "wildly ignorant and unqualified" that way.
     
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  5. teleamp

    teleamp Strat-Talker Silver Member

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    Looks cheesy...
     
  6. dirocyn

    dirocyn Most Honored Senior Member

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    Bleeding part of the signal to ground is exactly what passive attenuators do. While keeping the resistance the same, so you don't destroy the amp. It means you can cook your power tubes without sending so much power to the speakers, and it's effective albeit primitive.

    The problem people have is that ears work differently at low volume. Ya know that "loudness" button on a stereo, that gives you a lot more bass and a little more treble to make low volume playback sound more like high volume playback? Same issue, and this simple circuit hasn't got one. You might have to adjust the knobs on the amp a little to get the sound you want.

    An L-pad works the same way, except with a continual dial instead of a switch.

    Active attenuators are a whole lot more complicated, but usually account for the Loudness effect.