Should I make this attenuator?

Discussion in 'Shock Brother's DIY Amps' started by radiocaster, Feb 22, 2019.

  1. radiocaster

    radiocaster Poster Extraordinaire

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    The resistors are huge, so is the lpad (see jack). The box is pretty shallow and wider than most amp heads. Should I make holes in the sky for ventilation? Is that enough? The attenuation knob is on the front of the locomotive. Sorry for the crappy pics.
    att1.JPG
    att2.JPG
     
  2. archetype

    archetype Fiend of Leo's

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    Sure, go ahead and make it if you have all the parts. Punch a few holes the top so it doesn't get too hot.

    I'm sure that I've never seen one built in a cookie or biscuit tin, but you get a prize for doing so. That's unique.
     
  3. Doctorx33

    Doctorx33 Tele-Afflicted

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    I always put a fan in
    when I build attenuators like that.
     
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  4. dan40

    dan40 Tele-Afflicted

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    Check out this thread....http://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/ JohnH designed several different circuits in this thread. I built the version listed on page 4 but he does have a reactive version also if you read through the whole thread. It is very easy to build and to my ears, is much better than the minimass that I had been using. It uses switches instead of a rheostat for volume control, but this seems to be the reason that it does sound so transparent.
     
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  5. johnDH

    johnDH Tele-Meister

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    Thanks for linking that Dan.

    Yes that's mine. The designs from page 4 onwards are all based on the same basic ideas, which took a few pages to get right. The design is modular and can be adapted to different needs quite easily. The key thing is how each resistive stage works, and its not complicated but its different to most. I'm happy to help.
     
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  6. Mirrorshades

    Mirrorshades TDPRI Member

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    I eagerly await the commercial release of this brilliant attenuator!

    Or, at least a kit...
     
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  7. corn husk bag

    corn husk bag TDPRI Member

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    Drill a hole right above the smoke stack, get some of the Lionel smoke pills we used back in the 50s. Lay a pill on the resister, play Big Train by Ronnie Earl. Have fun! Watch it smoke!


    [​IMG]
    :p
     
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  8. radiocaster

    radiocaster Poster Extraordinaire

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    I didn't drill the smoke coming out of the locomotive because I thought it wasn't big enough, but I did drill the sky, then the waiting platform in the train station because I thought the sky might not be enough ventilation. Got this stand for my hand drill and it was fun to drill (probably not if I didn't have a stand). Didn't take pics of it because I figure that's when I'll finish it.

    I did come across some problems.

    The wire I used is too thick and doesn't bend easily. I broke the tab on a resistor.
    broken-res.JPG
    I have another resistor, but it was kind of expensive. Need to buy thinner wire.

    Also need to figure out a good way to fix the Lpad to the lid.

    I did try a couple of circuits with test leads, measuring the load (without an amp) and also trying them out with my Greta.
    att-schem.png
    The first one at the top is from the instructions of the Lpad. The resistance increases, then decreases again as you turn it.

    The second is the one I was going use, simply has a resistor in parallel and one in series, to provide better power handling and maybe a bit of attenuation as well. The resistance increases constantly.

    The third one I came across by mistake, experimenting perhaps, but it does work. I didn't measure the sound levels, but it seemed similar to the second circuit when the Lpad is turned hard left. It also attenuates when all the way to the right. The one advantage of this is that the values are much closer to what the amp would like to see, and the value does not change as much.

    The last one just attenuates a little, I just wanted to read the values without an Lpad and measure the resistors, which have a higher resistance than their nominal values.
     
  9. Paul-T

    Paul-T Tele-Meister

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    This thread interests me, as I made a similar circuit for my Park 75 (insanely loud) many moons ago. it was still insanely loud even at 1/4 the power.

    @johnDH , thanks for that thread, I wondered what is the effect of the speaker damping? For instance, I was planning to use a 10 inch 4 Ohm Jensen in a Princeton build with an 8ohm speaker output, simply putting a 4W resistor in series, I couldn't think of any downside - but am I right? TIA.
     
  10. chemobrain

    chemobrain Friend of Leo's

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    lol! that's very funny.
     
  11. johnDH

    johnDH Tele-Meister

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    Yes that might sound fine, or might sound thin.

    If you want to get close to a natural speaker tone from even a simple resistor set up, you have to take account of the real output impedance of the amp, and its this that affects damping. This is in addition to making sure the am sees the load that it needs. This output resistance varies according to the amp design, and how hard the amp is working, but it is generally several times the nominal ohms. My Marshall VM, on an 8 ohm tap, has a small signal output impedance of about 20Ohms. It would be about 10Ohms with its 4 ohm tap.

    The worst thing about simple attenuators based on LPads is when this gets ignored, and the speaker ends up seeing a very low ohms which damps out its highs and lows.

    So, consider what you are proposing, taking my amp as an example.

    The 4 Ohm speaker is used to seeing say a 10ohm output resistance. You will run it off an 8 ohm tap, with an extra 4 ohms. So the speaker will see around 24ohms. It should sound pretty lively, or it may sound a bit harsh/thin etc. Its safe to try. A refinement could be an extra resistor of about 20 across the speaker, and redo all the math to suit.
     
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  12. Paul-T

    Paul-T Tele-Meister

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    Ah, got it. Many thanks.
     
  13. mcentee2

    mcentee2 Tele-Meister

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    I think this touches on something I "nearly understood" when putting together a load box recently (thread is on here somewhere).

    The idea came from @robrob 5e3 mod page where he lifts the attenuated output resistor set up from the Clapton Tremolux in order to attenuate the second speaker out level.

    I looked at and researched what LPads were, how they worked and how they seemed to be used in simple guitar amp attenuator builds.

    I looked at, amongst many, the Weber calculator, and they all showed really strange resistor values Vs the Tremolux version.

    This took me age to understand why some had "large" resistorsv vsthe "small" calculator ones.

    A really good tutorial for the theory is https://www.electronics-tutorials.ws/attenuators/l-pad-attenuator.html

    It shows how the "large" and "small" pairs can be derived from the way that you use the Lpad circuit and "which direction" you are looking at, towards source or towards the load.

    So eventually I went back to the Tremolux circuit, and robrob's, and it struck me that when going from the OT to the speaker the attenuated output has the *parallel resistor first* then the series.

    Most Lpad layouts seem to always have the OT into a series resistor then have the parallel across the speaker.

    I haven't followed all the theory when it strays into variable AC impedance with frequency for both source and load, but I have a feeling this is where a lot of issues with using LPads withwthe "traditional" circuit layout come from re losing treble as you turn them down, or justjplain tone mismatches.

    Having seen this thread and the quoted post and now learning about output "damping" interplay between the OT/Amp output/source impedance Vs the speaker load impedance...

    I might be way off here, but my gut is saying there is something to be gleaned from it re how LPads do or do not work well.

    FWIW, my load box is built with two positions, a .25 power 16r parallel/ 8r series, and a .1 power12 parallel/ 16 series pairs and it doesn't seem to suck tone as far as I can tell over and above any derived from just volume loss.
     
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  14. mcentee2

    mcentee2 Tele-Meister

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    Double post!
     
    Last edited: Feb 28, 2019
  15. radiocaster

    radiocaster Poster Extraordinaire

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    Also tried a simple circuit without the Lpad with 2 50W resistors. Simply a 6.8 ohm in series with a 1 ohm in parallel with the speaker.

    I liked the sound of the Lpad circuit better with the Greta, but the results could be different depending on what amp you use the attenuator with.

    The simple circuit provided a little less attenuation than the Lpad circuit I tried earlier. Neither provided enough attenuation for quiet playing at night with a 12" speaker while others are sleeping at your place (except in a more isolated room).

    Note that the Greta (without any attenuator) with the 12" at even half makes furniture around the room rattle. With the attenuator it doesn't.

    You could probably reduce the 1 ohm resistor to something under an ohm for further attenuation.
     
  16. radiocaster

    radiocaster Poster Extraordinaire

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    Is there any problem if the case is grounded on this?
     
  17. johnDH

    johnDH Tele-Meister

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    Best to use plastic jacks and keep the case ungrounded. The amp taps are not intended to be grounded. But, im not sure of what problems occur if you don't.
     
  18. radiocaster

    radiocaster Poster Extraordinaire

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    That's going to make it so much more complicated to attach the L-pad to the lid if I can't ground the case. In fact, I don't know how to do it. It keeps spinning around when I turn the knob.
     
  19. johnDH

    johnDH Tele-Meister

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    Although an Lpad isnt quite like a normal pot, most pots have the lugs that go to the resistive elements not connected to the shaft and case. Is yours not like that though? Ive not used an Lpad myself.
     
  20. radiocaster

    radiocaster Poster Extraordinaire

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    It's like a gigantic potetiometer, but it's missing this bit.

    800px-Potentiometer.jpg

    Also when I screw down the nut all the way, it's not tight like on a pot, so it keeps turning.
     
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