Simple Attenuators - Design And Testing

Discussion in 'The Workbench' started by JohnH, Oct 21, 2017.

  1. mAx___

    mAx___ Active Member

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    Love this thread!
    Question: I need close-to-no attenuation because my 4x12 lives in an iso cab. -3dB would as much as I need to protect the speakers but I'd also like to try -2dB. Can those levels be achieved by adapting values in John's design?
     
  2. JohnH

    JohnH Well-Known Member

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    Sure, see page 11, Jan4th post. There are single srage designs down to -3db. Could do a -2 as well.
     
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  3. assaf110

    assaf110 Well-Known Member

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    JohnH,
    I'm in the process of final assembly, wiring the attenuator. A silly question, why SW1 is not bypassing stages 1-4, only 1-3?
    Is it to allow a minimum attenuation of -3.5db?
    Sorry if it has been answered before and i missed it.
    Also, just to clarify, is it a connection point between L1 & L2 (left of R2 on the diagram) or the cables (L1 to R2 and R9 to L2) bypass each other?
     
    Last edited: Jun 26, 2019
  4. assaf110

    assaf110 Well-Known Member

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    .
     
    Last edited: Jun 26, 2019
  5. JohnH

    JohnH Well-Known Member

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    hi @assaf110

    Yes L1, L2, R2 and R9 are all connected to each other at that point.

    As you describe, my bypass wiring was to give the option of running the -3.5 db stage on its own. Otherwise the first step is -7 db when the reactive Stage 1 is engaged but others are off. I figured that for some users who just want a small reduction for a gig, this could be useful. I wanted to try it in my build just to check it out. And it works fine, even though its just a resistive stage when on its own. You can hear it as the second-to-last stage in the sound clips a few pages back.

    But for many, including my own actual use, a full bypass is fine.

    I'm looking forward to hearing about how yours sounds with your SV!
     
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  6. matttornado

    matttornado Well-Known Member

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    JohnH

    I'm a little confused. I usually set my amp to 8 ohms when using a single 16 ohm cab or 2 16 ohm cabs (amps stays on 8 regardless if I'm using one or two cabs).

    So what do I build, the 8 ohm tap or 16 ohm tap? I still want to keep the amp at ohms but still use one or two 16 ohm cabs.

    Please advise?

    Thank you so much!
     
  7. JohnH

    JohnH Well-Known Member

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    Hi @matttornado

    Thanks for your post. Id say build the 8 Ohm version to use with your 8 Ohm tap. You can run 1 or 2 16 Ohm cabs out of that.

    The 8 Ohm version has, if you build it (parts in the dotted red box), an extra output socket for 16 Ohms and a couple of extra resistors to adjust the tone for use into a 16 Ohm cab. Its a subtle and optional tweak, and its also safe to use a 16 ohm cab out of the normal outputs too.

    What kind of amp do you have? Obviously, there is a nominal mismatch when you use it at 8 Ohms straight into a 16 cab, but clearly it works for you. The attenuator will take care of that though, so you can run the 16 cab and 8 ohm tap with no mismatch seen by the amp. That might be a good thing, since presumably with an attenuator, you'll be running the amp at higher power than before.
     
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  8. matttornado

    matttornado Well-Known Member

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    Thanks JohnH!

    i play a 1974 100 watt superlead. it is a monster!
    i have a 4x12 cab with 25 watt greenbacks & a 1x12 with a 150 watt Eminence Patriot speaker.
    sometimes i run a wet/dry rig along with my DSL too.
     
  9. JohnH

    JohnH Well-Known Member

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    Ok!
    Well I'd expect if that was my rig, the Superlead just into the 4x12 would be the main rig, and I'd run that all at 16Ohm with the 16Ohm attenuator. With that, you can still plug a second 16 cab in.

    The attenuator gets the most even attenuation steps and on paper, most consistent tone if attenuator, amp tap and speaker load all match. But both the 8 or 16 ohm versions, if matched to the amp tap, can run with 8 or 16 Ohm speaker loads.

    An advantage of the 16Ohm version, when running a very powerful amp is that there is less current flowing than with the same power at 8 Ohms. So less stress on switches and jacks.

    So what to build?

    If you need to use an 8 Ohm tap, then an 8 Ohm attenuator.
    Otherwise, build the one that matches the most important or commonly used rig, with a bias towards 16Ohm if no other preference.

    Choices!
     
  10. matttornado

    matttornado Well-Known Member

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    I think I'm going to go with the 16 ohm tap.

    Now it's time to start getting the parts. My boss & I just made the two inductors.

    Can I set the amp at 8 ohms using the 16 0hm tap?
     
    Last edited: Jun 28, 2019
  11. Jordan Prysmiki

    Jordan Prysmiki New Member

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    Hey guys just a quick follow up and a question for JohnH below.

    I had a chance to replace my R5 resistor and now my -14 switch works. Wow, what a massive volume reduction with all 3 switches (-7, -7, -14) it's amazing!!

    My goal was to be able to use the amps master volumes at higher levels and now I can absolutely achieve power amp saturation at bedroom volumes!

    I might have noticed a bit of a high treble roll off but I could be mistaken as their were a few other distractions in the room. If what I noticed was accurate I put probably make that up with the treble and presence controls. I will know more after more playing.

    With just a bit of playing I am now seeing the necessity for having that -3 switch in addition to what I have now, as you all said, for "fine tuning". Maybe I will build another.

    @JohnH, do you know what value resistors I would need for -5 or -10 reductions @8ohms? Let's call them R9 and R10 to stay consistent with the diagrams. Could I just add a hypothetical R9 to ground and R10 anywhere after the R5/R6 circuit (or even sooner in the circuit)?
     
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  12. JohnH

    JohnH Well-Known Member

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    A bit of clarifying needed to answer the question: By 'tap', Im meaning the output of the output transformer (eg, which is tapped for 4, 8, 16 Ohm speakers), ie it is the amplifier output socket. You should match that to the version of the attenuator that you build.

    So whats the plan?

    Also interested in your inductors, what kind of wire and wire gage did you use? A photo would be great!

    A couple of thoughts on the tone difference you are getting with 8 or 16 Ohm amp outputs: The way that the speaker impedance (which varies with frequency) shapes the tone depends on the real output impedance of the amp. The higher it is, the more the speaker will develop a bass resonance and a treble lift. Lower impedance out of the amp will, for a given speaker, reduce the highs and maybe the bass and favour the mids. I reckon this is what is happening when you plug a 16 Ohm speaker into your amp set at 8 Ohms. On my VM amp, the 8 Ohm output has an output impedance of about 18-20 Ohms for small signals, and with a 16 ohm output it would be 2x that.

    BTW, this effect is partly why some SS amps can sound lifeless, and why the usual L-pad speaker attenuators sound dull at low volume. Both of these get very low output impedance, squashing the ability of the speaker to develop its treble and bass tone.

    Our attenuator design takes account of these effects, and the current designs are matched to my amp measurements which I think are reasonable ballpark values for a range of amps. Whether you prefer the tone of a 16 or 8 ohm cab and which attenuator design it is into will likely depend mostly on the relationship between the attenuator and the speaker, rather than the amp and the attenuator. Luckily, once you are running the attenuator, it takes out any issues of mismatch to the amp and you can experiment.

    So out of that, Id still recommend to match the nominal output Ohms of the amp (ie 8 or 16)to the attenuator version, just for the safety of your classic amp when you get to run it at high power
     
  13. JohnH

    JohnH Well-Known Member

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    If you wanted to have a switched stage to do -5 db, or -10db, the values would be, for an 8 ohm attenuator:

    -5db
    R9 (to ground) 25 Ohm
    R10 (series) 8 Ohm

    -10db
    R9 10 Ohm
    R10 15 Ohm

    These could be wired in, like the other stages, anywhere after the first reactive stage
     
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  14. Jordan Prysmiki

    Jordan Prysmiki New Member

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    So I have a 20 watt amp. Are these calculations correct for the switches I went with?

    -7db = 2.86 watts
    -7db, -7db = .41 watts
    -7db, -14db = .20
    -7db, -7db, -14db = .03

    I believe so as my last calculation for -28db is close to yours for -30db w/20Watt amp
     
    Last edited: Jun 29, 2019
  15. JohnH

    JohnH Well-Known Member

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    hi @Jordan Prysmiki

    Not quite the right maths. db's are a log scale. Although -10db is a factor of 1/10th, -7db is not a factor of 1/7th but is in fact a factor of 1/5th. ( actually 10^-0.7)

    -14db is a factor of 1/25th (ie like two -7db stages: 1/5 x 1/5 = 1/25.)

    So starting with a 20W amp, you'll get....

    -7db = 20W/5 = 4W
    -7 -7db = 4W/5 = 0.8W
    -7 -14db = 0.8W/5 = 0.16W
    -7 -7 -14db = 0.16W/5 = 0.032W

    ...so you'll arrive in about the same place by the end! Each step will sound like a roughly equal reduction, since hearing perception of volume is also logarithmic.
     
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  16. Jordan Prysmiki

    Jordan Prysmiki New Member

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    Man you are wizard! I really appreciate that!

    It's amazing how loud the 4 watts really is still.

    Keith Williams from the "Five Watt World" youtube channel says 5 watts is all you need for your home studio or practicing. I know that is subject to opinion but I completely agree. We naturally think wattage and loudness are a ratio and it's really not. As you said it's logarithmic and the attenuator really helped bring this home for me with knowing just how loud different wattages actually are.
     
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  17. Jordan Prysmiki

    Jordan Prysmiki New Member

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    Alright alright. I'm as done as done can be. Here are the pics!

    The case I built the attenuator In has mounting ears on the back panel. I chose this so I could mount the attenuator in my custom ©stagecraft.com BluesJack 2x12. I made custom connections from the cabinet's input jack, to the input of the attenuator and back to the speakers. I did this to eliminate extra cables on the rear of the amp and for cool factor.

    IMG_4501.jpg
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    IMG_4520.jpg
    IMG_4521.jpg
    IMG_4526.jpg
     
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  18. matttornado

    matttornado Well-Known Member

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    So simply put, with an 8 ohm tap amp output, i should build the 8 0hm attenuator and use either one or two 16 ohm cabs, right?

    makes sense now. i will post some pics and info of the inductors.
     
  19. JohnH

    JohnH Well-Known Member

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    Yes, that sounds like a plan!
     
  20. matttornado

    matttornado Well-Known Member

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    here are the inductors my boss & I made. IMG_1669.jpg IMG_1670.jpg IMG_1671.jpg IMG_1670.jpg IMG_1671.jpg
     
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