Simple Attenuators - Design And Testing

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

  1. Jordan Prysmiki

    Jordan Prysmiki New Member

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    Is the first pic the finished product or the stated project for the inductors?
     
  2. matttornado

    matttornado Well-Known Member

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    Yes they are the finished products. They were measured wit hthe meter set at 100Hz
     
  3. JohnH

    JohnH Well-Known Member

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    Interesting! they are wound onto cores, which means you get the inductance with far fewer turns. All the other builds are using air-cored, and I started with those since that is whets used in loudspeaker crossover networks, which is a similar application. I think the reason there is to avoid core saturation, and at that point im all out of tech! I don't know what the consequences would be except I cant imagine there's any risk, Can any one speculate on using a ferrite or iron cored coil in a loudspeaker application?
     
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  4. Nik Henville

    Nik Henville Well-Known Member

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    If memory serves - cored coils start to behave non-linearly as the current increases. This in turn leads to distortion. Don't quote me, though - experts WILL be along soon...

    :hippie::uk:
     
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  5. TonyK

    TonyK Member

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    @Jordan Prysmiki Very nice looking work there. I wish I could get my wiring dressed as tidily as that!

    I've had a little time to try out the attenuator now with both my Hot Rod Deville 410 (60W) and my BassBreaker 15. So this is the first time I've ever heard my HRDv dimed! Disclaimer -- this ain't no Marshall!!! I've recorded a few samples so that people can hear the tone at different attenuation levels.

    This is a very abbreviated "Cocaine". I recorded the riff clean into a Boss looper pedal, and then played it through a Hot Rod Deville 410 dimed. Starting attenuation was -14db (too loud to start with no attenuation) and then the following segments are respectively -17.5db, -21db. -24.5db, -28db, and -31.5db. The spl in my room started around 92db and finished up at between 70 and 75db -- easily bedroom level.

    The second sample is the same but with each music segment normalised to -1db.

    I'm posting this little piece to highlight just how good the attenuator really is, and also to show what a Hot Rod sounds like dimed! Obviously this is the clean channel, and I use a 12AY7 in V1 specifically to manage the volume and tone better at lower levels, but now I will probably put back in the 12AX7 to see what the more normal breakup is like.






    Thanks a million to @JohnH :)
     
  6. JohnH

    JohnH Well-Known Member

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    Thanks @TonyK , very helpful and they sound great!

    I appreciate the full volume issue. When I do maxed out sound samples I need to put on the same ear muffs I use when working with my chainsaw

    This shows how you don't need to spend hundreds to get a small low powered amp with a tiny speaker to play at low volume, if you already have a full size amp and can wire up an attenuator circuit. Nor do you necessarily need an expensive load box plus digital IR processing and re-amping to simulate the tone of a real amp at low volume, when you cam just play a real amp at low volume. And as long as you can have a bit of volume (like at least vocal volume), you can mic it too.

    That being said, the latest digital solutions to working with valve amps (like, OX, Boss etc) are wonderful devices with awesome capabilities and technology. Its just that if you just want to play the amp, there are alternatives . I kind of like how the simple robust technology of our attenuators fits in with the simple analogue tech of a nice old amp.
     
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  7. Jordan Prysmiki

    Jordan Prysmiki New Member

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    JohnH well said bro!
     
  8. Jordan Prysmiki

    Jordan Prysmiki New Member

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    @TonyK , nice clips mang.

    I compared the -17db intro of your clip to the last -31.5db clip over and over on the -1 normalized version and heard what I thought were some tonal differences.

    The difference was that the initial first clip had a longer tail to each note and stop as well as some more openness. I then realized that since you were recording at louder volumes in the first clip that the amp probably generated more room ambience for the mic to pick up actually adding to the perceived tone.

    It sounded good regardless, just pointing that out.

    Have you cranked your master volume yet to hear the power amp tonal differences at lower levels? If so what are your thoughts on that?
     
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  9. Edchen

    Edchen New Member

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    I salute you John and all that contributed to the design of this fantastic attenuator!

    I am very proud to confirm that I’ve built the 16ohms version using 100w rated resistors and the result is truly superb!!!

    I cannot believe I almost forked out £1000 on The Ox when the cheap resistive attenuator I bought from Thomann ended up sounding extremely dull and lifeless.

    I am finally able to fully appreciate the tonal capabilities of Victory V30 MK2 without blowing off the roof!

    I have attached some pics of my build which will hopefully help others in the future.

    Best regards
    Eddie E8DCE210-0AB6-421F-922B-4588EFCAA3F5.jpeg attenuator 1.jpg attenuator 2.jpg
    E41884DC-C950-4C2D-B6CE-5808278D8160.jpeg
     
  10. JohnH

    JohnH Well-Known Member

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    Hi @Edchen
    Thanks, and that's a great looking build. Awesome graphic and I like the drilling too! I'm glad it worked out.
     
  11. Filipe Soares

    Filipe Soares Well-Known Member

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    can it be scaled up to 400w?
     
  12. JohnH

    JohnH Well-Known Member

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    All the theory scales up, but what kind of amp are you thinking of? is it a bass amp? Is it tube or ss? This design is based around tube driven guitar amps and guitar speakers.

    And if there is 400W at play, much more attention to cooling would be needed (eg fan cooling) plus much greater current capacity in switches, inductors and wiring.
     
  13. Nik Henville

    Nik Henville Well-Known Member

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  14. Jordan Prysmiki

    Jordan Prysmiki New Member

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    @Edchen, nice build man! How you get the holes so perfect?
     
  15. matttornado

    matttornado Well-Known Member

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    That looks awesome!
     
  16. Filipe Soares

    Filipe Soares Well-Known Member

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    It's a Marshall Major, 200w. I said 400w because there's the thumbrule (Don't know how true it is) : attenuator capacity = 2x amp power. so there's why the humongous wattage
     
  17. JohnH

    JohnH Well-Known Member

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    OK cool! (or hot) There'd never be an amp more in need of an attenuator than a Marshall Major.

    Just a watchit, I'm thinking that's a old amp. So if you use an attenuator to allow it to be pushed, it may be working a lot harder than it ever did

    The basic component power ratings for 50W are chosen to have plenty of capacity for an actual 50W amp. So for the 200W Major, a x4 scale up is indicated. If a 16ohm version would suit, I'd suggest that instead of 8ohm, since less current is needed.

    Where the 50W version uses a 25W resistor, use a 100W instead of the same value. Where 100W was used, you can use 4x 100W in series/parallel. Build it big and wide with lots of space and ventilation on a heavy Aluminium chassis, with all resistors chassis mounted using thermal grease

    Inductors could be based on 14ga wire. Do you need full bypass switching? Maybe not needed? Even if -7db is the least attenuation, that's still a very loud 40W unit for a 200W input.

    If you do need full bypass, with a 16 ohm version, you are switching nominally about 3.5A 10Amp switch rating would be good.

    If you build it, I wish you luck and I reckon it will be awesome.
     
  18. Filipe Soares

    Filipe Soares Well-Known Member

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    first of all, thank you for your help. I do not want to push too hard on the metal grandpa. just enough for the tubes to work a little. just enought to work at half power without me being arrested, divorced or kicked out of the building. I have no plans to ¨use it as a plexi¨ , all on 10 like yjm.

    my major has a master volume installed, so the idea is to put the master volume at half and be able to play it without committing any felony LOL.

    I´ll start sourcing the material, maybe I´ll put a fan to force ventilation through the box.
     
  19. tmingle

    tmingle Active Member

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    Sorry guys, I had to do it.

     
  20. TonyK

    TonyK Member

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    Jordan, you're right about the speaker output levels and the effect this can have on the perceived tone. That's exactly why I generally enjoy playing around a 90db level and not at bedroom levels. The whole experience is just... better:) But I am amazed at the overall tonal characteristics of John's attenuator design. I was planning on building a 5e3 (in a few weeks) but now I'm wondering if I'd be much happier with one of the larger tweeds given that I'd be able to push it into power tube saturation with ease!

    Relative to your question on the master volume, I think you're talking about the (in)famous Drive and More Drive channels on the Hot Rod (how bad are they, really). I recorded a riff with the master on full, starting with the Drive at 3 and then gradually increasing it plus also repeating the riff with the More Drive each time. But when I'm playing it back, for some reason I can hear no difference at all - certainly a recording error. I'll redo the experiment as soon as possible, including, once the amp is totally dimed, then lowering the attenuation by -3.5db for each segment, so that the clip will also give another "hearing" to the actual attenuation. If it works, and with John's permission, (and with a link back to this thread) I'll post some of the clips on another forum that is more Fender orientated as it would be of potential use to those readers.
     
    Last edited: Jul 12, 2019

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