Simple Attenuators - Design And Testing

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

  1. matttornado

    matttornado Well-Known Member

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    Hi John. Is there a way to do it so i can leave the resistors in the attenuator?
     
  2. Northtone

    Northtone New Member

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    Hi John, this sounds interesting.
    So, to use the attenuator as a reactive load, I would need a line-out.
    I can follow the post from the user matttornado, right (p. 41, post 814 - unfortunately I can't add links yet)?
     
  3. JohnH

    JohnH Well-Known Member

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    I think they look fine, the right contacts and plenty of current capacity. You just have to work a line of neat rectangular holes in your chassis, but the switch looks like it overlaps the edges a bit to hide any small inconsistencies.
     
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  4. JohnH

    JohnH Well-Known Member

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    Hi @Northtone , This was the post with @matttornado 's line out (and i agree with the comments):


     
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  5. JohnH

    JohnH Well-Known Member

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    Probably. The switch for each stage has three lugs and you could send it out with a similar 3 core cable that I was showing, using 5A flex. Youd build an extra set of resistors in the main box, and use a switched jack so its bypassed when you don't use the footswitch.

    The advantage of what I was showing was that you just have to add the jack insert in the main unit.

    I think the footswitched stage, wherever it is, is best as an additional stage so that you still have access to all the other stages and db settings

    Or, set up a relay system triggered by dc - Then the speaker signals aren't going out to the floor box. Untested though, Id worry about dc pulses being picked up by other cables.
     
  6. ThePanda

    ThePanda New Member

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    Just a quick few questions.

    What do you do with lug 3 of the pot? Is it not used? And what would the benefit be to have this Line Out be after stage 1 instead of seeing full power?

    If there is a significant difference I was thinking of adding a switch to go from 'full' to 'stage 1'.

    Thank you!
     
    Last edited: Sep 19, 2021
  7. JohnH

    JohnH Well-Known Member

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    Just to note that this isn't our diagram, so Id have drawn it differently, but its basically ok. The right lug of the pot needs to go to the 'cold' connection, ie the wire that goes across the base of our diagrams, and joins the jack outer barrels. We don't really have a ground to the chassis.
     
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  8. matttornado

    matttornado Well-Known Member

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    Hi John. I built the M4 version without output 3 in dotted red lines.

    I'm willing to add a footswitch with resistors so I need to:

    1. add an additional stage after the -3.5db?

    2. for a solo boost would I want a 7db or 3.5 db stage?

    3. what values would I use for the footswitch? I have a 16 ohm build.

    4. Do I simply just Follow the diagram above and add 7 db footswitch stage with same values I already used with the 1st 7 db stage?

    I really like this because I always struggle with a good volume boost for solos. This is my ticket!!!!

    Any help or additonal diagrams would be super appreciative. Once Iwrap my head around this, I'll be good to go. I looked at the diagram with the footswitch and that raised these questions.

    Thanks!!!!:)

    full.gif
     
    Last edited: Sep 20, 2021
  9. matttornado

    matttornado Well-Known Member

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    FYI: From my experience - I tried the line out at the input of the attenuator (100% amp power) and at the end of the attenuator (attenuated output) & it works best at the attenuator input at 100% because this way you'll always have enough line out signal. When I had the line out hooked up at the attenuated levels, my line level was really weak when heavily attenuated.
     
  10. ThePanda

    ThePanda New Member

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    That makes sense I will put it at the input.

    What are you using to capture your line out?

    Just ordered the parts this morning.
     
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  11. matttornado

    matttornado Well-Known Member

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    I run my line out into the FX return on my DSL40CR
     
  12. JohnH

    JohnH Well-Known Member

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    Hi Matt, I had a look back at your build pics, which look very classy indeed

    I note that its a 16 Ohm build. I'm not seeing a bypass switch either? is that the case? if so that helps and makes the addition of the footswitch more fool-proof. I can help with diagrams.

    Also, what are the usual levels of attenuation that you use?

    As for the footswitch db change, Don found that a -3.5db step worked best for him. Before that, i would have guessed -7db. Values for these would be the same as in the main box. It would be a good idea to try with the levels of change that you can get with the current switched stages to judge this for your own use. If you decide that something in between would be most ideal, its easy to work out values for that too. It is also possible to add another switch into the footswitched box to make its value changeable.
     
  13. matttornado

    matttornado Well-Known Member

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    Hi John! yes NO bypass switch so my amp constantly sees that huge 300 watt Resistor.

    I don't remember what level I had my attenuator set at the last time I gigged with it to be honest, sorry about that. I'd like to start with a -3.5db stage for now and maybe tweak later?

    I pretty much understand your footswitch diagram except for the added Footswitch female jack at the attenaator.

    I can't quite make out how it is wired.
    Are the female jacks switched? I would imagine the one at the attenuator would have to be so it can work without the footswich connected. Not sure about the one at the footswitch.

    I might hard wire the cord at the footswitch with a strain relief like a typical amp footswitch. I might use my footswitch that came with my DSL! HAHA!

    When I get this all together and working I will post many pics!

    Thank you so much!
     
  14. JohnH

    JohnH Well-Known Member

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    Yes the jack at the attenuator is a switched stereo jack with 3 pairs of lugs. When there's no plug, the main contacts each connect over to the respective lug on the other side With a plug inserted, this contact is opened. It's the kind of socket that might usually be used to cut out a pair of speakers on a stereo when you plug in headphones.
     
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  15. matttornado

    matttornado Well-Known Member

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    Thanks! I don't understand how that gets wired at the attenuator yet. not sure I understand the connection lines on the diagram.
     
  16. JohnH

    JohnH Well-Known Member

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    I'll draw something in the next couple of days
     
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  17. matttornado

    matttornado Well-Known Member

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    Can I get away with using 25 watt resistors in the footswitch since it will be so far down the attenuation chain? I believe I have 100 watt resistors for the 3.5 db stage inside the attenuator.
     
  18. JohnH

    JohnH Well-Known Member

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    On the M version that you have, the full diagram lets the -3.5db stage run on its own, so it would then take a lot of power and hence high ratings. But with yours, I think the -7db stage at least, is always on. So if you also have a -3.5 stage after it then, then according to numbers, and based on 100W coming in (i think you have a 100W head?) then the 75 Ohm will get 6W and the 12 Ohm will get 10W. My rule of thumb is x3 on that for the component rating. So based on that, 25W is just below for the 12 Ohm, but almost certainly not a problem, so long as its bolted with thermal grease etc to the aluminium box.
     
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  19. Graham G

    Graham G Well-Known Member

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    John, to avoid using a steel Bolt to fasten the Air Core to the case(aluminium), would it be OK to use hot melt glue on the plastic spool to attach to the case, or not advisable ?.
    Thanks.
     
  20. JohnH

    JohnH Well-Known Member

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    Hi @Graham G , hot melting glue may not be the best, since it could melt it it gets hot. A dab of silicone would be better, or zip ties. or a non steel bolt
     

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