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BWM Attenuator

Discussion in 'The Tone Zone' started by Markfresh, May 11, 2021.

  1. Markfresh

    Markfresh Active Member

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  2. PelliX

    PelliX Well-Known Member

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    Never seen it before, but I'll bet you two cold ones that it is in fact "Ohm specific". It states that it uses/contains a rheostat (a.k.a. L-pad), which is essentially just a beefy variable resistor. Think of it as a tough, high power handling potentiometer. Basically, 9 out of 10 valve amps will happily work with an ~8-16 Ohm load attached to either their 8 or 16 Ohm taps. Even 32 Ohms on the 16 tap will generally not result in immediate damage. My guess is they just slammed an 8 Ohm rheostat and some high power resistors in there, then have the cab/speaker attached in series, thus putting the impedance in the 'non catastrophic immediate failure' range. I wouldn't buy it without a look at the schematic. If my assumptions are (for the most part) accurate, then you might as well get something cheaper or build one yourself. A resistive attenuator is really one of the most basic circuits there is.
     
  3. JohnH

    JohnH Well-Known Member

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    It looks like a small business, having a go, I wish them luck. But I would not connect that to any of my amps. It must have some Ohms value and they cannot possibly claim that it is suitable for any given amp without any specs or other info either way.
     
  4. JohnH

    JohnH Well-Known Member

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    Just speculation, and I don't know .....

    The general arrangement if it is very similar to types by Pan/KLD and also Airbrakes. ie,

    one rotary switch, one pot
    true bypass then four switched -1.8db steps
    the final step engages the pot to go down to -30 db

    If that's what it's based on, then its better known. Airbrakes can sound quite good but don't pay much attention to showing consistent ohms to the amp.
     
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  5. Markfresh

    Markfresh Active Member

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    Thanks for the reply's ,
    I did read that yes it is a copy off the dr-z airbrake, which i cant find anything to show that they harm a amp.
    I have access to one at 1/3 of rrp used, but starting to think that no set ohms and used may be not worth it, as saving money here could cost me alot more in the long run.

    download.jpg
     
  6. AdrianDSL

    AdrianDSL Active Member

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    @Markfresh
    Does your amp have an FX loop? You could try a volume pedal, or even a graphical EQ in the loop, crank the volume around or above the sweet spot and control the overall volume from the pedal. That's how I've been approaching my tone settings recently and it really gives me that Marshall growl and grind at bedroom level. (Oh well, it did until 10 days or so ago when I managed to fracture a bone in my wrist, so yeah, taking the rubbish out can be dangerous business :-()
     
  7. PelliX

    PelliX Well-Known Member

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  8. Markfresh

    Markfresh Active Member

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    Hi,
    I do have a effects loop but i find the origin has its sweet spot with the master at about 2 o"clock, which is why im trying to take it , as even on the lowest power setting its loud.
    And by putting a volume in the effects loop this stops the power amp from being pushed etc.
    And damn what did u have in that rubbish? concrete?

    And
    Hi, im still a bit confussed about the lack of ohm selection, and just reading the link u posted , it says that that design is to run between 4-8 ohm,
    but then goes on to say that its needs to match a 8ohm speaker.
    So i imagine thats because it can handle 8ohm and then down to 4ohm.
    Which kind of makes sence to me with my lack of knowledge.

    But the BWM one im looking at doesnt state any of this and my speaker and head is 16ohm so i have no way to know what it can handle etc.
     
  9. AdrianDSL

    AdrianDSL Active Member

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    Tripped on kids' small trampoline, so small I didn't see it!
     
  10. PelliX

    PelliX Well-Known Member

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    In a nutshell, resistors in series increase resistance, and two equal resistors in parallel halve it. You could just modify the existing 'parts list' and get a 16 Ohm L-pad, double the resistance of the other resistors, then you would have your 16 Ohm setup. If you're confident you don't need other impendences, then skip the 'multiple input' part, obviously.
    Also, the design is intended to present 4 or 8 Ohms to the amplifier - your use of the word 'between' might be misleading. Not trying to nitpick here, just clarifying. The capacitor values would remain the same (if you actually want the bright switching - some people prefer to regulate that from the amp instead, etc).

    Take a look at the schematics, it's a 'penny drop' thing. Once the penny drops, it will be clear as day to you. If you're still unsure afterwards, I can see whether I can knock up a concise parts list and layout for you based on the 16 Ohm impendance you're aiming for. :)
     
  11. Markfresh

    Markfresh Active Member

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    Thanks for the reply,
    I might of been confusing in my post.
    I was just comparing your link to the BWM i was looking at.

    What i was trying to say is based on whether i should buy the BWM as the one in your link stats what it does which is fine... but the one i was looking at doesnt
    and i cant tell the l-pad rating so not sure whether to purchase it or not.
    I wasnt looking to build one, and there is also another great design on this forum but its a bit out of my build confidence, a pedal i can do with intrument signal, but with speaker single im not so confident in my work haha.
     
  12. PelliX

    PelliX Well-Known Member

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    Right, gotcha. Well, I wouldn't buy it without a schematic first, and if you get said schematic, I'd love to see it, too.

    Although the pedal might appear less 'risky' in the short term, the attenuator is a lot simpler as far as construction is concerned, and you can check whether you got it right by A) just probing with a multimeter or B) trying it on a solid state amp. Impedance mismatches matter much, much less on solid state so even if you've only got a 4 or 8 Ohm SS, you can check whether your attenuator is operating as expected.

    Totally up to you, though, but if you're looking for a cheap non-DIY solution, the Bugera PS1 is basically just a Jettenuator with a prettier case. Harley Benton make a clone, too. I wouldn't generally recommend those sort of products, but for this simple purpose...
    As far as I'm concerned there is no point in investing much more unless you move to the 'next level' of attenuators like the Webers which actually have real speaker loads, etc.
     
  13. Markfresh

    Markfresh Active Member

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    Yeh i was going to go for the Bugera ps-1 the reason i was looking at the BWM was that the bugera is $160 and the used BWM is $60 .
    But i will look again at the DIY as i do like a project :p
     
  14. Markfresh

    Markfresh Active Member

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    Also with that DIY one, would i swap out the 8ohm 50w resistor for a 16ohm and the L-pad for a 16 ohm?
     
  15. PelliX

    PelliX Well-Known Member

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    Mark: yes, though instead of a 16 Ohm 50W resistor, you could of course use two 8 Ohm ones in series ;)

    One thing that might not be obvious is that most L-pads differ from a pot in that the resistance measured at the (let's just call it the) input remains the same regardless of the wiper position. So, in theory, just an L-pad of that type would be sufficient to reduce volume. Also, don't go by the 'max' rating on them. I wouldn't recommend them being used over about 40-50% of the rating (so maybe put 40W on a 100W L-pad), but the more headroom you have, the better. I think the guy says something similar in the article.

    EDIT: afterthought, presuming the guy will tell you what L-pad is in it, why not buy it and make it what you want? You get a case, connectors, probably all the kit you need bar some solder and maybe thermal paste. Might work out cheaper and easier than making one yourself. This was partly my reasoning behind getting the Bugera. It works as a load box and has a nice case. When the time comes, it will be re-purposed. Nice female jacks and XLR, little transformer coil, some high power resistors and an L-pad... never bad to have around, right.

    Even if the L-pad is the 'wrong' resistance, you can chain it with a resistor. If it's something really high like 32 Ohms put one in parallel. These solutions will limit the range of attenuation and/or incur a certain drop in volume even with the L-pad dialed back, but that's actually how many (like the PS1) work, anyway.

    EDIT #2: To clear that up, the PS1, Jettenuator, etc incur a "50%" volume decrease by definition, and control the level between that and total silence. There's a schematic of the Jettenuator floating around on the interwebz somewhere, if you're interested.
     
    Last edited: May 12, 2021
  16. Markfresh

    Markfresh Active Member

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    Actually thats not a bad idea, i could grab it, open it up see whats in it and modify it if needed with some guidance of course.
    and if worst comes to worst i could take the guts out and keep the input/outputs and build a pedal into it etc.
    If thats where u were heading.
     
  17. PelliX

    PelliX Well-Known Member

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    We have a cache hit. That means we're on the same page :thumb:
     

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