Simple Attenuators - Design And Testing

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

  1. Gene Ballzz

    Gene Ballzz Well-Known Member

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    @JohnH

    Ya know, I'm betting that "PERFECT FOR OUR USE" switches are out there and available at reasonable prices. I'm just not certain of all the search criteria/details to find them. When dealing with companies from "Chinesiawanoreanam" the language barrier, variations and discrepancies can make things especially confusing and/or complicated!

    With that said,i've come to feel as John does about the "binary" toggle usage. While it may seem crude at first glance, they're laid out in a manner that keeps the user away from electronic trouble and/or possible amp damage! The only potential danger may be using any of these (16 ohm or 8 ohm amp tap version) with a different speaker impedance than the amp tap rating and then simply flipping a direct bypass switch. While the speaker impedance is not critical when attenuator is actually in use, an 8 ohm or 4 ohm speaker could potentially get connected to a 16 ohm amp tap, in bypass mode, etc!

    Considering all that, if anyone finds an appropriate 8 to 10 position rotary switch, at reasonable pricing, please share the information?

    Just My $.02,
    Gene
     
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  2. bodhi

    bodhi New Member

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    @Gene Ballzz Hi, thanks for the welcome! Regarding my last name, you could probably argue that you've guessed correctly, as the connection is made with the character the nickname is based on. There's a link to my actual name as well, but as it is with in-jokes, none of them get any better with explanation ;)

    @JohnH Thanks for filling in the math, now I know what to look for properly! Looking at listings on Mouser and similar, there seems to be some relatively economical 2.5A rated rotary switches by C&K (< 10€ / $ / £ each) that could be useful in some schemes.

    My end goal was to have something integrated into a combo amp, and in that case a rotary that can be easily found by reaching around to the back rather than individual switches would serve the purpose a lot better, I think. A separate bypass switch could still work, in that it's probably easy enough to find in practice with a bit of planning, so you wouldn't mistake it for a bypass or power switch.
     
  3. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Thanks for that and please understand that I had no intention of making fun of or mocking your or anyone else's spiritualism! Many (including myself) might enjoy a bit of enlightenment on our journey.

    Thank You Again!
    Gene
     
  4. bodhi

    bodhi New Member

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    Absolutely no lines crossed at least on my behalf, it's perhaps a somewhat irresponsible username by me to take, but similarly no ill intents meant. The pop culture reference is close to 30 years old by now, so even that is really quite hit'n'miss...

    Wrt. rotary switches, there also seems to be something like very high voltage rotary cam selectors used as spares for welding machines according to the description that might be possible to wire up properly with a full bypass option, these seem to be in the 10-20 $ range. Probably prohibitively large if one is going for compact box builds, but could again work in an amp chassis.
     
  5. JohnH

    JohnH Well-Known Member

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  6. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Yepper, $76US for me! Even though quite nice, it might be a bit overkill, particularly if doing a separate bypass switch. I'd be very interested in perusing some links links to the ones that @bodhi found?

    Just Askin'
    Gene
     
  7. bodhi

    bodhi New Member

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    This is the datasheet for the C&K switches, which seem to be the economical choice on the site. If I'm reading it properly, the A206 should be 2.5A @ 125VAC 2p6t with at least the possibility for make-before-break contacts https://www.mouser.fi/datasheet/2/60/arotary-1369275.pdf - but Mouser doesn't seem to stock that one...

    The cam switches are things like these https://www.aliexpress.com/item/32829955717.html .

    I had some first draft ideas about wiring schemes from a long time ago, but need to review my notes if I can find them as they were never verified. The thought was basically to use a rotary switch wired onto itself in different ways to ensure the proper combinations of components were connected in series...
     
  8. JohnH

    JohnH Well-Known Member

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    I did some thinking about, assuming suitable rotaries were available, what would the circuit look like?. My starting point is no compromise on the consistency of impedances and hence tones that the designs to date achieve.

    To get the full range of steps, and capabilities assuming a fixed stage 1 (not involved with the rotary), then 8 switch positions are needed if we want to go from -7db to -31.5 db in equal -3.5db steps.

    The easist version to think about has 3 poles, so each pole does the job of one of the toggle switches from Stages 2 to 4. So that's a 3-pole, 8-position switch with shorting contacts, rated at 2.5A at 125Vac, for an 8 Ohm build and up to 50 W amp.

    Then I wanted to see if a 2-pole switch could also work. I think it can, but not as easily as I'd hoped. To do it, I think many more resistors are needed, maybe 19, to replace 6 in stages 2 to 4. That's not so crazy, since they can be inexpensive (max power rating 25W), if it makes the switch more findable in reality.

    If you can only get 6 positions, all of the above could still apply, maybe stretching out the increment steps to -4db each, and not quite so much max attenuation. You could go -8, -12, -16, -20, -24, -28 db.

    Or, keep the -3.5 steps (or even reduce them to -3 db), and add another toggled switched stage, like a hi/low range switch.
     
  9. Marvelicious

    Marvelicious New Member

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    Hmmm... Yeah, more resistors should wind up being a lot cheaper than the switch in the end. I'm going to have to sit down with this again tonight... I'm old fashioned when it comes to circuits, so sitting down with a pad of graph paper and diagramming it out is how it makes sense to me.
     
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  10. JohnH

    JohnH Well-Known Member

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    I totally understand that! I leant to think 'schematic', using a pencil and paper, when I was an electronics nerd at high school in the '70's.

    I have this idea now about how to use a 2-pole switch, I'll sketch it out so we can discuss it.
     
  11. matttornado

    matttornado Well-Known Member

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    My attenuator is working great, however, its gets too hot to handle after about an hour of full attenuation. Does anyone know how to add a fan powered by the attenuator? I measured about 60VAC peak at the attenuator's input from my amp with my 100 watt amp cranked. Not sure how accurate that measurement is but.... I'm thinking maybe a 12 volt DC fan.
     
  12. JohnH

    JohnH Well-Known Member

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    I think that's a great idea. The idea is to pull a little a.c. power from the input, using a resistor, high enough value not to mess with the load, then bridge rectifier to feed the dc fan. Ideally, it would start turning slowly while still at moderate power, and spin faster at full. Need plenty of air vents in and out.

    I'll check out a schematic.

    60V peak is 42V RMS, or 112W into 16 Ohms! All as it should be. (EDIT: actually, is that right? or is it swinging -30V to -30V, which is 21V RMS?)

    I had a quick look at a few 12V fans. I saw current draw from 80 to 360mA at 12 V. I'd suggest to use one with low current. 80mA and 12V implies the fan would be around 150Ohm. Thinking the feed resistor from the amp input would be about 470 Ohms, but you'd need to test, it could be more or less, to get it running right. Then to a BR, with a 10uF smoothing cap, then the fan.

    80mA through 470 Ohm gives 3W in the resistor. Need at least a 10W capacity by those numbers.

    All of this needs to be tested in a mock up to get the values right. But at least, if it's all through that resistor of several hundred ohms, there's no risk to the amp to try it.
     
    Last edited: Dec 11, 2019
  13. tmingle

    tmingle Active Member

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    Im pretty sure I saw a schematic for 1 of the commercial attennators that powered a cooling fan from the speaker out. I think it may have been the Marshall Power Brake.
     
  14. Marvelicious

    Marvelicious New Member

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    Yeah, I'm jealous of people who can make electrical CAD programs work for them, but I wind up spending all my time figuring out how to make the program work and none of my time working on the circuit at hand. Still, now that I'm thinking it through, I'm pretty sure a 2-pole switch should work, I just have to draw it out to be sure.

    I had an idea to use a Seebeck generator (basically the same thing as a Peltier module, but technically, it's the Seebeck effect if it's used to make power and it's the Peltier effect if it's used to cool something) to power a small fan. The sell versions of this designed for wood stoves, this would just be the baby version.
     
  15. matttornado

    matttornado Well-Known Member

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    For the fun of it, I alligator clipped a a AC to DC wall wart P.S. rated at 120VAC / 12DC to the input of my attenuator. I measured about 6 volts DC from the DC Power supply & hooked it up to a 5 volt DC fan. It would not power the fan. It wanted to. The fan would slightly move just a little. Once the fan started to spin but not at it's rated speed. So it can work. Looks like it needed more current maybe?

    IMG_2235.jpg
     
  16. bodhi

    bodhi New Member

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    Regarding steps and decibels, one of the original incentives for me to look for attenuators (and the idea of integrating into combos) came from Tone King amps and attenuators.

    The one integrated into the Imperial (mk ii) amp has levels of 0db(bypass), -3db, -9db, -15db, -24db, -36db on a rotary switch. The separate attenuator in a box is called Ironman II, which has a Hi/Lo switch and a rotary
    • Hi range: -3db, -7db, -11db, -15db, -25db, -35db
    • Lo range: -6db, -10db, -14db, -18db, -28db, -38db

    So both of those have 6 steps, and I would assume the specific levels were not chosen at random... Food for thought, haven't built the proper M2 attenuator to figure out which levels might be the most useful ones myself yet.

    As for switching diagrams, realistically I don't have any time for drawing up plans that shouldn't really be spent elsewhere at least this week, but likely until Christmas taking the time of the year into consideration. Still haven't found my original wiring idea notes...
     
  17. JohnH

    JohnH Well-Known Member

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    Hi @matttornado , yes the idea should work! Different fans need quite different amounts of juice, so specific circuit values are TBC. But thanks for trying the concept.

    @bodhi I think a hi/lo range switch is a good idea. I don't quite get the odd increments of the Ironman though. I may be to do with some peculiarity of its internal circuitry. I think it would be best if each step around a rotary is the same increment, and the 3.5db steps that we have are pretty small in practice.

    6 position rotaries should open up the choices since they are available with two poles per wafer, with standard 30 degree rotation steps.

    You could have an overall bypass switch, a fixed -7db reactive stage, a high/ low toggle that did maybe 0db/-9db , then a 6-pos rotary doing say -3db steps 0db to -15db. That would give an overall range from -7db to -(7 + 9 +15) = -31db in consistent 3db steps. A -3db workaround is yo run the box in parallel with a speaker with amp set to half ohms.

    Still havent found a real, suitable, available switch though, and it's still a complicated build compared to the previous designs!
     
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  18. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Hey guys,

    This thread is so cool on so many levels!

    Just My $.02,
    Gene
     
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  19. Marvelicious

    Marvelicious New Member

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    @JohnH
    So, I'm doing a bit of sketching, but I'm having a little late night brain lock... You could just endlessly string instances of R7 and R8 for successive 3.5db drops, right? If this still makes sense when I look at it in the morning, it may actually be simpler in practice.
     
  20. Gene Ballzz

    Gene Ballzz Well-Known Member

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    So FWIW folks, I use one of these units almost constantly, with various amps and must say that it is as close to perfect as I think we can get, for a generally "passive" unit! With that said, a tiny bit of highs (the "sparkle" if you will) seems lost at attenuation levels of more than -21db, but I belive that to be more of a physical/air movement phenomenom than an electronic one. I'm betting that any attempt to correct for it "electronically" within the attenuator, will end up being a "hit or miss" situation, depending on the amp used as well as the attenuation level. This design is truly one of the:

    Greatest Things Since Sliced Bread! :fever:
    Gene
     
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