Aiken's Reactive Dummy Load.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by James Freeman, Apr 27, 2012.

  1. Husky

    Husky Gold Supporting Member

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    Randall is the man and his website is a wealth of solid information.

    But I went and had a look at his site and just for the record even though Inductors, Resistors and Capacitors are going to be in most devices like this we are not doing exactly what Randall shows in his white papers and our values are completely different.

    This is a problem I have been thinking about since the mid 80s, when I had my big Bradshaw rack and I used to slave amps. I quickly learned that resistive loads didn’t cut it. I just don’t have the rack anymore and didn’t find a need until IRs became popular. I mean the Juice Extractor was a load with an EQ to re-amp and slave amplifiers and the problem with that sounding flat which it did, was because it was resistive.

    What I find surprising is most of the Reactive Loads I have measured do not follow a true impedance curve at all.
     
    Last edited: Apr 7, 2019
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  2. mnemonic

    mnemonic Member

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    Has anyone put any kind of isolation between the line-out and whatever you’re plugging into?

    I’ve got most of my parts together to build one of these, but the more I think about it, the more nervous I get about just having just a resistor between the full amp output and my interface input. If something goes wrong I want to limit the amount of stuff that gets toasted.

    Will a simple op amp buffer work as protection, or do I need to look into audio transformers?
     
  3. PLX

    PLX MENSA member, Astronaut, Dated Your Mom Once Gold Supporting Member

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    Last edited: May 13, 2019
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  4. mnemonic

    mnemonic Member

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    Very nice. Does the transformer provide protection to whatever you’re plugging into, or is it mainly to avoid noise / ground loops?

    Quite the pricy transformer, at least here in the U.K.

    I’ve been thinking a bit more and I think a buffer pedal between the line-out and my interface would protect the interface, right? Like if something failed catastrophically and the full speaker signal went though the line-out, the buffer pedal would clip (and maybe burn out) but that full signal wouldn’t make it to the interface?

    This solution I could do for a couple bucks in parts that I probably already have, and I don’t really mind having a powered pedal in the signal chain.
     
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  5. PLX

    PLX MENSA member, Astronaut, Dated Your Mom Once Gold Supporting Member

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    I've build both resistive and reactive loads - both with and without transformer driven line outs.. doesn't really make any difference except the Jensen JT-11-FL affords me the ability to have either a balanced XLR or a balanced 1/4" output, or an unbalanced 1/4" mono output.

    The power resistors are absorbing the energy from the amp's output.

    I rated the resistive load in this project at 500W (Two 33Ω, 250W in parallel) so I don't have to worry about failures of the dummy load - since it is so grossly overrated.
     
  6. Guitar_matt

    Guitar_matt Member

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    Hi,
    I hope this thread is still alive! I'd really like to build one of these!

    I'm in the UK and I'm finding it hard to find all of the components listed here.
    Could someone look at the components I've chosen to see if they would be suitable?

    For the high power resisters, I'm getting from eBay - will the quality be good enough?
    50ohm, 75w - https://www.ebay.co.uk/itm/362590935070
    8ohm, 200w - https://www.ebay.co.uk/itm/332996533995
    50ohm, 50w - https://www.ebay.co.uk/itm/401514561019

    This is the capacitor I've chosen, will that work well?
    200uF bipolar - https://www.ebay.co.uk/itm/183388254384

    And for the inductors, I'm sourcing from falcon acoustics. Will they handle the current and be sufficient?
    11mA, 5A - https://www.falconacoustics.co.uk/a...r-air-core-audio-9mh-10mh-audio-inductor.html
    0.5mH, 5A - https://www.falconacoustics.co.uk/a...-power-air-core-041-050mh-audio-inductor.html

    Any thoughts?
    Thanks in advance!
     
  7. Johndh

    Johndh Member

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    I haven't built one of these, but I read up about them and use the thinking in my reactive attenuator (rather than load box) design.

    So, looking at those products, I think you should start from the amp spec you want it for. The design is based on a cranked 100W amp, and your resistors match the specs for that in the OP. But if you dont need a 100W amp, then they could be scaled back in power.

    But, if you do want to match the design and use a 100W amp fully cranked, then those caps don't look big and ugly enough! Its not certain since then dont spec the ripple current rating.

    Also, the inductors. The big inductor is wound with 1mm dia wire which is 18ga. I use that for inductors in a 50W build at 8 Ohm. Id want thicker wire to crank a 100W amp. Or, build with all values scaled to a 16Ohm design, which reduces current.

    And the small inductors are only wound with 0.7mm wire, half the area, seems too fine and there should be no prob at least getting them with 18ga.

    Just my thoughts. If what i put above is at odds with James or the pro's posting on this thread, Id respect their views over mine.
     
    Last edited: Aug 7, 2019
  8. Tony Bones

    Tony Bones Member

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    What diameter wire do you recommend?
     
  9. Johndh

    Johndh Member

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    What power amp do you want to use? And would it be to make an 8 ohm load, or a 16 ohm load?
     
  10. Guitar_matt

    Guitar_matt Member

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    I would like to build something capable of handling a 50w amp, but would be starting out with a 5w amp.
     
  11. Johndh

    Johndh Member

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    My opinion would be that for a 50W amp, you are OK with 1mm core windings which is 18 gage, which is what I use in my attenuators for 50W amps.
    But in this case, there's one thing more to consider which is that the big inductor has a resistance itself of a couple of ohms, which adds to the general impedance of the load box, at least at low frequency. if you hold off ordering parts for a day or two, you might get a comment from others as to what winding wire was used before, and also I could run some modelling to see if that is likely to make a difference.
     
    Last edited: Aug 8, 2019
  12. Mike Lind

    Mike Lind Member

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    John has given you some good advice but let me expand a little bit.

    Air coils are usually preferred as they will not saturate and will make for a stable circuit. Air coil for the bass resonance is not always practical as they either have high DC resistance or become big, heavy and expensive. Iron can be used for the bass coil as they will have much lower DCR and a size and price tag that might be more manageable. Any saturation is much harder to detect in the low frequencies.

    It’s usually the treble inductor that tends to get hot so use the heaviest wire as you can fit and afford. The bass coil usually has an easier life but if have a powerful amp and/or are a 7-8 string guitar player there will be more current produced at lower frequencies.

    You can use electrolytic bipolar caps but polypropylene is a better choice although more expensive. Always check AC rating and have a healthy overhead.
     
  13. Johndh

    Johndh Member

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    Good advice from Mike. An iron or Ferrite core bass coil with low DCR sounds like the way to go, which is what James seems have built (see page 2). I ran a SPICE sim and the DCR of 2.5 Ohms as for an air-cored coil of the right value, tended to lose about 1/3 of the bass peak height. (Not a show stopper though.)

    18ga = 1mm wire diameter air-core seems good for the small inductor, matching James' version
     
  14. Daniel G.

    Daniel G. Member

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    I've noticed that ERSE seems to be out-of-stock of almost all the recommended components mentioned throughout these posts.

    Here's what I have found for the inductors. They seem pretty close although they are 16ga instead of 14ga like had been recommended. The rated DC resistance is still very low so as far as I can tell they should still be fine. I do intend to potentially run up to 4x6L6 amps into the load so I'd like to be sure the power ratings are all on point.

    http://www.erseaudio.com/ESQ55-16-10000
    http://www.erseaudio.com/EAC33-16-510

    They're out of stock of all the capacitors too. The original 200uF ones aren't even listed as an option anymore, the 100uF's are out of stock, as are the 51uF ones. I have contacted ERSE to see if/when they'll be back in stock.

    Someone linked to Madisound, who do have the Solen caps in stock, however I would rather not have to waste shipping costs by having to go to a 3rd vendor.
     
  15. Mike Lind

    Mike Lind Member

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    These coils will do fine.

    Mouser has film caps suited for this application and reasonable shipping.
     
  16. Daniel G.

    Daniel G. Member

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    https://www.mouser.com/datasheet/2/212/F3303_C44A-1101424.pdf
    Kemet C44AFGP6200ZE0J

    I found these caps, they're listed as film capacitors and the ratings seem pretty close, although they're in a totally different form factor. The price is similar to the Solen cap, and it's tempting to save shipping costs, but I don't want to buy something that doesn't work. What do you guys think? There aren't any other film type caps on Mouser that have similar ratings or design as the Solen one.
     
  17. Johndh

    Johndh Member

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    I don't want to comment on those caps (they seem like they'd be fine to me but I'm not knowledgeable enough to be sure)

    But, it seems like the cap, plus the associated inductor and resistor, add up to the largest part in terms of both cost and bulk of the whole design. These create the bass resonance peak in response. I wonder if you could get very good result without these parts at all, just bypass that part of the circuit? I think the amp will be perfectly happy with that, still with the treble rise created by the smaller inductors.

    When you look at real measurements of cab impedance vs frequency, that bass peak is generally very sharp and focused at around 100 to 110 hz.and a couple of semitones either side (see for example, Mike Lind's measurements for a 4x12 Ax greenback cab :https://www.thegearpage.net/board/index.php?threads/attenuators-and-load-boxes.1947804/ ).

    Only the fundamental tones of the very lowest guitar notes can get there, and all harmonics (where the tonal character is) are well above. On some speakers, the resonance is even lower.

    So if you don't include these parts, you'll not notice a difference except maybe on low E and A strings on low frets, and you could lift these downstream in software or EQ if needed.
     
    Last edited: Sep 11, 2019 at 10:02 PM
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  18. Mike Lind

    Mike Lind Member

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    Go over to Parts Express as they have Erse coils and film caps from Audyn and Solen.

    This is not as sensitive as building a Hi-Fi speaker crossover although parts and reasoning is somewhat similar.
     
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  19. Daniel G.

    Daniel G. Member

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    That's an interesting idea. I'm planning on pairing my load with a Teensy 4.0 based convolution processor, and it will have a ton of horsepower leftover to add some EQ beyond just the speaker impulse response simulations. That one capacitor is the cost of the entire DSP solution so it's certainly tempting.

    edit:
    I may keep that section and just use some bipolar electrolytics, and arrange them to bulk up the voltage rating a bit. I'll leave room in case I need to upgrade to a better cap.
     
    Last edited: Sep 13, 2019 at 7:47 PM
  20. Mike Lind

    Mike Lind Member

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    Doing the impedance curve in software is possible and is used by several manufactures but it will not be the same. Another rout is to build a resistive load with a high output impedance as John mentioned and taking a line signal from the output instead of the common input. In this case your speaker cab will act as the reactive component. However, your amp will see a flat resistive load and therefore will not compress and distort in the same manner.

    Non polar electrolytes will work but check their AC and ripple current rating. Their lifespan will be greatly reduced in a hot environment so keep them away from any hot surface.
     
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