Aiken's Reactive Dummy Load.

7thString

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I'm just wrapping mine up, and doing the "more accurate" version is a total PIA build wise. The reason is that you want to have each of the inductive elements mounted at 90 degrees to the others in order to minimize mutual inductance, which for this project is a big deal. Obviously the inductors themselves, but also the power resistors if not using the more expensive non-inductive parts.

All to say, I'd recommend building Freeman's recipe for the simple version and call it a day. Someone in the thread says they have both versions and the differences are slight when playing them.

So back to mounting inductive elements at 90 degrees to each other, with the simple version you have three inductive elements, the two inductors, and as a group the wound power resistors. You also have the X, Y, Z axis to mount the three elements and spacing of them is not so critical (though more spacing the better). Add the extra elements of the "more accurate" version and you still only have X, Y, Z axis to work with but you have 4 inductive elements, therefore spacing at least two of them becomes a big deal.

Try to implement switchable main inductors in order to move the LF peak, and the physical layout becomes pretty hairy. I had to resort to iterative tuning of the physical spacing of the LF inductors placed on axis in order to leverage mutual inductance. Plus had to do the layout to get enough physical space between the two HF inductors placed on the same axis in order to keep the entire layout in compliance with the three axis principle. Total PIA for very little improvements. So build the Freeman recipe IMO.

That said, I would not skimp on the cap(s) for this design though I have found the lower priced 20awg inductors to be quite satisfactory even for 150wrms (sine) designs. If you go with lower priced inductors, be sure to model the design in SPICE in order to adjust the damping resistor values to compensate for the differences in internal resistance between the premium parts and the lower priced parts. If you need/want help with that, just post your inductor specs and I can model it for you and give you the revised damper resister values....



Personally I'm liking the latest from Own Hammer. I'll be shooting IRs of my homebrew 2x12 (a 1936 but 1" deeper) loaded with G12T75 in a couple of weeks. But I'm not holding my breath after hearing all of the DIY IRs floating around the net. There is an art to get something usable and LOL, my guess is I will be way off the mark even though I have great gear to capture the impulse... That said, would be happy to trade IRs with you and we can see how we did!
Thanks for all the info!! If a Suhr RL comes up used on eBay I might just call it a day, ha! (Have had it before to good effect, along with the two notes captor; pricey though they are, so is my time if I spend several days planning / building my own iteration)..

Regarding IRs, I have done a lot of testing to figure out what is important and what isn't - there's definitely a slight art form to it. Having a somewhat anechoic room, mics you like and good pres seem to help considerably. Unfortunately my current music/project space is small and untreated so shall wait and see how the next set turn out with the new 4x12. Will post a g drive link to the result IRs later next week, provided they aren't terrible.
 

Mike Sch

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If a Suhr RL comes up used on eBay I might just call it a day, ha!
After listening to a bunch of YouTubes I really liked what I was hearing out of the Fractal load box. But apparently I'm not the only one, because they are sold out and none to be found on the used mkt. I saw some sold listings for used ones going for new prices. LOL, all of which led me to this thread and building my own. Fortunately I've done plenty of this sort of thing before, but I would have gladly spent for a Fractal to avoid building it!
 

7thString

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1,345
After listening to a bunch of YouTubes I really liked what I was hearing out of the Fractal load box. But apparently I'm not the only one, because they are sold out and none to be found on the used mkt. I saw some sold listings for used ones going for new prices. LOL, all of which led me to this thread and building my own. Fortunately I've done plenty of this sort of thing before, but I would have gladly spent for a Fractal to avoid building it!
Yep, noticed that regarding used prices and yeh it sounds great, you may have seen Leon's video comparing to the Captor. I should have lent him my Suhr at the time but didn't know until much later that he had done that video. It's weird because John posts on TGP a fair bit especially in this and other loadbox threads and points out the importance of simulating the cab resonant impedance peak, and that the captor (among others) has been shown to be flat (no peak), but for some reason the sonic differences don't amount to as much as you would anticipate IME, though I never had both at the same time unfortunately.
 

Mike Sch

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16
It's weird because John posts on TGP a fair bit especially in this and other loadbox threads and points out the importance of simulating the cab resonant impedance peak, and that the captor (among others) has been shown to be flat (no peak), but for some reason the sonic differences don't amount to as much as you would anticipate IME, though I never had both at the same time unfortunately.
It may have to do with playing style. One question I wanted answered before finalizing mine is "does the reactance curve change based on applied power?" I did a test rig to apply 50W (used a solid state ADCOM amp for measurements) and determined that the resonant peak in particular gets squashed down quite a bit under high power for both the cab and the box (fortunately they track pretty well). So it may be that the dimed amp scenario sounds not so different while the cleans may be a different story?
 

7thString

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1,345
It may have to do with playing style. One question I wanted answered before finalizing mine is "does the reactance curve change based on applied power?" I did a test rig to apply 50W (used a solid state ADCOM amp for measurements) and determined that the resonant peak in particular gets squashed down quite a bit under high power for both the cab and the box (fortunately they track pretty well). So it may be that the dimed amp scenario sounds not so different while the cleans may be a different story?
Picked up a used captor. Now to try and build something that sounds better.
 

Mike Sch

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16
Picked up a used captor. Now to try and build something that sounds better.
LOL, BUILD THE SIMPLE ONE!

I'm still fighting with my "more accurate" build...had to order some heat sinks (yes plural) since the all aluminum chassis isn't enough of a cooling solution on its own for full wattage, though it is plenty good for 50 watt amps. But darn it, if I spec'd it for 150W I'm going to cool it for 150W, lol. Hope to post a clip next week if I have time.
 

2L man

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131
I put this loudspeaker output attenuator together fast but it works so good that it deserves a neater wiring and housing. It is basically a reactive load which I first saw on Aiken site. In the middle is an aluminium 10 ohm 200W series resistor. On left is high freguenzy slope circuit 1,8mH air core coil and green 100ohm resistor. Top right there are 100ohm, 15mH and four blue capacitors total of 220uF which form a resonant circuit.

Attenuation is done using a line transformer which belongs to 100V standard systems and it is on bottom right. It is parallel with this reactive unit and loudspeaker is connected to it. It has three inputs and three outputs which selection is done using just banana jacks which are in the middle of front panel which is on botton on this picture. However in practise there are only five different attenuations possible. I bought it after I tested how normal output transformer attenuates the signal and it sound good but it did attenuate too much because of higher winding ratios.

Esa
 

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Mike Sch

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I'm still fighting with my "more accurate" build...
To follow up on my "more accurate" build...despite what LT-Spice had to say about the matter, and despite very tight curves in REW compared to real cabs measured the same session with a SS reference amp, my designed sounded like poo on my tube amps.

So I retrofit just the treble section with a .5mH EARSE 18ga and a 50W 50 ohm and it sounds awesome. James Freeman nailed the values imo.

FWIW I left the bass section as I designed it since the bass and treble sections are about as separate as you can get in terms of one not affecting the other. All to say that the bass section can be modeled in SPICE to adjust for parts availability and pricing (which is what I did and saved about $40) and it should work fine. Also the cheaper grade EARSE laminated inductors for the bass section sound great and have plenty of power capacity...the more expensive Super Q is overkill imo though it will also sound great.

Also, concerning price and availability of the bass inductor, I found that a 5.6mH plus a 6.5mH wired in series and physically butted together on axis yields 24mH due to mutual inductance effects (and of course doubles the power handling capacity over a single inductor). I'm actually running a pair like this, only they are spaced 22mm to provide an air gap to yield about 12mH.
 
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Johndh

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400
It may have to do with playing style. One question I wanted answered before finalizing mine is "does the reactance curve change based on applied power?" I did a test rig to apply 50W (used a solid state ADCOM amp for measurements) and determined that the resonant peak in particular gets squashed down quite a bit under high power for both the cab and the box (fortunately they track pretty well). So it may be that the dimed amp scenario sounds not so different while the cleans may be a different story?
I think that's a true observation, Ive found it too in my attenuator designs. But I don't think its the cab/attenuator/load box that is changing or needs to change, its the effective output resistance of the amp which can be very high at clean levels, particularly if NFB is low and gets very low at saturation levels. This then fully damps the effects of the bass resonance and treble rise. So I reckon that in modelling these speaker parameters in a load box or attenuator, its ok to set them based on small signal levels, then let the amp drive it from there.

In your recent tests, was it the treble section you were mainly adjusting? which values did you try but didn't like? I match my designs to Mind Linds measurements of a G12M cab, and it gets a good result.
 

Mike Sch

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I think that's a true observation, Ive found it too in my attenuator designs. But I don't think its the cab/attenuator/load box that is changing or needs to change, its the effective output resistance of the amp...
I agree its the amp, what I was trying to answer was whether the measured curves change under high output conditions, which they do, and would the speaker load and the load box exhibit similar changes in their measured curves, which they do. Which of course means you're correct in assuming you can match them at low outputs and call it a day. Only now we know that for a fact since I compared them at various drives from the miliwatt level to 50 watt level and the curves change similarly for the cab and load box.

In your recent tests, was it the treble section you were mainly adjusting? which values did you try but didn't like? I match my designs to Mind Linds measurements of a G12M cab, and it gets a good result.
What I did not like at all in the treble section was my design for the "more accurate" version using two treble inductors despite a perfect curve match, so I ditched it and changed the treble section to Freeman's single inductor values. It was much better, but I still heard too much top end (4KHz and up) using various commercial IRs, so ended up changing the treble section resistor from 50 ohms to 33 ohms. I do still need to verify whether my physical layout is causing the treble inductor value to increase due to proximity of a ferrous part. Because if so, then the resistor change I made would compensate for that phenomenon. The physical layout of these boxes is critical in order to avoid mutual conductance and increased inductance due to ferrous parts.

As for the bass section, I just modeled in LT Spice to tweak values to deal with parts price and availability and it works fine.
 
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Johndh

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400
Early on i also sometimes found an unreasonably harsh upper treble. It was mainly with my dsl401 which, like an AC30, uses el84's and no negative feedback, making it very sensitive. Getting some resistance around the inductance seems to keep it under control. I work my tones out mainly by looking at the difference between about 500 and 5000hz. Above that I flatten off the response relative to Mike's impedance measurements. Guitar speaker responses are falling away up there anyway.

I've also been surprised by what a huge difference even a small M3 steel bolt makes to inductance.
 

303_trancer

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11
I've not red the whole tread and maybe someone suggested it already... I want to make a reactive load with just 2 speakers in a closed box that are wired in anti phase so they do cancel each others sound for the most part. This way the amp sees real speakers but the sound will be manageble. If it is too loud, I can build a box in a box. But maybe this whole thread is about a portable RL then my idea is of no use at all.
 

Mike Sch

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Early on i also sometimes found an unreasonably harsh upper treble. It was mainly with my dsl401 which, like an AC30, uses el84's and no negative feedback, making it very sensitive. Getting some resistance around the inductance seems to keep it under control. I work my tones out mainly by looking at the difference between about 500 and 5000hz. Above that I flatten off the response relative to Mike's impedance measurements. Guitar speaker responses are falling away up there anyway.

I've also been surprised by what a huge difference even a small M3 steel bolt makes to inductance.
I think part of the issue is the idea that IRs are a perfect response. I noted several scenarios not related to load boxes where IRs seemed to bleed in a lot of sizzle, leaving me the impression they are less than ideal technically speaking. So I suppose that is why I had no problem jumping from graphs to what my ear is telling me and just squash the highs a bit in the load box by replacing the 50 ohm treble with a 33 ohm. It actually sounds much nicer this way than using an EQ to roll it off.

LOL, yes I found out the hard way that even a small ferrous item anywhere close to an inductor can change its working value in a big way. I also found out the hard way about mutual inductance. I was building it as a switchable design using two bass inductors and had them mounted side by side. Kept blowing fuses in my measurement amp when I turned it on. That's when I studied up and found that mutual inductance turned my two side by side inductors into a transformer! Placed them end to end and all is well.

So, since the wire wound resistors are somewhat inductive (unless using the more expensive non-inductive), as are the film capacitors. I ended up mounting the bass inductor(s) on the X axis in the chassis, the caps and resistors on the Y axis, and the treble inductor on the Z axis in order to mitigate the effect of mutual conductance. Mounting at 90 degrees is what all of the literature was saying when I studied up. It works.

These effects also indicate that where you choose to set your load box may very well reek havoc on the working inductor values. I.E. probably not a good idea to set it on top of an amp head with massive transformers inside. I haven't tried doing that and always place mine a few feet away from everything. But then after my "side by side inductor experience" I'm paranoid that setting it on the amp head will affect the values so much I'll have an amp repair job on my hands....
 
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Mike Sch

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I've not red the whole tread and maybe someone suggested it already... I want to make a reactive load with just 2 speakers in a closed box that are wired in anti phase so they do cancel each others sound for the most part. This way the amp sees real speakers but the sound will be manageble. If it is too loud, I can build a box in a box. But maybe this whole thread is about a portable RL then my idea is of no use at all.
I'd say that the load box in this thread will accomplish your purpose much better, much cheaper, much smaller, much lighter, and with a volume knob and a line out to go to your other gear.

The biggest problem with your idea is that the cab design greatly affects where the bass spike shows up on the plot. Doing sound canceling speakers in close proximity is going to produce a very unnatural placement or even remove it altogether due to cancellation. And as a result it won't sound the way you expect.

Just build one like presented in the first post and you'll be happy imo. If it has too much top end sizzle for your tastes, just lower the 50 ohm to 33 ohm.
 

303_trancer

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11
I want to use this only at home and connect a DI box between the amp and box. The signal from the DI box is going to an audio interface and I will use IR's in the DAW.
Could you explain why the bass spike will move due to cab design or remove it all together due to cancellation? Wouldn't this be when there was a mike in that box? I don't know this. If the cab design is influencing the load for the amp, what could I do to compensate it?
I don't care about the sound of the speakers themself at all. They are only there for a RL. I'm only interessted in the signal from the DI box to my DAW and my thought was that speakers will form the best RL in this scenario.
 

Mike Sch

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Could you explain why the bass spike will move due to cab design or remove it all together due to cancellation?

I'm only interessted in the signal from the DI box to my DAW and my thought was that speakers will form the best RL in this scenario.
The position of the bass peak depends on the speaker design and the resonant internal pressures of the cab. This is why a closed 4x12 has a higher bass peak than an open back. Your design to cancel the physical speakers will have way more effect than a normal cab on the movements of the cones and thus the reactive load curve.
 
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303_trancer

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So than it will be better to wire them in phase and build a box in a box. Any tips on placement of the speakers (next to each other on one baffle or opposite to each other) and do I need extra hole(s) in the baffle?
Sorry for hijacking this thread.
 

Mike Sch

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So than it will be better to wire them in phase and build a box in a box.
IMO it's much better to build the load box in the first post. But that said, after thinking about what you posted, are you talking about a traditional 2x12 cab with the speakers wired out of phase? If so, its not such a problem with respect to the impedance curve. The bass peak will be similar to that of an open back since the internal pressures in the cab will never change since when one speaker is pushing, the other is pulling. I thought you were talking about building a small cab with speakers physically opposing each other... Still, way better to build the Aiken box imo, its an eloquent solution to the exact requirement you cite.
 

303_trancer

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The idea was a closed box with a baffle in the middle of the box. I wanted them out of phase so the sound in the room would be minimal. If it is still to loud it can be put in another box. That should do the trick. And in this arrangement it is pretty easy to get the sound out of phase. But if I could put 2 speakers opposite to each other the box could be much smaller but in this arrangement I don't think it is possible to get the sound out of phase.
To get it very small (and light) I was thinking about Ampeg 86-126-04 4ohm 6,5" 50 watt speakers instead of the usual 12" speakers. As I said, the sound of them will not matter.
And this way I don't have to calculate some components and test it because this is the real deal and not a replacement circuit of a speaker.
 
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Mike Sch

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And this way I don't have to calculate some components and test it because this is the real deal and not a replacement circuit of a speaker.
Just be aware of how the cab influences the reactive load curve. You can get REW software (free) to plot the curve of a real cab and your new cab (or load box if you end up building one) in order to compare to reality and verify your design.

A bit of history is that people tried the idea you have several decades ago with less than great results which is what led to these types of circuits being developed.

The nice thing about this design is everyone has already done the fiddling around and you don't have to calculate anything. Plus its all tried and true technology...there is good reason why you cannot buy a Fractal load box on the used market, and if you do find one it will cost more than new. And good reason the Suhr costs so much. This design will equal either in terms of sound quality.

Which ever way you go let us know how it works out!
 




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