Aiken's Reactive Dummy Load.

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

  1. James Freeman

    James Freeman Member

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    I see the AXE-FX guys starting to get into this impedance curve effecting power amp frequency response thing.
    http://forum.fractalaudio.com/axe-f...e-release-monster-speaker-resonance-page.html
    I have read the whole thread and couldn't find a single impedance plot of a real 4x12.
    Just wanted to tell them that a 2x12 is around 120Hz and as I read a 4x12 is 110Hz and not 75Hz like in free air.

    My first attempt was based on that (75Hz) but now its 110Hz as in a 4x12.
    To go higher from 75Hz to 110Hz I had to unwind the inductor from 20mH to 11mH.
    It made the re-amped dummy load resonate with my 2x12 (110 and 120 are close enough) so the natural springy bouncy feeling is back.
    Now when I reamp the dummy into a solid state power amp the sound is indistinguishable from the straight to speaker sound.

    But I'm still determined to make it 1:1.
    Raise the Resonance peak impedance to match, and raise the highs impedance a little too.
    Soon, I promise.

    Here is mine with the latest tweaks:
    Purple is my Framus 2x12 (120Hz Fc), Green is my Dummy Load (110Hz Fc).
    [​IMG]


    Useful Info.

    If anyone thinks they can fix this curve with an EQ after a simple Resistive dummy load is wrong.

    A tube amp (poweramp) is a "Constant Current" amplifier.
    It means the only thing that changes is the Voltage and it has a limit (B+).
    This limit is when the poweramp has no more voltage to swing and amplify the signal resulting in = Clipping/Distortion.

    Guess what frequencies get to distort first?
    That's right, the Resonant peak and the Inductive treble hill (They are higher in impedance therefor higher in Watts).
    As you go higher on the master volume knob the middle section "catches up" with the already distorted
    resonant peak and the Inductive treble hill, eventually resulting in a flat line.
    (I have tested all of this).

    This is CRUCIAL to how your amp sound and distorts at any volume, and you can't fake it with any kind of post EQ.
    It gets some famous attenuators out of the game right there.


    :beer
     
  2. teemuk

    teemuk Member

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    That's likely because it would vary a lot depending on used speakers and overall dimensions. Not all 12-inch speakers have the same resonant frequency and cab's properties would alter alter fr even further.

    The cabinet type will also have an effect. As mentioned, in sealed cabs the resonant frequency will change somewhat because the speaker is no longer in "free air". Electromechanical characteristics of porting, on the other hand, actually create a whole another "peak" - except that this peak is "negative" and works on to decrease the impedance. Properly tuned ported cab usually more or less tries to match that peak with speaker's own resonant peak (or more properly, the other way around) so you end up with a plot more or less similar to this:
    [​IMG]

    This is also the key issue of what "tuning" is about: matching electromechanical characteristics of a resonating system (cabinet) to another resonating system (the speaker). You can't just combine any random speaker with any ported cab of random dimensions and expect things to work properly.
     
  3. James Freeman

    James Freeman Member

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    Useful Info:

    About Impedance Mismatch and certain attenuators.

    First, let me say that a tube amplifier is Current Driven or "Constant current" and solid state is Voltage driven.
    It means that in a tube amp the current is constant and what changes to drive the speaker is Voltage.
    In solid state the voltage is constant and the current swings.

    If your 100W amp supposed to produce these 100w into a 8Ω load,
    Then 3.53 amperes squared times 8 will result in 100 Watts.
    All is OK.

    What if you mismatch and load your 100w 8Ω amp with a Lower 4Ω speaker?
    3.53^2*4 = 50 Watts.
    Your amp will run cooler with half the power rating.
    Besides that everything is a OK.

    What if we mismatch Higher, say 30Ω?
    3.53^2*30 = 373 Watts.
    Your 100W amp will not be able to produce this amount of power.
    Your will be louder faster, distort sooner and will not exploit its full range.
    It will run at "full throttle" and the power tubes will die sooner.

    Moreover, there is a real danger connecting a higher load.
    I'll just quote it:
    In short a dead Output Transformer and burned Power Tubes Sockets.
    Just Google: impedance mismatch flyback voltage.

    A Reactive Attenuator and a real speaker has higher impedance in the
    Resonance peak and the inductive hill but not across the whole frequency range.
    The safest Dummy/Attenuator will be an equal Resistive load with the right resistance (4Ω, 8Ω, 16Ω) but we know it sound like poop.

    Lesson learned:
    For tube amps (Current driven/Constant current).
    Always match Impedance, or Lower if its the only option.

    For Solid State its just the opposite (Voltage driven/Constant voltage).
    The Higher the load, less watts are produced, the cooler the amplifier runs.
    The Lower the load the higher the power demand that can overheat and destroy the transistor.

    :)

    EDIT:
    Fixed some misleading info.
     
    Last edited: Aug 16, 2012
  4. James Freeman

    James Freeman Member

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    Updated the last post.

    And tweaked the dummy load some more.
    I think this is it, I'm Done !!!! :rockin:banana:beer:band:bonk:JAM:love::phones :dude

    Blue = 2x12, Green = final Dummy Load.

    [​IMG]

    Final Values.
    Big resistor 8 ohm.
    Treble Resistor 50 ohm.
    Treble Inductor 0.5mH.
    Resonance Capacitor 200uF.
    Resonance Inductor 11mH (unwound from 20mH).
    Resonance Resistor 75 Ohm.


    No more bumping this thread from me.
    :bumpbump
     
    mesa3077boogie likes this.
  5. DT7

    DT7 Member

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    Any chance of getting rid of that dip from 2-4k?
     
  6. James Freeman

    James Freeman Member

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    No.
    Unless I change the Inductor and start tweaking again (higher inductor 0.6mH and lower resistor 40 ohm).
    It just 2 ohm difference and its completely inaudible.
    Believe me, its more than close enough, closer than any attenuator on the market to date.

    Why do you ask?
     
  7. torquil

    torquil Member

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    It is not self-evident that a traditional guitar speaker is the gold standard of guitar tone to any given listener's ears. I think it could be interesting to experiment with radically different impedance curves. Is this something that is possible to do in the Axe-FX or other devices? Or are they restricted to always approximate traditional guitar speakers?

    Of course, it is very nice to have the opportunity to attenuate while keeping a traditional impedance curve, and the vast majority of guitarists would probably prefer that approach.
     
  8. James Freeman

    James Freeman Member

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    If the Axe-Fx changes the actual Cab impedance and the emulated power amp reacts like a real one would to this changing impedance,
    its a major step forward for them in terms of the realism of the power amp distortion.
     
  9. yfeefy

    yfeefy Member

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    Thank you Ilya, Randall, and everyone else for this great and very interesting info.

    If one was to buy something rather than build it from scratch, it seems like the marshal power brake is the best (inexpensive and readily available) device out there which incorporates these ideas.

    Looking at the chart in the Harry Kolbe fake book site... do you think the marshall device could be modded to present an impedance curve closer to the Aiken-Ilya device...?

    I ask because I'm mostly interested in an attrenuator, not a full dummy load.

    Great discussion!!!
     
  10. teemuk

    teemuk Member

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    It does this. Then again, generic solid-state guitar amps have done the same from mid 1960's - early 1970's as well. Faking high output impedance in a solid-state power amp is pretty much common knowledge for all the guys who design those things.
     
  11. James Freeman

    James Freeman Member

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    Yes & No.
    You can closely match the internal Impedance to any specs you desire.
    It has this internal Impedance Response in PARALLEL with the attenuated speaker you plug in.
    It uses a transformer for the attenuation levels thus making it behavior not so predictable.
    So it will not maintain the perfect curve you want, as what you see in the Harry Kolbe fake book site.

    On the other hand Aracom uses the same principle but with a rheostat to slide between a reactive dummy load and the real speaker,
    thus maintaining the impedance curve and smoothly lowering the volume.

    Aracom is currently hard to beat if you want all passive.
    Unless you don't mind a reamping approach with a dummy load.

    I think you meant Low output impedance and low Damping factor that they tried to build into SS amps to behave like tube amps.

    Peavey Transtube series & Randall Amplification is an example for using a SS Current Driven power amps to make it behave like a tube amp.
    I also happen to have the Peavey Bandit II (USA made) and I have done all sort of measurement and testing on it since I've built the dummy load.

    From my tests on the Bandit I can tell you its close but no cigar.
    It indeed has a Partial Current Drive but not fully current driven like a tube amp.
    I even took my modeded valveking tube preamp from the zero loss fx loop I installed, into the power amp of the Bandit into the Framus 2x12.
    *(Used the Bandit as Poweramp only).
    It still don't sound like a tube power amp.

    Randall RG amps are known for using a Current Driven topology in their amps.
    Fender, Marshall, Peavey, etc... "borrowed" it later on.
    As I stated earlier, Close but not yet there.


    Info about Constant Current and Tube amps vs SS amps:
    http://lenardaudio.com/education/12_amps_8.html
    http://sound.westhost.com/project56.htm
     
  12. yfeefy

    yfeefy Member

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    Thanks for your comments Ilya , what you've done with this is incredible.
     
  13. DT7

    DT7 Member

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    First off, it looks more like ~7 ohms rather than 2. Second, the inflections downward (prior to) and upward (afterwards) in that band-width would indicate phase shifting in that frequency range...shifting that doesn't appear to exist in the 2x12 signal (according to your graph). Such a phase shifting most likely WILL change the tone...and in a tone-critical area. I'd be more concerned about that than getting the resonance peak exact....but maybe that's just me.
     
  14. tamivox

    tamivox Member

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    Thanks for mentioning my page; feel free to archive and mirror it anywhere. Suggestions and questions are invited, because the subject is capable of considerable development.

    - - Dave Barber, aka tamivox
     
  15. James Freeman

    James Freeman Member

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    First off, your the first one here to include negative criticism in this thread.
    Second, Its more like ±2Ω.
    Third, ....Its just you.
    Forth, Let my see YOUR build then we'll talk.

    Moreover,
    The low resonant frequency is MORE important than the high frequency hill because its what makes the Bouncey/Springy feeling of the Tube amp along with a very low Damping factor.
    A lot of well known Reactive attenuators lack this Resonance circuitry; They do have the Inductive Hill but still lacking the Bounce that the Resonant peak creates.

    As I typed earlier, In certain point of the Master Volume (about 2 and up) where the amp begins to distort these peaks (they reach the amps 100W capacity first).
    By the time I get to about 5 on the MV the whole frequency response is flat lined, the hill and peak are gone (they are clipped).
    From 5 to 10 on MV the amp just get squishier and more distorted, and the audible influence of the impedance curve on the power amp gets less and less.
    The most audible effect of the Impedance curve is in lower volumes (0-3) when the Peak and Hill are NOT so distorted.

    So a little 2Ω difference is nothing Capiche?
     
    Last edited: Aug 17, 2012
  16. DT7

    DT7 Member

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    Reactive loads can be used for many things, and somebody else may find something more criticial than what you find critical. Your point of view is no more and no less valid than anybody elses in this regard. You act like a threw a turd in your personal punch-bowl. No such thing.

    As for what makes a difference and what doesn't, a graph can give you a rough idea...but until you've listened to it and compared with the real thing, you have no grounds to state what does and what does not "make a difference"...especially for folks other than yourself.

    Sorry I even bothered to ask a question.
     
  17. James Freeman

    James Freeman Member

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    Sound Clips:


    https://docs.google.com/open?id=0ByFNLR6xWTuXQ1hITVl2TF9rM2M


    Preamp set to JCM800 gain mode (3 gain stages).
    EQ: Bass 7, Middle 4, Treble 7, Presence 5, Resonance 0.
    Master Volume on 10.
    Channel Volume from 1 to 10 (10 sound clips).
    SM57 on my Framus 2x12.

    Nothing changes except the Channel volume from clip to clip.
    I balanced the volumes to match each other.

    There is an audio clip of the amp on 1 without the dummy load (I am not turning it louder that that in the apartment) for refernce between With/Without the Dummy load.
    There are two folders there Resistive & Reactive, total of 20 clips for each volume.

    I will do the clean channel on the weekend which you'll hear what a pure power tube distortion sounds like, because my Clean preamp channel
    dosn't break or distort AT ALL its purely clean to 10 so what you'll hear is 100% 6L6GC power tube distortion with all the bells and whistles.

    Hope it helps someone somehow.
     
  18. yfeefy

    yfeefy Member

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    Is it silly to think that a bunch of those little epoxy inductors that look like resistors could be put in parallel to make one with decent current capacity? They have a spec. of a couple hundred mA, so 10 in parallel would be 2A possibly. I was thinking of saving space, weight and money were I to build something like this, but I guess they would have to be spread apart somewhat so the fields wouldn't interfere? Thanks.
     
  19. James Freeman

    James Freeman Member

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    yfeefy.

    Save your cash, time & effort.
    Buy the Inductors and Capacitor from "ERSE Audio" and be done with it.

    10mH & 200uF will resonate around 112Hz perfect for 2x12 and 4x12.
    (I had to un-wind the big inductor which originally was 20mH).
    0.5mH will match the Celestion guitar speaker inductance.

    For people who love Tube Amps, TONE, Recording, Electronics, Measuring amplifiers SS or Tube.
    Worth every penny, believe me.
     
  20. yfeefy

    yfeefy Member

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    Thanks Ilya - those just didn't look like they would work, but thought I'd ask...

    I'll do the erse when I do this project....
     

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