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Thread: dummy speaker load.

  1. #1
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    dummy speaker load.

    I know to make a dummy load to simulate speaker is hard. But what is a simple way to make a dummy load that is "similar" the the impedance of a speaker? I am not an expert, the most simple thing I can think of is a resistor of something like 4 or 5 ohm in series with a parallel combination of an inductor and capacitor. The inductor has value similar to the speaker and the capacitor the get the resonate frequency of a typical speaker.

    This is the simulate a resonance circuit with peak like a typical speaker, then the resistance of the resistor at off peak frequency. May be a resistor Rp in parallel with the LC tank to lower the Q.

    What do you guys think? If you have schematic of existing dummy speaker load, please post it for me.

    Thanks
    Attached Thumbnails Attached Thumbnails speaker-simulator.jpg  
    Last edited by Alan0354; 01-08-2014 at 05:13 AM.

  2. #2
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    This thread will tell you all you need to know...

    Aiken's Reactive Dummy Load. - The Gear Page

  3. #3
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    Thanks, I looked at the thread, the OP want to load the tube amp so the line out sounds like an amp with speaker in the direct recording. That is not what I am after.

    What I am trying to do is using it as a dummy load. I have a Pro Reverb that has two 8 ohm speaker in parallel. All I want is to replace one of the speaker with the dummy load and using one speaker and reduce the output loudness by half. Basically I want to have a switch to switch between one speaker or dummy load so I can get the equivalent of either a 40W or a 20W amp. I just need to have a load that does not change the sound of the one speaker. So I would expect the requirement will be a lot more relaxed and the design should be simpler.

    I tried using a straight resistor, it is really not too bad. I seemed to loss a little of the rawness and a little high. Maybe that is due to the lack of the resonance peak to divert more power to the speaker at resonance frequency. I think all I want is the impedance graph of a speak and design a load to curve fit the impedance.

  4. #4
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    If you don't think the HF impedance curve is important your diagram is fine. Using values readily available at Parts Express Crossover Components in the Speaker Components Department at Parts Express | 21 you can build an 8R "active" load either way like this.

    I've noticed that using air core inductors for the HF inductor can cause EMF feedback with single coil pickups and possibly amp circuitry. I solved for this by using two 250mH inductors in series, out of phase stacked together with a copper plate in between to minimize cross induction. Then the EMF from the 500mH circuit is humbucking.
    Attached Thumbnails Attached Thumbnails 8rdload.jpg  
    "I should have been born sooner. Of course, if I had been, I might be dead now." trem

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    Your requirements are much higher than his, who worst case may "cheat", load the amp with *anything* (a cheap simple resistor) and tweak low level signal afterwards, while you want "same response" at full power, and listen to it through the remaining speaker, a much harder task .
    And lowering power by 3 dB is very little.
    I'm more and more convinced that Power Scaling is the way to go, only consistent way to *really* reduce power and nothing else.
    Juan Manuel Fahey

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    Juan, I think it's important to note that the amp would indeed only be 3db less loud at half power, but we're also taking half the speaker surface out of the equation too! I think the difference should be worth the effort. Lot's to consider if you really wanted to nail the frequency response and the amplifiers reactance though. Roughing it will have to do. Plotting the actual speaker being bypassed would mean using multiple components in series or parallel to nail the actual impedance curve and then, of course, the other speaker doing the rest of the work now has the whole cabinet to itself, which will change things too. Getting closer than a plain old resistor is worth the effort IMHE but a generalized guitar speaker-ish plot should be fine.
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    "I should have been born sooner. Of course, if I had been, I might be dead now." trem

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    Senior Member Enzo's Avatar
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    Remember, the OP didn't say half power, he said half loudness. A resistor will show how well that will work, all adding the L and C components will do is massage the tone.
    Education is what you're left with after you have forgotten what you have learned.

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    Absolutely. Good point on the plain ol' resistor giving the general clue on actual loudness.

    I can't speak as to exactly what "half as loud" even means. Half the dB's would probably seem waayy less than half as loud. And of course half power is waayy more than half as loud. There may be an actual formula for it but I think it mostly comes down to a perception by the user.

    I do know that back in the day, when the Fender Twin Reverb was the almighty stage king for well healed players, a common trick was to pull two tubes and then disconnect one speaker to correct the load. It did indeed sound like half the amp was shut down.
    "I should have been born sooner. Of course, if I had been, I might be dead now." trem

  9. #9
    Senior Member Enzo's Avatar
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    When we speak of loudness in any sort of practical sense we are concerning ourselves with SPLs. Perceptions are not measurable.
    Education is what you're left with after you have forgotten what you have learned.

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    I want to clarify, I should have said "half power loud". Yes, using a dummy load for one of the two speaker just make my Pro Reverb loudness down to a Deluxe Reverb......from 40W to apparent 20W. Not much lower in loudness.

    But as everyone experience, no one method can lower the loudness all the way down and still sounds good. I did a few power scaling, if you go down beyond 1/3 power, it will sound different. So are attenuators.

    I even tried unplugged one speaker and using my 8 ohm THD Hotplate to replace the speaker when put it in high attenuation setting. It still does change the sound a little. But remember, attenuator are made to optimize the sound when you plug the speaker through it, not necessary a dummy load simulator.

    So far, I tried a simple resistor, THD Hotplate as dummy load. Seems they are no better if even as good as just using the THD Hotplate and use it at -4dB ( more than reducing half power already).

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    Anyone have experience with the Weber attenuator. Their Mass supposedly have a speaker type of load. Anyone know how's the inside looks like?

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by Alan0354 View Post
    ...Anyone know how's the inside looks like?
    It's a speaker magnet structure, voice coil and spider mounted in an abbreviated speaker frame. Basically a "speaker" without a cone and the large part of the basket that supports the cone. I assume that a mass (weight) is added to take the place of the cone.
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  13. #13
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    Quote Originally Posted by Tom Phillips View Post
    It's a speaker magnet structure, voice coil and spider mounted in an abbreviated speaker frame. Basically a "speaker" without a cone and the large part of the basket that supports the cone. I assume that a mass (weight) is added to take the place of the cone.
    But that still change the power handling capability and the reactance. The resistance of the real cone change everything. Or else it will be too simple by just cutting the cone off a cheaper speaker and use it as a dummy load. That was my first thought way at the beginning and I dismissed that out right!!! I thought make the shoot absorber to simulate the speaker cone will be too hard.

    It is like comparing an loaded motor and an unloaded motor, the impedance and the impedance profile is totally different.
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  14. #14
    Old Timer Tom Phillips's Avatar
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    Yep. The Weber mass is not a perfect simulation of a real speaker. There is a photo of the "Speaker Motor" at Attenuators . It appears that they have simplified since the original design. A while back they also stopped selling the stand alone speaker motor and I believe it is now only available as part of their attenuator products.

  15. #15
    Old Timer olddawg's Avatar
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    I use a Weber Mini Mass 50 watt in clubs all the time. It has a 3 position treble compensation switch on it. I had mine made with a foot switch to turn it on and off. Sometimes I use it like a boost for leads. It also has a line out. It does color your sound a bit, especially if it is turned way down. It is best used just to take the "edge" off of an amp not squash it. I hear you can use them for a dummy load with no speaker attached although I have never done it. I would research that. Generally I use mine in practice and in small clubs for the first set while things are filling up and settling in. The 50 watt version for me, with the foot switch option was $125. I could not build anything near that quality for that price. It is very rugged in a metal case with switch craft jacks and such. Very usable and a great bang for the buck. Many times I have it on turned all the way up. Then switch it off for leads. Keeps the stage volume down and the amp cranked. Works great on my 18 watt Marshall and my VJ. That's another idea. You might want to pick up a VJ for $100 and play through it and one 12" speaker. You might be surprised. Nobody believes I'm playing through a $99 5 watt amp in the clubs but I do it all of the time. I have played the HOB with that VJ head through a 2x12 cab. The sound guy loved it. Same thing for 1200 people at The Coach House. Just got miked up.

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    I went ahead and just use a 7.5 ohm resistor and a SPDT to choose between one of the speaker and the resistor. I am surprised how good the result is. I had the amp up to 5.5 with resistor and about 3.5 with the speaker. It has more base with two speaker, I verify by using a 4 ohm resistor to even take more load off the speaker. I got even less bass with 4 ohm. So the observation of using resistor reduce the bottom a little. With a 10 ohm, it's just very little difference.

    Two speaker do sound fuller than a single. But I think that is normal as I was in a small room with the amp less than 3' from me. So having two speaker do widen the wall of sound quite a bit. It's like you crank the balance of the stereo to one side. Even though you know the stereo sounds exactly the same, just because you lose one side, it's just sound not as full. I think in order to have a more representative test, I have to have it in my living room where I can stand at least 10' away.

    As for the sound, I actually think it sound slightly better than using my THD Hotplate attenuator at -4dB. The THD seemed to lose a little punch even at -4dB.

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    This topic made me ask myself a question to which I do not know the complete answer, I'm hoping one of you smarter guys will be able to fill in the blanks.

    Assuming the two speakers in the amp are in parallel, for a (modern) solid state amp we could just disconnect one speaker and get a theoretical 6dB reduction in level (assuming the voltage output stays constant from the amp); 3dB decrease in voltage sensitivty changing from 4 to 8 ohms and 3 dB decrease from mutual coupling of the speakers. If we did the same in a tube amp we wouldn't get the same results because the load reflected back to the power tubes would double as a result of the speaker impedance doubling. Will that cause the output tubes to generate more voltage or less voltage? I assume it will be more voltage? How much more? I can't imagine it would double. How much would you anticipate the real-world reduction in output would be? For a solid-state amp, real world is usually somewhere between 4-5 dB.

    Also, since most guitar amps seem to tolerate running with the load doubled (8 ohms on a 4 ohm tap, for example), why not just unplug one of the speakers instead of putting any kind of dummy load in place? I know that at very high levels that can be a problem for the OT, but it seems like it is something done successfully quite often.

    Please educate me!

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    Quote Originally Posted by cbarrow7625 View Post
    This topic made me ask myself a question to which I do not know the complete answer, I'm hoping one of you smarter guys will be able to fill in the blanks.
    I do not the complete answer either, so we will both see what that is...

    What I can say is that changing the load impedance, by removing one speaker (or swapping cabs, or whatever), will cause the load line on the output tubes to rotate. The primary effect this has is to change the *character* of the sound, as the operation of the tube(s) is now different than it was. For a doubling of the load impedance, the primary Z will also double, causing the load line to flatten. What that will do to the voltage depends on the actual load line and operating points, but it will likely go up a little bit, nowhere near double though.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  19. #19
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    I can tell you the amp does not go down much in loudness when I disconnect one speaker. The main thing that the sound change is more like it is not as "wide" for obvious reason..........you cut out half the area that produce sound!!! I think sound change from increasing impedance is very minor, but you don't lower the loudness much by cutting out one speaker.

    On the other hand, replacing one speaker with a resistor do lower the loudness more obvious. Tube amp is not like SS that you truly half the power by doubling the load impedance.

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