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Dummy speakers to alter impedance?

Taylor Livingston

Taylor Livingston

Commercial User
Dec 25, 2002
3,210
2,312
Oregon, US
Disclosures
Owner, Iron Ether Electronics
#1
This post will probably reveal that I don't have a great grasp of the physics involved in amplification. Here's what I'm wondering: Is it possible to have a dummy speaker coil inside a cabinet, that can be switched in or out of the circuit so as to switch between 8 ohms and 4 ohms? It seems like this would achieve what the "Accuswitch" was trying to, but I've never heard mention of anything like this, so I'm guessing it isn't possible, or wouldn't work as I'm imagining for some reason. Any of you knowledgable folks have some thoughts on this?
 
#2
Yes, you can do it, but there's no point because the dummy load will dissipate power without producing any useful output. Here's an example:

Suppose your head puts out 300 W into 8 ohms and 500W into 4 (pretty typical that most heads won't put out double their wattage into half the impedance).

Take your 8 ohm dummy load and put it in parallel with an 8 ohm speaker. Now you have a 4 ohm load and your amp is producing 500W -- great, right? Not so! Because that 500W is being divided between an 8 ohm speaker and an 8 ohm dummy load -- so the SPEAKER only gets 250W -- and the other 250W is being dissipated in the dummy load as useless heat.

AT BEST, you'll break even if the amplifier power doubles with a halved load impedance. But then what's the point?

Asad
 
#6
anyonefortennis said:
As stated before, this is the equivalent of filling the trunk of your car with concrete so you can use all the horsepower in your car.
Click to expand...

Not really the best analogy (since a car engine produces the same amount of power no matter what the weight is, and there's nothing stopping one from using all of that power in any case), but the end result is the same :)

Asad
 
#8
Bob Lee (QSC) said:
No, it doesn't. It's very rare for a driver to use all the power available from the car's engine.
Click to expand...

I totally disagree with this statement... I own a perfomance shop... I know that the above statement is untrue.

As for the switching impedance... Don't bother. Guitar guys are always looking for Hotplates to do it, and all you get is lower volume, and it really doen't work right...
 
#10
asad137 said:
(since a car engine produces the same amount of power no matter what the weight is, and there's nothing stopping one from using all of that power in any case)
Click to expand...

Absolutely false.

A typical automotive internal combustion is MAYBE 25% efficient. The biggest loss of power is heat, followed by the internal frictions of all the moving mechanical parts, followed by friction.

Then you lose power in the transmission as you try to gain mechanical advantage through gear reduction, then you lose it again at the axle's differential, followed by the wheel bearings.

Cars are incredibly inefficient in relation to the true energy release of combustion of gasoline.
 
#11
Bob Lee said:
No, it doesn't. It's very rare for a driver to use all the power available from the car's engine.
Click to expand...

Even so, the amount of power the ENGINE can produce is not a function of the WEIGHT of the car. And plenty of teenagers (and non-teenagers) use all the power available from their car.

alexclaber said:
You only get full power from an engine when the throttle is fully open and the engine revs are at the power peak (usually somewhere within 1000 RPM of the redline).
Click to expand...

Quite true. Also still not a function of vehicle weight.

Bryan316 said:
Absolutely false.
...
Cars are incredibly inefficient in relation to the true energy release of combustion of gasoline.
Click to expand...

:rolleyes: You know, this is why I rarely ever pay attention to your posts. You've completely missed the point and are arguing a point that is not only off-topic but also completely and totally irrelevant. The power rating of a car is the amount of usable power the car produces, AFTER all of the inefficiencies. Yes, engines are not thermodynamically very efficient. But who cares? Nobody compares BSFC numbers.

Please. I'm not a ****ing idiot. I know how engines work. I have built my own turbocharger system for my own car. I have had it dyno tested (using ALL of the available power from the engine). I know plenty of people that use ALL of the available power from their cars, be it on the dragstrip, track days, autocrosses, or on the street (though I don't condone the last). And in ALL OF THESE CASES, MAXIMUM ENGINE POWER IS NOT A FUNCTION OF VEHICLE WEIGHT.

Thank you. Have a nice day.

Asad
 
#12
asad137 said:
Even so, the amount of power the ENGINE can produce is not a function of the WEIGHT of the car. And plenty of teenagers (and non-teenagers) use all the power available from their car.
Click to expand...

True, the amount of power the engine can put out is not a function of the weight of the car.

How much power it does put out can be related to the load--the weight--and how hard the driver is trying to make it accelerate, though. An engine with no load, for example, delivers no power (except that which overcomes friction, runs the electrical system, et al).

It is in fact rare to use an engine's full power. Most of the time when we drive we're using just a fraction of the available power.

You can use more engine power by increasing the load on it (increasing the vehicle's weight, dragging a heavy anchor behind it, etc.), similar to increasing the load on the power amp by adding a non-transducing load impedance. But you won't gain anything from it; it's just wasted power.
 
#14
There CAN be a reason to do this.

Guitarists are often looking to get maximum tone out of tube amps without running at maximum volumes. A Marshall 100 watt head dimed out (run tull up) sounds great, but DAMN it is loud, usually too loud to use live. This is why guitarists with tube amps often play too loud - to get the sweet spot for tone, you need to push the amp hard (not just the preamp, but the power amp, too), which means you often need to crank it up! And that's very loud if your guitarist is running a high watt head. Running a dummy load or other power attenuation / power scaling can help this. Lots of guitarist use separate devices (THD Hot Plate, Marshall Power Brake, etc) and some are integrated in amps (Suhr Badger, Fargen, etc). Some devices work well in reducing volume but comprimise your tone, other work very well but are quite expensive.

Just FYI...
 
#17
heath_r_91 said:
Couldn't it be useful in tube amps? Because can't say, SVT-CL's only run on 4 or 2 ohms? So adding a dummy load to an 8 ohm cab could make it so you could use it with that amp.
Click to expand...

Sure, but it's even worse in a tube amp than a solid state because the power output doesn't increase as the load impedance drops. And you're still wasting power in the dummy load.

Asad
 
#18
billfitzmaurice said:
Or play through one twelve instead of eight. :rolleyes:
Click to expand...

Yes, that'll help but...

To get my Vox AC30 30 watt 1x12 combo sounding good (to my ears) I turn the pre-amp volume to between 5 & 7 and the master volume to between 5 & 7, which is VERY LOUD - too loud for practice, OK for some stages, fine for recording.

To get my Sovtek MIG-50 50 watt head into my Avatar 1x12 sounding good (to my ears) I dime everything on the head (all up to 10) and it's too loud for anything but recording.

Now sometimes I use the Sovtek MIG-50 into a Hartke 4x10 to record distorted bass. Again, I dime the amp to get it sounding really great, but it's loud (not as loud as the guitar stuff, though). Awesome for recording, too loud for live.

:D

asad137 said:
Sure, but it's even worse in a tube amp than a solid state because the power output doesn't increase as the load impedance drops. And you're still wasting power in the dummy load.
Click to expand...

But sometimes that can be a good thing, as I described, when you want to push the amp without the volume.

:D
 
#19
Hi, conical johnson.

I haven't tried it myself, but a compound, (or isobaric for some folks) dual driver combo should do the trick of changing the impedance. The dual voice-coil drivers might also work, but the physical construction of the dual coils is seldom ideal and I haven't seen any MI dual voice-coils. Only auto Hi-Fi and Hi-Fi ones, and those aren't IMHO feasible alternatives.

Bryan316 said:
A typical automotive internal combustion is MAYBE 25% efficient.

Cars are incredibly inefficient in relation to the true energy release of combustion of gasoline.
Click to expand...

And compared to the ~5% of a typical MI speaker and ~1% of a full range Hi-Fi one? Still a bad efficiency?

Just my 0.02€
Sam
 
#20
T-Bird said:
Hi, conical johnson.

I haven't tried it myself, but a compound, (or isobaric for some folks) dual driver combo should do the trick of changing the impedance. The dual voice-coil drivers might also work, but the physical construction of the dual coils is seldom ideal and I haven't seen any MI dual voice-coils. Only auto Hi-Fi and Hi-Fi ones, and those aren't IMHO feasible alternatives.



And compared to the ~5% of a typical MI speaker and ~1% of a full range Hi-Fi one? Still a bad efficiency?

Just my 0.02€
Sam
Click to expand...

So is anybody pursuing this for musical instrument speakers? Are there any really negative attributes for these dual coil speakers? It seems like there would be somebody doing this if it were viable.
 

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