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Old 07-24-2012   #51 (permalink)
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Default Re: Possibly the best way ever to clamp an amp cheap.

You might also be able to figure out the Power factor angle from the measured DC resistance versus the impedance when your amp is playing. From there you could figure out the real power
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Old 07-24-2012   #52 (permalink)
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Default Re: Possibly the best way ever to clamp an amp cheap.

Quote:
Originally Posted by SINTORMAN View Post
The problem is the phase shift so the voltage and amperage are not aligned. Hence not a true reading of power.
but if the impedance is close to the DC resistance then they are mostly in line. I don't know how accurate you want to be but for most people that would be close enough
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Old 07-24-2012   #53 (permalink)
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Default Re: Possibly the best way ever to clamp an amp cheap.

Quote:
Originally Posted by mporettim View Post
but if the impedance is close to the DC resistance then they are mostly in line. I don't know how accurate you want to be but for most people that would be close enough
I can't really comment on that since I don't know enough to disagree or agree lol. I am sure fallen will chime in.
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Old 07-24-2012   #54 (permalink)
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Default Re: Possibly the best way ever to clamp an amp cheap.

He'll agree

The DC resistance sets the minimum for impedance, the rest comes from reactance (inductive or capacitive).

This image shows what Power factor angle is. The power factor is the cosine of the angle:
http://www.generatorguide.net/power-triangle.JPG

In power systems the goal is to reduce the power factor angle so more real power is delivered. This is often done by adding a cap in the system to cancel out the inductance from generators, etc:
http://www.electronics-tutorials.ws/inductor/ind69.gif

So at tuning the impedance is minimum means the power factor angle is the smallest getting you closest to directly measuring the real power.


Earlier I was assuming the resistance of the coil wouldn't change much during testing but it would be worth measuring again right after testing to see is the resistance has changed from heat
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Old 07-25-2012   #55 (permalink)
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Default Re: Possibly the best way ever to clamp an amp cheap.

Yup I agree, unless the VC heats up. Then the DC resistance will rise, but it'll look like the impedace rose. (well unless you periodically check the DCR of the sub).

But at tuning the load should be resistive. That's actually an interesting solution.

The only issue I could see is when things start to become nonlinear (port compression)

In a random testbox in winISD with a port Q of 100 the impedance magnitude is nearly DCR at tuning and impedance phase is nearly 0.

Change the Q to 1(really crappy port) and the phase jumps up to 20degrees, and the impedance is now over double what it was before. This is still at "tuning"

So ideally it would work. Problem is we're testing amps at their limits, so we'll need a box that has near ideal properties, with a crapload of Power being thrown at it. We're talking worst case scenario, this is where port velocity is at a maximum.

UP SIDE! by choosing 60Hz, you don't need nearly as long of a port compared to a typical box. Or conversly you can fit a crapload of port area if needed.

So the main thing is to measure the DCR periodically if you do this method. VCs will heat up.



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Old 07-25-2012   #56 (permalink)
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Default Re: Possibly the best way ever to clamp an amp cheap.

All of that is controllable though and building a box is fairly cheap.

And for sure measuring the resistance immediately after testing would be best. I'm curious as to how much heat will affect the resistance.
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Old 07-26-2012   #57 (permalink)
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Default Re: Possibly the best way ever to clamp an amp cheap.

Quote:
Originally Posted by mporettim View Post
All of that is controllable though and building a box is fairly cheap.

And for sure measuring the resistance immediately after testing would be best. I'm curious as to how much heat will affect the resistance.
While it's true a box is cheap. 200' of magnet wire is cheaper. The effort in making a box vs wrapping the wire around a suitable former and encasing it is probably similar.

I don't 100% agree on it being controllable. With an ideal port yes, but things aren't ideal. If there was no resistance to the air flowing through the port it would be a perfect solution. But that resistance to the airflow will show up as a rise in impedance.

It's definitely not a bad solution though. I just think it would be better to use essentially a power resistor for benching amps.


As far as how much does temperature rise affect copper wire?
Temperature is in Celsius (25C is roughly room temp)
RT2 = RT1 * (1 + α(T2 - T1))
R25 =1Ω
R200 = 1Ω * (1 + 3.95E-3(200 - 25)) = 1.69Ω

To figure out where the DCR doubles
2Ω= 1Ω * (1 + 3.95E-3(T2 - 25))
2= 1 + 3.95E-3(T2 - 25)
1+25*3.95E-3=3.95E-3T2
T2=(1+25*3.95E-3)/3.95E-3=278C (532F)

So if your VC hits 278 degrees Celsius, the DCR will double. Also a lot of bad things happen at this point. Check out the site below for more info.
Anyways this needs to be accounted for in either the power resistor or subwoofer method of benching the amp. As the DCR rises less current will flow, and an easier load will be presented to the amp. Luckily it takes time for the temperature to rise, (no I'm not going to get into heat equations that's waaaaaaaay beyond this thread), so if you are quick enough with the measurement it shouldn't matter too much.





I modified it for 1 ohm, originally was for 6:
Power Handling Vs. Efficiency



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Old 09-07-2012   #58 (permalink)
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Default Re: Possibly the best way ever to clamp an amp cheap.

who the hell voted my sub should be moved to the no question is dumb forum lol?
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Old 09-08-2012   #59 (permalink)
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Default Re: Possibly the best way ever to clamp an amp cheap.

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