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Working around impedance rise


k58.cross

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So my system isn't making the power I want, even though I have enough current supply and big enough amp and sub. The setup is stock 125 alt, stock starter battery group 36, XS power XP2000 in parallel with stock battery, 1.5kW mono block amp, 1.75kW sub wired to 1 ohm, ported box tuned to approx 35 Hz. I have measured voltage and current to the sub and not seen 1.5kW yet. Here enters impedance rise...

I know the Z-curve for a ported box has 2 peaks and the tuning freq of the box is between them. So that means you get the most output right at box tuning, weak output just outside box tuning, and moderate output everywhere else?

For a sealed box, is the Z-curve the same profile as if the sub was in free air? If so, does that mean output would be weakest at the resonant frequency of the sub, and moderate everywhere else?

With all that in consideration, is the only way to get more power out of my sub to get a bigger amp or different sub and wire lower?

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And yeah I know it's a small build. But this is in a -Ford Fiesta- the smallest god damn sedan you've ever seen lol.

I am just trying to learn as I go and get the most I can out of the system. With the extra battery, I should have all the current supply I need for 1.5k. But I want to actually measure that much output. Seems like the system is getting limited by impedance rise. Driving the amp to clipping, I am not maxing out my current draw, so it does seem like current is simply not drawing heavily enough. If you have any suggestions on how to tweak this further besides just going bigger...

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So you are making a couple of false assumptions.  The first is that frequency response and impedance are related, the second is that frequencies where your amp puts out more watts is where it will be louder, both of those are incorrect. 

You are correct that a ported box has an impedance graph with two peaks, between those two peaks is a dip in impedance at the tuning frequency and this is where your sub will allow the amp to flow the most current and thus the most watts, it may or may NOT be the point were you get the most output.  With sealed boxes, the peak in impedance where the amp is producing very few watts there is no dip in output around those frequencies at all.  The point is output and impedance are not directly correlated.  

On 1/24/2019 at 7:09 AM, k58.cross said:

... is the only way to get more power out of my sub to get a bigger amp or different sub and wire lower?

This may be true, depending on your box.  Since its a ported box, if your port is undersized you will get port compression which negatively affects output and raises impedance.  What are the specs of your port?

"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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Well that killed what I thought I knew lol. I definitely did think it was straightforward like impedance > current draw > power draw > SPL. But since this is both an electrical (power circuit) and a mechanical system (air circuit), not surprised it's more complicated than that.

My port is 23" long and 6" diameter aero with flared ends, and the box is approx 2 cubes net.

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1 hour ago, Triticum Agricolam said:

You are correct that a ported box has an impedance graph with two peaks, between those two peaks is a dip in impedance at the tuning frequency and this is where your sub will allow the amp to flow the most current and thus the most watts, it may or may NOT be the point were you get the most output.

In my experience I have cabin gain that affects the loudness more than the tuning of my box. Is this what you are talking about here? Why else would you get less output when the amp is putting out the most watts?

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1 hour ago, k58.cross said:

Well that killed what I thought I knew lol. I definitely did think it was straightforward like impedance > current draw > power draw > SPL. But since this is both an electrical (power circuit) and a mechanical system (air circuit), not surprised it's more complicated than that.

My port is 23" long and 6" diameter aero with flared ends, and the box is approx 2 cubes net.

Your port area is a bit low for the amount of power you are running so you probably are getting moderate port compression, but its definitely not what I would call severe.    If you ever upgraded to a larger amp would want to build a new box with more port area. 

"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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28 minutes ago, Bret said:

In my experience I have cabin gain that affects the loudness more than the tuning of my box. Is this what you are talking about here? Why else would you get less output when the amp is putting out the most watts?

Cabin gain will effect your frequency response significantly, but it has nothing to do with the amount of power your amp puts out.  

So what I'm referring to above is the fact that you subwoofer is more efficient at some frequencies than it is at others and your box is a big part of that.  Here are some pics to help explain what I'm talking about.

image.png.fbf258f2e413cb27e446646d683048f0.pngThis is the frequency response of a sealed 12" sub in open air (no cabin gain).  This is what the sub will output assuming it is sent the same signal level (voltage).  Pretty flat response that rolls off significantly below 30 hz.  

 

image.png.2f96a67799681503c16f403abd9ba85a.png

This is the impedance of that same sub, its a sealed box so there is only one peak, its at the subs sealed resonance frequency (44 Hz).  Notice that this graph looks nothing like the frequency response graph.  

 

image.png.3915cad23c9396d24a3ba184d5f8a9bb.png
Here is the amplifier apparent load graph assuming 1000 watts nominal input power.   You will notice that the amp never actually puts out 1,000 watts, there is a dip around 44 Hz in amp output watts due to the impedance spike.  Please note that over this whole frequency range the amp is putting out the same output voltage, the impedance of the sub is limiting the current, and thus watts, but the output voltage stays the same.  

"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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30 minutes ago, k58.cross said:

So the frequency response stays even because the mechanical resonance of the speaker is allowing/causing the amp to do less work?

Yes pretty much.  The system naturally wants to resonant at/near its sealed resonance frequency, so it requires very little effort from the amp to get it to do so.  

"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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