Impedance Rise

[Kian]

OK yesterday I was reading about the Stetsom amps and read this...

"Also did some .5ohms spl testing in our spl car NSB G3100 battery. Impedance rise needs to be close to 2ohms so amp won't go into protect but expect at least 4300w with a single good group 31 batter with a voltage drop to 11.4v. We tried it at .5ohms with the music box but the impedance rise was only to 1.4ohms from .5ohms and amp went into protect even with mute roll. It's a bad little MO FO."

I head people talking about it all the time, i've tried to search for it (nothing), so what is a Box impedance rise

because this is a pet peeve right now.

Even if there is a website, i'll read it if no one feels like explaining it.

Thanks

kian.

[AI James]

The DC resistance is the measurement of the coil at rest. When the cone starts moving, the magnetic field associated with the magnet and the moving coil increases the resistance of the voice coil. Usually the more resonant the enclosure, the higher the impedance rise. Most competitors try to factor rise into the equation so they can get the most power out of their amps.

[Kian]

Usually the more resonant the enclosure, the higher the impedance rise.

so lower the tuning of the box would lower the impedance rise. which would be better?

[Torres]

there's a few factors that come into play, like size of the box and number of amps also

[Kian]

ok lets say i built a new box around 4^3 and running 1 amp. because there are people out that swear there are running there kicker zx2500.1 amp which is stable @ 2 Ohms @ .7 Ohm i only want to use this for burps at competition. and then go back to 2 ohms. does what its tuned to make a difference?

Because my amp keeps going into protect at 1/2 volume.

[bkolfo4]

You have to test to understand where you are with your setup. It may have a higher impedance, you may be closer to your "rated" impedance.

Also, impedance changes with frequency, so that adds more issues. When I first finished my van, I had one song that just would not play without sending the amps into protect. . .I guess the bass in that song was right in my low impedance point. (Before I fixed the problem, all I had to do was play the subs at 1/2 volume for 15-20 seconds and let the coils warm up before going full volume, since a warm coil has a higher impedance).

Some amp manufactures rate their amps knowing there will be some impedance rise.

edit - for burps, your amp also has to handle the instant high power hit of a resting coil. When power first hits the coil, the impedance is pretty close to the DCR. But you can always do a rolling burp.

Brian

[Kian]

i Figured it out. WOOOOOT

it's voltage divided by amperage

Ohm's Law states that E=I*R

R is Resistance(Ohms)

E is Voltage(Volts)

I is Amperage(Amperes)

P is Power(Watts)

R=E/I

R=E^2/P

R=P/I^2

Measure you nominal resistance with power off. Now put the clamp on the + output wires from the amp to the speaker. Put the voltometer on the + and - outputs of the amp. Power everything thing up. BURP!!!! Quickly write down the voltage and amperage. If you have a termlab, THEN write down the SPL. Calculate power by multiplying Volts*Amps. Then divide Volts by Amps for reactive impedance. Subtract the nominal impedance, and you have your impedance rise for that frequency, power, and enclosure. Change one of those factors, and the rise and final impedances will change.

Every few burps, get another nominal impedance measurement, since hot coils may read different

If you have a second clamp and voltometer, you can measure amplifier efficiency at the same time.