Random probably stupid impedance question

[white4d96]

I'm trying to figure out why my '1500rms' MTX amp isn't putting out like any power and am looking at things to see if I can find the problem.

First off, you know how you can check the impedance of your drivers from the speaker terminals when they're hooked up? Is it normal for them to go from 2.1 ohms to 28+ ohms just from the amp turning on, no signal through it?

[Mobile Chassis Dyno]

I'm trying to figure out why my '1500rms' MTX amp isn't putting out like any power and am looking at things to see if I can find the problem.

First off, you know how you can check the impedance of your drivers from the speaker terminals when they're hooked up? Is it normal for them to go from 2.1 ohms to 28+ ohms just from the amp turning on, no signal through it?

I think you are measuring both the impedance of the subs and amp at 28 ohms.

Waiting for the experts to chime in.

Edited January 29, 2010 by Mobile Chassis Dyno

[Percy]

how are the subs wired up

Edited January 29, 2010 by Percy

[fischy fisch]

im sure its just impedence rise that your reading cause ive seen subs jump to like 64ohms and like 108 but the amp waqs messing up on that one but wait for someone who can tell you more

[HHR Ed]

for a proper impedance reading, you should disconnect the subs from the amp.

BTW, the reading will change if the woofer cones moves, even if it only moves a little when wind blows or backround noise causes it to vibrate.

[bkolfo4]

im sure its just impedence rise that your reading

You are kidding right?

As for th eproblem, as already stated, you can only measure the subs with the amp off (and they really should be disconnected). The DMM will not register ohms correctly with an external voltage applied to the circuit. . .

[white4d96]

I just didn't know if the spike in impedance could hint at an issue with the amp.

I was hoping it would point to something because I've heard my '1500rms' system and a similar one in a similar car and they were miles apart. It's really annoying me. I just don't get it. Even a friend's 2 12's sealed off 1200w is louder than me with two 12's off '1500' in a ported box (although said box doesn't have much port area, so maybe part of issue?

I just think my system should be able to get tons louder than it is. Everyone I've talked to has said I should be able to hit '30's no problem, but even to my noobish ear it sounds like its barely in the hundreds.

[bkolfo4]

I just didn't know if the spike in impedance could hint at an issue with the amp.

I was hoping it would point to something because I've heard my '1500rms' system and a similar one in a similar car and they were miles apart. It's really annoying me. I just don't get it. Even a friend's 2 12's sealed off 1200w is louder than me with two 12's off '1500' in a ported box (although said box doesn't have much port area, so maybe part of issue?

I just think my system should be able to get tons louder than it is. Everyone I've talked to has said I should be able to hit '30's no problem, but even to my noobish ear it sounds like its barely in the hundreds.

What woofers? Can you post up all of the dimensions of the box?

[white4d96]

Woofers are two Alpine SWR-1222d's, series-paralleled to 2 ohms. They read about 2.1 on my POS meter. Box is all 3/4" MDF, divorced chamber, 1.3 cubes each with 4" PVC ports, one per chamber. Overall, box is 36x15.25x15.25? IIRC; I built it last year (it's either 15.25 or 16.) Each side has one pound of polyfill from me messing around with ideas for new box volume. IIRC gives it an effective volume of 1.8 per and tuning of 28Hz (without fill, 32 and change.) Doesn't act like its tuned that low though; there's a lot of rolloff down low. The system I heard was two AB 12's off an AB 1500rms amp in a Buick Century. That's all I know; a local shop demo'd it for me an a friend who was woofer shopping. It seemed to be tuned a little higher, but there was much less rolloff than my box.

[Boon]

im sure its just impedence rise that your reading

Dude, don't be a spanner. If you don't know what you're talking about, don't post.

It's a class D amp right? If it has an output relay you're probably seeing the impedance of the driver in parallel with the impedance of the low-pass filter on the output of the amp.

I wouldn't ever measure the impedance of subs while they're hooked up to the amp, especially not when the amp's running. Do you know how a multimeter checks impedance? It pushes a small current through the circuit and measures the voltage drop across it. Do you really want to be pushing DC voltage through the output section of your amp? If you want to find out impedance, check the voltage and current and calculate it.