Why's my amp put out 1/4 of rated power?

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beady

Copper - Posts: 65

Joined: August 08, 2004

Location: United States

Posted: August 08, 2004 at 9:11 PM / IP Logged

I have two car audio amplifiers; an old school Punch 150 I've had since college, and a newer Alpine MRV-T505 V12 Expert amp. Both are two channel, both are bridgeable.

I've had my Alpine running my Boston Acoustic RC61 component mids and the Punch running sub duty. I have never been really happy with the level of power the Alpine delivered to the BA's, but they sounded clean at low levels.

I decided to hook my sub (one JL Audio 10W3-D4 wired for 8 ohms) to the Alpine bridged to see how it compared to the Punch. The Alpine barely moves the cone of the 10W3, even at higher volumes.

I decided to hook my AC voltmeter across the sub to measure voltage and calculate power. I realize that this is not a fully accurate method to determine power, but I figured it would at least give me a quantitative power comparison between the Alpine and the Punch.

The Alpine maxed out at 13 volts AC, usually a bit closer to 10 or 12. The sub measured 7.6 ohms after being run with the amp. This figures to about 22 watts of power. That is from the Alpine running tri-mode, mixed-mono, whatever you want to call it. With an 8 ohm sub on it bridged, the output should be about 100 watts.

I switched the crossover on the amp to low pass at 80 Hz and 13 volts was the max I could get. That works out to about 11 watts per channel at 4 ohms per channel. The amp is rated 50W per channel at 4 ohms. I thought Alpines were underrated on power output also?

Why would the amp be putting out only 11 watts per channel?

That would explain why the volume from my mids was never that high. I did set the amp's gain by playing a CD at a HU volume of 30 out of 38 and turning up the gain until I got distortion, then turned it down a hair.

For comparison, I hooked the same JL10 up to my Punch 150 bridged, and it beats the crap out of the JL10 compared to the Alpine. All kinds of cone excursion, HUGE difference in sound. I measured 50+ volts at the speaker terminals. Again, I measured the resistance of the sub after the run and it was 7.6 ohms still. This works out to 330 watts or so. That's 15 times the power output of the Alpine! The Punch is rated at 75 watts per channel at 4 ohms, so 150 watts into an 8 ohm bridged load.

I figured the difference between the 50 watts/ch of the Alpine and the 75 watts/ch of the Punch would not be that different.

I figure there has to be something wrong with the Alpine becuase it's power output is so far below its rated power.

What can I do to troubleshoot or double check?

Thanks for any help,

Mike

Teamrf

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Joined: January 13, 2004

Location: United States

Posted: August 08, 2004 at 9:57 PM / IP Logged

That sub is barely going to move at 8 ohms.

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beady

Copper - Posts: 65

Joined: August 08, 2004

Location: United States

Posted: August 08, 2004 at 10:11 PM / IP Logged

Teamrf wrote:

That sub is barely going to move at 8 ohms.

"For comparison, I hooked the same JL10 up to my Punch 150 bridged, and it beats the crap out of the JL10 compared to the Alpine. All kinds of cone excursion, HUGE difference in sound. I measured 50+ volts at the speaker terminals. Again, I measured the resistance of the sub after the run and it was 7.6 ohms still. This works out to 330 watts or so. That's 15 times the power output of the Alpine! The Punch is rated at 75 watts per channel at 4 ohms, so 150 watts into an 8 ohm bridged load."

beady

Copper - Posts: 65

Joined: August 08, 2004

Location: United States

Posted: August 08, 2004 at 10:34 PM / IP Logged

At 8 ohms bridged on the amp, it should see 100 watts. That is plenty to move the sub.

Besides, did you not notice that it put out 15 times less power than my Punch? The punch is rated at 75 watts per channel, the Alpine 50 watts per channel. The Alpine should not put out 15 times less power, something is wrong.

Mike

dpaton

Copper - Posts: 141

Joined: July 19, 2004

Location: United States

Posted: August 08, 2004 at 11:45 PM / IP Logged

That 15x factor indicates that either something was very wrong with the measurement methodology, or your Alpine is b0rked. The former sounds infinitely more likely...

Unless you were measuring a 60Hz test tone, your meter readings are probably less than worthless. Most meters (the ones that aren't True RMS types like the expensive Flukes, Teks, HPs, and WaveTeks) will give really wonky readings with complex signals (music) or things much outside their design range (not 50-70Hz).

The only way to know for sure is to put a dummy load on the amps, feed them a known signal, and watch it on an oscilloscope. Turn it up until the sinewave just starts to flatten or distort, then back off until it's clean. Check the gridlines and from there you can get your peak to peak voltage. Figure RMS. Run it through P=(V^2)/R and voila, you have the max power the amp can produce.

-dave

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beady

Copper - Posts: 65

Joined: August 08, 2004

Location: United States

Posted: August 08, 2004 at 11:57 PM / IP Logged

dpaton wrote:

That 15x factor indicates that either something was very wrong with the measurement methodology, or your Alpine is b0rked. The former sounds infinitely more likely...

-dave

Yes, I realize you need to use a dummy load for the power you calculate to be accurate. The meter is good DVM. I used a song with a repetitive base line that played over and over and used the same song to test the amps.

But you're missing the point.... I was not trying to get an "accurate" measurement of the amps' power, I was trying to get a "quantitative" measurement of comparison between the two amps. I did accomplish this, and regardless of how accurate my actual power calculations were or were not, they do show that the Alpine is making vastly less power then the Punch. My ears and eyes confirm this.

Mike

auex

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Posted: August 09, 2004 at 12:32 AM / IP Logged

If all else is equal and the alpine doesn't move the sub then your alpine is f**ked, simply put.

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beady

Copper - Posts: 65

Joined: August 08, 2004

Location: United States

Posted: August 09, 2004 at 1:09 AM / IP Logged

auex wrote:

If all else is equal and the alpine doesn't move the sub then your alpine is f**ked, simply put.

While this may be a possibility, I don't think (hope) so. I bought the amp new, in the box, from an Alpine dealer. I installed it carefully with RCA cables down one side of the center console, speaker wires up the other side of teh console, and power cable down door sill. The amp has always sounded "good" when not driven to extremes. I have the amp pushing a set of Boston Acoustic RC61 components, they should be able to take all this amp puts out and then some since it is clean power and they are good components, right? Components rated at 90 watts continous if I remember.

Point being that in 4 or 5 years of ownership, the sound of the amp has never changed, nor the volume level.

If the amp was malfunctioning, wouldn't I hear it in some fashion? I.E. the volue would be less, one channel out, noise, etc?

Are there any measurements I can take from the amp to narrow down any problems?

Mike

heavilymedicate

Silver - Posts: 328

Joined: August 05, 2004

Location: Texas, United States

Posted: August 09, 2004 at 2:26 AM / IP Logged

I was not able to find that amp when I looked to get the specs but... If your numbers are right your only question is how much money do you have and what subs do you want. Personally i prefer JL's but there are a lot of good subs to be had if you have the cash! So, how much ya got ? Why's my amp put out 1/4 of rated power? -- posted image.

Why's my amp put out 1/4 of rated power? -- posted image.

heavilymedicate

Silver - Posts: 328

Joined: August 05, 2004

Location: Texas, United States

Posted: August 09, 2004 at 2:31 AM / IP Logged

DOH! Ignor that shiznit, i had multipule pages open and got lost. I accualy have something for you though, have you tried changing the amp set up? I mean taking the Alpine and moving it so that you use all the conections from the Rockford. Maybe its somewhere in the wiring your using for the Alpine, if nothing else you can rule out the wiring because you know the rockford wiring is good.

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