Why would an amp put out 1/4 of its rated power? >> - diyAudio
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Old 9th August 2004, 02:13 AM   #1
beady is offline beady  United States
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Default Why would an amp put out 1/4 of its rated power? >>

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
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Old 9th August 2004, 04:59 PM   #2
maylar is offline maylar  United States
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I couldn't find any specs online for that Alpine amp, so I don't know what it's rated for. But I'm sure it wasn't intended to run 8 ohm loads (no car amp is).

Alpine amps are not known for poor performance. I'd have to ask a couple questions:

1) Are you using a fixed frequency sine wave for the source?
2) What's the input level to the amp (HU preout voltage)?
3) What's the sensitivity range of the amp?
4) Where was the amp's gain control set?
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Old 9th August 2004, 06:03 PM   #3
beady is offline beady  United States
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Figured out what the problem was.

Originally I set the gain using a "musical" CD, set HU at 30 out of 38, and adjusted gain on the amp until I got audible distortion. The gain setting showed about 2.25 volts input, my RF 8120 deck specs say it puts out 2.4 volts, so I'd say the gain was close to where it needs to be.

After I read the JL Audio tutorial i searched the internet and found some test tome MP3's and burned a .wav of the 50 Hz tone and took it to the car and measured amp voltage without the speakers hook up as per JL. I tested the amp bridged and got an AC voltage of 12.4 volts, which looks close to correct based on the numbers for the other amps shown on JL's site, so my gain setting using a music CD seems to be very close to what it should be.

I decided to play with the parametric EQ, which I had not touched before. Turned it ON, maxed out the Q, set the center frequency at 50 Hz and maxed boost. Low and behold, AC voltage with 50 Hz test tone jumped to close to 50 volts! Hooked up the JL10W3 and it confirmed what I just saw. Wonderful bass, cone excursion :-)

Solution for future reference: Parametric EQ needed to be on and adjusted.

I probably don't want to leave the Q gain maxed on one frequency if I want SQ, right?

Manual link for reference:

http://www.nrg1.com/Alp_bruksanvisni...90664W91-B.pdf

Thanks,

Mike
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Old 9th August 2004, 06:10 PM   #4
beady is offline beady  United States
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Join Date: Aug 2004
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Quote:
Originally posted by maylar
I couldn't find any specs online for that Alpine amp, so I don't know what it's rated for. But I'm sure it wasn't intended to run 8 ohm loads (no car amp is).

Alpine amps are not known for poor performance. I'd have to ask a couple questions:

1) Are you using a fixed frequency sine wave for the source?
2) What's the input level to the amp (HU preout voltage)?
3) What's the sensitivity range of the amp?
4) Where was the amp's gain control set?
I am running the sub voicecoils wired in series for 8 ohms. the 8 ohm load in bridged on the amp. The amp sees 4 ohms per channel, just as it is designed to.

Mike
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