Why’s my amp put out 1/4 of 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

Teamrf wrote:

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That sub is barely going to move at 8 ohms.

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"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."

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 wrote:

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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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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]I wrote:

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

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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

um, the alpine is 50w/ch@ 4 ohm which is about 100w mono @ 4 ohms bridged, which is about 50w mono @ 8 ohms

the rockford is 75w/ch@ 4 ohms which is about 150w mono @4 ohm bridged, which is about 75w mono @ 8 ohms.

Sorry this doesn't help your problem at all.

As long as you have the multimeter, go to https://www.jlaudio.com/tutorials/index.html and check out the "setting gain" tutorial. It's quick and easy.

If either of these amps are 2 ohm stable while bridged, I would run it that way to the sub (more power eeeyaahhh)

Alpine MRV-T505 manual:

https://www.nrg1.com/Alp_bruksanvisninger/Owners/M/MRV-1505/68P90664W91-B.pdf

Seems to be a somewhat rare amp, hard to find info on it.  It's larger brother the MRV-1505 gets all the press. 

Notice that the MRV-T505 and the MRV-1505 are stable bridged to a 2 Ohm load, i.e. 1 Ohm per channel stereo.  It says you must move the switch to the low impedance position to run this way.  See swithc "b" in the manual.  Does anyone know if this will increase the power over output over a bridged 4 Ohm load, or is it just to give you more wiring options without altering the power output?

I tried running the amp with only the 10" bridged and the other speakers disconnected, no change.  Also tried the input mode selector switch in stereo and 1+2 and it didn't make much difference.  Am running with the amp's crossover set at about 80 Hz low-pass.

My amps are wired in a "daisy chain" fashion with the Alpine getting the wires first.  As in the power wire runs into the Alpine, then out of the Alpine to the Punch.  Same with the ground wire so they are grounded to teh same location.  RCA cables are obviously seperate as they come from an electronic crossover to split the frequencies before the amps.  So, if the Punch is getting adequate power, the Alpine should also since it is "upstream" of the Punch on the wires.

Thanks, any more ideas?

Mike

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?

Thanks,

Mike