Amp testing/comparison

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stevdart

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Joined: January 24, 2004

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Posted: July 17, 2004 at 7:30 AM / IP Logged

Sorry, dude, but if you want to believe that you made some kind of realistic comparison of those three amps, go ahead and believe it. But it's flawed. If you don't make an accurate measurement, then you are obviously relying on the volume that you perceived to hear. That's no basis for an anouncement of amplifier power vs. published ratings.

I suggest you study up on the fundamentals of audio electronics. There's a lot to be learned in this forum from some knowledgeable people, who insist they are learning something new every day. (I am so far behind many of them that I learn 100 things a day.)

And welcome to the forum!

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Leif

Copper - Posts: 71

Joined: July 15, 2004

Location: United States

Posted: July 17, 2004 at 9:56 AM / IP Logged

I think there's been a misunderstanding of my test methodology, let me clarify again. If I'm mistaken, please enlighten me, but be specific.

I played three different amplifiers with the same program material into identical loads (the same set of woofers) and looked at the test signal with a scope. (Woofers aren't acceptable for absolute power measurements, but fine for direct comparisons)

When testing the first amp, I adjusted the scope so that the amp's clip level (clearly visible on the scope) ended up exactly on the -50% and +50% lines on the scope. I subsequently didn't touch the scope at all, but just hooked up the different amps and adjusted the gain until I saw clipping. (I wasn't listening to the woofer output, so my ears are out of the equation for this one.) This is where there accuracy comes in - if the scope shows that the amps all clip at the same level, they're producing the same power.

Which part are you disagreeing with exactly?

///Leif

stevdart

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Posted: July 17, 2004 at 3:11 PM / IP Logged

When you proclaim that three amps, from two companies, all with different power ratings, are identical in power output because you devised your own way of testing, which is not an accurate testing procedure, someone is going to call you on it. You have to use a proper testing procedure (please read the link I provided to you) to make such a proclamation on a forum that is read by as many people as this one is.

Frankly, I don't care that you think the three amps produce the same power. But somebody reading this may get the impression that the test was proper. When you said that the voltage reading was "about 12 volts peak to peak", that gave you away. You know just enough about the terminology, and have a fine grasp of the English language, to sound almost convincing. But anybody who has tested voltage output knows better.

You have an oscilloscope, now you should obtain the rest of the equipment described in the link and bench test those amps properly. You'll be glad to learn it the right way. Then you will be able to speak with authority.

Sorry to call you out like this, but when you brush off a knowledgable expert like customsuburb, and announce conclusions that are false, it has to be done. He gave you the best advice in this thread. You should re-read it. And good luck with the testing.

If I am wrong, somebody on the forum call me on it. Then that'll be another new thing I've learned. I'm done with this topic, unless I get called out. Then I'll apologize to you and then be done.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

DYohn

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Posted: July 17, 2004 at 3:26 PM / IP Logged

To second Stevedart, if you actually read 12 volts AC on the output of the amplifier into a 4-ohm load, that corresponds to 36 watts of power. Ohm's Law. This fact alone should clue you that there is some flaw in your testing methodology, especially if three different amps gave you the exact same results. Besides the fact that this is way below the ratied power on any of the amps, three amps with the same make/model numbers will not often produce the exact same results.

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kgerry

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Joined: February 07, 2004

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Posted: July 17, 2004 at 5:41 PM / IP Logged

just to chirp in as it hasnt come up yet....when doing any type of measurement like this try to do it with a quantifable known source signal, like a solid 1K test tone, etc...measuring a music floor is rarely accurate because the source signal is bouncing all over the place.....some may say it's a more realistic measurement re: dynamics but i always use either a 400 Hz or 1 Khz test tone.......

Kevin Gerry

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Leif

Copper - Posts: 71

Joined: July 15, 2004

Location: United States

Posted: July 17, 2004 at 11:53 PM / IP Logged

Fair enough, time for me to put up or shut up. (I must admit I am a little curious to see if the standard test methodology will yield the same results.)

I have a 500ft spool of 18GA wire (stranded single conductor, insulated), its resistance is 3.6 ohms according to a digital multimeter. Stevdart, do you have any problem with me using this wire as the dummy load? Also, should I perhaps unwind it into a disorganized pile to make it less of a coil?

Furthermore I'll pick up an Optima Yellow Top battery (there's no way I'm carrying all the gear out to the car again, way too much work) to use as the power supply. I'm planning to keep a 10A 13.8V supply (radio shack) hooked up to it to at least mitigate any battery discharge between testing the different amps.

I will use a modern discman (digital volume control) with continuous test tones recorded on CDR, a digital multimeter in AC mode to monitor the output voltage, the same oscilloscope to keep the gain just below amp clipping, and another multimeter in DC mode to monitor the supply voltage.

If there's anything wrong here, please let me know quickly so that I can get it right this time.

///Leif

Leif

Copper - Posts: 71

Joined: July 15, 2004

Location: United States

Posted: July 18, 2004 at 8:43 PM / IP Logged

Okay, the measurements are done. I scrapped the idea of a long-wire resistor (research showed that it would be a highly inductive load) and instead combined 25 watt high power resistors to a total resistance of 5.0 ohms, 150 watts. (Resistance increased to 5.3 after they heated up from the testing).

Supply voltage was a brand new Optima Yellow Top battery - voltage measured at 11.9 volts during testing (full signal load).

I tested with a clean 50hz sine wave, and used the oscilloscope to make sure it was just a hair from clipping. Just to get it out of the way, I also looked at the amp input signal on the oscilloscope - no clipping.

The results are:

Sony XM-7547 - spec says 2x300w into 4 ohms - actual voltage was 29.2 volts 50hz AC at 5.3 ohms, yielding 160.8 watts.

Alpine MRV-F407 - spec says 2x160w into 4 ohms, actual voltage was 27.7 volts 50hz AC at 5.3 ohms, yielding 144.8 watts.

Alpine MRV-F1507 - spec says 2x225w into 4 ohms, actual voltage was 29.5 volts 50hz AC at 5.3 ohms, yielding 164.2.

Stevdart, this is to me very close to my original figure of 150 watts per channel. Sure, it wasn't absolutely accurate - now it is.

So, 4 hours and $200 later, now that the test methodology is out of the way, would someone please answer the original question, which is, is it normal for amplifiers of such different specified wattages (and different purchase costs) to produce for all intents and purposes identical output power?

(The difference between 164 and 144 watts is much less than one single decibel, and inaudible.)

Thank you.

///Leif

haemphyst

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Joined: January 19, 2003

Location: Michigan, Bouvet Island

Posted: July 18, 2004 at 9:39 PM / IP Logged

I'm pretty sure I am on Leif's side... c'mon guys! What is wrong with his methodology?!? Sounds to me as though he standardized pretty well... I am curious to know what you think he has done wrong NOW...

Additionally, I am pretty sure he didn't have to go through all of this TWICE. I got what he was saying from the beginning. I am curious to know what you feel the real answer is as well.

The Sony, in my opinion is the poser in this scenario. From what I have seen the Sony stuff is NOT rated at an RMS output, so the specified 300 watts is a bit (OK, a LOT) misleading, and the 160 is probably the real deal as far as RMS output. As far as the Alpine stuff is concerned, I think those numbers are probably pretty close to what they can really do. They are close to the specified ratings of the amplifiers, so I would say "Take those as the real numbers" By the time you were to actually have a 4 ohm load on the outputs, you would see the numbers (as far as actual power output) go up a little bit, bringing you closer to the specified numbers. The peak numbers at clipping may sag a little bit from your 5.3 ohm levels, but it shouldn't be much more than a few tenths of a volt.

If you were to figure it backwards, Leif, the 407 would be 25.3v at 4 ohms, and the 1507 would be 30v at 4 ohms. I think the numbers you came up with are pretty close to the mark.

Good job on the testing, BTW...

It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

stevdart

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Joined: January 24, 2004

Location: Pennsylvania, United States

Posted: July 18, 2004 at 10:13 PM / IP Logged

haemphyst wrote:

If you were to figure it backwards, Leif, the 407 would be 25.3v at 4 ohms, and the 1507 would be 30v at 4 ohms.

Ohm's law with these numbers gives the Alpines exactly what the published ratings are. 25.3*25.3 /4=160 30*30 /4=225

Considering they published the ratings using a 14.4V source, I would say their ratings are as accurate as can be, even somewhat underrated. Good follow-through, Leif.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

stevdart

Platinum - Posts: 5,816

Joined: January 24, 2004

Location: Pennsylvania, United States

Posted: July 18, 2004 at 10:21 PM / IP Logged

OH, I see now how you came up with those numbers, hemp. You used the ratings to determine the voltage, I thought you were figuring a difference in voltage based on the different ohm load.... Amp testing/comparison - Page 2 -- posted image.

But seriously, Leif, you know that original guess of 150 was a shot in the dark, or 144 or whatever you came up with.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

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