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#162514 - 04/03/07 11:30 PM Re: Emotiva MPS-1 shutting down? Amp gurus help [Re: SirQuack]
SirQuack Offline
shareholder in the making

Registered: 01/29/04
Posts: 13344
Loc: Iowa
Since the kids are in bed, I could not turn it up very far as their rooms are right above my HT area.

Anyway, I turned my meter on the 2 Volts selection and attached the black (COM) clip to the negative wire, and the red (volt) clip to the positive wire terminals.

When playing a particular song, obviously the readings bounced around, but the highest I saw it jump was 1.60.

So would this be correct using your equation?

P=Vsquared/R
P=2.56/4
P=.64 watts

Thanks, Randy
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#162515 - 04/03/07 11:34 PM Re: Emotiva MPS-1 shutting down? Amp gurus help [Re: SirQuack]
MadScientist Offline
regular

Registered: 04/03/07
Posts: 8
Perhaps I can shed a little light here. You will need a stable 4 ohm load bank and not the speakers themselves. Speakers are a reactive device and not a purely resistive load. So you will need a solid resistive load to get an acurate measurement. Next you need to check and make sure your meter is frequency compensated. Most are not. Lastly you will need a CD with test tones on it. Music is all about the dynamics and you will not get an accurete ready.

Connect the load bank to the output of the amp, bring up the volume a test tone, probably something around 1Khz and measure the voltage at the speaker terminals. Then it is just a little math to get the power output.

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#162516 - 04/03/07 11:56 PM Re: Emotiva MPS-1 shutting down? Amp gurus help [Re: SirQuack]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10412
See, I told you that you didn't need all that power! Okay, I was looking for something in the area of 20 volts peak. The suggestion of maybe using a test tone disc at a specific frequency(a few hundred Hz, say)rather than music is a good one. Then the frequency and the level won't vary(except when you vary it), and the specific M80 impedance at that frequency(from the NRC measurements)can be used and should be adequate.
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#162517 - 04/04/07 12:03 AM Re: Emotiva MPS-1 shutting down? Amp gurus help [Re: JohnK]
SirQuack Offline
shareholder in the making

Registered: 01/29/04
Posts: 13344
Loc: Iowa
Thanks, yeah I had to leave it at like -30 on the volume knob, nowhere near the +5 when the amp shuts down. I will do some more testing when I have more time. As you mentioned the m80's are a very stable 4ohm load, unlike many other speakers that are all over the place. For what I'm trying to research this will be adequate. I do have the Realtraps test tones from when I graphed my room acoustics, I may try that as well.

Thanks again, I will keep you posted.
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M80s-VP180-QS8s-EP600-2xEP350 Denon3808 Outlaw7700
M22-OWM22-VP100-Denon2805
Audio Nirvana

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#162518 - 04/04/07 12:09 AM Re: Emotiva MPS-1 shutting down? Amp gurus help [Re: SirQuack]
Mojo Offline
connoisseur

Registered: 01/21/07
Posts: 3292
Randy,

MadScientist is essentially right. But good luck finding that "stable" load bank. By stable, he means that it can't change resistance as it heats up (I suppose you could rig it up in your freezer ). This is what I mentioned earlier about the testing that Emo is doing.

And he's also right about the speaker not always being 4 Ohms. The impedance changes with frequency. Now how much it changes I don't know. And he's also right about a frequency-compensated meter. So, you could go about this in two ways to get a very gross indication:

1. Put on some pink noise. Avoid a test tone as you could end up over-heating and damaging one of your drivers!!! Ask Axiom to tell you what the impedance of the M80 is with pink noise. I wouldn't be surprised if it's somewhere around 4 Ohms. Set your volt-meter to 60V AC (not DC). Find some way to clamp the leads on to the speaker terminals (DON'T ATTEMPT TO HOLD THE LEADS AS 15 THOUSANDTHS OF AN AMP IS ALL IT TAKES TO KILL YOU!!!). Turn up the EMO and take readings of the voltage. Your power will be E squared over the speaker impedance.

2. If Axiom won't give you the impedance, then you have to measure both the voltage and the current. So you will need two meters set on AC. The voltmeter will go across the terminals as in 1. and the ammeter will go in series with the wires. Set the ammeter to 15 amps. Clamp the ammeter on (DON'T ATTEMPT TO HOLD THE LEADS AS 15 THOUSANDTHS OF AN AMP IS ALL IT TAKES TO KILL YOU!!!). Take both measurements and multiply.

Take SPL readings as well if you can. I have to tell you that I really don't know what kind of accuracy you will get from these measurements as it all depends on the meter characteristics. Remember: DON'T ATTEMPT TO HOLD THE LEADS.

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#162519 - 04/04/07 12:10 AM Re: Emotiva MPS-1 shutting down? Amp gurus help [Re: SirQuack]
MadScientist Offline
regular

Registered: 04/03/07
Posts: 8
Quote:

Thanks, yeah I had to leave it at like -30 on the volume knob, nowhere near the +5 when the amp shuts down. I will do some more testing when I have more time. As you mentioned the m80's are a very stable 4ohm load, unlike many other speakers that are all over the place. For what I'm trying to research this will be adequate. I do have the Realtraps test tones from when I graphed my room acoustics, I may try that as well.

Thanks again, I will keep you posted.




No matter how stable the impedance is, it is still not a resistive load. The M80 have a complex crossover with both resistive and capacitive elements which will reck havac on your readings.

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#162520 - 04/04/07 12:13 AM Re: Emotiva MPS-1 shutting down? Amp gurus help [Re: SirQuack]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10412
My first impression was that something was wrong with the hookup, but if you were listening at that moderate a level, maybe that was a reasonable result. Using about .64 watt would result in about a 89dB output from the M80 at 1 meter(neglecting room effects), which is comfortably loud. It'll be more interesting when you're able to turn it up into the problem levels.
_________________________
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Enjoy the music, not the equipment.



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#162521 - 04/04/07 12:19 AM Re: Emotiva MPS-1 shutting down? Amp gurus help [Re: Mojo]
MadScientist Offline
regular

Registered: 04/03/07
Posts: 8
Mojo, Good suggestion. Measuring the voltage and the current would give a reasonably close measurement.

The only variable not accounted for is distortion. I would recommend that you use a Piezo tweeter wired across the speaker terminals of the amp. Play a tone that is well below the frequecy response of the tweeter (like 100hz). Turn up the volume until you hear a buzzing in the tweeter. The buzzing you hear is the harmonics generated by the amplifier clipping and squaring off. Then you could take your measurements and run the math.

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#162522 - 04/04/07 12:33 AM Re: Emotiva MPS-1 shutting down? Amp gurus help [Re: MadScientist]
Mojo Offline
connoisseur

Registered: 01/21/07
Posts: 3292
The piezo tweeter is actually a very good idea. So he would disconnect his speakers and connect the piezo tweeter, apply the low frequency tone, turn up the gain until the piezo starts shrieking, remove the piezo, connect the M80s and measure.

What kind of a load does the piezo present to the amp?

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#162523 - 04/04/07 12:37 AM Re: Emotiva MPS-1 shutting down? Amp gurus help [Re: Mojo]
MadScientist Offline
regular

Registered: 04/03/07
Posts: 8
Somewhere in the neighborhood of 2Kohms at low frequencies. So you would really need to apply some other kind of load to the amp like a passive sub. Otherwise the amp wouldn't be loaded properly. With such a light load the power supply rails would not sag.

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