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#151989 - 11/24/06 11:28 PM Re: Is this bad for my amp? [Re: Thasp]
Wid Offline
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Registered: 06/22/03
Posts: 6789
Loc: The Peoples Republic of Il.
Quote:

I don't get 86 at 1 watt 1 meter




Just being curious. When it is stated 86dB/W/m (2.8V) does this mean it is tested @ 2.8 V ? If this is the case then wouldn't it require a doubling of watts @ 4 ohm to produce the 86 db rating and not 1 watt.
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#151990 - 11/25/06 12:24 AM Re: Is this bad for my amp? [Re: Wid]
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10621
Rick, Thasp hasn't checked back in and didn't give full details of how he measured and calculated, but yes, what you're suggesting is correct. The applicable form of Ohm's Law is that power equals the square root of (Vsquared/resistance), so the standard measure is 2.83V input into 8ohms, which is equivalent to 1 watt. If 2.83V was instead input into 4ohms, the power would be 2 watts rather than 1 watt. To actually get a 4ohm number for 1 watt the input would be 2.0V rather than 2.83V(or the result would be adjusted down by 3dB to compensate for the lower impedance).

Another point is that the basic sensitivity measure is supposed to be taken under anechoic conditions, and as we discussed with Gena quite a while ago, measured in room at normal listening distances the reverberent sound field can be as strong or stronger than the direct field(so the 6dB loss per doubling of distance isn't the complete picture and less power is required than that measure would imply).
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#151991 - 11/25/06 12:34 AM Re: Is this bad for my amp? [Re: JohnK]
Thasp Offline
devotee

Registered: 03/27/04
Posts: 339
I measured the impedance myself, this is quite easy with a multimeter. I did not measure sensitivity myself, someone else did. I didn't ask him to but he was curious, since he also noticed the amp getting hot. He plays live music and has an SPL meter in his bag with a relatively nice mic compared to the radioshack crap, but I don't know how he got exactly 1 watt out of the amp.. or maybe he didn't. It happened after he noticed it was hot when he walked past the amp.

I still see a jaw drop to everyone I demo these to.


Edited by Thasp (11/25/06 12:35 AM)

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#151992 - 11/25/06 01:34 AM Re: Is this bad for my amp? [Re: Thasp]
Haoleb Offline
connoisseur

Registered: 09/25/02
Posts: 1488
Loc: Maine
heatsinks will get much hotter than the outside of the amp, Depending on the design you can touch the heatsinks, my old NAD was like this, if you stuck your finger up the bottom vent and touched the heatsink directly after cranking it you would be greeted by a little too much heat.

The sound that keeps playing when you unplug the amp means that first of all you have got a decent amount of filter capacitance, and that you also dont have a speaker protection relay that will usually come on when you do something like unplug the amp. This is perfectly normal for amps without a relay.

Also keep in mind that you cannot measure impedance with a multimeter by just sticking the probes on the speaker terminals. Speaker Impedance is the average of the different resistance at different frequencies.


Edited by Haoleb (11/25/06 01:36 AM)
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#151993 - 11/25/06 02:10 AM Re: Is this bad for my amp? [Re: Haoleb]
St_PatGuy Offline
axiomite

Registered: 03/07/05
Posts: 7463
Loc: Glendale, Arizona
Quote:

if you stuck your finger up the bottom vent and touched the heatsink directly after cranking it




Brandon, you're making this way too easy. . .
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#151994 - 11/25/06 06:34 AM Re: Is this bad for my amp? [Re: JohnK]
Wid Offline
axiomite

Registered: 06/22/03
Posts: 6789
Loc: The Peoples Republic of Il.

Thanks John.
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"A fear of weapons is a sign of retarded sexual and emotional maturity." Sigmund Freud


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#151995 - 11/25/06 08:06 AM Re: Is this bad for my amp? [Re: Thasp]
real80sman Offline
connoisseur

Registered: 05/16/02
Posts: 1175
Loc: Ontario, Canada
Quote:

I can tape the corners of the fan to the two back corners of the amp and it'll blow cool air through the fins.




Someone with more smarts than me can chime in, but I thought that you were supposed to be drawing the hot air out. Not trying to blow air in.
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#151996 - 11/26/06 01:58 AM Re: Is this bad for my amp? [Re: real80sman]
Thasp Offline
devotee

Registered: 03/27/04
Posts: 339
PC cooling designs always have the fan pointing down to bring cool air down to the heatsink. VGA coolers, active chipset coolers, hard drive coolers(if your PC has these, doubt it does unless you put them in yourself).

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#151997 - 11/26/06 03:45 AM Re: Is this bad for my amp? [Re: Thasp]
BrenR Offline
connoisseur

Registered: 12/10/03
Posts: 3602
Loc: Winnipeg MB Canada
And amps and rackmount equipment take hot air out with filter elements on the intakes.

Slit throat, slit spine : same-same.

Bren R.

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#151998 - 11/26/06 10:00 AM Re: Is this bad for my amp? [Re: real80sman]
bridgman Offline
axiomite

Registered: 08/25/04
Posts: 6052
Loc: Bowmanville, Ontario, Canada
>>Someone with more smarts than me can chime in, but I thought that you were supposed to be drawing the hot air out. Not trying to blow air in

This isn't a cast-in-stone rule... it's just that you normally want the fan to be assisting natural convection, not fighting it. As others have said, though, sometimes there are good arguments for doing something different.
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