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Old 10th January 2008, 12:41 AM   #1
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Default Not a lot of Amps drawn, not what I expected!

Hello all, My sub doesn't draw much current, why?

To be honest, I'm indoors with it, and I'm not sure where I should post it? I could have asked in the Power Supply forum or Subwoofers.

I have a 12" 4ohm woofer on 1 pair of a 4 channel amp, bridged and being fed Linkwitz Transform. The car amp is 4 x 100w so i guess 4 x 35 rms, not sure of power or believe what they say, but anyhow, I'm using this amp because I havn't bought a mains jobby yet. I've set the Transform @20hz and this comes via 40hz 24db lowpass filter. My power source, a PC 300w psu. I've been happy with it, but have been curious about the power my amp's drawing. So I installed my DMM in 10A range and although its late, its displaying 0.70 Amp +- 1Amp. Even when theres no music playing, it reads the same. I began turning it up to my normal daytime level, and it read 0.80Amp. The test signal was Dance music on a satelite tv channel and then some Hip Hop, with which it managed 1.02Amps. I deduce that 99% listening time, the system isn't doing much. The gain on the Amplifier is maxed, without you being able to listen, its hard to ask, but, do you think with modern music and the use of a Linkwitz Transform, I should be drawing more current? The 12" cone is moving no more than my 6" mains.

At 12 volts 12 x 0.7 = 8.4 W

Confused.

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Old 10th January 2008, 01:00 AM   #2
Eva is offline Eva  Spain
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Consider these facts:

A woofer in a proper sealed enclosure exhibits a wide high impedance peak around 50..60Hz, it may be well above 10 or 20 ohms across one entire octave. This is particularly true if Qts is low and/or the enclosure is big. You can try modelling your system with some software.

A DMM does not measure peak current, it only does a slow average.

Music demands high peak power but low average power. A 100W amplifier may be delivering only 20W or 30W of music when it starts to clip.

Most music has actually little content below 50Hz, most of the stuff we perceive as bass is in the 50Hz to 400Hz range.

Anyway, such a low reading suggests a lack of DMM precision.
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Old 10th January 2008, 02:11 AM   #3
jol50 is offline jol50  United States
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You will also find at high volume the load starts to pick up faster. It can go quite loud with little power, then it ramps up.
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Old 10th January 2008, 05:11 AM   #4
ppia600 is offline ppia600  United States
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Where do you have the meter connected to measure current? Is it between one of the 12v power leads going into the amp?
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Old 10th January 2008, 07:16 AM   #5
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Default Hello

Yes meter inbetween 12v and batt in on amp.

Cheers iUSERTLO72p
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Old 10th January 2008, 07:17 AM   #6
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Default Also

See here
Does room gain at 20hz mean a bad thing?

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Old 10th January 2008, 08:22 PM   #7
jol50 is offline jol50  United States
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I just ran a 150wrms x2 amp on a battery with a 2A charger on it. It has about 12.6-12.8v with the amp and HU idle. I put my meter on the amp and it draws 1.26A at idle and 1.46A playing at quiet volume inside the house but loud enough to hear fine. This was running a sub full range bridged, actually a DVC so 8 ohms. I turned it up louder and it went to 2A, a good listening level but far from blasting so maybe close to 1 watt (often what a standard TV is WOT), and the sub does not sound the best full range but is making bass well...that way I could tell what song it was. So then I take the meter off and start cranking on it harder, a little 10", getting the cone moving good. The amp quits. I look and had forgot to remove the 10A fuse I had in it to start it up. I put the two 30A back in and run it louder yet, but with no subsonic filter it starts getting to xmax of this 400w 10 in the ported box. I put the LP on and it sure has some port huffing, lol, but doubt one would ever hear it in use.
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Old 11th January 2008, 01:32 PM   #8
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tl072 wrote:

Quote:
At 12 volts 12 x 0.7 = 8.4 W
That sounds reasonable for average power consumption.

Eva wrote
Quote:
A DMM does not measure peak current, it only does a slow average.

Music demands high peak power but low average power. A 100W amplifier may be delivering only 20W or 30W of music when it starts to clip.
Absolutely.

Transient power is what your speakers are using the most of and average power or current doesn't reflect that.

Run a 30 Hz sine wave through it and you should see larger numbers on the meter. I'd be curious how your PSU holds up, too.

tl072:
Quote:
I've set the Transform @20hz and this comes via 40hz 24db lowpass filter
(snip)
The test signal was Dance music on a satelite tv channel
Eva:
Quote:
Most music has actually little content below 50Hz
And dance music qualifies for that statement. PA systems have a hard time filling a large room with lower frequencies at significant volume. 50Hz wouldn't be an unreasonable F3 for a club PA, as dance music doesn't go much below that.

To add to that, your LP is at 40Hz with 80Hz at -24dB. Which should work very well if your mains can meet the subs way down there. I always prefer the lowest possible crossover frequency for subs.

Tim
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Old 11th January 2008, 03:20 PM   #9
jol50 is offline jol50  United States
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Old dance music used to get down there, like janet jackson/madonna/etc. Yeah old, certain songs I could count on to blow my amp fuses. Lots of sine waves in there more or less except it was actually music.
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Old 11th January 2008, 03:39 PM   #10
ppia600 is offline ppia600  United States
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ATB and Ian Van Dahl = Bass
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