Nathan Short wrote on Mon, 22 February 2010 14:31 |
I don't see how it is a cheap shot, just the truth.
The quick and easy method to reproduce my findings.
Take an oscillator, Run it into a midas venice, set the level to 0.
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doesn't matter
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Buss the channel. Run Masters to 0. Masters are plugged directly into the amplifier.
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bus.. Console outputs, I presume. Same as level coming from the Oscillator? IIRC Midas uses peak meters, but I'm not sure how they calibrate for sine waves. Probably doesn't matter.
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Amplifier was tested on 2 dummy loads. I will explain the first. A 4 element 4x 7.8-8.2 ohm 2000w water heater elements suspended in a bucket of water. This provides an easy impedance that doesn't seem to drift more than .2ohm when hot.
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nominal 2 ohm load.. one on each channel I assume.
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Test leads from a BnK multimeter were attached to the individual leads one one of the resistors , and at the amp different times.
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Average AC volts (even if RMS it doesn't tell us the full story since it's clipping).
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The amp was plugged into a military grade distro, with a current meter. The distro was plugged into two seperate 100amp breakers, 1 per circuit of the 2 circuit unit. The unit has 40amp breakers. both circuits went back to the same panel ground, and seperate neutral bus.
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I assume nominal 115-120V mains?
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Channel 1 was found, when starting to engage the clip led by driving the output of the midas past +4, to start putting out a lot more voltage than Channel 2. Channel 2 behaved normally like almost every other amp at the test. Channel 1 could be made to put out 81v when the clip light was moderately active. The current meter on the Distro showed 33amp draw.
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81V/2 ohm is 40A. A perfect amp would require 24A line cord draw for just this one channel, if mains were stiff 120V. Since the amp isn't 100% efficient (even class D amps aren't) the mains draw needs to also supply current to account for losses.
A clip light being "moderately" active on a sine wave means IT'S CLIPPING.
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What else can I tell you. A few of my EE friends, as I am not an EE, gave the testing a thumbs up, and a great way to check for thermal limits also.
The 3602 was found to exibit this behavior at 30Hz and 15kHz, I did not push the test further for fear of ruining the amp.
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My EE friends would ask you for more data.. like current draw and output voltage at 1kHz for the same input voltage.
I wouldn't be surprised to see clipping earlier at 30Hz than 1kHz. More data might help reveal a problem (or not). In general the clip light being on, makes the actual (clean) output voltage rather difficult to know from a clipped waveform measurement.
If your EE friends approve voltage measurements with clipped waveforms, why stop at only moderate clipping? You could double the power with a full square wave.
JR
PS: Inexpensive pots in a value amp... I'm shocked.