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Amp Tests (1 of 2)
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<blockquote data-quote="Langston Holland" data-source="post: 32124" data-attributes="member: 171"><p>Re: Amp Tests (2 of 2)</p><p></p><p>Hi Ian:</p><p></p><p>I don't think you're going to have the slightest problem with the ITech groaning with that load and a 120v supply. My tests used a massive 4Ω resistor and sine waves to induce that kind of vibration from the amps' power supplies. A sine wave has a 3dB difference between its average and peak. Lab Gruppen's engineers did a wonderful study recently that they discussed at InfoComm this year that measured the crest factors all four passbands of the amp outputs at several major concerts and they found that 15dB was about worst case. This is consistent with previously reported studies, but it was fascinating to see their plots on a per passband basis with time on the X axis.</p><p></p><p>What does this mean? In reference to your question it means my test setup was insanely harsh relative to what we actually experience in live sound work. We have (3) variables here:</p><p></p><p>1. Output level. It's reasonable to assume worst case: the amp clip lights flashing the entire set.</p><p></p><p>2. Output crest factor. It's not reasonable to assume 3dB continuous sine waves. The amps' power supplies will have a great deal more down time than in my tests.</p><p></p><p>3. Output load. It's not reasonable to assume a flat 4Ω impedance that the amps will be forced to deliver power to. I don't have a TH118 impedance curve handy, but it's likely to be very similar to the 4Ω version of the TH115. The blue circles are the only places that the amp will see anything approaching 4Ω's (the heavy black horizontal line). The amp will be delivering very little power to the remainder of the passband, thus your power supply will be loafing compared to what my tests required of it.</p><p></p><p>[ATTACH]149778[/ATTACH]</p><p></p><p>You might devise your own test that would still be much worse than real world by making a high power 4Ω resistor with (3) 4,500 watt 240v water heater elements wired in parallel and submerged in a bucket of water. Instead of a sine wave, bring out your worst-case music samples and push the amp into hard limiting. Make sure you have the processing and limiters set for TH118 use. Bet it'll be quiet other than fans.</p><p></p><p>Bottom line? I'd try to feed a large amp 208v/240v whenever practical, but wouldn't fret about it - unless it's paired with a sub with (4) 21" drivers intended to cover 2,000 people at a rave. <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite1" alt=":)" title="Smile :)" loading="lazy" data-shortname=":)" /></p></blockquote><p></p>
[QUOTE="Langston Holland, post: 32124, member: 171"] Re: Amp Tests (2 of 2) Hi Ian: I don't think you're going to have the slightest problem with the ITech groaning with that load and a 120v supply. My tests used a massive 4Ω resistor and sine waves to induce that kind of vibration from the amps' power supplies. A sine wave has a 3dB difference between its average and peak. Lab Gruppen's engineers did a wonderful study recently that they discussed at InfoComm this year that measured the crest factors all four passbands of the amp outputs at several major concerts and they found that 15dB was about worst case. This is consistent with previously reported studies, but it was fascinating to see their plots on a per passband basis with time on the X axis. What does this mean? In reference to your question it means my test setup was insanely harsh relative to what we actually experience in live sound work. We have (3) variables here: 1. Output level. It's reasonable to assume worst case: the amp clip lights flashing the entire set. 2. Output crest factor. It's not reasonable to assume 3dB continuous sine waves. The amps' power supplies will have a great deal more down time than in my tests. 3. Output load. It's not reasonable to assume a flat 4Ω impedance that the amps will be forced to deliver power to. I don't have a TH118 impedance curve handy, but it's likely to be very similar to the 4Ω version of the TH115. The blue circles are the only places that the amp will see anything approaching 4Ω's (the heavy black horizontal line). The amp will be delivering very little power to the remainder of the passband, thus your power supply will be loafing compared to what my tests required of it. [ATTACH=CONFIG]149778.vB5-legacyid=1618[/ATTACH] You might devise your own test that would still be much worse than real world by making a high power 4Ω resistor with (3) 4,500 watt 240v water heater elements wired in parallel and submerged in a bucket of water. Instead of a sine wave, bring out your worst-case music samples and push the amp into hard limiting. Make sure you have the processing and limiters set for TH118 use. Bet it'll be quiet other than fans. Bottom line? I'd try to feed a large amp 208v/240v whenever practical, but wouldn't fret about it - unless it's paired with a sub with (4) 21" drivers intended to cover 2,000 people at a rave. :) [/QUOTE]
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