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Junior Varsity
QSC CMX500Va Amp in Bridge mode problem
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<blockquote data-quote="Alan Hamilton" data-source="post: 10867" data-attributes="member: 1799"><p>I'm trying to see if I am missing something before deciding the fault is in the amp. The QSC CMX500 recommends bridged mode to drive 70V speakers. </p><p></p><p>I was asked to troubleshoot the system because the amp was cutting out. I assume it was going into protect because it would go silent and signal meters go dark... then after a bit audio would return. When it did the RIGHT side of the amp was showing heavy clip. Not the same as the right side at all. Right side (ch 2) input attenuator was all the way down (fully counter clockwise)</p><p></p><p>Input is balanced line feeding the barrier strip channel 1 input.</p><p></p><p>Speakers are tapped at 480W IIRC (16 x 30W ea).</p><p></p><p>When speakers were working I heard no audible distortion, but I'm listening to 70V horns so I wasn't expecting hi-fi... but they sounded fine. Just cutting out.</p><p></p><p><span style="color: #141823"><span style="font-family: 'Helvetica'">Dip switches set exactly as shown in manual page 12 (FYI, unshown switches are set to the LEFT side):</span></span><a href="http://qsc.com/files/8713/8911/9180/CMXa_Series_User_Manual_5-Language.pdf" target="_blank">http://qsc.com/files/8713/8911/9180/CMXa_Series_User_Manual_5-Language.pdf</a></p><p></p><p>The speaker wires were connected to the bridge output section of the rear terminals.</p><p></p><p>I disconnected one leg of the speakers and there was no change at the amp as far as metering goes (still heavy clip showing on the RIGHT side).</p><p></p><p>If I switch the amp out of bridge mode, there is no clipping on the left input (just normal signal meter). And if I connect the speaker line to that side all is still fine. If I put it in parallel mode, channel 2 metering behaves identically with channel 1 (I did not put a load on channel 2 to test past this). This tends to indicate to me there is no problem in the speaker line.</p><p></p><p>As far as input level goes, what was being sent to amp when I took the amp out of bridge mode was the same as what was sent when it was in bridge mode. So the signal was not hotter being fed to it... Yet no clipping in parallel mode. Big clipping on RIGHT side in bridged mode.</p><p></p><p>Bringing down the the channel 1 input knob on the amp didn't take the right side out of clip either when it was bridged. It either had no or little effect.</p><p></p><p>I was working in combat audio conditions just to get them back working so I patched around the problem and didn't get to test every last detail. Between the amp and the speakers there is an Altec 70V level control (similiar to these: <a href="http://www.mcmelectronics.com/product/ATLAS-SOUND-AT100-/555-7342" target="_blank">http://www.mcmelectronics.com/product/ATLAS-SOUND-AT100-/555-7342</a>)</p><p>I'm not sure if that somehow throws a monkey wrench into the equation or not... but it doesn't when the speakers are connected to the amp in normal mode (versus bridged). But I would've liked to have tested with this level control out of the circuit in bridged mode. But I did disconnect a leg of the speakers at one point directly from the back of the amp with no change so I'd think that would eliminate this line of thinking anyway.</p><p></p><p>Sooooooo Is there a fault in the amp? Did I miss something? Does the level control throw a monkey wrench into the equation in bridge mode? Something I should try before I pull this? Does this point to an external fault I'm not thinking of?</p><p></p><p>The gist of it is, is there anything that would cause the RIGHT meter to be in hard clip and not registering the same as the LEFT meter in bridged mode?</p><p></p><p>-Alan Hamilton</p></blockquote><p></p>
[QUOTE="Alan Hamilton, post: 10867, member: 1799"] I'm trying to see if I am missing something before deciding the fault is in the amp. The QSC CMX500 recommends bridged mode to drive 70V speakers. I was asked to troubleshoot the system because the amp was cutting out. I assume it was going into protect because it would go silent and signal meters go dark... then after a bit audio would return. When it did the RIGHT side of the amp was showing heavy clip. Not the same as the right side at all. Right side (ch 2) input attenuator was all the way down (fully counter clockwise) Input is balanced line feeding the barrier strip channel 1 input. Speakers are tapped at 480W IIRC (16 x 30W ea). When speakers were working I heard no audible distortion, but I'm listening to 70V horns so I wasn't expecting hi-fi... but they sounded fine. Just cutting out. [COLOR=#141823][FONT=Helvetica]Dip switches set exactly as shown in manual page 12 (FYI, unshown switches are set to the LEFT side):[/FONT][/COLOR][URL]http://qsc.com/files/8713/8911/9180/CMXa_Series_User_Manual_5-Language.pdf[/URL] The speaker wires were connected to the bridge output section of the rear terminals. I disconnected one leg of the speakers and there was no change at the amp as far as metering goes (still heavy clip showing on the RIGHT side). If I switch the amp out of bridge mode, there is no clipping on the left input (just normal signal meter). And if I connect the speaker line to that side all is still fine. If I put it in parallel mode, channel 2 metering behaves identically with channel 1 (I did not put a load on channel 2 to test past this). This tends to indicate to me there is no problem in the speaker line. As far as input level goes, what was being sent to amp when I took the amp out of bridge mode was the same as what was sent when it was in bridge mode. So the signal was not hotter being fed to it... Yet no clipping in parallel mode. Big clipping on RIGHT side in bridged mode. Bringing down the the channel 1 input knob on the amp didn't take the right side out of clip either when it was bridged. It either had no or little effect. I was working in combat audio conditions just to get them back working so I patched around the problem and didn't get to test every last detail. Between the amp and the speakers there is an Altec 70V level control (similiar to these: [URL]http://www.mcmelectronics.com/product/ATLAS-SOUND-AT100-/555-7342[/URL]) I'm not sure if that somehow throws a monkey wrench into the equation or not... but it doesn't when the speakers are connected to the amp in normal mode (versus bridged). But I would've liked to have tested with this level control out of the circuit in bridged mode. But I did disconnect a leg of the speakers at one point directly from the back of the amp with no change so I'd think that would eliminate this line of thinking anyway. Sooooooo Is there a fault in the amp? Did I miss something? Does the level control throw a monkey wrench into the equation in bridge mode? Something I should try before I pull this? Does this point to an external fault I'm not thinking of? The gist of it is, is there anything that would cause the RIGHT meter to be in hard clip and not registering the same as the LEFT meter in bridged mode? -Alan Hamilton [/QUOTE]
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QSC CMX500Va Amp in Bridge mode problem
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