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Author Topic: Connecting oscilloscope to QSC DCA Amplifiers Danial Simmonds Expert Film Handler Posts: 107 From: Kota Damansara, Selangor, Malaysia Registered: Jan 2008 posted 07-19-2009 09:52 PM                        Is it possible to connect an oscilloscope to a QSC DCA Amplifier to see the output wave? Anyone got any experiece or advice care to share... Thank you so much Dan.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 07-19-2009 10:38 PM                     As long as the amp is not in bridge mode just hook the scope probe's ground wire to the black terminal and the scope probe itself to the red terminal. Be sure the scope input AC-DC switch is set to AC. Adjust the gain on the scope to the 10 volt scale and then get the system playing... turn the gain up in sensitivity a click at a time till you fill the screen as you'd like for measurement purposes, perhaps 2 volts full scale or there abouts (depends on if your probes are X10 or not). Also remember that a scope doesn't display RMS AC voltage it displays peak to peak AC voltage. Hope this gets you started... Mark  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 07-20-2009 01:28 PM                     I'd like to add to Mark's excellent advice with the following cautions: First off, don't even attempt to measure the output of ANY amplifier if you don't have 10x probes!! It is very easy to fry the scope or the amp if you're not extremely careful. (Don't ask how I know this.. ) Do NOT ever try to measure the bridge mode outputs with any scope that is connected to AC power. As you WILL end up grounding one side of the bridge output and possibly kill the amp or scope. IF, and ONLY IF, your scope can operate on batteries (WITHOUT PLUGGING IT IN TO AC) you can safely scope the output of a bridge mode amp. Be very careful as the voltage on the speaker terminals of a bridge mode amp can be quite high. Finally, DO NOT do the test without the appropriate dummy load resistor or a speaker you don't mind blowing up connected. Most amps do not like operating into an open circuit.  |  IP: Logged Robert Minichino Master Film Handler Posts: 350 From: Haskell, NJ, USA Registered: Dec 2005 posted 07-20-2009 02:30 PM                        Amps without output transformers (damn near all modern amps) will work happily with no load on them, but because you're not loading the power supply down you won't get meaningful results if you're trying to measure power output. After getting bit from floating my 'scope, I got a differential probe. Much safer, and better results to boot.  |  IP: Logged Eric Robinson Jedi Master Film Handler Posts: 538 From: Santa Rosa, CA Registered: Jan 2005 posted 07-20-2009 11:41 PM                     quote: Tony Bandiera Jr Do NOT ever try to measure the bridge mode outputs with any scope that is connected to AC power. As you WILL end up grounding one side of the bridge output and possibly kill the amp or scope. Interesting...Can you explain why this is so?  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 07-21-2009 12:35 AM                     Eric, it is simple: One side of the probe (the clip lead) is referenced to ground in virtually all scopes. When the scope is connected via its AC power, and you try to read off the bridge outputs (where BOTH sides of the speaker wiring are HOT, but at opposite polarities) you will ground out whichever side you connect the clip lead of the probe to. It's kind of like shorting out any AC power source. If you're really super lucky the amp's protection circuits will shut it down before something blows up...but don't count on it. The scope will usually get damaged first, and in one case I am aware of, the probe cable melted down and the scope's circuitry caught fire! Using a battery powered scope there is no reference to the AC ground and hence you can check the output of a bridged amp, BUT you have to be careful to make sure you're not grounded and touching the scope or either side of the probe leads or you will get a nasty shock. Check out the QSC USA manuals (Pages 15-16)on here, they explain bridge mode operation nicely. The MxA Series manual also has an explanation of how bridge mode works.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 07-21-2009 05:56 AM                     I use a transformer to connect across bridged or other balanced signals  |  IP: Logged John Walsh Film God Posts: 2490 From: Connecticut, USA, Earth, Milky Way Registered: Oct 1999 posted 07-21-2009 06:15 AM                     When I wanted to measure xenon lamp supply current ripple, I carried a ground adapter to isolate the third prong ground on the scope. I'm sure it would for this application also. Obviously, you must be careful and have some idea what you are doing before trying this.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 07-21-2009 07:02 AM                     I've always used our handy Fluke Scopemeter...never a ground reference and the case is plastic...no worries. It also allows one to quickly verify no clipping and what the power is (it will do the math for you and read directly in watts if you tell it what your load is). Furthermore, it will also read AC/DC volts at the same time so you can tell about any ripple or DC offset (Depending on what you are measuring). Quite the handy tool. Steve  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 07-21-2009 08:14 AM                     I still have and use my ORC current ripple/inrush meter. Not many were built and of course not many sold. Doesn't work with switchng type supplies except to read operating current. I agree on the Differential probe but they are expensive and somewhat hard to find(E-Bay) may be the best place to get a good deal on them. As for the Fuke Scope meter I see alot of those for sale for some reason or other, never investigated why so many are for sale but I sure thought it quite odd. Mark  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 07-21-2009 05:00 PM                     Yeah I've noticed a lot of Fluke scopemeters on fleabay too..wonder why? It sounds like a great tool..but I already have all the scopes and meters I can ever use anyways. And my scope is battery powered.  |  IP: Logged Robert Minichino Master Film Handler Posts: 350 From: Haskell, NJ, USA Registered: Dec 2005 posted 07-21-2009 05:07 PM                        The early Scopemeters had a low sampling rate and weren't quite as useful as analog scopes with the same bandwidth, so I imagine there was a lot of buyer's remorse. They're still great for anything in the audio bandwidth as well as for power applications, which I believe was their original purpose.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 07-21-2009 08:55 PM                        Most scope meters N.G. for Dolby Digital due to not showing much detail Under the waveform. Fluke said we were crazy and sent their factory team to check it out. We showed then an old analog scope. They were unable to get their equipment to show the detail. Louis  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 07-21-2009 09:56 PM                     Yep...that was the case with our Scopemeter. It was bought (for a tidy sum) before I came to the company and was sitting because it didn't really do what they wanted it for (including working with an Abacus RTA). I, on the other hand, adopted it as my bench scope and meter for most things. I've repaired many an amp with it as well as power supplies. Being completely isolated makes it quite handy and it doesn't take up much room. Having AC and DC volts on one display also is handy...catch bad caps real easy. Once I started using it, it seemed to find a new demand. It isn't a great scope but for analog audio troubleshooting, it is generally more than adequate. Just forget anything involving x/y...it is beyond worthless. Steve  |  IP: Logged Danial Simmonds Expert Film Handler Posts: 107 From: Kota Damansara, Selangor, Malaysia Registered: Jan 2008 posted 07-23-2009 12:11 AM                        Well guys, thank you for the very informative info... I will try it out soon... Thank you Danial  |  IP: Logged

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