Film-Tech Forum ARCHIVE: Connecting oscilloscope to QSC DCA Amplifiers

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Posted by Danial Simmonds (Member # 4638) on 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.

Posted by Mark Gulbrandsen (Member # 72) on 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

Posted by Tony Bandiera Jr (Member # 2365) on 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.

Posted by Robert Minichino (Member # 3495) on 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.

Posted by Eric Robinson (Member # 2966) on 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?

Posted by Tony Bandiera Jr (Member # 2365) on 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.

Posted by Gordon McLeod (Member # 33) on 07-21-2009, 05:56 AM:

I use a transformer to connect across bridged or other balanced signals

Posted by John Walsh (Member # 168) on 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.

Posted by Steve Guttag (Member # 268) on 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

Posted by Mark Gulbrandsen (Member # 72) on 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

Posted by Tony Bandiera Jr (Member # 2365) on 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. [thumbsup]

Posted by Robert Minichino (Member # 3495) on 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.

Posted by Louis Bornwasser (Member # 3063) on 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

Posted by Steve Guttag (Member # 268) on 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

Posted by Danial Simmonds (Member # 4638) on 07-23-2009, 12:11 AM:

Well guys, thank you for the very informative info... I will try it out soon...

Thank you
Danial

Posted by Michael Schaffer (Member # 1204) on 07-25-2009, 08:21 PM:

Are scope meters with higher bandwidths (e.g. 100 MHz) any good for digital A-chains?

Posted by Steve Guttag (Member # 268) on 07-25-2009, 09:08 PM:

Not in my opinion...but I've seen some techs use some Teks for it.

Posted by Mark Gulbrandsen (Member # 72) on 07-25-2009, 09:43 PM:

There are digital scopes with multi ghz bandwidth. When you get up into the 30K and up range there isn't much they can't display but they also use much different display screens that are capable of the task.

Mark

Posted by Steve Guttag (Member # 268) on 07-26-2009, 08:55 AM:

One of the things that helps one align the lens is that an Analog scope will have a dimmer area in the middle of the trace (where the bits are transitioning)...seeing this occur as the focus improves is a great aid. Mark, do the digital scopes do this? I suppose, with enough bandwidth, there could then be an advantage to merely magnifying the Scope's trace (using time and volts) to just focus in on that area.

Steve

Posted by Robert Minichino (Member # 3495) on 07-26-2009, 05:28 PM:

The Tek "Digital Phosphor" scopes do a good job of showing signal distribution by intensity.