How do the pros hookup differential amps to o-scopes?

Michael Rothacher · 28th March 2003, 11:54 PM

In the process of testing my balanced bridge power amplifier, I managed to blow out channel 2 on my PC scope. I was trying the 'ol invert channel 2 method to look at the output of my amp. How do the pros (I know a bunch of you are working on X-amps!) hookup their balanced amps to the scope?

Is a differential probe the only way to go?

Also, I thought I was having HF rolloff problems with my design, but now I'm wondering if I was loading the amp with capacitance by floating the scope. Does this sound like a reasonable consequence?

Thanks for your advice.

Mike

hitsware · 29th March 2003, 12:12 AM

if you isolate your scope ground from earth ground and everything else you can simply use the scope ground lead on one amp and the hot lead on the other.........

pinkmouse · 29th March 2003, 12:39 AM

Quote:

Originally posted by hitsware
if you isolate your scope ground from earth ground and everything else you can simply use the scope ground lead on one amp and the hot lead on the other.........

This is not advisable as if any fault develops the case of the 'scope could be at a potentially harmful voltage

If this is done in professional enviroment the 'scope has to be sheilded behind perspex to comply with health and safety regulations.

hitsware · 29th March 2003, 01:04 AM

If you're that concerned about earth ground, don't use your toaster anymore......

pinkmouse · 29th March 2003, 01:10 AM

Quote:

Originally posted by hitsware
If you're that concerned about earth ground, don't use your toaster anymore......

Sorry, I don't understand, my toaster has an earth connection to the case, and if it didn't it would have to be double insulated, which AFAIK, no 'scope is.

nw_avphile · 29th March 2003, 03:17 AM

Sorry you blew out one channel on your scope. Those can be tricky to fix (even impossible on some analog Tek scopes as the proprietary parts are no longer made).

Floating the scope is NOT a great idea because the chassis of the scope will have half the output voltage of your amp on it. You could zap yourself, or more likely, accidentally ground the scope and short things out and/or end up with the output voltage getting back into the input of your amp which can turn it into a very brief high-power oscillator (before something burns up).

Are you after really precise measurements with the scope or just an idea of when it's clipping? One option would be to use an inexpensive transformer. The ones sold for ground loop isolation of amplifier inputs will work and are about $8 for a pair of them. You can experiment with different resistor loading to get the flattest response out of it (they're usually around a few hundred ohms). They won't pass a 20khz square wave very well, however.

You might be able to limp along just measuring either half of the output circuit for critical measurements, and using the transformer when you need to view the bridged output.

hitsware · 29th March 2003, 03:47 AM

your toaster has a 2 wire plug (assuming USA) ... what happens if you turn the plug around?

jleaman · 29th March 2003, 04:36 AM

uh i think it burns the toast. and the pop up feature flings the tost into Flames. ! heh.

What does happen. thou

hitsware · 29th March 2003, 04:48 AM

Nothin' cause the chassis of your toaster is floating. The heating element is isolated from it. If you choose to stick your hand in the slot then you might get toasted ..........

jleaman · 29th March 2003, 04:52 AM

i onece put bread in mine. . All this time i thought they were for making Cookies.

LMAo jk.

28th March 2003, 11:54 PM	#1
Michael Rothacher www.audiohobby.com diyAudio Member Join Date: May 2001 Blog Entries: 3	How do the pros hookup differential amps to o-scopes? In the process of testing my balanced bridge power amplifier, I managed to blow out channel 2 on my PC scope. I was trying the 'ol invert channel 2 method to look at the output of my amp. How do the pros (I know a bunch of you are working on X-amps!) hookup their balanced amps to the scope? Is a differential probe the only way to go? Also, I thought I was having HF rolloff problems with my design, but now I'm wondering if I was loading the amp with capacitance by floating the scope. Does this sound like a reasonable consequence? Thanks for your advice. Mike

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29th March 2003, 12:12 AM	#2
hitsware diyAudio Member Join Date: Mar 2002	if you isolate your scope ground from earth ground and everything else you can simply use the scope ground lead on one amp and the hot lead on the other.........

29th March 2003, 01:04 AM	#4
hitsware diyAudio Member Join Date: Mar 2002	If you're that concerned about earth ground, don't use your toaster anymore......

29th March 2003, 03:17 AM	#6
nw_avphile diyAudio Member Join Date: Mar 2003 Location: Northwest	Sorry you blew out one channel on your scope. Those can be tricky to fix (even impossible on some analog Tek scopes as the proprietary parts are no longer made). Floating the scope is NOT a great idea because the chassis of the scope will have half the output voltage of your amp on it. You could zap yourself, or more likely, accidentally ground the scope and short things out and/or end up with the output voltage getting back into the input of your amp which can turn it into a very brief high-power oscillator (before something burns up). Are you after really precise measurements with the scope or just an idea of when it's clipping? One option would be to use an inexpensive transformer. The ones sold for ground loop isolation of amplifier inputs will work and are about $8 for a pair of them. You can experiment with different resistor loading to get the flattest response out of it (they're usually around a few hundred ohms). They won't pass a 20khz square wave very well, however. You might be able to limp along just measuring either half of the output circuit for critical measurements, and using the transformer when you need to view the bridged output.

29th March 2003, 03:47 AM	#7
hitsware diyAudio Member Join Date: Mar 2002	your toaster has a 2 wire plug (assuming USA) ... what happens if you turn the plug around?

29th March 2003, 04:36 AM	#8
jleaman Banned Join Date: Apr 2002	uh i think it burns the toast. and the pop up feature flings the tost into Flames. ! heh. What does happen. thou

29th March 2003, 04:48 AM	#9
hitsware diyAudio Member Join Date: Mar 2002	Nothin' cause the chassis of your toaster is floating. The heating element is isolated from it. If you choose to stick your hand in the slot then you might get toasted ..........

29th March 2003, 04:52 AM	#10
jleaman Banned Join Date: Apr 2002	i onece put bread in mine. . All this time i thought they were for making Cookies. LMAo jk.