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Correct (and safe) way to connect oscilloscope to audio amp
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Author:  spudnick [ 27 Feb 2018, 06:36 ]
Post subject:  Correct (and safe) way to connect oscilloscope to audio amp

Greetings! Allow me to introduce myself. I am a Moron who just blew up his smartphone by incorrectly connecting an oscilloscope. Problem is, this Moron does not fully understand what he did incorrectly, or what he should actually have done. This Moron hopes to be educated by seeking the advice of the wise ones on this forum, and hopefully not repeat his mistake... :|

Okay, so I bought one of those tiny inexpensive Class D audio amps from eBay. Then I wanted to see with an oscilloscope the input & the output of the amp superimposed on each other just to see the distortion, in time domain. Not having a function generator within my reach, I plugged my shiny pristine smartphone into the amp (well it's actually 2 years old and full of scratches but at least it was working fine). Then I went into Youtube and played some sine-wavish audio track.

1) Connected CH1 + and - of scope to + and - of amp output. Didn't know I should be putting a load at the output, was actually measuring the outputs directly.

2) Saw the sine wave was noisy so wanted to probe the input at the same time to check if it was just a noisy input source.

3) Connected CH2 probe + to - of amp OUTPUT.
CH2 Probe - was still sharing GND with CH1 -.

4) Connected CH2 probe + to - of amp input. Didn't see anything. Like there was no signal.

5) Connected CH2 probe + to + of amp input. Still didn't see anything. Like there was no signal.

6) Disconnected - and + of probe CH2 from amp OUTPUT.

7) Connected CH2 probe GND to - of amp INPUT.

8) Kaboomm! Sparks flew and the sine wave on CH1 disappeared.

9) Phone rebooted itself (protection feature I guess??)

10) Phone's 3.5mm jack is now dead.

11) Went out for a long walk

12) Cursed at himself profusely

13) Wondered what he had done wrong

14) Cursed at himself profusely

15) Wondered if phone would die eventually or it's just the 3.5mm jack foobarred

16) Cursed at himself profusely

17) Wondered if el-cheapo audio amp is actually still working (strange thing to be think about of is as it is a $600 phone vs $10 amp... LOL)

18) Signed up on

19) Posted this question...

Author:  Gio [ 27 Feb 2018, 21:01 ]
Post subject:  Re: Correct (and safe) way to connect oscilloscope to audio

Welcome to the forum.

It's definitely a grounding problem. Could be the house grounding. But more likely is the amp power supply and interaction with the scope. What kind of PS is used for the amp? I'll guess a ground loop with the scope PS. What kind of scope are you using? PC scopes can be really really bad for this. Is everything 3 prong?

Not to sound alarming. The voltage sounds like it was high so that could be a safety concern had you been holding the phone and been inadvertently grounded like by touching a chassis. So best to determine what the problem was before potentially repeating.

Author:  gofar99 [ 02 Mar 2018, 22:41 ]
Post subject:  Re: Correct (and safe) way to connect oscilloscope to audio

Hi, I am with Gio on this. I have fried two PC scopes (a while back...I was a slow learner :blush: ) because of ground difference problems with gear under test. One I sent back to see if it could be repaired....the response was that nearly all ICs in the amp portions were fried. Cheaper to replace it. Now I do a voltage check between the grounds just to be sure. Only exception is if the gear has a third wire permanent power ground. If it is bunged up the fault detection circuit in my shop will trip before I can test it.

Good listening

Author:  laurie54 [ 04 Mar 2018, 05:17 ]
Post subject:  Re: Correct (and safe) way to connect oscilloscope to audio

Which ClassD amp did you get? What type of output drive topology does it have? Does the board/amp have a negative voltage generator to create a split power supply? That is (V+ and V-) with respect to gnd.
Regarding the spkr outputs you say spkr+ and -. Is the output of the amp, differential? This would make spkr - not connected to gnd and would mean you just plugged 1/2 of a PWM output signal with enough potential to drive spkrs into the output of your cell.
Also then scope probe 2's gnd (common with probe 1's gnd) could have been a dead short from spkr- to amp boards power gnd at the input gnd when you connected probe 2's gnd lead.
Just spit balling. Brain is not ready to go to bed yet.

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