Phone generated frequency thru amp to O-scope

[gfmucci]

Is there any danger to my Android phone or tablet generating a tone generated by a tone generator app through my integrated amp feeding into my oscilloscope to determine any harmonics using the scopes FFT function?   How careful do I need to be with the amp amplitude/output setting to be gentle enough on the scope?  Can I use a speaker out (to test power amp) or just the high impedance outs to test the preamp?

I should probably check the phones output directly into the scope to see what harmonics the phone generates by itself, first.  And maybe the phone direct to the scope Channel A and the phone through the amp to the scope in Channel B for a direct comparison.

[macboy]

You can't realistically measure audio amplifier distortion using the FFT function of an oscilloscope. The scope samples the signal with an 8-bit ADC (10 bit if you are lucky), and the harmonics will probably (hopefully!) be well below the quantization noise floor of the ADC. In addition, the scope's front end and ADC will likely have much more distortion than an audio amp.

A high quality computer sound card can have low enough distortion to measure an audio amplifier. There are various well made software programs that do all the work for you, such as RightMark Audio Analyzer.

Look into audio distortion analyzers. There is a reason that these instruments exist, and have high prices.

[gfmucci]

Is there anything inherently dangerous to the phone/tablet generating the frequency signal, or to the amp or scope in the hookup I described?  Any particular safeguards to be taken, such as additional resistance, or using high or low level amp outputs into the scope?

[bill_c]

I did something very similar when I was breadboading a prototype and had no problems.
Then I got up from my chair, which slowly spun, which pulled on a cord, which knocked over something on my desk, which landed on a screwdriver, which spun ever so precisely to short the 30V rail to the audio output of my prior cell phone.   I actually like my new phone more even though its 1/4 the price.

[gfmucci]

You can't realistically measure audio amplifier distortion using the FFT function of an oscilloscope. The scope samples the signal with an 8-bit ADC (10 bit if you are lucky), and the harmonics will probably (hopefully!) be well below the quantization noise floor of the ADC. In addition, the scope's front end and ADC will likely have much more distortion than an audio amp.

Is there anything inherently dangerous to the phone/tablet generating the frequency signal, or to the amp or scope in the hookup I described?  Any particular safeguards to be taken, such as additional resistance, or using high or low level amp outputs into the scope?

I just found this which seems to contradict the idea that what I propose can't be done with any helpful result...



And another...



Lesson learned:

While using an 8 bit scope is not useful for measuring Harmonics below 0.2% of the fundamental signal (the range of quality audio components), both of these videos demonstrated that a scope can be helpful for measuring harmonic distortion of out of spec audio amplifiers, and cheaper audio amplifiers, such as of your phone, powered speaker, bluetooth speaker, etc.

[gfmucci]

From Audioholics forum...

How high can harmonic distortion in an amp be before even an audioholic can notice it?

Given the levels present in modern receivers I'd say it's a lot more than they are capable of producing, assuming you don't overdrive them.

Odds are you'll never hear it unless you overdrive the amp beyond all logic and reason.

If you really want to get anal, check out some of the harmonic distortion figures in speakers. Compared to that of electronics, they'll make the hairs on the back of your neck stand up.

Peter Walker when he did the double blind test with "Golden Ears" who denied that solid state amps sounded ad good as tubes, did something naughty the Golden Eared crowd never forgave him for.

He used his fine Quad II tube amps, the solid sate 303 and the solid state 405. He then told them he had a new experimental amp. Peter had resigned the amp to add 2% THD across the board. The Golden Ears doubled blinded could not distinguish any of the amps.

In Kessler's book the Closest Approach, he is asked why bother to make amps with 0.1% or less THD. He said, because it does not cost any more to make an amp with 0.1% THD, and he could not sell an amp with 2% THD!

[macboy]

I don't want to get into whether a bit of harmonic distortion from a well-behaved amp is audible, or if it is, if it is objectionable.
Measurement of distortion can have very real practical uses. Zero-cross distortion is clearly audible and objectionable sounding (it's an example of distortion that is not well-behaved). Tweaking an amplifier's output device bias while monitoring distortion is one practical use of distortion analyzers. Comparing amplifiers based on the merits of distortion measurements in the realm of hundredths of a percent is not such a practical use. Verifying a repair, especially when a device is substituted, is another very practical use.