Electronics > Repair
Audio… (Amps, THD, channel separation) with analog equipment (scopes, etc.)
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CaptDon:
There is a program called spectrum lab and several others, perhaps RoomEq that use your sound card and software FFT to show spectrum analysis of your signal. When I compare 'those' results to a real purely analog 'swept I.F.' style analyzer the results differ and there must be at least four types of FFT which all show different results. I think my Tektronix TDS644B scope has at least three FFT calculation algorithms. Anyway, the simple point being if you analyze the input signal to the amplifier and note its cleanliness and harmonic content and then analyze the output most forms of distortion show up as addition harmonics or elevated harmonic levels. If the sinewaves look good on the scope and sound good to your ear then you are in the ballpark. As you raise and lower the output level of the amplifier look for three important signs of problems. 1. Zero Crossing Distortion, Shows up as little flat segments where the trace crosses zero. Usually at low volume levels and they sound really bad!! 2. Flat Topping Clipping, especially if it occurs well below the max power output level. 3. Weird Kinks in Sinewave. This is sometimes found in so called 'Class G' type amplifiers where there are multiple voltage rails in the output stage and the BJT or MOSFET devices are 'stacked'. Many of the Bob Carver amplifiers are built this way and the transfer of voltage output level can see a small anomaly as the next higher device in the ladder begins to source current and voltage (P=I X E). Remember, even harmonics are musically pleasing, odd harmonics sound horrible and are very evident to the ear!!
Fried Chicken:
This classic A/AB amp should reference to ground across everything, including the speaker terminals, right?

I can safely probe around bing bing bing bing bing?
Fried Chicken:

--- Quote from: vk6zgo on December 27, 2024, 11:16:28 am ---
Oscilloscopes can be used to measure (for example), distortion , just by looking at the shape of a sinewave, but the distortion has to be quite profound before aberrations become visible.

You could compare the sinewave signal at the input with that at the amp output by using the "A" minus "B" function of an analog 'scope, or equivalent on a DSO, but you will run into problems with differing phase.

Any "tricks"  delivering usable results become increasingly complex to perform, as they require additional circuitry which you will have to design yourself

I earned "my daily bread" measuring distortion in Broadcasting for years, & later in testing hearing equipment, & believe me, if there were easy tricks to satisfactorily measure THD using a standalone Oscilloscope,my various employers would have used them!
The two "filter" methods I mentioned give repeatable results, so are used, the N & D set in Broadcasting, & the Selective voltmeter in the testing of equipment used in hearing testing.


--- End quote ---

Well... what other "tricks"?

Does the A minus B work off the displayed signal or the fed signal?  Can I set the voltages so that the two signals match on screen and then do A minus B?  I can alter the phasing by altering the trigger, no?  Or do I need a tunable RC circuit to change the phase?
vk6zgo:

--- Quote from: Fried Chicken on December 27, 2024, 03:57:58 pm ---
--- Quote from: vk6zgo on December 27, 2024, 11:16:28 am ---
Oscilloscopes can be used to measure (for example), distortion , just by looking at the shape of a sinewave, but the distortion has to be quite profound before aberrations become visible.

You could compare the sinewave signal at the input with that at the amp output by using the "A" minus "B" function of an analog 'scope, or equivalent on a DSO, but you will run into problems with differing phase.

Any "tricks"  delivering usable results become increasingly complex to perform, as they require additional circuitry which you will have to design yourself

I earned "my daily bread" measuring distortion in Broadcasting for years, & later in testing hearing equipment, & believe me, if there were easy tricks to satisfactorily measure THD using a standalone Oscilloscope,my various employers would have used them!
The two "filter" methods I mentioned give repeatable results, so are used, the N & D set in Broadcasting, & the Selective voltmeter in the testing of equipment used in hearing testing.


--- End quote ---

Well... what other "tricks"?

Does the A minus B work off the displayed signal or the fed signal?  Can I set the voltages so that the two signals match on screen and then do A minus B?  I can alter the phasing by altering the trigger, no?  Or do I need a tunable RC circuit to change the phase?

--- End quote ---

There may be tricks you can do with a DSO, but with an analog 'scope the RC phase change circuit would be the way to go.
The difference in shape of the two signals would show up as the result of A minus B, which may require some analysis to work out the relationship between that display & distortion in % (or dB).

It will, in fact be a result of THD + Noise, but usually the latter is low enough to be ignored.

In TV testing, we expressed both THD + Noise & Noise by itself in dB, making it easier to mathematically remove the latter to give pure THD.
TimFox:

--- Quote from: Fried Chicken on December 27, 2024, 03:19:59 pm ---This classic A/AB amp should reference to ground across everything, including the speaker terminals, right?

I can safely probe around bing bing bing bing bing?

--- End quote ---

Usually, that is true.  It is the exceptions that might kill you, for example an analog amplifier with an H-bridge output.
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