Oscilloscope wave form

[Sancho49]

Hi Everyone

I've a little question about measurement of an DIY amplifier First Watt with an Oscilloscope ...

I've build a dummy load ( 8Ohm / 50W ) , and buy Hameg Oscilloscope for testing Amp.

::: Test situation :::

View attachment 772979

::: Square form evolution :::

100mV Offset / 1 min
View attachment 772980

60mV Offset / 10 min
View attachment 772981

20mV Offset / + 20 min
View attachment 772982

View attachment 772983

Square form look like (J) exemple on the last pic.
It's a normal wave form for you with 1Khz square under 8Ohm load ?

Thank you

Sancho

 

[ubergeeknz]

It seems like your attachments aren't working, sorry.

 

[Sancho49]

::: Test situation :::

::: Square form evolution :::

100mV Offset / 1 min

60mV Offset / 10 min

20mV Offset / + 20 min

 

[Sancho49]

Square form look like (J) exemple on the last pic.
It's a normal wave form for you with 1Khz square under 8Ohm load ?

 

[Mooly]

Look at what is going into the amplifier before worrying over what is coming out. A laptop used as a generator may not produce good squarewaves. Check that first.

 

[Sancho49]

1Khz Sin Wave form / 5V/DIV
Look like perfect ...?!

 

[Mooly]

Ah! I thought you were trying to generate a squarewave test signal. OK, so that's fine as a sine wave test input.

Most amplifiers need only a small input voltage and so you need an attenuator (volume control) at the input. At 5 volts per division you have 30 volts pk/pk.

The scope image of the squarewave looks like some form of clipping but the amplitudes you mention don't make sense (20mv, 60mv etc).

 

[Sancho49]

The 3 first pics is the sqare wave at the output of the amplifier. ( @1Khz / .5v/DIV)
The last pic, is the sine wave at the output of the amplifier. ( @ 1Khz / 5v/DIV)

 

[Sancho49]

With full input power, 10Khz Square wave, +/-28W, I don't have a Square look like the next pic ...

My Square Look like this ...

 

[Mooly]

I'm not really understanding what you are trying to do here Both those images say 10kHz at 28 watts.

The first picture shows a typical squarewave from an amp that is well behaved and deliberately (in a good way) bandwidth limited perhaps by having a low pass filter at the input.

The second picture shows classic ringing and pre ringing such as you see from a typical CD player and its DAC/Filtering.

Gibbs phenomenon - Wikipedia

 

[DF96]

Yes, what does the input 'square wave' look like?

 

[paulebappie]

Did you calibrate the probe?

 

[Mark Tillotson]

Soundcards can't produce decent square waves, their output is band-limited to the audio range by a brick-wall digital filter.

 

[Sancho49]

Hi Everyone,
Yes I've check the probe calibration, but same problem.
Finaly, I've replace my computer with function generator software to an function generator, and ... Work !

 

[Mooly]

that looks much better. I suspected it could be the method of generating the signal that was the issue.