Maker Pro
Maker Pro

Oscilloscope Probe at Audio Amplifier Output Causes HF Oscillation

epsolutions

Sep 7, 2019
96
Joined
Sep 7, 2019
Messages
96
I built a John Hood 1969 type, single ended audio amplifier. It works fine into a speaker. But when I connect my Atten digital oscilloscope across the output, a 2Vpp 5MHz sine wave appears on the screen. It does not affect the audible sound quality. I tried swapping probes, channels, etc. to no effect.

Does this indicate a defect in my scope? Is there anyway to test? Any advice would be appreciated.

jlhnotesfig2.gif
 

Audioguru

Sep 24, 2016
3,650
Joined
Sep 24, 2016
Messages
3,650
It is class-A so its heating power is more than its feeble output power even when it plays very low levels.
It oscillates because it is missing a frequency compensation capacitor.
 

epsolutions

Sep 7, 2019
96
Joined
Sep 7, 2019
Messages
96
Asked the wrong question and got the right answer. Made my day.

I connected a 104 ceramic across the amp's output pins. Problem solved. I should mention there was a 10K resistor also in parallel, not shown on the previously attached circuit.
 
Top