oscillation 101

Equilibrium · 9th January 2003, 01:09 AM

How can you tell if an amplifier is oscillating without using an oscilloscope?

hitsware · 9th January 2003, 01:38 AM

it may exhibit a hiss or whistle
it may run hot at idle
it may burn a small resistor accross the output
if you can monitor the mains current it may raise with a load
if it's at a low level it's hard

Kilowatt · 9th January 2003, 02:53 AM

Unfortunately, smoke is usually the first and only sign of the bad sine.

In fact, even with a good scope, such oscillations may be too high for it to see.

Of course if the amp has a stable design to begin with...

Neutron Bob · 9th January 2003, 05:43 AM

Might I suggest mysteriously blowing tweeter fuses too. Sounds like an amplifier is trying to be a transmitter, eh?

daatkins · 9th January 2003, 07:53 AM

A lightbulb in series with a 100nF -> 1uF capacitor on the output works. The cap blocks any DC content in the output but AC oscillations pass straight through lighting the bulb for all to see.

Obviously, you need to size the bulb to your power rails.... A 12v car bulb with a series resistor would work.

Nice one,
David.

9th January 2003, 01:09 AM	#1
Equilibrium diyAudio Member Join Date: Oct 2002 Location: Mad Dog, Texas	oscillation 101 How can you tell if an amplifier is oscillating without using an oscilloscope?

9th January 2003, 02:53 AM	#3
Kilowatt diyAudio Member Join Date: Nov 2001 Location: Harlowton, MT, USA	Unfortunately, smoke is usually the first and only sign of the bad sine. In fact, even with a good scope, such oscillations may be too high for it to see. Of course if the amp has a stable design to begin with... __________________ - "With power amplifiers, negative feedback is a good thing, and positive feedback can induce destructive oscillation, but with people, positive feedback is a good thing, and negative feedback can induce destructive oscillation"

9th January 2003, 05:43 AM	#4
Neutron Bob diyAudio Member Join Date: Jul 2002 Location: San Clemente, CA	Oscillation Might I suggest mysteriously blowing tweeter fuses too. Sounds like an amplifier is trying to be a transmitter, eh?

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9th January 2003, 01:38 AM	#2
hitsware diyAudio Member Join Date: Mar 2002	it may exhibit a hiss or whistle it may run hot at idle it may burn a small resistor accross the output if you can monitor the mains current it may raise with a load if it's at a low level it's hard

9th January 2003, 07:53 AM	#5
daatkins diyAudio Member Join Date: Aug 2001 Location: Bath, UK	A lightbulb in series with a 100nF -> 1uF capacitor on the output works. The cap blocks any DC content in the output but AC oscillations pass straight through lighting the bulb for all to see. Obviously, you need to size the bulb to your power rails.... A 12v car bulb with a series resistor would work. Nice one, David.

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