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How to monitor a speaker for open/close faults?

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Advanced Member level 3
Jun 19, 2001
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Hello, I have a little audio amp that is being designed into a circuit, it has a small 8 ohm speaker in and I need to be able to monitor the speaker for both open circuit and close circuit.

Open circuit is relatively easy to monitor, but close circuit a bit harder, Google searches bring up little.

I was wondering if any of you guys can recommend any circuitry or websites that would help?


Do you need to monitor the loudspeaker while it is producing sound?

If you only need to monitor while it is silent and you have a signal availabe to know if you are in a silent period then it can be done in a simpler way because you don't need to filter out the audio.

Yes and no, it needs almost constant monitoring, but not monitoring that the audio is correct.

Basically monitoring that the speaker is still there & in a working fashion.

There are several possible approaches including

1)Produce an low level ultrasonic tone from the loudspeaker and have a microphone
or an ultrasonic receiver next to it to check that it is working. Using an ultrasonic
transducer wil reduce the need to filter out the audio. amplify and rectify the tone
to get a DC level representing the amount of tone then use a comparitor to give
a good/bad output.

2)Putting a DC voltage on a loudspeaker will move the cone off centre and reduce the volume you can get out of it before it clips. If you use DC monitoring you either need to keep the current very low.

I'd want to keep the current down to 1mA but you might get away with 5mA or 10mA.

Feed some current to the loudspeaker either with a resistor from a regulated supply
or with a constant current regulator.

Assuming the loudspeaker has a DC resistance of 8ohms a 1mA monitoring current gives
you 8mV across the loudspeaker. That low enough for offsets in opamp circuits to be
a pain but it's doable.

An RC filter may be good enough to remove the audio.

I'v designed a few circuits like this. This diagram is what I'd start with.
It's guaranteed to need some work.
**broken link removed**

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