Output Amp Blown Up

[Chet T16]

I was working on something with a tablet where i was feeding an audio signal (pure sine) into an opamp (as a comparator) to get a square wave to be read by an attiny85. For testing i was using the headphone jack. When i was looking to wire it permanently i desoldered the onboard speaker and took the output from there, which resulted in the output amp toasting itself. The headphone amp was seperate and still works fine when connected to the opamp.

There is about 6 feet of wire between the tablet and the opamp.

Am i missing something as to why the speaker output is different than the headphone one? Or is it just possible i shorted the output unknown to myself?

Thanks

[IanB]

I always remember being told in the old days that you should never operate an amp with the speakers disconnected as the amp could be damaged. I wonder if that's what's happened here?

[Chet T16]

It would definitely have been operated without the speaker connected and some stage while i was getting it together so thats a possibility 

[madires]

I always remember being told in the old days that you should never operate an amp with the speakers disconnected as the amp could be damaged. I wonder if that's what's happened here?

That's what most protection circuits do. They disconnect the speakers by a relay in case of too much DC or at start-up to prevent the "blob" sound.

[jnd]

I only read one datasheet for Class D filterless amplifier where they say that powering up the amp with external filter and without speaker load may damage the amplifier. As I understand it, these devices rely on the speaker inductance to work properly but they shouldn't fry themselves unless there is already some resonance from the PCB/wires/output components.

[madires]

Am i missing something as to why the speaker output is different than the headphone one? Or is it just possible i shorted the output unknown to myself?

Is the amp for the speaker a push-pull or bridge type?

[Chet T16]

Am i missing something as to why the speaker output is different than the headphone one? Or is it just possible i shorted the output unknown to myself?

Is the amp for the speaker a push-pull or bridge type?

I have no idea, given that this is a $45 chinese tablet i'll go with "whatever is cheapest"!

[SeanB]

Given it works off 3V6 it will be a bridge type. You normally use those to drive by placing a resistor across the terminals, almost any value from 10-33R and rated at 0.5W will do, then feed the signal via a 10uF series capacitor to the outside. Add a 100r resistor in series with the output and connect the shield of the cable to the chassis as a ground. The series resistor along with cable capacitance makes a low pass filter to remove a lot of the switching noise. Otherwise a 10uH series coil as well with a 1k parallel resistor ( or a really low Q coil) will tame a lot of the noise, though it will affect the frequency response.