The use of coupling capacitor when injecting signals.

Thread Starter

Yami

Joined Jan 18, 2016
279
I know its an old thread but just wanted to ask @Roque - were you able to get the unit fixed?
Also @MrChips I was hoping that you could explain to me why the 10uF (polarized? if so which way round should it be used) capacitor should be used as a jumper (suggested on post #17).
Thanks so much
 

MrChips

Joined Oct 2, 2009
21,121
A capacitor is used to inject a test signal into a circuit in order to trace what is working and what isn't.
The reason for a capacitor is you do not want DC currents flowing between your signal source and the circuit under test.
The capacitor is called an AC coupling capacitor.
You want a capacitance value that is not too small and not too large, since you are essentially creating a high pass filter.
A low capacitance (under 100nF) poorly passes low frequencies. The sound will be very tinny.
A high capacitance (over 10μF) will pass more low frequencies but also could carry a large charge around which we don't want.
Remember, this is just a temporary test. We don't care too much about the voltage rating and polarity at this point. A non-polar 1μF capacitor will do fine.
 

Thread Starter

Yami

Joined Jan 18, 2016
279
A capacitor is used to inject a test signal into a circuit in order to trace what is working and what isn't.
The reason for a capacitor is you do not want DC currents flowing between your signal source and the circuit under test.
The capacitor is called an AC coupling capacitor.
You want a capacitance value that is not too small and not too large, since you are essentially creating a high pass filter.
A low capacitance (under 100nF) poorly passes low frequencies. The sound will be very tinny.
A high capacitance (over 10μF) will pass more low frequencies but also could carry a large charge around which we don't want.
Remember, this is just a temporary test. We don't care too much about the voltage rating and polarity at this point. A non-polar 1μF capacitor will do fine.
Thanks for clearing that up for me. Could there be a detrimental effect if the coupling capacitor was not used during troubleshooting, I am guilty of doing it sometime back.
So using a polarized cap with either polarity should not be an issue right? In case a non-polar cap is not available on hand at that moment.
Should I be using a coupling capacitor always? For eg: Say I was injecting some audio from my phone output to a node in the preamp section or the power amp section?
Thanks :)
 

MrChips

Joined Oct 2, 2009
21,121
Thanks for clearing that up for me. Could there be a detrimental effect if the coupling capacitor was not used during troubleshooting, I am guilty of doing it sometime back.
So using a polarized cap with either polarity should not be an issue right? In case a non-polar cap is not available on hand at that moment.
Should I be using a coupling capacitor always? For eg: Say I was injecting some audio from my phone output to a node in the preamp section or the power amp section?
Thanks :)
If you do not use a coupling capacitor you are at risk of

1) destroying the equipment being tested (DUT - device under test)
2) destroying the signal source, e.g. your phone or mp3 player
3) destroying both your signal source and the DUT.
 
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