AC coupling and hum question

[elsewhere]

Hi guys,

I'm learning about audio circuits and please forgive me if this question is elementary. I understand that most modern line outs use AC instead of DC voltage. Amplification is done using Voltage differences that are created by the signal. Because it is voltage and not the frequency of the current that is amplified, the 60 hz hum in AC is irrelevant so long as it does not introduce a variation in the voltage. If it does that is what causes the hum in your signal.

Please let me know if I got this correct. Thanks!

 

[JonSnell Electronic]

Almost.
If the two phases are 180 degrees out of phase, the differential amplifier amplifies the difference in the signal and rejects the summed signal. So, if the balanced line has 60hZ on it, the 60hZ will be on both lines and in phase. The differential amplifier will not amplify this signal, it will reject it. So, if you have a 60hZ hum after your differential amplifier, the 60hZ was introduced before the balanced line amplifier.

 

[elsewhere]

Thanks! I'm learning the single ended stuff first, but it's good to know the significant variation of the differential amp

 

[DF96]

I understand that most modern line outs use AC instead of DC voltage.

I'm not sure what you are saying here. You may be saying that line outputs use coupling capacitors to block DC but pass the AC signal. This is generally true, but has more or less always been true.

Because it is voltage and not the frequency of the current that is amplified, the 60 hz hum in AC is irrelevant so long as it does not introduce a variation in the voltage. If it does that is what causes the hum in your signal.

Again I am not certain what you are saying. Hum is either present or not present - or at least not noticeable, as it will always be present. If the music signal is being carried by a voltage (which is the usual arrangement) then you need 60Hz voltage too to get 60Hz hum. This is not actually saying anything, so I guess it is not what you are saying?

Amplification is done using Voltage differences that are created by the signal.

Good. You seem to have grasped that voltage is always a difference between two points. Many people never get this.