Input pin for a LTP

[Fusion916]

Are there any issues with "floating" input pins in an LTP input picking up random noise? Since there is no DC path to ground, as for example here (from ESP):

An externally hosted image should be here but it was not working when we last tested it.

 

[MagicBox]

There's an issue yes, noise may actually build up DC on the 100pF cap. It can be fixed by adding an extra resistor to ground straight from the input. 47K - 100K will do just fine.

 

[AndrewT]

The transistor (Q1) sends current OUT of the base.
This current must flow somewhere.
It usually flows to Signal Ground and from there back to PSU Zero Volts via the Main Audio Ground.

This is the input offset current.

This current flowing through the resistances creates the input offset voltage. Typically 50mVdc to 500mVdc. Positive for the PNP LTP you have shown.
This brings the emitters up a further ~600mVdc. The bottom end of R1 could be sitting as much as 1Vdc above Signal Ground.

 

[DF96]

There should always be a DC path to somewhere for both LTP inputs. If not, the result is lack of bias not noise pickup. Noise pickup would result from being open circuit for AC, not DC.

 

[Fusion916]

There should always be a DC path to somewhere for both LTP inputs. If not, the result is lack of bias not noise pickup. Noise pickup would result from being open circuit for AC, not DC.

That's really what i'm talking about, stray bias at the input due to lack of DC path to ground.

 

[djk]

Without bias the output will probably slam to the rail.

To minimize offset, the bias currents must be the same for both inputs (equal value resistors).