I am finishing a 4 channel tube microphone preamp build. The audio channels are based on EF86 and ECC88 tubes, and there is a solid state power supply with a regulator for high voltage and one for filaments.
If one of the input transformers is left without input load termination, there is noise on all the preamp channels.
With no load resistor across the input transformer secondary, noise is 2Khz and upper harmonics. With a 130K load resistor, noise is 20KHz.
How should I properly deal with this situation ? Obviously it's not acceptable to have all the 4 microphone input plugged in at all time in order to maintain noise free operation.
Do I have to fine tune the secondary load resistor, or perhaps adding a zobel network ?
If one of the input transformers is left without input load termination, there is noise on all the preamp channels.
With no load resistor across the input transformer secondary, noise is 2Khz and upper harmonics. With a 130K load resistor, noise is 20KHz.
How should I properly deal with this situation ? Obviously it's not acceptable to have all the 4 microphone input plugged in at all time in order to maintain noise free operation.
Do I have to fine tune the secondary load resistor, or perhaps adding a zobel network ?
