D104 Low Impedance Conversion
D104 conversions on the web seem to be exceptionally prone to error. Some pages have FET's backwards, some pages have needless RF chokes and parts, and some pages have incorrect bias.
RF chokes are common used to decouple leads, but we have to be careful when using them. A series RF choke and shunt bypass capacitor actually behaves like a network. Above a certain frequency, it can indeed reduce voltage passing though. On lower frequencies, it can increase voltage passing through! If possible, we should really use a suppression ferrite (appearing resistive rather than inductive) or a resistor in RF filters. This prevents the circuit from having resonances, and from stepping up or increasing RF feedthrough on some frequencies. Because of current flowing through a bypass or decoupling system, it isn't always possible or practical to use a resistance. In such cases suppression beads over a suitable conductor or an inductor shunting the resistor can limit unwanted RF coupling peaks through the RF bypass/filter while still passing considerable dc or low frequency current with minimal voltage drop.
Here is a circuit that works!
R1 affects frequency response. Higher values increase bass response
R4 is RF decoupling
C3 can be reduced in value to roll-off low frequency response
Keep leads on C4 and C5 short, and keep R4 and C4 near the audio cable output where the shield is grounded
C5 can be increased in value to roll-off highs
V1 is the mic element
Note: use either R7 or a battery. Switch the battery off when not using the microphone