KC2QMA wrote:
-What is the TRS balanced mic input reference level? (Typical is -10db or +4db)
You are referencing the standard maximum line input levels, more specifically -10dBV (+2dBu) which is typically found on consumer equipment, and +4dBu (+1.8dBV) which is typically found on professional equipment. (Note: the only difference between dBV and dBU is that dBV is referenced to 1V and dBu is referenced to 0.775V).
Unfortunately there are no equivalent standards for mic preamp standard maximum input levels. As it happens, many mic preamps have maximum standard input levels similar to line inputs.
Looking at the G2 schematics (which Apache has asked not be publicly posted, but as an owner you can request a copy), it appears the maximum allowable mic input level is -21dBV (-18.7dBu).
This is based on the circuit design being comprised of an NE5534 op amp in differential, non-inverting, single supply configuration and having a gain of 10. This is followed by a TLV320AIC3204 CODEC chip. This is significantly different than the TLV320AIC23B used in previous ANAN designs. The 3204 is in a configuration that allows a maximum, single-ended 0.9V audio input.
-What is the TRS balanced mic input impedance? (Typical is 100-600ohms)
Typical professional, balanced mic, differential input impedance is in the 1 to 3Kohm range. Unfortunately the G2 design has a differential input impedance of 20Kohms. That's pretty high, and may have something to do with it's sensitivity to RFI or EMI.
Side note: I believe the TRS balance input on the G2 may need some internal RF choking with ferrites or small capacitor across the input as I believe I am detecting a low frequency audio oscillation below 30hz in the mic preamp even into a dummy load with no mic connected.
The schematic shows a L-C Tee filters (FL1 and FL2) on both differential mic input lines. Unfortunately there are no component values shown on the schematic so its properties cannot be calculated. A Tee filter is present on nearly every input on all Apache-built SDR boards from the Hermes to the Saturn. It's a good bet they are all the same. This includes the 3.5mm mic input on the G2. Since that filter seems to work well otherwise, any RFI problems are more likely to due to other reasons, such as the aforementioned high input impedance, or perhaps the susceptibility of the NE5534.
My own side note: the schematic shows the balanced mic preamp board having a 5 pin header as the audio input. It makes provision for a phantom power (mic bias) input on pin 4. Assuming FL1 and FL2 are appropriately rated it might be possible to feed a proper 48V professional phantom voltage to a condenser microphone. The DC blocking cap's are shown on the schematic as being rated for 60V, so that's a good sign. Motivated individuals might be interested in adding a 48V phantom supply internal to the G2.