Help understanding KM84 circuit (and other basic electronic principles…)

[JohnnyM]

Hi all,



Let me preface this by saying I have a really hard time understanding electronics and circuits. Or rather, envisioning how they work. If anyone could help me wrap my head around some of the roadblocks my brain has come up against, that’d be appreciated.



For some reason my brain won’t let me just build the circuit, it has to know how it works as well

1. 48v to polarise the capsule- you have two sources of +48v DC from pins 2+3 as it enters the mic. These pass R9+R10, and per the diagram you have +48v (which goes onward to R5 towards the capsule and to R6 towards the JFET. What exactly is happening here? a) the two 48v sources sum to 96v, but then R9 and R10 act as a voltage divider with equal values, so it’s halved back down to 48v? b) these are parallel circuits so there’s no voltage division, it’s just 48v in parallel c) something else
2. Carrying on towards the capsule, the voltage stays at 48v because the resistors in the rest of the path are all parallel circuits? So it’s current division rather than voltage?
3. The website I took that image from says “Look at the circuit from the FET’s power source at C5 to ground and trace current flow.” How is C5 the power source of the FET? Is that a typo?
4. Between R4 and C2 the diagram is marked 10v, from 21.5 between r4 and R7. Is this because R4 acts as a voltage divider with R3 through the drain and source of the FET?
5. What is the zener diode doing?

Sorry if these things are so basic it numbs your brain to have to explain them, I just find that every explanation I read for electronics leaves me with more questions than answers…



Thanks all!

 

[rock soderstrom]

This is a phantom voltage of 48V which is coupled into the XLR (pin2/3) cabel via the microphone amplifier (or dedicated PSU) with two resistors of exactly the same size and then decoupled again via R9 and R10. This is quite clever and not so easy to understand. Google it!

This 48V DC is then used to polarise the capsule (to charge it with a voltage, the capsule is a capacitor) and to provide the operating voltage for the FET impedance converter (from high to lower impedance together with the step down Xfrmer).

The polarisation voltage is then ripple-filtered by the two RC filters R5/C6 and R8/C10. As almost no current flows here, no significant voltage drops across the resistors (U=R x I) even though they are gigantic.

They have to be this big otherwise they have no effect at all due to the extremely low current flow in this branch of the circuit. Imagine the polarisation voltage as a static, currentless voltage.

In the other branch of the circuit, R6/C8 and R7/C5 also form ripple RC filters for the operating voltage of the FET to make it as clean as possible. As there is slightly more current flowing here, the resistors are correspondingly smaller and the voltage losses greater.

The Zener diode stabilises the voltage of the FET, C8 and also R7/C5 filter the Zener noise from the operating voltage. The Zener diode acts as a simple "voltage regulator", which works quite well because the current flow is very low.