Amp In A Box input & output impedances

[PRR]

OK one more question...

That one is complicated.



R4 does not go to a groundy point, but to an amplified inverse of the input (Q1 Drain).

Read "Miller Effect". Works for resistors also.

If we assume Q1 has voltage gain of 10, then R4 acts-like around 1Meg/10 or 100K, as far as the input signal knows.

It is not easy to predict the gain of a MOSFET used as an amplifier.

Also here the gain can be reduced with a Gain knob. When way-down the gain is about unity, and R4 acts-like about 500K.

If this bothers you, on a MOSFET R3 and R4 can be MUCH higher, maybe 10Meg. Now the input is likely 1Meg for most cases.

Parallel with R2, yes.

[antonis]

1) As I understand, the default output impedance is 2K7

Actually, I was trying to set a reference value to start - that's why I've wrote "as it is" - and then proceed it with  transconductance calulations... 

Output impedance of an Emitter follower (with specific components values) isn't "absolute" but it's ruther expessed in relation with previous stage output impedance Rs and it's calculated by r_e in series with Rs/(h_FE+1)
(paralleled by the usually omited in calulation R_E..)

More simplified, in case of FET, it's just 1/g_m, provided Source resistor (alone or in parallel with load) >> 1/g_m

IMHO, that's a compomise - like setting sole r_e as the output impedance of an Emitter follower..

But, as usually, they already are well said by Greg.. 

[POTL]

Today Marvel Drive assembled and decided to warm it up with AMZ Mosfet Booster, there was a problem - the tuning of the first transistor was lost.
Solved the problem by installing a shunt resistor at the output of the booster (set it between the output capacitor and ground).
The question who does not work correctly is the AMZ circuit, since it does not have a shunt resistor at the output (for example, in ZVEX SHO or EQD Black Eye, it is either a normal resistor or a volume control).
Or is there something wrong with the input impedance of Marvel?