Minibooster high pass rolloff calc

[nooneknows]

Hi,
there's something I don't understand in the Jack Orman's minibooster and I'd be very happy if someone could explain me.
I thought the capacitance between gate of the upper transistor and the junction source-drain of the two fets formed a high pass filter with the input impedance of the UPPER transistor itself.
Since there are no other components I also thought the rolloff frequencies had to be given by 1 /(2*PI*(Ra//Rb)*C) where Ra and Rb are the two classic 1M resistors between  +9Vcc and ground, right? Mmmmh, it sounds a little strange, because if I put a 1uF cap the rollof could be 0.31 HZ (?!) and with a 0.1uF the frequency rise to 3Hz...
Ok, I'm undoubtedly wrong, because to my ears the differences are far greater... so, where am I wrong? How the input impedance of the upper transistor work in this circuit?
Thank you in advance.
Marcello

[Ansil]

hmm see although i am not the transistor or fet person, i was assuming the  r2 and r1 were forming a simple voltage bias to source half of the v+ to the G of the q1. and i was asuming that the input went through c1 into q2 and through c2 into q1 and back out the S making a sort of  gain loop  and finally exiting through c3

hmm maybe i missed your point, but i am unsure where you are getting the 1uf and the .1uf  is this the circuit you are lookin at???


http://www.muzique.com/amz/mini.htm

[nooneknows]

hi,
no no, you didn't missed the point,  I was talking about C2.
You're right about the resistor R2 and R3, I have to consider Q2 and R5 too at this point.
Mmmh, still I can't figure how to do...  :-)
thanx
M

[Jered]

The bottom JFET is your gain stage the upper fet acts as a constant current load. The value of C2 determines the MB high pass characteristics, but I like to use C3 and C6 to shape the sound even further, fine tune it so to speak.
 Jered

[RDV]

There's some really good info on mini-boo techno stuff in this by R.G.

http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm

Regards

RDV