Idle current through Aleph1.7 gain FET???

[Klaus]

Hello Aleph P1.7 friends,

I´m right now prepared to match my FETs for the Aleph P1.7, but the more I think about the confidence level of my information about the idling current through the related gain FET, the more I run into deep doubts...

So could please anyone who really knows it tell me what this current is about? I have to admit not to understand the circuit with its two current sources and gain bridge good enough, to be capable to check it out .

Thanks!

Klaus

[stefanobilliani]

I just look at the Aleph P and L schematics.
The upper current source are the same for both circuits .
In the Aleph L you can see signed a voltage reference across R111 of 0.7 volts. Well this resistor do the same work in Aleph P but its name is R44. From Ohm's law we know the current that flows that is : for Aleph L 0.7/15 = 46,6mA
for Aleph P 0.7/22.1 = 31,6mA
15 is the value of R111 (Aleph L) ; 22.1 is the value of R44 (AlephP)
Hope this will help ,

[Klaus]

Hi Stefano,

thanks for your reply. Your caculated 31mA will be close to the truth. The BE-voltage of the ZXxxx will be around 0,6V, causing at 22.1ohms 27mA. BUT: in the lower current sink of the Aleph 1.7 we see a 33.1ohm resistor in almost the same configuration - sucking just 18mA?! Here I get lost - which source is feeding what and how do the influence each other...

Let´s wait for the electronics freaks - or the one and only itself - to advise.

regards

Klaus

[uli]

This 33.1 R resistor leads to about 20mA bias thru Q16,17,19,20
or about 18mA as you calculated. The difference to the about 30
mA = ~12mA flows thru R53,56,68,71 thus leading to about +40Volts at the junction of those resistors and the inputfet!
They are a resistive load for the gain devices and help to control
open loop gain and therefore control of the amount of negative feedback. BTW with those resistors you can define the DC-
offset of the output before the coupling C´s.
Uli

[Klaus]

THANKS A LOT, ULI!

I had heared 40V and 20mA round about, but it was to easy to beleave...

[uli]

Hi Klaus,
NP´s circuits ARE simple and as such that well sounding.
A. Einstein:" So simpel wie möglich und nur so kompliziert wie
unbedingt notwendig!"

Uli