Voodoo Lab Overdrive - Source´s Resistor question

[lepra85]

Hi All,
maybe this is a silly question but i am not solving out how it works.
Seeing the Voodoo Lab Overdrive ( http://www.diystompboxes.com/smfforum/index.php?topic=64238.0)
, I noticed that the diodes are tied to Vr. (in other similar overdrive/ditortions, they are tied to GND).
When the signal that reach the diodes is about 9V (this is an approximation) the current througth the diode (and from Vr) it is about 0,5mA.
If that is correct, the current through the resistors of the source (R11 & R12) should be around 10 times the mentioned current (5mA), as is suggested in http://www.geofex.com/circuits/Biasnet.htm
To achieve that, R11 & R12 should be 1k each one.
In the circuit, R11 & R12 are of 47k. So, I expect that Vr varies during the clipping.
Am I wrong?

thanks,

[antonis]

You are a bit confused about DC & AC currents.. 
(and where do they go to..)

R11/R12 create Vref for DC bias..
(and you're correct about 10/1 rule of thumb but in case of Fet Input bias current amp, rather noise than current is the restrict factor for resistive voltage divider size..)

Vref ISN'T biasing clipping diodes..!!
(there are other ways for that - if we want some "raise" on clipping threashold..)

AC current throught diodes goes "straight" to GND..!!
(via C10 which is an effective short-circuit for AC..)

So, clipping diodes AREN'T load for Vef..!!
(no room for superposition theorem..)


P.S.
IF your thought could stand, you should have to worry more about C4 because current throught it would pass more "freely" to Vref - without diode forward voltage drop offset.. 

[lepra85]

Antonis,
Thanks for your explanation.
if i take out C10, does the current flow throught the resistors?

[antonis]

Of course it does..

It actually does it even with C10 in place, because C10 DOES introduce a small series resistance (it's value depends on AC frequency..) so diodes AC current is actually splited to GND through C10, R12 and (R11 is series with C8//C9) in series with R13 in series with power supply internal resistance..