What do the 1M resistors R1 and R2 do in this ampeg scrambler circuit? What is their function?
http://www.tonepad.com/getFile.asp?id=31
Thanks.
Q2 is the start of the actual effect. It has a low and wonky impedance. We want to buffer that from the guitar.
Q1 is a common Emitter Follower. It is a very common buffer stage; if you are going to peep schematics, you should be able to spot one in a crowd.
Every amplifying stage has to be "biased" somewhere between no current and infinite current. Often we can do this by twiddling a voltage near its input.
For maximum peak signal, we want to bias a stage's output roughly half-way between the rails. A CF's output "follows" its input. So we try to set the input roughly half-way between the rails.
R1 and R2 form a voltage-divider. The unloaded voltage is exactly halfway between the 9V and 0V rails. 4.5V.
When loaded with Q1 base the R1 R2 junction will sag a bit. That's OK. Our 9V battery is much-much more than a CF follower stage needs to pass guitar signal.
Actually Q1 is specified as Darlington, so sag will be negligible. A Darlington's emitter "follows" about 1.2V lower than base. So 3.3V at emitter. With Q2's loading, maximum CF output might be 1.6V. That's still a heap more than any normal guitar puts out.
Interesting...thanks!
:)
Reviving this nearly decade old thread to go a little deeper.
(http://i.imgur.com/Lx9QLJ9.jpg)
I feel like I have a pretty solid understanding of this circuit. But can someone clarify the interplay between the voltage divider R11/R12 and R8/R9? It seems like the R11/R12 voltage divider is primarily to bias Q2 at half Vcc, but what's going on with those two voltage dividers and the rectifier diodes?
Thanks
Afterthought: Looking at it again, is the R8/R9 voltage divider feeding off the R11/R12 voltage divider to determine the bias of Q4 in the differential amplifier, therefore determining the level of the octave up signal?
C6 stops that lot from doing anything to Q2 bias.
R11/12 make a bias voltage divider. R8 and R9 supply this to the bases of Q3 & Q4 separately - they are bias feed splitters rather than voltage dividers.
R8 is adjustable to balance the threshold/amplitude of one of the rectified half waves against the other. Octave up effect sounds better when this balance is set right. Best done by ear I think.
Thanks Jim. I also just found this old thread, which fills in a lot.
http://www.diystompboxes.com/smfforum/index.php?topic=85733.0