ampeg scrambler 1M resistors

[Smoky Barnable]

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.

[PRR]

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.

[Smoky Barnable]

Interesting...thanks!

[turdadactyl]

Reviving this nearly decade old thread to go a little deeper.




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

[turdadactyl]

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?

[anotherjim]

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.

[turdadactyl]

Thanks Jim.  I also just found this old thread, which fills in a lot.

http://www.diystompboxes.com/smfforum/index.php?topic=85733.0