Pulldown resistor from positive to negative

[armdnrdy]

I'm having a hard time wrapping my head around part of a circuit!

The circuit is the modulation section from the WEM Hyperflanger.

The flanger comes equipped with a Manual/Auto switch (SW2) and a CV input jack (J4) for external control of the modulation. The Manual/Auto switch is depicted in the drawing in the auto position.



The drawing and the user manual call for 0 to +5 volts for the control voltage. The part I don't really understand is when SW2 is in the Manual position..... +5 volts is getting sent through R76 (22K) and a connection is made between R75 and the negative supply.

This looks to be a pull down resistor to negative voltage....but I'm not sure what purpose it serves. The same type of arrangement is seen to the left at P4 (Speed control) with +V coming in and R70 to -VS.

I would like to set up the circuit internally to accept an off the shelf expression pedal but I need a little better understanding of the CV section first.

Any thoughts?

[PRR]

Do you understand opamp summing networks? Do you care? Or do you just want a yes/no?

R75 against R76 mean that EXT must rise above 0.33V before "D" crosses zero (and presumably does its thing). This compensates for external pedals which don't quite go to dead-nuts Zero. You have to come off the end a few percent before anything happens.

R70 is emphatically NOT same type of arrangement. Study the LM13600.

[armdnrdy]

Do you understand opamp summing networks? Do you care? Or do you just want a yes/no?

R75 against R76 mean that EXT must rise above 0.33V before "D" crosses zero (and presumably does its thing). This compensates for external pedals which don't quite go to dead-nuts Zero. You have to come off the end a few percent before anything happens.

R70 is emphatically NOT same type of arrangement. Study the LM13600.

No...I do not have a great understanding of summing networks.

"Do you care? Or do you just want a yes/no?"

I actually do care. I research every build that I take on. I study the circuit intensely to try to understand the different sections before I commit to a circuit board. When I come across something that I can't identify or am unfamiliar with, I ask questions for clarity.

I do appreciate your reply. 

"R75 against R76 mean that EXT must rise above 0.33V before "D" crosses zero"

Just so I understand fully......how did you come up with the .33V?

[slacker]

The opamp stage sums what ever you feed into it through it's various inputs. It's an inverting amp so the gain applied to each input is minus (feedback resistor/input resistor) so for the signal from the ext jack -(R77/76) = -(68k/22k) = -3 so a 0 to 5 volt signal from the ext jack gives 0 to -15 out or 0.33 volts in = -1 volt out.
For the R75 input the gain is -(68k/1000k) = -0.068 x -VS so assuming -VS is -15 volts you get +1 volt out. This is then added to the 0 - 15 volts so you end up with an output that swings from 1 + 0 = 1 to 1 + -15 = -14.
This is ignoring the R78 input any voltage from this is also summed in the same way.

Hope that makes sense.

[armdnrdy]

Thanks Ian!

That is the explanation that I was looking for.

I'll roll this around for a while until that light bulb is illuminated  

Oh...and yes...the supply is -15/0/+15

[PRR]

I shudda showed my math.

> 0.33 volts in = -1 volt out.

No. -15V/1Meg balances 22K with +0.33V across it.

-15V/1meg= -15uA
0.33V/22K= 0.015mA = 15uA

Ignore R78 and the Depth network. (If Depth full-down, no effect.)

EXT has to rise above about 0.33V to overcome the 15V/1meg from R75 and move IC16B's output through zero.

I would ass-ume this gives sensible results with external controllers which don't go *quite* to zero.