Cavedweller mod question

[jonboper]

Just saw this video, and can't find anything on forum search. Does anyone know what kind of a mod this momentary switch is:

https://youtu.be/46kGWQA-4OQ

[midwayfair]

short lugs 2 and 3 of the feedback pot. It's the same thing on any PT2399 delay.

If you want something more sophisticated that doesn't ramp up immediately look up the build report for the Joshua Tree Delay.

[jonboper]

Thanks Midwayfair, I'll check that document out.

Total beginner here - what does that do, if it's possible to boil down.

[midwayfair]

Thanks Midwayfair, I'll check that document out.

Total beginner here - what does that do, if it's possible to boil down.

The repeats (Feedback) pot in a delay is a voltage divider. (A volume control is also a voltage divider.) The feedback pot works by returning a small amount of signal back to the input of the delay. Since the signal degrades each time it goes through the delay chip, if it doesn't feed much back (that is, if the pot is turned down), you won't get more than a certain number of repeats.

A voltage divider is two resistors. The intersection is the wiper of the pot:

~~R1~~|<-- take the output from here
             |
             R2
             |
            Ground (no signal)

When the voltage divider is a pot, as you turn the pot clockwise, R1 decreases while R2 increases. The result is more signal is allowed to go to the output rather than (in this case) ground. The opposite results in less signal.

This forms a ratio. If R1 = R2, the ratio is 50/50. This is a loss of 6dB of volume, half the voltage, or any other way you want to express it.

If R1 is, say, 9K and R2 is 1K, that's a ratio of 1:9. Put in say 10V and you only get 1V out.

If R1 is 1K and R2 is 9K, that's a ratio of 9:1. Put in 10V and you get 9V out.

Before you go any further, what's the ratio if R1 is 0?

[jeopardy theme....

...

...]

Shorting legs 2 and 3 of the voltage divider makes R1 0. Now, as long as R2 has any resistance, the ratio is theoretically infinite. Put in 10V and get out 10V.

I say "theoretically" because in electronics, there's no such thing as a signal source with infinitely low impedance. I won't get into it too much, but just know that there is still some resistance in series with (adds with) R1. However, for all intents and purposes, there is really only one knob settings where shorting R1 will not result in immediate infinite repeats. (See if you can figure out what it is.)

Since we're talking about voltage dividers, I'm going to repeat one of the most useful phrases in electronics, which long ago was passed down from the mountain by PRR (he's over on DIYstompboxes and a couple of other places):

Everything useful is a voltage divider.

Just knowing this, and learning to recognize how voltage is being manipulated throughout a pedal, will make modifying and eventually designing your own circuits far easier.

PRR also drew me gorilla butlers to illustrate how buffers work. Like I said, a very wise man.

[sonnyboy27]

If there is any possible way to see the gorilla buffers explanation then I would be forever grateful.

[midwayfair]

If there is any possible way to see the gorilla buffers explanation then I would be forever grateful.

http://www.diystompboxes.com/smfforum/index.php?topic=105399.msg949114#msg949114

[jonboper]

Thanks again Midwayfair, after reading that a few times, thinking about it, reading it again I think I understand the concept. That loopy feedback sound he gets from hitting the momentary switch is the result of the feedback pot being shorted out and feeding all of the signal back through the circuit. Correct?

I have a lot to learn, there's basically no end to what I don't know about all of this. I have some CaveDwellers on order and this mod is really intriguing to me...all of the momentary switch mods are, they're so much fun to play with.