Note: I'm using Logic Pro X and referring to the "Echo" delay plugin specifically, but I'm sure all delays are the same. The "Echo" plugin is seen below:
Does the "wet" percentage refer to the volume of the first repeated signal? In this case, would the first repeat be at 54% of the volume of the dry signal? And then would the feedback percentage make each subsequent repeated signal 35% of the first repeated signal? Basically I'm trying to figure out how feedback and the wet knob work together.
Wet is the volume of each repeat. Dry is ONLY the volume of initial sound. If you are on 100% wet, then you would hear nothing when you hit the note, then only the repeats. If you were 100% dry, you would hear no echoes. I often put the delay on a return track and set it to 100% wet. The initial sound plays from the track it's on, and the delay is separate. The reason I do this is so I have one plug-in for all of the tracks I want to delay, and changing one setting will effect them all the same.
Feedback is how much of the signal is fed back through the effect, creating more echoes. At 100% feedback, the signal will continue echoing forever, getting louder over time, until the feedback loop turns it into noise. If you want the sound to die out faster, keep the feedback lower.
We call 'wet', the return from an FX processor. 'Dry' is the input signal to the effect. So if you turn 'wet' down and 'dry' up, the signal will pass through the effects unit unchanged.
The reason many effects units have a dry/wet setting is so you can either use them as insert effects, or send effects. An insert effect is one where all of a channel's signal passes through the effect and comes out the other side (output) - mixers have an input/output in a single jack for this use. Send effects on the other hand are those where you want to be able to hear the direct sound + the effect - mixers usually have potentiometers named 'Aux(illiary) sends' for this use (among others). There are also auxiliary return channels on most mixers where you can return the sound from the effects unit.
So if you used this delay effect as an insert you would usually turn the dry level all the way up and add as much of the effect as you like using the 'wet' control. But if you used it as a send effect, you would usually turn the dry level all the way down - something I kept forgetting in my early days - to avoid doubling your level. Then turn the wet level all the way up and control the final level of the effect from the aux return.
In the diagram bellow, these are the two signals mixed on the output mixer (+).
The picture above is the diagram of a simple, mono delay unit. The input signal going directly to the output is your 'dry' signal. It also goes into the delay line and some of the delayed signal is fed back and added (=mixed) with the input to be delayed again and again and again...
If you set the feedback level to 100%, you will hear continuous repetitions until you turn it back down. If you turn feedback all the way down, you will only hear 1 repetition.
There is also a 'color' slider which I suspect controls the cutoff frequency of a filter. This filter is commonly placed in the feedback loop (where that triangle is) to make each repetition sound a bit different. If it's a low pass filter, the signal will sound duller with each pass through the feedback loop. So if we assume this filter is an LPF, even with 100% feedback, the repetitions would not last forever in some cases.
It is worth mentioning that by returning the delay output on a normal channel instead of an aux return, you can have an external feedback loop with any (insert) effect you wish inside it including the very useful EQ - just send it back the the delay using the aux send on the return channel. But beware. Anything including the insert effects that increases the level in the feedback loop can make your levels explode, as each repetition will be louder instead of quieter than the previous one. Still, this technique is very useful in making more complex repetitions like pitch-shifting them for example.
