what is a "cancel" trimmer?

[nognow]

I was looking at that schematic:
http://3.bp.blogspot.com/-mR_I9Qjux6w/U8UGotv7rKI/AAAAAAAAINQ/_MG8DE-dMw0/s1600/Electro+Harmonix+Clone+Theory+Rev4.png

I didn't quite understand what is a "cancel" trimmer
what in the world does it do?
Thanks!

Also,I wanted to add a 3 band eq to the circuit ,what is the best way to do so?
Thanks!

[DrAlx]

The BBD delay chip has two interleaved outputs that are added together.
That trimmer lets you balance the two outputs to minimize overall clock noise on the audio.

See page 31 of armdnrdy's mutron flanger instructions for some pics of how the trimmer can minimise clock noise ...
https://dl.dropboxusercontent.com/u/53299166/DIYstompboxes/Pedal%20Flanger%20Build%20Document.pdf

[njkmonty]

CANCEL – Sets the mix of the two output waveforms

[nognow]

Thanks!
what about the EQ?

[Mark Hammer]

The good Dr beat me to it by seconds!

But I'll extend his note in the hopes of improving your understanding maybe a little more.

BBDs generally require two complementary clock signals to drive the chip through its paces.  Think of them as tick and tock from the same master clock. When path A gets tick, path B gets tock, and vice versa.  The clock pulses sort of contaminate the audio signal in a way.  If tick and tock are combined, you get silence, because they are perfectly antiphase.  This is very much like the way the two coils of a humbucker pickup both pick up the same hum, but since they pick it up in opposite fashion, those two hums cancel out and all you hear is the strings.

Normally, pedals that employ one or more BBDs will use a pair of equal-value resistors to mix the two parallel interleaved versions of the delayed signal coming out of the chip, to achieve that cancellation of the clock.  Of course, these are frequently 5% tolerance resistors (to keep costs down), such that the cancellation may be imperfect.  Again, like the pickups, if one coil is different from the other, you'll get hum reduction, but maybe not perfect cancellation.  The manufacturer probably could opt for 1% resistors for micing, but then you never have guarantees that each channel in each chip carries the signal at the same amplitude with the exact same output level.

So, what some manufactuerers do/did was include a trimpot to achieve perfect mixing and cancellation of the clock signal.  Other manufacturers might adopt a less elegant approach and simply use a lower cutoff frequency for the lowpass filtering that normally accompanies BBD-based effects.  However, if you can virtually eliminate any audible clock noise atthe output of the chip, you have the latitude to aim for wider bandwidth and roll off the frequency content a little higher.