fOXX Phaser - Brighter/Darker

[nickbungus]

Hi

Just a quick question.  if I wanted to change the overall brightness or darkness of this phaser, would it be best to play with the values of C16 and C21 (where the wet and dry meet), or C20 at the output stage?


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[anotherjim]

I'd try a treble boost at the input amp. Actually the amp is attenuating 100/270 (x0.37 gain). 2.2n in series with 100k and both across the 270k R1 will remove the attenuation over 700hz. To stop it boosting high noise, a 100p cap can go across R3.
If it sounds too bright or hissy then C20 could be increased. 2.2n will cancel out all the treble boost added in the input (but you will have added a noise reduction feature). 220p would probably have no audible effect in a guitar rig. A good value for C20 will probably be somewhere between 200p and 2.2n.

[nickbungus]

Thanks Jim as always.  Actually I've got one of the 10 stage version that's lacking presence that I'm trying to brighten up.

In the 10 stage, where c20 is,  it's 1nf instead of 180pf.  It's also a pick and place smd board I've had manufactured.  The others in the batch seem fine so I'm guessing it's to do with component tolerances.

[anotherjim]

If C20 is 1n then it will be dark. Cutting over 1.5khz if the feedback R is 100k. It could either do with a treble boost on the input or a smaller C20. If you only make C20 smaller, you may get too much hiss.

[nickbungus]

I'd try a treble boost at the input amp. Actually the amp is attenuating 100/270 (x0.37 gain). 2.2n in series with 100k and both across the 270k R1 will remove the attenuation over 700hz. To stop it boosting high noise, a 100p cap can go across R3.

Ah, so are we saying take R1 out and replace it with a 100k with a 2n2 in series?

Thanks

[anotherjim]

No, keep R1 in. The extra R & C I suggest will bypass the higher frequencies and make them louder. Low frequencies where the extra C will have a high impedance will work as the original R1 only does.
Most music signal amplitude is in the low-frequency components. A FET phaser can be easily clipped by a large signal - but, having too small a signal means more circuit noise when it gets amplified.
Boosting only the higher frequencies has a better chance of making it brighter without clipping the FETs.
Treble boosting only at the output would make it brighter without increasing risk of clipping, but also increase circuit noise.

[nickbungus]

Brilliant - got it.

Just out of curiosity, what would the affect of changing C16 and C21 (where wet and dry mix) have?

[amptramp]

C20 at 180 pF and R49 at 100K combine to give you a -3db rolloff at 8846 Hz which should get rid of some noise.  You could afford to raise the value of C20 but don't go overboard.