Adding a knob to a phaser

[imzuker]

Hi
I would like to make a modded phase 90 or a small stone that will have a "depth/range" knob.
someone have an idea about how to do it?

thanks,
imzuker

[Seljer]

Theres a 'width' control in this schematic which affects how wide the sweep is http://www.geofex.com/FX_images/p180plus.gif as well as the the 'mix' control (which I've seen on some phasers labeled as 'depth').

[Mark Hammer]

By varying the bias current coming from the bias trimpot through the 1M fixed resistor, you can vary the range of sweep.  I think I've used a 330k fixed resistor in series with 1M pot/variable-resistance.  That does not affect how much sweep there is within that range, however.

In the drawing linked to, you will see that the bias current, coming through the 1M pot, is summed with the LFO, to feed the gates of all the JFETs.  You can alter the LFO amplitude by varying the value of that resistance.  In the schem shown, it is 3M3, however in other MXR P90 schems you will also see 3M9, suggesting that it is also open to modification.

In one of mine, maybe two, I've installed a 3-position toggle to select between several different resistance values and sweep widths.  I do this, rather than install a pot, because the resistance values one needs to achieve are well outside the range of the pots I have in my parts bin.  It's just easier to use a couple of paralleled resistors to pick a few presets.  Besides, the sweep width interacts with the range/bias setting, unnecessarily complicating matters if one uses 2 pots, and the range control tends to have more audible impact on the resulting sound in my experience.

[Electron Tornado]

I've got a phase 90 on the bench right now, so I'll have to try those ideas.

Take a look at one of the phaser kits from BYOC, and they have something they call a depth control that you might be able to fit into a phase 90. Another mod I found I like on the phase 90 is to remove the trim pot and install an external control.

[Mark Hammer]

The "Phase Royale" (as opposed to the Phase Royale with Cheese  ) has a slightly different LFO circuit and provides a buffer stage before the LFO depth pot.  The core of the LFO is the same as a P90, but there is that extra stage to support use of a depth pot.  I won't say that adapting an existing P90 pedal to that configuration is impossible, but it will involve some sort of "floating daughter-board" if one is not using a BYOC PCB.  How comfortable you are doing that is your call.

[imzuker]

Oh you just reminded me that i wanted to ask that question:
I looked on the ROSS phaser and i saw this LFO and REGEN, so what do they mean?

[Mark Hammer]

I'll assume you are referring to the Tonepad schematic drawing of the Ross Phaser issue that uses LM13600 chips.

The entire circuit is split into two separate chunks, simply fro ease of drawing and to fit neatly onto one page.  The encircled labels are simply points that are meant to connect but would make the drawing too confusing if the actual connections were drawn in as lines.  The box marked LFO is the LFO output and is hardwired to the point where the phase-shift sections receive the LFO control signal.  The point labelled REGEN goes to the point on the circuit labelled REGEN.  On the board layout itself, you don't have to look for those points since the connections are built into the layout.