Phaser/Filter mod on Phase 90? Calling Mark Hammer!

[frequencycentral]

I have a spare populated P90 board. I need to do something 'different' with it to justify boxing it up. So it it possible to perform the phaser/filter mod seen on OTA phasers with a P90....?

I'm also considering putting an LDR across the width resistor, stopping to LFO, and running it from an envelope detector......

[Mark Hammer]

Unfortunately, the phase-filter mod is specific to OTA-based phasers.  I mean I suppose a person could insert one or two OTA-based lowpass stages to a FET-based phaser, but the standard mod is a simple matter of redirecting one end of two caps...which is a whole lot simpler than inserting two lowpass stages and bypassing them.  So, if you conceive of a "mod" as a simple and minor change to an existing circuit, then phase-filter is a mod to OTA-based units, but not FET or LDR-based.  So, Ross, Small-Stone, DOD FX-20, yes, and P90, Bad Stone, Phase 100, Bi-Phase, no.

But if you want "different", then consider:

• varying the 1M resistance from the trimpot to shift the range up and down.  I used a 680k resistor and 500k pot on mine last night and had a pleasing variation in range
• varying the 3M9 resistance off the LFO to get different sweep widths.  I used a 3-position toggle on one of mine last night to get 2M4, 2M8, and 3M9, for three audibly different sweep widths - very helpful for fast bubbly settings

I mean, it's not exactly outrageous or other-worldly, but it makes for a nice 4-knobber (speed, resonance, range, width).  If you want, tack on a switchable pair of fixed stages between stage 2 and 3 (so as not to monkey with the feedback path).

[slacker]

If you disconnect the resistor that goes to the inverting input of the phase shift stage from the output of the proceeding stage and connect it to vref instead, you should get a high pass filter. No idea what it sounds like or whether it even works though.

[puretube]

Though not being Mark...

The
Search-
-Function
helps!

[edit]: in response to slacker`s reply#2...

[Mark Hammer]

I had originally tried to describe some additions which didn't show up in the earlier post.

Both the speed and suggested Range pots are 500k.  If you tap the output of the input buffer stage, you can feed a simple half-wave rectifier, and use that to drive an optoisolator.  The LDR half of the optoisolator can go either in parallel with the Speed pot or the Range pot.  A DPDT on-off-on toggle will let you assign the LDR to the one, the other, or neither.

My sense is that you'd want long lag/decay for such an envelope-follower, since you'd want this to produce a "trend", rather than a short-lived immediate response.

[Earthscum]

I haven't had enough fets to board up any of these, so I've always been wondering about stacking amps. Would this have any effect on the notches from being driven with a higher current?

[Mark Hammer]

I haven't had enough fets to board up any of these, so I've always been wondering about stacking amps. Would this have any effect on the notches from being driven with a higher current?

Not to my knowledge.