This Week on the Breadboard: Phase 90

[Chuck D. Bones]

I've been meaning to breadboard the Phase 90 for a long time. Anyone who has played thru one of these knows about the ethereal, liquid, tone.
I recall tracing one of these when I was an engineering student back in 1976. I had read about using JFETs as variable resistors, but this was the first time I had seen someone actually do it.
I built the breadboard in accordance with the PPCB XC Phase schematic with the following exceptions:

1. I connected a Throb LED to IC3-7.
2. Q5 is a 2N5087, just like in the Phase 90 I traced.
3. C8 is 10uF tantalum. I was out of 15uF caps. MXR uses a tantalum cap and for good reason. A leaky LFO cap could cause the LFO stall.
4. C10 is 22uF aluminum. This cap filters Vref and it's value and material are not critical.
5. The bias trim is a 100K trimpot with a 100K resistor at each end. It is much less touchy this way.
6. I omitted the STAGES switch.
7. I used an ON/OFF/ON switch for the SCRIPT switch to get two levels of feedback (or no feedback).
8. I omitted D1 because it doesn't do anything.

The four phase shift stages in the Phase 90 make up to 720° phase shift (at certain frequencies). Since the input and output of the phase shift stages are summed, the resulting freq response has two notches. Those notches sweep up and down in frequency as the LFO varies the gate voltage on the four JFETs. I matched four 2N5952s for Idss & Vp @ Id ≈ 1.8μA. Matching at one point isn't guaranteed to get a good match. So what's the point of matching JFETs in a phase shifter? If the mismatch is large, then there is no setting of the Bias Trim that will satisfy all of the JFETs. Some might be stuck at max or min resistance and we get less sweep, or an uneven sweep. If the JFET matching is only off a little bit, then the spacing between the two notches will be different. A slight mismatch is no big deal. I'm going to try some other JFETs to see which ones work well. The ones I used had Idss between 4.7mA & 4.8mA. Vp was close to 1.75V.

All four blue rails are ground. The bottom two and top red rails are Vcc. The 3rd red rail is Vref. SCRIPT switch is on the right. Center is Script Logo, down is Block Logo and up is high Q (stronger feedback than Block Logo). I rearranged the opamps a bit. The chip on the far right is the input buffer and the 1st phase-shift stage. Middle chip is the 2nd & 3rd phase-shift stages. Chip on the far left is the 4th phase-shift stage and LFO.


There is a good analysis of the Phase 90 over at ElectroSmash and also a cool interview with Don Morris, one of MXR's engineers.

 

[cooder]

Swirlz!

 

[Coda]

Don’t worry, it’s just a phase…

 

[Coda]

…get it?

 

[Chuck D. Bones]

…get it?

If ya gotta ask...

 

[Chuck D. Bones]

Next up is a close cousin, the PH-1. Then maybe some mods...

 

[Robert]

…get it?

It's funny.

 

[jimilee]

Now you’re talking. I love me some phazerz.

 

[steviejr92]

Looking soooooo forward to this!

 

[KR Sound]

I love me some phasers.

 

[Feral Feline]

Like a zarglott to a laser-flamethrower...

 

[Chuck D. Bones]

I've tried 2N5484, 2SK30A-Y & BF244A so far. They all work well. But...
When subbing other JFETs, it's necessary to adjust the modulation depth. To accomplish that, I added a Range Trim to my breadboard. I tweak the Bias and Range by ear. With the Bias and Range adjusted to sweep down to the low-E string, we get a pretty good UniVibe throb. Ref des are per the XC Phase schematic.


Next, I'm going to try stagger-tuning the phase-shift stages like a UniVibe to see how that sounds. Sounds pretty damned good as-is.

 

[cooder]

The "stagger-tuning the phase-shift stages like a Uni-Vibe" could be on a 3P4T switch for phase 90 and two different staggered UniVibe settings. Me thinks. I have done that before with UniVibes and gives a nice extra variety of soundz.

 

[Chuck D. Bones]

I've seen schematics that have switches on the capacitors. My sense it that having both a phaser and a Uni-Vibe is just as good, if not better, since one may desire different speed settings on the two.

The stagger tuning sounds very good and pairs nicely with my 5-knob CMOS OD.

 

[lrgaraujo]

The phase 90 is the reason I fell in love with guitar pedals!
Why does the feedback go back to the 2nd all-pass stage and not the 1st one?

 

[Chuck D. Bones]

Why does the feedback go back to the 2nd all-pass stage and not the 1st one?

Good question.
Either way will work, but the feedback is more effective when it is around an odd number of stages. With an even number of stages, we need a capacitor in series with the feedback resistor (R27) to break the DC path.

 

[lrgaraujo]

Either way will work, but the feedback is more effective when it is around an odd number of stages. With an even number of stages, we need a capacitor in series with the feedback resistor (R27) to break the DC path.

Thanks for the answer!
I have had a lot of fun with a 3-stage phaser (3 stages for me is the right amount of effect) based on the phase 45 topology, if I decided to add some feedback would I need the cap?

 

[Chuck D. Bones]

Short answer: yes.
Long answer: breadboard it first because that feedback point may not sound good to you.

AFAIK no one sells a phaser with an odd number of stages. The wet/dry mixing comes out different with an odd number of stages. You get a lot of bass cut when the notches sweep up in freq.

 

[lrgaraujo]

Short answer: yes.
Long answer: breadboard it first because that feedback point may not sound good to you.

Will do, thanks!

AFAIK no one sells a phaser with an odd number of stages. The wet/dry mixing comes out different with an odd number of stages. You get a lot of bass cut when the notches sweep up in freq.

There's one from the late Blackout Effectors, called Sibling Phaser. I think it's the same phaser circuit (or at least super similar) found in the Blackout Effectors Crystal Dagger

 

[Chuck D. Bones]

Got a schematic?