This Week on the Breadboard: Phase 90

[lrgaraujo]

Got a schematic?

DeadendFx has a board for the Crystal Dagger. It's the Shard: https://drive.google.com/file/d/1ocYb4a0HaWZYs3LlNG0tKL1VbXGSi3le/view

 

[jimilee]

The VFE phaser is 3 as well, I believe.

 

[Chuck D. Bones]

Peter Rutter went all-out with the VFE Tractor Beam. I dig all of the knobs and switches. I'm going to have to try out the 2-way FEEDBACK control.

 

[cooder]

Looks more like we will see a chuckified phase 90 here... getting giddy!

 

[jimilee]

Peter Rutter went all-out with the VFE Tractor Beam. I dig all of the knobs and switches. I'm going to have to try out the 2-way FEEDBACK control.

It’s really neat. I built one on a board I bought from Mdanean.

 

[cooder]

@jimilee knows phasers. all of them pretty much.

 

[jimilee]

@jimilee knows phasers. all of them pretty much.

If I don’t have it, it isn’t made.

 

[Chuck D. Bones]

I'm baaaaaack. I took a few days to go on a road trip with some friends. We drove up to Santa Cruz to see Zeparella.

Anyway, before I left I was playing with the capacitor values in the phase-shift stages, trying to get a UniVibe-ish tone. I have the caps set as follows:
C2 = 47nF (stock value)
C3 = 470nF
C4 = 18nF
C5 = 3.3nF
The cap values are larger than in the UniVibe because the associated resistances (Q1/R6, Q2/R9, Q3/R12, Q4/R15) are smaller than we would get with the LDRs in a UniVibe. With the Bias and Range trims dialed-in, it's a pretty convincing UniVibe tone. The LFO waveform is completely different from the UniVibe, as is the transient response of JFETs vs. an incandescent lamp & LDRs, but everything taken together gets us surprisingly close. I leave the feedback (Script switch) turned off because the UniVibe doesn't use feedback and the stagger tuning does not respond to feedback in the same way that synchronous tuning does. If you're breadboarding this, it's definitely worth exploring.

 

[cooder]

If we put this on a 3P4T mini rotary we would have two more options for caps left, assuming we leave C 2 stock value and don't switch that.
Any useable suggestions for alternative vibe sound values?

 

[Chuck D. Bones]

Nope.
We might want a 6 or 7 pole switch so we can also change the rate, range & feedback when we select between Phaser & UniVibe modes. By making all of the phase-shift stages the same, phasers concentrate the phase shift over a relatively narrow range of frequencies (upper graph). The UniVibe spreads the phase shift evenly over the entire audio band (lower graph). I don't know if there is much use for something in between.

Synchronous tuning


Stagger-tuned

 

[music6000]

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.
View attachment 60381

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.

I love me some Phaser, I grew up listening to Waylon Jennings & from 70's & beyond it was his trademark sound.
He used a Maestro PS-1A, Fender & MXR at different periods.
I have The Boss PH-1r on my Board that I purchased new way back but I pull out the EVH Phase 90 which I think is great also!
Fender have rcently released a Waylon Jennings signature Phaser.

 

[Chuck D. Bones]

I have The Boss PH-1r on my Board that I purchased new way back but I pull out the EVH Phase 90 which I think is great also!

Those two circuits are very similar. The PH-1r does a couple of extra tricks to increase headroom and reduce distortion caused by the FETs. That distortion is just part of the tone in the Phase 90 and is subtle as long as you don't drive the Phase 90 too hard.

 

[Chuck D. Bones]

I'd like to see the Fender Waylon Jennings Signature Phaser schematic. Haven't found anything on the 'net so far...

 

[Chuck D. Bones]

I'm finding the Bias and Range trims are critical to getting the right UniVibe tone. These knobs should probably be on the front panel. Still messing about with it...

 

[cdwillis]

The bigger caps for a more vibey sound is really interesting to me. I built the Circulator a while back and while it was cool I wondered what I could do to make it a little thicker. Even though it's a not a jfet based phaser, I figured I could do something similar with the 2.2nf caps. I think I saw 3.3nf there in a schematic for the Small Stone or Bad Stone. One of these days when I'm bored I might pull it out of the enclosure and put some sockets in for those caps.

 

[cooder]

I'm finding the Bias and Range trims are critical to getting the right UniVibe tone. These knobs should probably be on the front panel. Still messing about with it...

Would be great to get the range and bias so that they are not too touchy and then have them on dash, probably with the resistor either end configuration you mentioned above. In my stock phase 90 the bias trim is so narrow useable.

 

[giovanni]

I’m a bit confused by the Electrosmash analysis. The math for the output voltage looks wrong to me, there is a wrong sign somewhere. Besides that, when discussing the phasing element, at first they say each stage shifts the phase 90 degrees, but then later they say 45 degrees? Am I reading it wrong?

 

[Chuck D. Bones]

Each stage shifts the phase between 0° and 180°, depending on freq and FET gate voltage. This is an LTSpice plot of the phase shift in one stage at various gate voltages. The green trace is -2.0V on the gate. The gate voltage is increased to -1.5V in 50mV steps for each successive trace.


There are four stages in series, so the total phase shift looks like this:



If the total phase shift at the output of the fourth stage is 180° or 540° (360+180), then the wet & dry signals cancel and we get a notch. If we divide 180° by four (four stages), we get 45°. Therefore, if the phase shift in each stage is 45° or 135°, then the total phase shift at the output of the fourth stage is 180° or 540°. Since there are two conditions that cause cancellation (180° or 540°), we get two notches. This is the output when the gate voltage is -1.9V.



Does that make sense?

 

[Chuck D. Bones]

Would be great to get the range and bias so that they are not too touchy and then have them on dash, probably with the resistor either end configuration you mentioned above. In my stock phase 90 the bias trim is so narrow useable.

That's do-able, but requires trimpots or SIT resistors for R46 & R47 to match the circuit to the particular JFETs. Here's my latest schematic. This is what's on my breadboard. It works pretty damned well as either a Phase 90 or a Poor-Man's UniVibe, depending on the phase shift capacitor values. DEPTH is usable across its entire range. COLOR is usable across maybe 1/2 of its range. Which part of the overall range is is usable depends on the DEPTH setting. We can reduce ths interaction of COLOR and DEPTH by inserting a 3.3uF film cap in series with R24. I'm considering replacing R101 & D1 with a 78L05 for a more stable Vref. Not absolutely necessary. Notice the feedback network around each JFET (R36, R6, C37 & R37 for Q1). It helps linearize the JFET, which reduces distortion and increases headroom. The Phase 45 has this configuration and the PH-1r has an equivalent config. Not absolutely necessary, but recommended.

 

[cooder]

But... but... where would I get a 3u3 film cap!?!