This Week on the Breadboard: Phase 90

[cooder]

Kidding aside it looks fab, Sir!

 

[giovanni]

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.

View attachment 61617
There are four stages in series, so the total phase shift looks like this:

View attachment 61618

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.

View attachment 61620

Does that make sense?

I was under the impression that the phase shift was constant at the cutoff frequency. And that the jfet voltage controlled the cutoff frequency, giving the effect of the notch filter moving in frequency?

 

[Chuck D. Bones]

I'm still fiddlin'. Might make RESONANCE a knob instead of a switch.

 

[Chuck D. Bones]

I was under the impression that the phase shift was constant at the cutoff frequency. And that the jfet voltage controlled the cutoff frequency, giving the effect of the notch filter moving in frequency?

Now you know better.

Here is a plot of the notches for 11 different JFET gate voltages. It's a busy plot for sure. Pay close attention to the trace colors. There are two notches for each trace. A stock Phase 90 does not swing the gate voltage quite so far up and down.

 

[jimilee]

Here’s something was asked. Is it possible to easily add a mod to switch between 2 and 4 stages? I said it would require reviewing the LFO. What do you think @Chuck D. Bones ?

 

[Chuck D. Bones]

It's very easy to switch between 2 and 4 stages. Robert did it on the XC Phase, take a look at the Build Docs. No LFO mods req'd. Bear in mind that with 2 stages, there is only one notch and the notch does not sweep as far up and down as it does with four stages. Here's the XC Phase set for two stages.

 

[cooder]

Isn't that what the XC phaser here does, switching between phase 45 and 90, switching between 2 and 4 stage if I'm not terribly mistaken.

Edit: as always he beat me to it even with a shiny LTspice graph. I shut up for the rest of day.

 

[jimilee]

It's very easy to switch between 2 and 4 stages. Robert did it on the XC Phase, take a look at the Build Docs. No LFO mods req'd. Bear in mind that with 2 stages, there is only one notch and the notch does not sweep as far up and down as it does with four stages. Here's the XC Phase set for two stages.

View attachment 61629

Well, how bout that. Thank you sir.

 

[Big Monk]

I toyed with the idea of a DIY phaser for a while, but the MXR Script w/ LED Phase 90 is just perfect for me.

 

[giovanni]

Now you know better.

Here is a plot of the notches for 11 different JFET gate voltages. It's a busy plot for sure. Pay close attention to the trace colors. There are two notches for each trace. A stock Phase 90 does not swing the gate voltage quite so far up and down.

View attachment 61628

I think we are saying the same thing but in slightly different ways as the voltage changes the position of the notches changes as I expected.

 

[Chuck D. Bones]

More-or-less. The phase shift is constant with freq only at the extreme ends of the spectrum.
Yes, the gate voltage sweeping up and down moves the phase curves left & right which causes the notches to sweep up and down in freq.

 

[Chuck D. Bones]

I toyed with the idea of a DIY phaser for a while, but the MXR Script w/ LED Phase 90 is just perfect for me.

Seems that the original Script Phase 90 is the Gold Standard for phase shifters. And for good reason.

 

[Big Monk]

Seems that the original Script Phase 90 is the Gold Standard for phase shifters. And for good reason.

This is the second time I’ve owned the Custom Shop LED and DC model.

It has true bypass, DC jack and LED and it just has THAT phaser sound.

It’s also available used for less than $100 all the time. Both times I purchased used for around $80-$90.

 

[jimilee]

I toyed with the idea of a DIY phaser for a while, but the MXR Script w/ LED Phase 90 is just perfect for me.

Blasphemy! There is always more phaser.

 

[Big Monk]

Blasphemy! There is always more phaser.

A really good sleeper phaser is the Joyo Vintage Phase.

 

[jimilee]

A really good sleeper phaser is the Joyo Vintage Phase.

That’s what I hear, I need to check it out. Just a phase 90 clone? Among the dozens I’ve built is the BYOC classic phaser. I also built a 45 and 90 in one enclosure. It’s pretty cool for a phased tremolo type sound. One speed up and one speed about halfway.

 

[Big Monk]

That’s what I hear, I need to check it out. Just a phase 90 clone? Among the dozens I’ve built is the BYOC classic phaser. I also built a 45 and 90 in one enclosure. It’s pretty cool for a phased tremolo type sound. One speed up and one speed about halfway.

The Joyo sounds like a Phase 45 until about 12 o’clock and the. Shifts to the more intense Phase 90 sound.

It’s a nice unit. Built like a brick shithouse with through hole stuff and the right JFETs.

Panasonic style caps and everything.

 

[jimilee]

The Joyo sounds like a Phase 45 until about 12 o’clock and the. Shifts to the more intense Phase 90 sound.

It’s a nice unit. Built like a brick shithouse with through hole stuff and the right JFETs.

Panasonic style caps and everything.

Very cool.

 

[Chuck D. Bones]

Latest & final schematic. I presently have 47nF caps for C2-C5 for the true Phase 90 tone. The Phase 90 is a great phaser in its own right. This is just my take on some tweaks to improve headroom and add some versatility. Do not go under 22K for R27 or the circuit will break into high-freq oscillations when RESONANCE is dimed.

 

[jimilee]

Latest & final schematic. I presently have 47nF caps for C2-C5 for the true Phase 90 tone. The Phase 90 is a great phaser in its own right. This is just my take on some tweaks to improve headroom and add some versatility. Do not go under 22K for R27 or the circuit will break into high-freq oscillations when RESONANCE is dimed.

View attachment 61760

Now, who’s gonna build it first?