Phase 90 mod - Univibe mod?

[ItsGiusto]

Hey, I'm finishing up a phase 90 clone build that I put on hold for a couple of years, due to frustration. I'm almost there, but I'd like to follow through on what I originally wanted, which was some sort of switch that makes it sound more like a Univibe. I tried making it with a switch that switches out all four .047uf phase-capacitors for the standard univibe-valued caps (I think .015uf, .22uf, .0047uf, and 470pf), but it wasn't successful - it just sounded muddied, and like the phasing wasn't matching up well. It didn't really sound like a univibe.

I've been told that the new MXR M68 is just a phase 90 with some mods to make it sounds like a univibe, but I have yet to see a schematic of it. Does anyone know what those mods might be?


Also, one such suggestion that I found for a univibe mod is listed here: http://stinkfoot.se/archives/975
But the sound-clip Stinkfoot posted doesn't really sound much like a univibe. Maybe if I try it with different cap values, or swap different caps in the circuit, instead of the first two, it could be better though? Not sure which ones might be good to swap

In general, I see a bunch of videos and listings of phase 90s with some supposed "univibe mod", and some are even moderately convincing at getting a univibe sound. But none seem to actually link to what the details of the mod actually are. For example, this phase 90 doesn't sound bad in getting a vibe-y sound: https://www.youtube.com/watch?v=BddBDNxI-TA
He says that the guy who modded this "started with the Stinkfoot mod", but I definitely think that the univibe character of the pedal in that video sounds better than Stinkfoot's audio clip. Do you think that they actually just did that Stinkfoot mod, of changing two capacitors for the values mentioned, or did they put more stuff on top of it in the mod, or change it around in some way?

In general, any advice for getting such a mod going in my clone would be appreciated!

[Mark Hammer]

A true Univibe produces broad shallow dips, rather than focussed notches, as a phaser does.  The staggering of cap values is an essential component of what produces that.

But, much like the way the character of a phaser can depend on where the notches are located in the spectrum, the pleasingness of a vibe will depend on where those broad shallow dips are situated.  An optimal vibe will sweep in the range you want it to.  Sweeping wider or sweeping in the "wrong" part of the spectrum will be suboptimal.

The trimpot on the P90 provides a bias voltage that directs the JFETs to a range of changes to drain-source resistance that can provide an audible sweep to where the notches are situated.  Some of that trimmer's range at the extremes yields no audible sweep, some yields limited halting sweep, and some yields decent sweep.  Within the decent-sweep range, however, there is still some adjustability to nudge it higher or lower.  Similarly, the 1M fixed resistor from the wiper of the trimmer to the FET gates can be adjusted in value to move things up and down.  Finally, the width of the sweep is part of what makes for a nice vibe.  The 3M9 (or 3M3) resistor coming off the LFO sets the width of sweep.  Larger values limits sweep and lower values yield wider sweep.

[Kipper4]

My opinion for what its worth.
vibrato and phaser are two completely different beasts.
I'd just build one of each.
Or just build a Mangatone to compliment your P90.


Then theres this Bi Phase.

http://www.ozvalveamps.org/repairs/circuits/stomps/biphase.gif

Put a phaser on one side. A vibe on the other, switching outputs. 1 input,two effects,1 output.

I'm already getting mission creep. Envelope detector anyone???

[Kipper4]

One more post and you hit 50 posts

[ItsGiusto]

My opinion for what its worth.
vibrato and phaser are two completely different beasts.
I'd just build one of each.
Or just build a Mangatone to compliment your P90.

Vibe and phaser are definitely two separate things. For what it's worth, I consider vibe and vibrato to be two separate things too. I also have a neovibe build that's been waylaid for a couple of years due to frustration, but I'm not ready to pick that up again just yet. And I also have a Jam Pedals Retrovibe which I don't think I'll ever replace, since I think it's probably the best sounding vibe, at least currently on the market. But I just thought it'd be cool to have a Phase 90 that I could switch on the fly to make it sound more like a Univibe, or maybe a cross between the two, since I like having modded things.

I'm already getting mission creep. Envelope detector anyone???

Hah, what, can't I multi-task?
Haha, yes, I'm actually waiting on some parts to come from Mammoth and ebay before I start my autowah build (most parts should arrive this afternoon). But while I was waiting, I suddenly had an urge to finish this phase 90 build that I had to put on hold once I graduated and got a job in 2015 (also put it on hold because I was frustrated and didn't know why it wasn't working, and didn't think I could actually debug it - turned out to be that my 250k trimpot didn't have enough precision, so I got a 50-turn screw-turnable one, and it works much better now). But now that I have some more free time (not unemployed, just better work-life balance) I'm going back and finishing my old projects.

[Kipper4]

One more post and you hit 50 posts

Congrats Justin.

Keep the iron hot.

[Mark Hammer]

My opinion for what its worth.
vibrato and phaser are two completely different beasts.
I'd just build one of each.
Or just build a Mangatone to compliment your P90.

1) I don't think they are entirely different beasts, but I do think that there are enough differences in how they are optimally set up that a simple flick of the toggle will not convert the one instantly into the other.  So yeah, making one of each, and fine-tuning it to do what it does, as well as it can do it, is a sensible idea.  Not unless someone makes an 8-pole double-throw toggle.
2) Isn't the Mangatone a pedal that has ridiculously large eyes and is Pokémon-based?

[Kipper4]

2) I know jack about Pokemon Mark. Except for some people getting into trouble over them.

Like the young couple caught out on rock in a rising tide.That had to be rescued by the lifeboat crew.
Whatd'ya know they carried on playing in the rescue vessel.

"8-pole double-throw toggle." Where's Stephen when you need him?
Link to a beaut.

http://www.diystompboxes.com/smfforum/index.php?topic=114745.msg1065553#msg1065553




Theres some Magnavibe chat here.
If you have 8 pages of time to spare.

http://www.diystompboxes.com/smfforum/index.php?topic=114118.0

Also try the Forum search for p90 vibe mods.

[Mark Hammer]

Things that are different abut a Uni-vibe, compared to a phaser:

1) Speed range: slow speeds are pointless with vibes, but quite reasonable with phasers.  Cut the value of the Speed pot in half (or even less) for a vibe, if using the same LFO as a phaser.

2) Feedback: There is NONE in vibes, while phasers can have feedback verging on oscillation, deliberately.

3) Sweep width: Wide sweep widths, with focussed highly resonant notches (and adjacent peaks) are what permit very slow speeds to work in a phaser.  Vibes want a throb, facilitated by a narrower sweep range and faster speed.

4) Sweep range: A vibe whose dips move around between, say 1khz and 3khz, is unlikely to be thought of as a vibe, whereas a phaser whose notches move around in that space may not be ideally suited for a given tune or desired tone, but it is still identifiably a phaser.  Vibes want to swirl around where the note fundamentals are.  That's how they animate the quality of distortion when placed before some sort of clipping circuit.  They bring the note fundamentals alternately closer to and farther from the clipping threshold, giving it a livelier quality.

[ItsGiusto]

That all sounds reasonable, that they're just too different to have on one switch. I can find some other switchable-mod to put into the hole I drilled. Thanks for the advice!

[Kipper4]

Put a switch between r16 and - input Ua1
Here

https://www.electrosmash.com/images/tech/phase-90/mxr-phase-90-script-logo-schematic-parts.png.

Or a 47k series r16 and a 100k pot wired as a variable resistor to the - input.

[ItsGiusto]

Put a switch between r16 and - input Ua1
Here

https://www.electrosmash.com/images/tech/phase-90/mxr-phase-90-script-logo-schematic-parts.png.

Or a 47k series r16 and a 100k pot wired as a variable resistor to the - input.

Already done - unfortunately, the vibrato sound from the phaser isn't amazingly pronounced, but like I said, I like to cram in the mods, so I put it a switch to take r16 out of the circuit. It's an okay vibrato sound.

[ItsGiusto]

Of course, I also have already added the obligatory script/block switch, but I also took it one step further:

It's an ON-OFF-ON switch with the middle OFF of course, being the script sound, with no feedback resistor. The top ON is the standard 22k "block logo" feedback resistor. But the bottom ON goes to a 15k resistor for a highly resonant deep-filtering sound. I originally tried it with a 10k, but half the time that caused it to actually get into a positive feedback loop, causing horrible deafening siren sounds to come out.

[Mark Hammer]

Put a switch between r16 and - input Ua1
Here

https://www.electrosmash.com/images/tech/phase-90/mxr-phase-90-script-logo-schematic-parts.png.

Or a 47k series r16 and a 100k pot wired as a variable resistor to the - input.

Already done - unfortunately, the vibrato sound from the phaser isn't amazingly pronounced, but like I said, I like to cram in the mods, so I put it a switch to take r16 out of the circuit. It's an okay vibrato sound.

To make it a little more "okay", you need to increase sweep width and keep the speed high.  So drop the value of that 3M9 resistor to around 3M or 3M3.

[ItsGiusto]

Put a switch between r16 and - input Ua1
Here

https://www.electrosmash.com/images/tech/phase-90/mxr-phase-90-script-logo-schematic-parts.png.

Or a 47k series r16 and a 100k pot wired as a variable resistor to the - input.

Already done - unfortunately, the vibrato sound from the phaser isn't amazingly pronounced, but like I said, I like to cram in the mods, so I put it a switch to take r16 out of the circuit. It's an okay vibrato sound.

To make it a little more "okay", you need to increase sweep width and keep the speed high.  So drop the value of that 3M9 resistor to around 3M or 3M3.

Cool, thanks, I'll try that. I already am using a dpdt for that switch so it should be easy to add in the extra resistor swap.

[ItsGiusto]

Ooh, actually maybe I can make that last drilled hole in my pedal into a sweep width knob. Change the 3m9 resistor to a 1m pot plus a resistor, or something.

[ItsGiusto]

4) Sweep range: A vibe whose dips move around between, say 1khz and 3khz, is unlikely to be thought of as a vibe, whereas a phaser whose notches move around in that space may not be ideally suited for a given tune or desired tone, but it is still identifiably a phaser.  Vibes want to swirl around where the note fundamentals are.  That's how they animate the quality of distortion when placed before some sort of clipping circuit.  They bring the note fundamentals alternately closer to and farther from the clipping threshold, giving it a livelier quality.

About this: would it sound good for a phaser to swirl around the lower region, around where the fundamentals are? Maybe I could try that out as a separate switch instead -have my switch swap out the 1M fixed resistor that biases the range of the sweep. If I wanted to move it down, would I want to lower the resistor value or increase it?

[ItsGiusto]

Sorry, one more question about this!

According to this, the depth of the sweep isn't controlled by the 3M9 resistor, it's controlled by that 1M resistor you mentioned before:
http://tagboardeffects.blogspot.com/2014/05/mxr-phase-90-with-script-switch.html
They added a depth knob by only using a 0.5M resistor, and having a 0.5M pot go the rest of the way.
You said, however, that the 1M resistor coming off the trimpot wiper actually controls the bias of the sweep, not the width.

So which one of these is correct? Or are you just using different terminology than they are?

[Mark Hammer]

When I make a P90 clone, I always include an "Offset" pot as a panel control, to adjust where in the spectrum the notches are going to be situated.  Phasers can sound great when the notches are down low or up high.  One is gurgly and the other is swirly.  The stock P90 picked some make-everybody-happy values for offset, sweep width, and feedback, fixed them and only left rate controllable, as was the custom for many pedals of that era.

That doesn't mean they can't or don't sound interesting or musically valid or beautiful with offset, width and feedback set to other values.  But MXR picked something that wasn't "niche", stuck with it, and it became a signature sound.  That doesn't make it best, or worst, merely VERY familiar.

As noted earlier, the trimmer can be used to move the sweep range around, though somewhat imperfectly.  There is a good chance that the resulting sweep will not be symmetrical (e.g., cutting off at one extreme or the other).  But dickering with the trimmer will let you try out different offset values/ranges to see if you like them, without having to install or modify anything. Just twiddle the trimmer.  Once you find that higher or lower ranges appeal to you, set the trimmer to find a sweep range that is as low as you can get it and still get a decent symmetrical sweep. Once you have that, play with the value of the 1M resistance from the trimmer wiper to the FET gates. Values lower than 1M will move the sweep downwards, while values greater than 1M will move it upwards. 

A simple, and relatively foolproof mod is to install a 1M2 resistor in place of the stock 1M.  You can then use a 3-position SPDT toggle to add a 5M6 or 2M2 resistor in parallel with the 1M2, to achieve effective resistances of 1M2, 988k and 776k.  That will get you a bit higher, a bit lower, and very close to stock, with minimal fuss and little panel space.

The 1M resistor from the trimmer  sets where the sweep begins.  It limits the current from a stable bias voltage applied to the FET gates.  The FETs are being used as voltage/current-controlled resistors, each of which is in parallel with a 22k (24k on some issues) fixed resistor.  When turned "off" the FET resistance is much much higher than 22k, so we'll consider the parallel resistance of the 22k+FET to be = 22k when the bias voltage is minimal and the LFO is not adding to that.  22k and .05uf at the non-inverting pin provides a steady 90-degree phase shift at 144hz.  As the FET resistance is pushed lower, its combined parallel resistance with the 22k resistor starts to move that phase shift point upwards.  So when the FET+resistor = 18k, maximum phase shift is at 176hz.  When  FET+resistor = 15k, max phase shift is at 212hz, 318hz when it drops to 10k, and 636hz when it reaches 5k.

The 3M9 resistor sets how far the LFO will move the sweep above that minimum starts point.  So the 3M9 sets the sweep width.  Insomuch as words like "depth" and "width" can get thrown around without any further clarification, I can see how one might be confused.  The key thing to remember is where the voltage source each resistance in line with is coming from.  The 1M feeds a stable voltage, while the 3M9 is in line with a changing voltage.

[ItsGiusto]

Cool, thanks for all the advice, and the technical explanation! Though I thought that FETs were open when a voltage wasn't being applied, and that applying a voltage to the gate closes it. Maybe I'm getting it confused with a sub-type, like JFET or something?

I ended up using my extra drilled-hole to put in a 1M log pot (log was all I had) wired to be a variable resistor. The pot actually measured at a little over 1.1M, which worked out well. I connected it to a couple of resistors summing to 2.75M. That way, with the pot completely rolled off, it'd be 2.75M for the widest sweep, and with the pot dimed, it will come to 3.85M for the smallest sweep. I'll calibrate the trimpot again and try it out tomorrow.