Phase 90 substitutes and uni-vibe mod

[Ell]

Hi there,

I plan on putting together a Phase 90 using the Tonepad layout. I live in Mexico and don't have all the correct values available to me. I can't find a 15uF polarized capacitor, do you think it would be better to use a 10uF, or a 22uF? According to ElectroSmash's article, this capacitor is part of the LFO.


I'd also like to have the option of sounding more "uni-vibey". I found a video on youtube that says:
"Uni-Vibe : Connect C1 and C3 to a DPDT switch. One side will have the stock .047uf caps and the other will have .015 for C1 and .22 for C3."
However, I'm not sure which capacitors "C1" and "C3" on the original board might refer to. Does anyone have any idea which capacitors they could be?

I'm not sure if I'm allowed to link other sites, so it's a video on youtube by Frank Martin called "MXR Phase 90 mods vs Deja-Vibe : Uni-Vibe, Depth knob, Scipt, Vibrato and Unity Gain"

I'm sorry, I know my questions are terrible. If anyone knows a good place to get parts in Mexico I would appreciate the recommendation.

[Ell]

So, reading around this forum, I think I understand that a larger capacitor value (22uF) in the LFO will make the speed slower, and a smaller value (10uF) will make the speed faster. By this logic I reckon I'll try out a 22uF.

In terms of shops that ship to Mexico, I've found taydaelectronics, but I don't know any other options. If anyone else has recommendations I would still appreciate them. I have been buying everything in Steren, and they aren't that bad, but it would be good to have a wider selection of parts.

As for the "Uni-vibe" mod, I still have no idea. My current suspicion is that they might be two of the 0.05uF capacitors in the phasing stage, but I don't know which they might be.

[Mark Hammer]

1) Uni-Vibes differ from phasers in a few ways: a) they don't sweep quite as slow, b) they produce wide and shallow dips, rather than steep focussed notches, and c) they never use feedback.

2) Keeping that in mind, you would probably want 10uf, rather than 22uf, if you want to be able to dial in useful speeds for Vibe mode.  You would also want to be able to lift/remove feedback between stage 4 and stage 2 (normally either a 22k or 24k resistor).

3) The traditional recommendation for FET-based phasers is to "match" the JFETs.  This is important for achieving wide, slow, and dramatic sweeps that use lots of feedback.  If a person is only trying to achieve medium-width "wobbles", where you're not asking the FETs to run a long distance side-by-side with the others, matching isn't quite as crucial.  The width of the sweep can be reduced by making the 3M9 resistance off the LFO higher (e.g., 4M3).

4) I suppose what matters is whether you want the unit to be "best" at phasing, "best" at Vibing, or as good at one as the other.

[Ell]

1) Uni-Vibes differ from phasers in a few ways: a) they don't sweep quite as slow, b) they produce wide and shallow dips, rather than steep focussed notches, and c) they never use feedback.

2) Keeping that in mind, you would probably want 10uf, rather than 22uf, if you want to be able to dial in useful speeds for Vibe mode.  You would also want to be able to lift/remove feedback between stage 4 and stage 2 (normally either a 22k or 24k resistor).

3) The traditional recommendation for FET-based phasers is to "match" the JFETs.  This is important for achieving wide, slow, and dramatic sweeps that use lots of feedback.  If a person is only trying to achieve medium-width "wobbles", where you're not asking the FETs to run a long distance side-by-side with the others, matching isn't quite as crucial.  The width of the sweep can be reduced by making the 3M9 resistance off the LFO higher (e.g., 4M3).

4) I suppose what matters is whether you want the unit to be "best" at phasing, "best" at Vibing, or as good at one as the other.

You are a superstar! Thank you Mark! I was actually reading your recommendations in this thread: https://www.diystompboxes.com/smfforum/index.php?topic=125704.0

I havebuilt a pretty stock Phase 90 before, but I don't have it with me, it just had depth, rate LED, pause, and feedback switch.

With this in mind, and because I haven't been able to afford or build a Uni-vibe and have always wanted one, I want to go as far in the Uni-vibe direction as possible with this build.

Right, so, based on your recommendations, no feedback, 10uF capacitor in the LFO.
I have lots of JFETs with me that are marked for their values, from previous builds. I have some very close values, should I go with those, or just go wild?

The width of the sweep can be reduced by making the 3M9 resistance off the LFO higher (e.g., 4M3).

This is another value that I can't find, but I can get two 2M2 resistors, and have a switch to either go with just one, or both in series. That might let me have two choices, but is 2M2 too low on it's own? Alternatively I can get 3M3 and 1M.

I have a large enclosure available for this project, so lots of pots and switches is okay.

[Mark Hammer]

On some P90 builds I've made, I find I can reduce the value of the current-limiting resistor on the LFO output down to maybe 2M7 at the very least.  Keep in mind that the LFO current sums with the bias current set by the trimmer.  If the sum of those two is too high, the FETs "choke" (and maybe even distort).  4M3 is probably a hard value to find easily, so just stick a 390k-470k in series with the usual 3M9.

If the LFO output is modest, then the 1M resistor on the trimmer output can be varied to move the range of the sweep upwards and downwards, from low and gurgly to higher and swirly.  On my builds, I use a 510k fixed resistor in series with a 500k (really 470k) variable resistance, to replace the standard 1M fixed resistor (this was an idea originally from RG Keen).  For calibration, set the resistor+pot to max resistance (around 1M), set the speed to about the midpoint, and then adjust the trimpot until you get something nice.  Play with the 500k "range"/offset pot, and adjust the trimmer as necessary.  Limiting the current from the LFO means you have more room to play around with the bias.

[Ell]

On some P90 builds I've made, I find I can reduce the value of the current-limiting resistor on the LFO output down to maybe 2M7 at the very least.  Keep in mind that the LFO current sums with the bias current set by the trimmer.  If the sum of those two is too high, the FETs "choke" (and maybe even distort).  4M3 is probably a hard value to find easily, so just stick a 390k-470k in series with the usual 3M9.

I can't actually find 3M9, so I might have to do 3M3 with a switch for 1M.

If the LFO output is modest, then the 1M resistor on the trimmer output can be varied to move the range of the sweep upwards and downwards, from low and gurgly to higher and swirly.  On my builds, I use a 510k fixed resistor in series with a 500k (really 470k) variable resistance, to replace the standard 1M fixed resistor (this was an idea originally from RG Keen).  For calibration, set the resistor+pot to max resistance (around 1M), set the speed to about the midpoint, and then adjust the trimpot until you get something nice.  Play with the 500k "range"/offset pot, and adjust the trimmer as necessary.  Limiting the current from the LFO means you have more room to play around with the bias.

This is all excellent stuff, thank you so much Mark!

So, I've been taking notes, and this is the summary of changes so far.

LFO capacitor - change from 15uF to 10uF for slightly faster rate (and because of availability)

Switch 1 - FEEDBACK remove
With the feedback resistor I will get a stronger phasing sound (like a modern Phase 90), with it removed I will get a more subtle effect (like a script Phase 90 or a Uni-vibe)

Switch 2 - Sweep WIDTH switch
3M3 resistor in series with 1M resistor. Just 3M3 will give wide phase 90 sweeps, 4M3 will give shallower Uni-vibe sweeps.

Sweep RANGE control
Replace the 1M resistor with a 560k resistor in series with a 500k pot.
Am I right in understanding that at full resistance I will have a high swirly phaser sound, and at lowest resistance it will be a low gurgly uni-vibe sound. Is that the correct way round?

Are there any other changes that you would make to get closer to a Uni-vibe?

[Ell]

I appreciate all of your wisdom on this.

Do you have an idea which capacitors this might refer to, and if it's even necessary?
""Uni-Vibe : Connect C1 and C3 to a DPDT switch. One side will have the stock .047uf caps and the other will have .015 for C1 and .22 for C3." "

[PRR]

.... shops that ship to Mexico...

Mouser has a whole website for Mexico. I'm seeing prices in USD, but maybe there's an option?
https://www.mouser.mx/
Customer Service Office
    01 33 3612 7301
    mexico@mouser.com
    Av. Moctezuma 3515, M1
    Zapopan, JAL – CP 45050
    Mexico

DigiKey Mexico:
https://www.digikey.com.mx/en
https://www.digikey.com.mx
001-888-598-1476
spanish.support@digikey.com

These may just be order-taking offices. One is in a giant hotel (big hotels rent office space) the other does not list an address. But both have power-house fulfillment operations near major shipping lines.
https://www.youtube.com/watch?v=G3V5Oi9fdr4
(Mouser, but DigiKey is similar)

[Mark Hammer]

Am I right in understanding that at full resistance I will have a high swirly phaser sound, and at lowest resistance it will be a low gurgly uni-vibe sound. Is that the correct way round?

Don't quote me, but I think it's the reverse.  The stock P90 is low and gurgly-sounding and that uses the full 1M.

Note that an actual UNi-Vibe will switch between what they call "chorus" (dry+wet) and "vibrato" (wet only).  Vibrato is achievable on the P90 by lifting one end of the 150k resistor that mixes in dry signal.  Having a slightly faster range of speeds is helpful, because vibrato is really only noticeable when it is fast enough.

The cap values are often copied from the actual Uni-Vibe (15nf, 220nf, 470pf, 4n7).    The staggering of values - in that order - results in an overall distribution of phase shift that produces two broad and shallow dips.  Remember that it is the cumulative phase shift over stages at specific frequencies that produces notches. 

BUT the cap values to be selected with an op-amp-based phaser, should be chosen in conjunction with the resistance of the FET+parallel resistor.  So, when a P90 uses .047uf, and 22k in parallel with each FET, that means 90 degrees of phase shift will be produced in that stage for content above 154hz.  As the drain-source resistance of the FET gets lower, the point where 90 degrees of phase shift is produced gets higher and higher.

Why am I telling you this?  I think what I am suggesting is that you try the "classic" values, and play with the added 500k offset pot to move the sweep upwards or downwards as required.  The "classic" values may just work, or perhaps work with some adjustment of the bias.

[Rob Strand]

I can't find a 15uF polarized capacitor, do you think it would be better to use a 10uF, or a 22uF?

You can always put a 10uF in parallel with a 4.7uF to give you 14.7uF.

[Ell]

Mouser has a whole website for Mexico.

Thank you very much PRR, these stores will really open up my options with what I can build.

Don't quote me, but I think it's the reverse.  The stock P90 is low and gurgly-sounding and that uses the full 1M.

Note that an actual UNi-Vibe will switch between what they call "chorus" (dry+wet) and "vibrato" (wet only).  Vibrato is achievable on the P90 by lifting one end of the 150k resistor that mixes in dry signal.  Having a slightly faster range of speeds is helpful, because vibrato is really only noticeable when it is fast enough.

Hmm, okay, I will probably skip the feedback switch (just not have it), and do a switch for vibrato mode instead, that's a good idea.

Why am I telling you this?  I think what I am suggesting is that you try the "classic" values, and play with the added 500k offset pot to move the sweep upwards or downwards as required.  The "classic" values may just work, or perhaps work with some adjustment of the bias.

Yeah, you are right. I reckon the sound will already be pretty cool, without playing with capacitors.
What I'm after is sort of an interesting, but subtle sound for my own playing that I can maybe leave on all the time. So I don't want to exact clone, or to copy Hendrix or anything like that. So this I think the current changes will be perfect for my needs. Thank you

I listened to an interview of yours Mark. I appreciate your help greatly. I used to study Psychology, so it's interesting to hear that that's your field, and about the crossover.

[Ell]

I can't find a 15uF polarized capacitor, do you think it would be better to use a 10uF, or a 22uF?

You can always put a 10uF in parallel with a 4.7uF to give you 14.7uF.

Cheers Rob! That's helpful to know. I'll try out the circuit with 10uF, I could try crocodile clipping on 4.7uF when it's done and see if I'm missing much by not having the exact value.

[Ell]

I put it together, but I'm having trouble getting the effect working. I've simplified the circuit back to the basics while I try to troubleshoot this, but I still have some substitutions. I have a 300k trimpot instead of 250K. I also have tried both 10uF and 20uF instead of 15uF, but neither made any difference. I have combined a 3M3 resistor with a 470K to try to get closer to 3M9.

I think I am getting the effected sound, because if I pull out jfets I can a different out of phase sound for each transistor I pull out, but there is no sweep. Has anyone got any ideas what the problem could be? I will try to attach some photos.

I am using a rechargable battery for these readings

IC1
1 4.31
2 4.30
3 2.97
4 0.01
5 3.22
6 3.8 - 5.0 (rises with time)
7 7.48 - 1.36 (suddenly changes)
8 8.11

IC2
1 4.31
2 4.3
3 4.29
4 0.01
5 4.3
6 4.3
7 4.3
8 8.11

IC3
1 4.3
2 4.31
3 4.31
4 0.01
5 4.31
6 0.01
7 0.01
8 0.01

Q1
Gate 3.08
Source 4.31
Drain 4.31

Q2
Gate 3.07
Source 4.31
Drain 4.31

Q3
Gate 2.98
Source 4.3
Drain 4.3

Q4
Gate 2.82
Source 4.3
Drain 4.3

Q5
Collector 2.52
Base 3.71
Emitter 4.29

EDIT:
Nevermind, I came back later and it was mysteriously working. I cleaned the circuit board and maybe that was the magic touch.

My new readings are as followed, just because I couldn't find any 3x Opamp Phase90 correct readings for reference.

IC1
1: 4.3
2: 4.3
3: 2.9
4: 0
5: 3.2
6: 4.3
7: 3.9 - 4.7
8: 8.0

IC2
1: 4.3
2: 4.3
3: 4.28
4: 0
5: 4.27
6: 4.29
7: 4.29
8: 8.0

IC3
1: 4.3
2: 4.29
3: 4.28
4: 0
5: 4.26
6: 4.29
7: 4.29
8: 8.08

G  1.32
S  4.28
D  4.27

G  1.31
S  4.3
D  4.3

G  1.34
S  4.3
D  4.26

G  1.32
S  4.3
D  4.26

Collector 2.49
Base 3.69
Emitter 4.29

Also, I think I figured out which capacitors change in the schematic and the Tonepad layout for the "uni-vibe" mod. I might try them out, and if I can verify that they work I will post an image.