Joyo Phaser mod

[Viral Rock]

Looking to perform a simple mod on my Joyo phaser.  Seems to go against the general consensus on the internet, but I prefer the block phase 90 sound to the script sound (I don't do subtle).  My ears tell me the Joyo phaser doesn't have a 22k feedback resistor, so I'd like to add one.  Anyone traced the board to identify where the feedback resistor would go?  I'm guessing not, since most people prefer the phase 90 sound without the feedback resistor, but I'd take an educated guess if anyone cares to offer theirs. 

[Mark Hammer]

Difficult to identify from the pics, but it will be a 22k or 24k resistor bridging the ouput of one op-amp and the inverting input of another one.

So, depending on how Joyo did it, that could be a resistor linking pins 1 or 7 (the outputs) with either pins 2 or 6,  That could be from 1 to 6 on the same chip, or 1 to 2 on a different chip.  Or it could be 7 to 2 on the same chip or 7 to 6 on a different one.

Note that there will be 22k resistors in parallel with each of the JFETs, so you may be able to more easily identify the outlier by looking for the 22k resistors paralelling the JFETs.

[Viral Rock]

Just to clarify - I'm not looking to identify the feedback resistor because I don't think it's there.  I'm looking to add one.

I think it's just a matter of identifying which op amp half is last in the phase shifting stage and linking that output to one the input of one of the earlier op amps.  Just having trouble following the traces to figure out which op amp is which.

[Mark Hammer]

I suppose it's possible that feedback was omitted, but given that they've done everything else to clone the Phase 90, I don't see why they would omit one little resistor.  Note that some issues use a 22k feedback resitor while others use 22k.  The 24k will have a less pronounced impact, so maybe that's what you are hearing.

If you can locate the relevant component, replacing it with an 18k in series with a 25k-50k pot will yield a wider range of usable resonances (and non-resonant sounds) than either the block or script versions.  Just note that at highest resonances (min feedback resistance) you will experience more hiss.

Just adjacent to the trimpot is what appears to be a 1meg resistor, as per the original.  Replace that with a 680k, and a 500k pot in series with it, or simply adding another resistor in parallel with the 1meg (2M2 or 2M7) to drop the effective resistance down to the 700k-ish range will get you the original sound plus a higher swirlier phasing range.

Finally, the range of sweep is set by a resistor that is 3M9 on one issue and 3M3 on another.  Varying the value of this resistance changes how wide or narrow the sweep is.  YMMV, but I'm satisfied with a wide/narrow toggle, or a wide/medium/narrow switch, if I have a 3-position toggle handy, to rein in the width when setting the unit for bubbly speeds.  I wouldn't go much below 2M7 for that resistance.  If your base resistance is 3M9, placing 22M in parallel drops that down to 3M3, and placing 10M in parallel drops it to 2M8.  That represents three very usable sweep widths.

[Viral Rock]

Thanks Mark.  I'll count the number of 22k/24k resistors to confirm.  Should be easy once I identify what markings I'm looking for.  I was making an educated guess that it was omitted based on how it sounds compared to a new block letter phase 90, but since I've never heard a phaser with the 24k feedback resistor I suppose it's possible it's there.  I was guessing they left it out since it seems to be such a popular mod to remove it, and fewer parts means cheaper for Joyo.  I'll check back after I confirm.

[Mark Hammer]

You might be right, but the lighting in the photo makes it hard to tell.  I know I see what I can only assume are 22k resistors across the two transistors in the upper left of the first picture.  Reasonably safe to assume they are both JFETs.

A real P90 has 5 transistors, 4 of which are JFETs and the 5th of which is for mixing the wet and dry signals at the output.  It is a reasonably good guess that the solo tranny to the right of the pot is the mixer one, especially since I see a couple of 150k resistors within reach.

That would make the pair closest to the trimmer JFETs, their nearby chip the basis of two phase-shift stages, and the chip nearest the Joyo legending the one that does the LFO and input buffer stage.  I'm pretty sure I see two 22k resistors near that pair of JFETs, but am not sure if I see others, given the lighting and photo resolution.

[LightSoundGeometry]

im thinking about buying a joyo phaser now ..they are getting nothing but good reviews everywhere i read; and for 30 dollars shipped to me it sounds like a good deal ..I might purchase one

would I be able to get the cold shot rotary sound or would you recommend a chorus for that ? or do you need a mix of both phase/chorus to get a spinning leslie ?  i wish i had 700 bucks to just buy a real leslie cab

[Mark Hammer]

The Line 6 Tonecore Roto Machine does a decent Leslie emulation.  Behringer makes a clone of one that can probably be had for $50.

[LightSoundGeometry]

headed to youtube to start watching demos now ,thanks MH !

[Viral Rock]

Still working on tracing this out.  I've confirmed there are only 4 22k resistors, and no 24k resistors.  Seems to confirm there is no feedback resistor in this circuit.  The 22k are marked with the orange dotted lines, and the 10k resistors are marked with the white dotted lines.  There appears to be one more 10k resistor than I would expect (11 as opposed to 10).  Any further thoughts on where to try adding feedback to the phase shifting portion of the circuit would be appreciated.

[Mark Hammer]

That's going to require identifying the order of the 4 phase shift stages.  That begins with identifying the first one connected to the input buffer, and then the last one before the mixing stage.

Interesting that they elected to omit the resonance, yet claim to mimic the P90.

[Viral Rock]

Going to try to find the first and last stages tonight. 

Which stage does the resonance feed back into in the current issue block phase 90?  The schematics I'm looking at are inconsistent - some show it going to the input of stage 1, but some show the input of stage 2. 

[Mark Hammer]

In general, feedback paths in phasers should travel an odd number of stages.  So in the case of a 4-stager, it travels 3 stages back.  That could be from stage 3 to 1, but is more typically from stage 4 to stage 2.

Exceptions include those instances where the feedback path itself contains an additional phase shift stage (see the orange Ross phaser), or where the feedback path straddles the mixing node back to the input.

The P90 goes from stage 4 to stage 2.

[Viral Rock]

Finally had a chance to trace this one out.  Mark was correct about all the op amps - the lower right one is the buffer and LFO, and the other 2 are the phase shifting stages.  I've marked the insertion points for adding a feedback resistor from stage 4 to stage 2 in the picture below. 

One question on the variable feedback/resonance mod Mark described (18k in series with 25k pot) - with the pot at max resistance, will the phaser sound the same as if there was no feedback loop at all?  If not, I think I'll go with a switch.  While I think I'll like the circuit with the feedback in there, I still want the option to cut it out completely and go back to stock.

[LightSoundGeometry]

I checked out a joyo but ended up buying a TC viscous vibe despite the puke vomit 70's green carpet color 

[Mark Hammer]

Finally had a chance to trace this one out.  Mark was correct about all the op amps - the lower right one is the buffer and LFO, and the other 2 are the phase shifting stages.  I've marked the insertion points for adding a feedback resistor from stage 4 to stage 2 in the picture below. 

One question on the variable feedback/resonance mod Mark described (18k in series with 25k pot) - with the pot at max resistance, will the phaser sound the same as if there was no feedback loop at all?  If not, I think I'll go with a switch.  While I think I'll like the circuit with the feedback in there, I still want the option to cut it out completely and go back to stock.

You are certainly welcome to consider a 3-way toggle that provides for high, mild, and no feedback.  An 18k, in series with 25k would give you a 43k feedback path, that would still provide for a bit of resonance, but much less than what any of the various issues provide.  I suppose it would sound different from one where there is NO feedback, but not by much.

[flatrockmobile]

Was this mod ever completed?

[flatrockmobile]

Progress report...strapped a 20k resister covered with heat shrink tubing to the points you marked.
Sounds much better now. There's more intensity without being overbearing. It now has more of a wow-wow-wow sound.
I can actually hear it working at lower speeds.
I guess the word to describe the sound now is "chewy".
Thanks for this.

[Jayadams]

Hey sorry I know this thread is old but there's not a whole lot of info about the joyo phasers.  I was just wondering if anyone who had traced it out could tell me which of the op amps is stages 1/2 and which is 3/4?   I'm trying to switch out some of the caps for the "Univibe" mod

[RickL]

After buying another one of these I dug this thread up and started doing some of the suggested mods, one at a time. I've added a three position resonance switch, a dry lift switch for vibrato and I strapped a 330R resistor across the resistor in line with the speed pot to get the maximum possible speed out of it.

My next try was Marks suggested range mod to drop the resistor going from the speed pot to the FETs. The relevant resistor on this board appears to be the 4.3M (instead of the 3.9M or 3.3M in a Phase 90) located just to the upper left of the speed pot in the picture. I strapped a 10M resistor in series with a 10M pot across this resistor to see what two values I would like to add to the stock resistor, but couldn't hear any difference in the sound from 20M down to 10M across the 4.3M (3.5M to 3M). What am I doing wrong? Do I need an equivalent resistance closer to 4.3M? And frankly, I'd like to increase the range, not decrease it. Should I be going larger on this resistor instead of smaller?

Edit: I've also added a ramp up/ramp down switch for the LFO by using a SPDT centre-off switch to add a diode either direction across the speed pot. It's most noticeable at medium speeds but sounds a little like a square wave on faster speeds.