Slight distortion w/ MXR Phaser 90 build

[Joe_Shroe]

I'm trying to build an MXR Phase 90 but using J112 as the JFETs instead of the original 2N5952. All the J112's are matched, but the output is slightly distorted. It's not as noticeable when lightly strumming some chords, but picking strings with some amount of force will produce some ugly bass-y distortion. I've also tried different FET configurations according to these designs but none of them seemed to help. I noticed on the data sheets that the J112 I'm using has a forward gate current of 50mA while the original 2N5952 has only 10mA. Would it be worth it to get another set of JFETs more in line with the 2N5952, or does that spec not matter?


Here are the voltage readings if this could be a problem with another part of the circuit:

VREF  4.25v
VBIAS  0.66v - 0.81v

INPUT OP-AMP (NTE941)
IN-  4.25v
IN+  2.85v
OUT  4.25v

PHASE STAGE OP-AMP (TL061)
IN-  4.26v
IN+  4.26v
OUT  4.28v

OUTPUT SIGNAL  3.95v

[duck_arse]

Joe - please can we see the layout you built to, and the actual thing you have built? does your Vbias vary when you adjust the bias trimmer?

I'll say this but you already know it - the 2N5952 gate is on the opposite side to the J112 gate pin, so it would need to be reversed in a board layed out for the 2N5952.

[Myampgoesto12]

My phase 90 build from GGG has the 2n5952s, and LM741 opamps, and with the feedback resistor engaged in the circuit a humbucker clips the signal in an ugly lofi way. To my understanding that's the nature of these and the original opamps in this beast, at least without any passive compensation pre-circuit.

I did some digging around the web before building mine and looked for ways to eliminate or decrease the severity of this clipping, some involved better opamps with adjustable gain added, or the ideas posted here:

https://www.google.com/amp/s/duoglassix.wordpress.com/2015/04/14/mxr-phase-90-modifications/amp/

I just built it to see what it would to and settled with it. Do you have really hot pickups?

[thermionix]

A Dynacomp before a Phase 90 is a nice thing.

[PRR]

> I noticed on the data sheets that the J112 I'm using has a forward gate current of 50mA while the original 2N5952 has only 10mA.

That's the BLOW-UP rating.

In normal use the gate current is about zero.

Random sub of JFET is liable to trouble, but the J112's dynamics are a good fit for the 2N5952 specs.

[Joe_Shroe]

Joe - please can we see the layout you built to, and the actual thing you have built? does your Vbias vary when you adjust the bias trimmer?

I'll say this but you already know it - the 2N5952 gate is on the opposite side to the J112 gate pin, so it would need to be reversed in a board layed out for the 2N5952.

It's only breadboarded so I don't know if you'd want a picture of that. The Vbias does vary from 0.66v - 0.81v as noted on my voltage readings. I did make sure to match up the JFET pins correctly, and I'm not sure it would even have worked if I hooked up the wrong pins.

My phase 90 build from GGG has the 2n5952s, and LM741 opamps, and with the feedback resistor engaged in the circuit a humbucker clips the signal in an ugly lofi way. To my understanding that's the nature of these and the original opamps in this beast, at least without any passive compensation pre-circuit.

I did some digging around the web before building mine and looked for ways to eliminate or decrease the severity of this clipping, some involved better opamps with adjustable gain added, or the ideas posted here:

https://www.google.com/amp/s/duoglassix.wordpress.com/2015/04/14/mxr-phase-90-modifications/amp/

I just built it to see what it would to and settled with it. Do you have really hot pickups?

I'm not using a feedback resistor here so that's not the issue. That blog post about mods definitely seems interesting from a quick glance, and it looks like he did solve distortion issues. I'll have to go through this in detail and see if this works for me too. As for pickups I'm just using a Jazzmaster with its single coil pickups and they've never sounded particularly hot with anything else, so the issue must be my pedal build.

A Dynacomp before a Phase 90 is a nice thing.

I don't have an actual Phase 90 to compare the sound to, but from youtube demo vids I'm not hearing the same level of distortion that I'm getting. I might build a compressor later on as well but since I don't have one now I'd rather focus on the phaser. The only phase pedal I have to compare it with is the Small Stone which by comparison has very clean and smooth phasing, and even though it uses OTA's I was hoping to get that same level of cleanliness.

[Myampgoesto12]

Something is odd there. I have to dig in pretty hard to get my neck PhatCat with 2 A5s to clip my phaser..

By chance did you measure your resistors before building? May be a low chance of it, but I've had a few metal film 1% resistors be more than 1% out of spec. One could possibly be short changed.

Maybe try the sort of phase 45 mod where you shunt two of the phase stages and see if it decreases the clipping, that could reveal the location of the problem.

Considering the cost of transistors I'd say its worth getting some 2n5952s to see if it make any difference. If for nothing else they could be used later on for another build.

[duck_arse]

It's only breadboarded so I don't know if you'd want a picture of that. The Vbias does vary from 0.66v - 0.81v as noted on my voltage readings. I did make sure to match up the JFET pins correctly, and I'm not sure it would even have worked if I hooked up the wrong pins.

we always want to see. the more information you give us all, the better.

IF you pull all the fets from the breadboard, and then measure the Vbias as you adjust, you should see it move from 0V ~ 5V1 or so, not from 100 mV to 200mV. and if this is the case, it would suggest your fets we incorrectly installed on the BB, "shorting" the Vbias to ground.

[Xavier]

I built mine with matched 2N5952's and stopped using it for the same reason.....

[Myampgoesto12]

Just putting this out here, I just got a super comp and messed with placement, thermionix is totally right about placing a comp before a phase90.

I hope your build turns out well!

[thermionix]

totally right about placing a comp before a phase90.

I only do it for clean phaser sounds.  With dirt I don't bother.  I like the P90 before overdrive or distortion, or after a fuzz (pretty extreme but very fun).

For clean wah tones, put the comp right after the wah.  Funk city.

Since we're talkin' Phase 90s though, I just finished my 3rd today, this one for my buddy James.  I tried a polished 1590B from Small Bear, looks better than I expected.  GGG board, script specs.  I snapped a quick crappy pic before James took it home (the lines are a reflection of my amp's grill cloth).

[Myampgoesto12]

Looks great!! I now have my phase 90 before my dirt, but still after my comp. I'm currently leaving the comp on all the time, but that's also because of the LFO extra depth mod and slightly hotter feedback I have set up in my phaser. Without the comp the phaser gets pretty muddy into my dirt.

I'll be posting pics of my phaser build soon. It needs a little tidying up first though. I keep digging into it to try things so I see no point in making it pretty inside yet hah.