What Does the Foxx Phase?

[nickbungus]

Hi Guys

This is my first attempt at anything like this so please go easy on me, but after a few months making a few distortions, treble boosters and the Causality phaser, I decided to try my hand at creating a layout.  Being a super massive Queen / Brian May fan, I've always loved his 70s tone and especially the Foxx Phaser.  Given the number of BM tone sims, Deacy Amps etc available I was shocked that I couldn't find a layout for the Foxx.  The schematic is available on DIYStompboxes but no PCB, Vero or perf layout.

So after many hours of grafting and so many failed attempts, I believe I have something.  Please note, I haven't made it myself as I do have a host of questions I hope some-one can help me with.

The Schematic...


IT IS UNVERIFIED!!




And heres the BOM
http://dirtmonkey.org/foxx/FoxBom.csv

[nickbungus]

So my questions are...

1.  I can't get the original transistors so I ordered 100x J112 for Fet matching, will they work.  I have based this guess on the following:


2.  I'm not sure on what diodes to use.  I've seen a few phasers (Mr Multi and MXR100 I think), have a zener in them.  I can't see one on the Foxx schematic so I was going to socket these and try 1n4148s for all except D1 which I was going to place a 1N4001. 

3.  Not being an electronics engineer with 0 training, I got a bit confused in the schematics with the 9v, 6v and 1.8v nets.  Can anyone confirm if I got these right in my layout.

4. Lastly, the Opamps I went for were the ones recommended by Digi2it, he recommended LM324's as used in a layout for the Korg Mr Multi you can find on this forum.  I've gone for this for all three opamps, is this right?

I will be etching this week so I'll keep this post posted.

[Mark Hammer]

Took me a bit, but I finally got the pun.  Nicely played! 

[Digital Larry]

The 1.8V bias at the op-amp at the upper left is going to make the signal distort on one side pretty abruptly.  I'm not familiar with this device - is that part of the sound?

[nickbungus]

Thanks Larry

There is a 1.8v bias going into pin 12 on Opamp 2 and Opamp 3, as well as 3, and 10 on Opamp3.  Oh and 10 on Opamp 2!

Heres a Youtube clip of a guy playing one with a clean tone:

.

The Schematic was posted by Greg Covington of Covington Effects http://www.covingtonguitareffects.net/Products.html.  I know he was making and selling a variation of these without the treadle pedal a few years back, so I'm hoping the schematic is correct.

[anotherjim]

LM324 can work all the way to ground, so 1.8V bias gives 3.6v p-p signal before clipping. Should be enough?

[antonis]

Propably "m" for capacitors refer to "μ" (or "u" for those who haven't Greek keyboard)...

[nickbungus]

Thanks Antonis, I got the mfd for microfarads in my BOM.

AnotherJim, are you saying the LM324's should be fine here?

[digi2t]

Propably "m" for capacitors refer to "μ" (or "u" for those who haven't Greek keyboard)...

My Dad's got one. He get's his mu on. 

What?

[Mark Hammer]

Propably "m" for capacitors refer to "μ" (or "u" for those who haven't Greek keyboard)...

My Dad's got one. He get's his mu on. 

What?

Given your own age, I'm assuming that you either have one VERY hip dad (should he own a Foxx Phaser), or you mean he has a Greek keyboard. 

[GGBB]

µ = Alt-0181 for Windows users (although it doesn't work in all software - Inkscape being one program I know of).

[anotherjim]

It would make a lot of sense for the amps to be LM324's given that low bias voltage.

It's an odd looking Schmitt/Integrator that LFO - the diodes and the centre tap on the bank of switch-able timing caps. Looks more complex than needs to be - unless there's something special about the waveform? Would love to see a 'scope trace.

[nickbungus]

Anotherjim, I don't know what you just said but you are my hero.  Do you think the J112's will be ok if I match them, instead of the original PN4302's?  If I get one working I would happily send it down to Wales so you can do a scope trace!

[nickbungus]

I've modified the layout - fewer jumpers and also smaller.  I've binned the jumpers I had on the signal path and now I'm just using them for the power lines.

[Tony Forestiere]

I've binned the jumpers I had on the signal path...

Even the nice ones? 

[anotherjim]

It looks like the J122 should do it. You have a trimmer R39 that should help. Matching them would give the best chance of a good phase effect.

Socket the IC's!
Thing with the LM324 is, it was designed to work all the way to ground, which is good for many non-audio path jobs. Most op-amp outputs swing within about a volt of either supply, but the 324 can swing almost exactly to ground, however, it pays for this by not being able to swing very close to the + supply. Expect within about 1.5v of the + supply voltage. This doesn't mean that it can't be used in the audio path - it can and often is, but the asymmetric output swing has to be considered. The phase shift amps have 6volts bias, so this is close to clipping at the positive swing.

The first amp the signal gets to actually reduces the guitar signal by almost a 1/3rd (100k/270k), so this design doesn't want a big signal at all (9volt JFET phasers are notorious for distortion with strong signals), so I'd expect the LM324 would still be ok for headroom except with a run down battery, and you'll probably use an external 9volt feed anyway.

"If I get one working I would happily send it down to Wales so you can do a scope trace!"
Sure, if it has a passport 

[anotherjim]

Another reason to conclude the intended amps for it are LM324. This chip is subject to crossover distortion. Biasing the signal off-centre is a cure  - hence the 1.8v or 6v biasing level with a 9v supply instead of the usual 4.5v.

[vigilante397]

I've binned the jumpers I had on the signal path...

Even the nice ones? 

And you've made me laugh in the middle of my math class. I just had to explain to those around me that triple integrals aren't actually that funny

[nickbungus]

AnotherJim, thank you again!  I've made my FET testing circuit and I'm ploughing through 100 J112s! 

One last question if possible?  The Fet biasing trimmer is not rated, any ideas as to what I should try?  Also, the diodes aren't rated in the schematic either, do you think I can get away with 1n4148s?

[nickbungus]

Sorry Jim, that's 2 questions.