What Does the Foxx Phase?

[nickbungus]

Without the scientific electronics knowledge I was wondering the same thing too. I'll crack on with my definitive versions and play them side by side.  Shame I soldered in the extra components. 

I think I'm going to label the 100k pot 'Rate' going forward.  It means the same but at least I won't have two 'Speed' controls.

[anotherjim]

I can lose sleep over what name to put on a knob. On the usual stomp box, I would probably put 2 related controls side by side, put "LFO" centred over them and "Range" and "Rate" under each. Rate could be speed but I use rub-down lettering. Applying the lettering is tedious so I want little words! If I was running out of "R" on the sheet I'd use "Speed" instead, unless I was running out of "e".

I made a delay with an LFO that would change between fast & slow rate, rotary speaker fashion, over variable time. Simulating the fact that the mass of a rotor cannot change between speeds instantly. Took me ages to decide what to call it. I settled on "Inertia".

[nickbungus]

I'm going to add a few more knobs: tempo, pace, velocity, rapidness  anti-slowness, fastness, and quickness.

[armdnrdy]

I found something interesting today.

A decent gut shot of the reissue.

There is a Blue, Grey, Green, Gold (6.8M) resistor, with a 1M resistor next to it.

Since there are no other 6.8M resistors in this circuit....I would imagine that these are R34 and R38.

So...the reissue uses the same values for the gain of IC3. (LFO signal gain)



Also....I was comparing the 1482 drawing and my drawing....

Did you use a 270K for R1 as in the 1482 drawing?
I've verified R1 as 100K on the reissue board as well as the older model 7 boards.
A 100K for R1 will give you gain of 1, the 270K will give you about a 1/3 of that.
That may have been the cause of your volume drop.
I recall you addressing that issue by raising the value of R49
The differences that I see between the 1482 and the model 7 all appear to be improvements or fixes.

[nickbungus]

Actually Larry,  because it was early in the build, before I'd got myself in a soldering frenzy,  I actually socketed this.  I've currently got a 270k in but, I believe I did try the 100k,

Because I tend to do this stuff late at night with minimal volume,  I can't really report any difference but I don't believe it affected the tone at all, which is what I'm listening for.

I'm away on business until Friday, but would happily do a side by side test when I get back.

[nickbungus]

I'm on a build, using Larrys spec.  Although I'm pretty sure I bought loads of them, I can't find a 180pf cap for C20.  I've got a 150pf but not 180pf.  Will this would make much difference?  I could put 150pf in parallel with a 47pf for 197pf.  Would this be better?

I usually measure all components before soldering and I have noticed the tolerance on caps is a joke.

[armdnrdy]

You can use the 150pf.

This is a filter to cut some of the harsh highs.

[nickbungus]

Thanks Larry, excuse my ignorance.  What's the ramifications of using a higher or a lower value?

[armdnrdy]

If you raise the value of C20, the cut off frequency will be lowered.

Less highs will pass through the filter.

[nickbungus]

This isn't a question or anything, just a note if anyone is interested (I doubt it) and for me.

For what I need and want, I don't need the treadle aspect of the pedal but if you do, here is how to wire the dpdt switch to toggle between auto-phasing and manual. (Going off the original schematic and not Larrys much improved one).

2 | 1
5 | To Treadle Pot (100K)
3 | 4

So if, like me, you don't want to use the treadle and just have a 100K pot, place a jumper between 2 and 5 and then wire 1 to the 100K pot, grounding the other 2 pins on the pot.

[nickbungus]

Thanks to Larry (armdnrdy) and a big congratulations for the brilliant tracing he has done on the fOXX Studio Model 7.

I've just built it using my board but his specification and it sounds great.  I'll post a video soon.

I had a few issues which for the first time ever on a fail, I managed to debug.  Larry also helped considerably with that.

Thanks again!

[nickbungus]

Going of Larrys schematic:



Do I really need C2?  Larry traced C25 as an offboard component, I presume because C2 wasn't cutting it.  On my current board I've added it along side C2.

It would free a bit of space up on my board if I could bin C2 and then lay flat C25.

[anotherjim]

I like to distribute capacitance rather than have just one big one, so maybe C2 could be close to the LFO chip supply pins. I don't think this particular LFO is a "clicky thumper", but it can't harm.

C25 at 220uF isn't particularly large value for it's duty, but if space is tight you can...
Hunt for smaller caps. A 16V rated one would be smaller that a 25V rated one. Maybe at most 6mm dia 11mm high.
Radial lead or axial  could offer different space usage options. Hight versus diameter?
Have 4x 47uF distributed around the board (and lose C2).

Absolute panic solution.. a surface mount Tanatalum cap on the solder side of the board.

[nickbungus]

Thanks Jim.

What is the purpose of these caps?  I'm sure this is week 1 stuff at pedal building school.  My guess is they smooth the DC (which probably means to level out any fluctuations).

[armdnrdy]

The board layout that I drew shows the placement of C2 and C25.

C25 (220µf) is the incoming power filter cap.

C2 is the local op amp decoupling capacitor.

C2 handles IC1 and IC2. (the signal op amps)

I would say that it is needed. While IC3 is oscillating up and down, C2 keeps a power reserve for IC1 and 2. If you delete C2...there might be issues with ticking in the signal section.

[nickbungus]

Thanks Larry

On your schematic they are in parallel, going between V+ and ground.  So are we saying they do 2 different jobs?  From what I have read, wouldn't they just work together to give 230uf capacitance? 

"The board layout that I drew shows the placement of C2 and C25." (Note that the text editor tools and icons don't work with MS Edge/Windows 10).

Are you saying Larry that schematics don't always show the desired position of components even when it matters?

Remember I am a novice trying to understand a little so please go easy on me!

[anotherjim]

They do 2 jobs, smoothing out any noise and ripples in the supply voltage is one. That resistor R53 is making a low pass filter with C25 to do that, but there's another effect and that's helping to stop other parts of your circuit that share the same power supply from interfering with each other via that common power supply.

All paths in a circuit have some resistance - be it in fractions of an ohm. When something switches on and pulls current there is going to be a voltage drop across the path proportional to the resistance. Other parts of the circuit downstream will "see" that volt drop and it could be audible. Capacitors across the power rail are storing electrical charge. If placed close to parts of the circuit that draw significant current, they directly supply that current without the resistance of the upstream supply path causing a volt drop. In IC based circuits, the chips are often the only things that draw current, so the chips power pins are the place to situate the cap.


Where all the circuit is only audio and especially one using op-amps, the supply input cap is all you need (and you will probably get away without one at all - it just isn't best practice not to) and Op-amps have very good immunity to power supply noise. With an LFO present, there is often an element that switches on or off every time the sweep changes direction. That sudden switching, if passed via the power rail to the audio side, will be heard as a ticking at the LFO rate. Very annoying. It needs a "local" supply cap.

I would raise the component count with a 100nF ceramic cap across each of those electrolytic caps. Electro's are good, but at high frequencies start to slow down  due to the way they're made. Ceramic are cheap and nasty but are fast. So I think of the electro as catching the hum and audible noise and the ceramic stopping the radio noise.

[armdnrdy]

Are you saying Larry that schematics don't always show the desired position of components even when it matters?

I am saying exactly that.

There are many drawings that show IC power pins connected "directly" to the output of a voltage regulator...when in reality...they are connected to the "power rail" on the other side of the board.

Same holds true with voltage divider resistors for voltage reference.
The resistors will be depicted on the schematics as being directly connected to the voltage regulator output....when once again...they might be on the far side of the board.

Schematics are the drawings of the circuit...a board layout can be something completely different.
When you are dealing with LFOs and clocks....placement of components and traces make a difference between a nice "quiet" effect and complete failure.
I learned this from mistakes in my early board layouts. 

[nickbungus]

Thanks Guys.  I always enjoy the lessons.  Slowly it is getting in. 

Good news then, with the way I have got it on both my 1482 and Studio Model 7 boards I have no clicking/noise!

[nickbungus]

Probably one for Jim as he coined the term when he had my v1 but all suggestions are welcome.

When Jim had my v1, he said that with the Feedback pot at minimum (or maximum - depending on which way it was wired), you could get a really 'swampy' sound.

I've built 4 fOXX phasers now, 1 with the original schematic (Sears 1482), 1 with mostly the original schematic and then 2 with the components that Larry brilliantly traced for the Studio Model 7,

The Studio Model 7 has a much better sweep and a clearer sound but I can't get them to get that swampy sound that I could get from the other 2.  Listening to Queen bootlegs from the 70s, the swampy sound is what makes it.

Is there any way I can mod either design to increase the swampyness? 

I've got a new board/Layout that I am currently building that accommodates both the Sears 1482 and the Studio Model 7 plus all the suggested enhancements from you guys.