fOXX Phaser Stages

[nickbungus]

Hi All

Its been a while since I started a new topic and I could have appended it to my original fOXX Phaser topic but didnt want to muddy the water.

Anyway, I had a few fOXX's on eBay as a good friend designed me a 2 sided 1590b friendly board and I got 20 fabbed. All was good but I got a passive aggressive email from a chap who was saying the 6 staged phaser is v2, the original as used by Brian May has 10 stages, hes built clones of v1 etc.  So I asked him to share the schematic or send me a pic and low and behold, he didnt want to share.  So I put this down to internet rumour.

Anyway, this week another chap from a Brian May forum sends me through a gutshot of what looks just like a fOXX Phaser but with 4 Quad Opamps and 10 trannies.  10 Stages! 

Heres the image


It looks very similar to the 6 stage.  A sea of 10k and 100k resistors with some 10nF caps, LM324s and some fets!

So i decided to just build a motherboard for an extra 4 stages and append them to a working board.  Sounds pretty good but the phase seems to dwell at each end of the spectrum.  Its a deep phase but the sweep doesnt quite sound right.

Heres the original schematic for the 6 stage.  When building a fOXX I always change C16 and C21 from 10nF to a 100nF and swap the 2M2 to a 6M8 for R34.  This was due to Larry (Armdnrdy) reverse engineering a 70s 6 Stage version.


Now I remember my first build and the sweep was a bit dodgy until R34 was changed from the 2M2 to a higher value.  I might try changing the 6M8 back to the 2M2 for the 10 stage.

Going off the schematic above, is there anything I should be looking at to make this work better with 10 Stages??

[anotherjim]

Wait for someone more phaser savvy...
...but is it possible the extra stages are not in additional series connection (as an extension), but rather in the dry or feedback paths to create either extra density or a pre-phase anti sweep to increase depth?

[nickbungus]

Jim.  Thanks.  I bet you thought you'd were done helping this idiot.   I've got both versions set up so I'll try and do a quick recording in the morning.

Also there's a very noticeable volume drop with 10 stages.

[Mark Hammer]

I honestly don't know why anyone wants more than 8 stages for a guitar effect.  I've heard phasers with as much as 24 stages, and they sound amazing...if you use them with white noise.  Guitar has MUCH less bandwidth, so all those extra notches are really more hypothetical than actual.

That's not to say that such a pedal MUST be used with guitar.  I'm sure some folks use them with keyboards, synths, etc., in which case the extra stages can be of some use.  But when it comes to guitar there just isn't much impact of additional stages beyond 8.  I have a Boss RPH-10, which has up to 12 stages, but I'll be damned if I can hear the impact of more notches in the 10 and 12-stage mode.  It sounds different, of course, but that may be more a product of the location of the notches that are audible than a sound arising from more audible notches.

[Myampgoesto12]

Keeping an eye on this as well. I was thinking about adding another 4 stages to my phase 90, and I'm curios if I'd need to change cap/resistor/fet values to accommodate.

I was also thinking of cloning the foot phaser..

Thanks!

[nickbungus]

Hi Chuck.   Try this

https://drive.google.com/file/d/1o5g83AXMsbRjBXlRRaHMLL-STz4NTdOy/view?usp=sharing

[Myampgoesto12]

Sweeeeet. Whenever I get etching materials I'll totally do this.

So.. What is the actual advantage to additional stages? Like 6 or 8 as opposed to the 4 in a phase 90?

[nickbungus]

Jim, looking at the gutshot it does seem that the stages are all in series following round the ICs.   (!Hopefully!)

I take it bottom right what look like .4W metal film resistors are actually 3 old skool diodes.  There's 2 bottom left too.  These match the 5 in the foxx.  The three in series going to ground and the 2 clipping diodes in the LFO.

Can only see one trimmer.  That must be the one to bias the fets.  Perhaps the feedback trimmer wasnt included and just had a fixed resistor.  I'm just jazzing out loud here.

I think theres enough for me to start a trace.  I'll probably come unstuck sharp, depends on how similar it is to the schematic we have.  The best thing is, where its really hidden, I can probably guess as its just 4 of the 10 stages obscured.

[Mark Hammer]

Sweeeeet. Whenever I get etching materials I'll totally do this.

So.. What is the actual advantage to additional stages? Like 6 or 8 as opposed to the 4 in a phase 90?

Having only 2 notches results in a slightly more "focussed" effect, insomuch as one can perceive the notches and adjacent peaks as being here at this moment and there at another.  Having more notches somewhat spreads the effect out, though not all that far, compared to a flanger.  I also find that adding more stages seems to move the overall effect lower down in the spectrum, but that may just be my builds.

Once you get above 4 stages, you will also often find a cap in the feedback loop of one of the phase-shift stages.  Not that 4 is somehow "magic", but the more phase-shift stages, the more likely hiss is to accumulate and become audible and objectionable.  So the cap is included to roll off some of the thighs in the phase-shift/wet signal.

Also, note that traditional phase-shift stages are supposed to be unity gain (e.g., 10k input and feedback resistances).  But resistors being what they are, it can result in one or more stages unintentionally adding gain.  If the input is 9.90k and the feedback is 10.10k, the resistors meet a 1% tolerance, but the stage has a "gain" of 1.02x.  Multiply that by more stages (and remember that gain is multiplicative), and the risk then becomes that the cumulative gain of 6 or 8 stages can be well above unity.  Why does this matter?  Because if you provide feedback from the last stage back to the input, and you add gain along the way, you run the risk of oscillation.  You can always dial back the amount of feedback to eliminate oscillation, but reducing the feedback is not what you really wanted from the pedal, is it? 

[Myampgoesto12]

Thank you again Mark! Always helpful!

[nickbungus]

Lol.  Read this.  I contacted the guy who first told me about the 10 stage foxx seeing if I can get more info.   This is his reply.

I already know your contacting many people and offering a replacement for info. That's your business. 

I stopped building because people like you cheapen the effort and cost it takes to make an exact clone. You don't make clones, you copy schematics. You need a reference pedal to do this. 

You want to flood the market with cut price original Brian May fOXX phasers but you won't get get my help at any price. You advertise and use Brian's name. 

By doing this you are open to RohS and WEEE compliance plus more.

Yes it's true I've sold a few on eBay with the boards I've had fabricated but it's hardly a great business model and I won't be giving up my day job anytime soon.

Well I did try to get more info

[Myampgoesto12]

It does seem a bit stiff.. Especially for such a thing as this..

But that doesn't make the statement untrue. Perhaps another means of learning about the device will show itself soon.

[duck_arse]

I stopped building because people like you cheapen the effort and cost it takes to make an exact clone. You don't make clones, you copy schematics. You need a reference pedal to do this.

this is a very strange thing to say. if you have a schematic, or circuit diagram, why would you need a reference pedal? isn't a clone a copy, of a schematic? weird; people.

[anotherjim]

You don't make clones, you copy schematics.

That statement has me completely baffled on the baffle bus all the way to Baffled town.

[Mark Hammer]

Sounds to me like you have a bad case of baffle-itis.  Sadly, it cannot be cured by more cowbell.

[Myampgoesto12]

Perhaps this person means that instead of going the full lengths of finding exact spec and era parts, finding a boomerang enclosure, a nos pot, etc is what makes this simply a copy of the schematic placed into the most available enclosure, and not a true clone.

[nickbungus]

I believe the guy used to go by the name of Mr Foxx on a Brian May forum and once repaired one for another member.

I think he now believes he owns the pedal and is a bit put out. I've had some great feedback on the ones I have sold and even some fan art.  Well they've sent me videos.

[duck_arse]

I believe the guy used to go by the name of Mr Foxx on a Brian May forum and once repaired one for another member.

not Mr. Crack Foxx, from the mighty boosh?

[StephenGiles]

There was a Folk Rock band called Mr Fox around 1970:

https://en.wikipedia.org/wiki/Mr._Fox

[nickbungus]

He sent me some more abuse, he has a real issue.  I dont know what he expects me to do if its just as cheap to get 20 boards fabbed as it is 1.  Unless he expects me to have a pedal board with 20 Phasers on it!

Anyway on a more pleasant note.  This guy sent me through a quick vid of him using one of my phasers and he really nails the Brian May live 70s sound.  I've messed on with a 10 stage for the last week or so and I think I might be wasting my time as this is the sound. Maybe I'm chasing something I dont need to.