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PostPosted: Sun Oct 07, 2012 5:47 am 
This post is the result of discussions on another thread about problems with the Phase Royal. I built a SPICE simulation of the pedal to see if I could help out and this seems to have revealed enough about the behaviour of the Phase Royal that I thought is worth posting here in the Mods section (really this is just an excuse to post some geeky graphs).

I would stress that I haven't built the BYOC Phase Royal for real and therefore I haven't tried any of my suggestions on a physical circuit. On the other hand SPICE simulation has been used in the electronics industry for many years so I reckon my sims are probably pretty reliable. If I wasn't short of cash right now I'd buy a Phase Royal kit and try this out.

BYOC Phase Royal improvements

1. The single transistor output stage is poorly biased and can cause distortion.
2. The Vref zener diode is current starved resulting in a low and unpredictable Vref (edit - also impacts on output stage bias/distortion)
2A. Edit on 24/04/13 - If your kit was supplied with a 1N4733 zener diode it should be replaced with a 500mW or 300mW 5.1V zener diode. The 1N4733 is unsuitable for use in this application.
It is a 1W diode that is only specified for its stabilised voltage of 5.1V at 49mA. With any sensible value of current limit resistor and a 9V supply the 1N4733 will be operating in the 'knee' portion of its reverse voltage curve, resulting in a low and unstable Vref, which is why most PR kits show a low Vref of only 3V or so.
3. The Resonance feedback network actually degrades the depth of the phase cancellation notches.
4. The FET bias trimmers useful range is limited to the top 1/5th of its rotation, making it very sensitive to setting.

5. (Recent edit) Low end is a bit lacking. Solution - take C11, C12 47nF up to taste. 220nF should be more than sufficient.

1. The output transistor can be re-biased by fitting a 68K resistor between base and emitter. This output stage is powered from Vref on its emitter and a low Vref exacerbates the poor biasing. The current biasing scheme limits the output stage headroom before clipping. Some kit builders have complained of distortion.

2. A typical 500mW Zener diode (BZX79C5V1 for example) might have a Zener voltage tolerance of 5% - so between 5.355 and 4.845 volts. In the BYOC kit the Zener is fed from a 10K resistor from a nominal 9V battery. So the current flowing through the Zener is between 0.52mA and 0.39mA (depending on the actual voltage across the zener) which is really too low to produce a full and stable Zener voltage.
This type of Zener is tested at 5mA for its rated voltage.
The Zener is used to provide Vref to numerous points throughout the Phase Royal circuit - it sets the bias voltage point for all of the modulation FETs, all of the op-amps and the transistor output stage.
If Vref is low it has a critical effect on the operation of the entire circuit. Kit builders have reported Vref readings of 3.8V and 4.1V. The suggested fix is to change R37 from a 10K to a 2.2K (NOTE - if you make this change as a retro-fit on an existing Phase Royal, perhaps by simply tacking a resistor in parallel with the 10K you will have to re-adjusted the Bias trimmer).

Edit - 24/04/13 - As noted in the short mod list at the top of this post, the zener shipped by BYOC for this kit is a 1Watt 1N4733, so the situation is even worse than I at first thought. This part is only specified to reach its 5.1V zener voltage when passing 49mA, clearly not a sensible current for a guitar effects pedal that might run off a 9V battery. With a 10K current limit resistor it is at worst only passing 0.39mA and really does not have a chance of producing a stable Vref at 5.1V. A lower power (300 to 500mW for example BZX79C) zener is the right choice for this circuit and an 820 ohm limit resistor would allow such a zener to run at roughly 5mA. You might get away with 1.5K if you really want to save on battery drain.

Unfortunately low power zeners with axial wire leads suitable for use in these pedals are getting hard to find (its all SMT these days) which may explain why BYOC are shipping unsuitable 1W zeners. The zener could perhaps, with a little extra circuitry, be replaced by a low power three lead 5V regulator.

The LFO bias will need re-adjusting for any change to the zener / Vref.

3. A SPICE simulation of the Phase Royal circuit shows that the simple Resonance circuit connected between pin 2 of IC3a and pin 1 of IC4a performs poorly. It introduces an unwanted phase shift that degrades the phase effect.
The suggested fix is to rewire the input buffer IC1a so that it has an inverting input and to use the currently unused half of IC2 to build an inverting buffer for the Resonance control. The output of this Resonance buffer would then feed the inverting input on the effects input buffer. This follows the feedback scheme used on the Soaring Skillet which performs well in simulation.
An extra switch can be added that switches the Resonance feedback between the 4 stage output and the 6 stage output to give further resonance peak options. Or even, as I have just seen a question about this in the mods section - a switch to go to 2 stages as well. (NOTE - Edit 21/12/12 - the feedback resistor values shown in my following circuit diagram provide too much loop gain. My R44 should be 270K and my R43 should be 91K in parallel with a 330pF cap. My modded circuit then echoes the gain structure of the Soaring Skillet as far as the Resonance feedback is concerned - see discussions on page 2 of this thread)

If people are interested enough I can post a circuit diagram of the modified circuit, but right now all I have is the SPICE circuit, which does not include all the jacks and switches and represents all the variable controls as two fixed resistors.

4. The adjustment range for the FET Bias trimmer is cramped up near one end of its rotation and the trimmer is sensitive and requires fine adjustment. The suggested fix is to replace the 250K with a 50K and to fit a 270K fixed resistor between circuit ground and one end of the trimmer. The typical bias range for the FETs is between Vref and Vref - 0.9V so it is quite small.

The attached plots below are for 6 phase stages and the Resonance first at minimum - mod on top, as stock below, then Resonance at max - mod on top, as stock below.


Attachments:
File comment: Resonance at min.
Phase Royal compared Res at min.png
Phase Royal compared Res at min.png [ 26.59 KiB | Viewed 19017 times ]
File comment: After and before (my proposed mods) response plots first with the Resonance at max then at min.
Phase Royal compared.png
Phase Royal compared.png [ 25.3 KiB | Viewed 19021 times ]


Last edited by Tark on Wed Apr 24, 2013 12:48 pm, edited 7 times in total.
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PostPosted: Sun Oct 07, 2012 12:15 pm 
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Hey Tark, thanks for doing this!!

I for one would be very interested in some schematic diagrams to try these mods out (especially 1 and 3). I built a PR a couple years ago now but never use it, as I was underwhelmed by the phasing, and the distortion in the circuit made it sound like it was in series with an overdrive pedal. If you could post some schems, I could attempt the mods and then post pics and maybe even some audio samples for anyone else who wants to try them.

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PostPosted: Sun Oct 07, 2012 2:37 pm 
Here's a screen capture of my SPICE circuit. I hope it is useful. I'll gladly help with any questions.


Attachments:
BYOC Phase Royal circuit mods.png
BYOC Phase Royal circuit mods.png [ 77.26 KiB | Viewed 18997 times ]
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PostPosted: Sun Oct 07, 2012 3:15 pm 
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thanks! any chance you'd post a capture of the "stock" schem too? it would be easier to toggle back and forth to see the exact changes...

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PostPosted: Sun Oct 07, 2012 4:23 pm 
OK ... here is the original circuit of the Phase Royal as entered into my SPICE app. Note by the way on the modified version of the circuit that all the mods are indicated / annotated by small text labels in blue. Also note that I didn't have a convenient SPICE model for a 5.1V Zener, so for the simulation I replaced the zener and is associated components with a 5.1V battery.


Attachments:
Phase Royal original circuit.png
Phase Royal original circuit.png [ 65.24 KiB | Viewed 18977 times ]
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PostPosted: Tue Oct 09, 2012 10:19 am 
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Though I didn't try the modification of the Resonance control per the Soaring Skillet scheme, I did try just disconnecting it, as Tark suggested in another thread. Makes a very noticeable sonic impact. Results/observations here: http://byocelectronics.com/board/viewtopic.php?p=352826#p352826

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PostPosted: Tue Oct 09, 2012 12:54 pm 
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I've made the first two modifications to my Phase Royal. Observations:

1. Added a 68K resistor between the base & emitter of Q7 (ran the new resistor from the north end of R27 to the south end of R3). Result - much less clipping in the output section, which brings out deeper phasing probably because the mixed signal is not being clipped as much. I had already increased 3 of the coupling cap values to let more bass through the dry signal. With the 68k mod, the dry signal is pretty much as transparent as can be with the mix knob turned fully clockwise. Turning the mix to 50% for full phase effect results in crisper, clearer phasing than before. Awesome mod! :D :wink:

2. Tacked on a 2.7k resistor in parallel with R37 to give a 2.13k total value in that spot (I would have just replaced R37 with a 2.2k, but that resistor sits underneath the IC2 socket). Result - Vref changed from 3.8vdc to 4.4 vdc. Better, but :? Audible result - probably a little better, but hard to tell. Definitely not worse. It also made the bias much more sensitive and difficult to dial in.

Here's a pic of the 2 mods:
Attachment:
PA100286.JPG
PA100286.JPG [ 130.44 KiB | Viewed 18895 times ]


I plan to try out the bias trimmer mod next.

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PostPosted: Tue Oct 09, 2012 2:42 pm 
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Hmmm, maybe I'm going to have to try that 68K resistor on the output tranny!

Just to make it easier to correlate some of the component references to the actual BYOC Phase Royal layout locations, here are a labeled PCB and a schematic, courtesy of our own Stephen. Keep in mind that these component numbers do NOT correlate to those shown in Tark's SPICE schemos--you'll have to compare the BYOC schematic to his to figure those out.:

Image
Image

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PostPosted: Tue Oct 09, 2012 7:30 pm 
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@Morgan - I notice your C12 is a 100n, whereas mine is a 47n. I was actually shorted 47n film cap and just used a box cap but it's the same value so I don't imagine there would be a difference. Is yours tied to a mod or perhaps an older build guide?

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PostPosted: Tue Oct 09, 2012 7:36 pm 
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Tried the same output tranny bias as Morgan, hooked up the same way. In my case, this made only a slight difference. Then again, I have never noticed a distortion problem with my Phase Royal, nor have I had any problem getting a good, deep phase cycle on it. So maybe the normal variation inherent in component tolerances (what my engineer friends call "stack up") can result in these apparently significant differences between builds?? I dunno...just a theory.

Just for grins, I recorded a very simple demo clip including this modified Phase Royal and three other phasers. See which you prefer here: https://www.box.com/s/cow54154zqtlr952eav8

The clip has five repeats of the same simple chord sequence--first time through clean and then once with each phaser. Phasing depth and speed were set to be approximately the same, at least to my old ears. Which one do you like best and why? Feel free to rank your preference, 1 - 4. I'll tell ya which is what as soon as we have enough unbiased (has nothing to do with the FET :wink: ) opinions to make it interesting....

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PostPosted: Tue Oct 09, 2012 8:20 pm 
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slacker775 wrote:
@Morgan - I notice your C12 is a 100n, whereas mine is a 47n. I was actually shorted 47n film cap and just used a box cap but it's the same value so I don't imagine there would be a difference. Is yours tied to a mod or perhaps an older build guide?

It's a mod. You might (or might not) notice that when you turn your mix knob fully clockwise, the dry signal may lack some base. C1, C11, and C12 are coupling caps in the dry signal path. Raising their values can keep the lows intact. If you look at Tark's schematic, you can see he has some different values in there. I socketed them and 100nf sounded good to me.

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PostPosted: Wed Oct 10, 2012 9:19 am 
Thanks for doing the sound samples DVM.

I think
1. is a bit diffuse, a bit weaker and slightly muffled
2. is quite clean and sharp
3. has more low end
4. is reasonably clear but sounds a little dirtier to me.

Do I win a cigar? :D I hope not, I don't smoke. I think I like 3 best but mostly because it has more bass.

It would be interesting to hear from other Phase Royal builders as to whether their PR's sound like any of those samples.

As to those cap values - I was messing with the simulation and had been quite sloppy with those input and output caps. I only slapped those screen captures of the sim layouts up there quickly to the forum on request. C12 might benefit from being larger than a 47nF if you run the pedal into a lowish impedance. A 47nF has a reactance of 67.7K at 50Hz and C12 is pulled down by a 150K pop stopper. If you then run into an input with say a 200K input impedance (some pedals do go that low) then I think you would notice some low end loss.


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PostPosted: Wed Oct 10, 2012 1:44 pm 
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duhvoodooman wrote:
Tried the same output tranny bias as Morgan, hooked up the same way. In my case, this made only a slight difference. Then again, I have never noticed a distortion problem with my Phase Royal, nor have I had any problem getting a good, deep phase cycle on it. So maybe the normal variation inherent in component tolerances can result in these apparently significant differences between builds?? I dunno...just a theory.

I believe your theory to be correct. It seems like just about all DIY Phase 90 clones have had this inherent "one works great, the next one doesn't" issue going back to the beginning of the century. You can see it in build reports of the Tonepad version, and it used to show up on DIYSB for the GGG project. Some builds would have this ~ 6 dB boost and the typical 'fix' would be to change the 150k feedback resistor on the output amp to a 120k. My phase royal had this when I first built it, I swapped it for a 120k and it got rid of the boost, but it still clipped too much and the phasing wasn't very deep. I ripped it out of the enclosure and experimented on it for about a year, tweaking different parts of the circuit, measuring & replacing fets, until it was this f-ed up POS. I finally recently returned it back to dead stock, and curiously the boost was gone but the clipping remained. The 68k mod patched it up to the best it ever has sounded.

Also, I built 5 or 6 of the previous BYOC phase 90 clones and I got exactly one build that sounded like a really great vintage script Phase 90. The others were good, but lacking in depth. All of them had matched fets. I think Tark is onto something with the vref adjustments and the output stage bias fix.

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PostPosted: Wed Oct 10, 2012 1:50 pm 
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duhvoodooman wrote:
Just for grins, I recorded a very simple demo clip including this modified Phase Royal and three other phasers. See which you prefer here: https://www.box.com/s/cow54154zqtlr952eav8


1 - very clean and deep with a big mid scoop. Way too deep and vocal for my personal preferences. This is the Soaring Skillet.

2 - Not quite as deep, similar mid scoop, still too vocal for my tastes. I'll guess this is the wild card; an exceptional sounding old BYOC phaser perhaps?

3 - Very similar to #3. I'll guess this is a Phase Royal in 6 stage mode.

4 - Mid scoop is almost gone, which lessens the phasing depth, and it sounds lo-fi, somewhat subtle, and how a vintage phaser should sound IMO. 8) I'll guess that this is the Phase Royal in 4 stage mode.

I'll be shocked if I got one of these correct.... :lol:

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PostPosted: Wed Oct 10, 2012 6:28 pm 
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duhvoodooman wrote:
I'll tell ya which is what as soon as we have enough unbiased (has nothing to do with the FET :wink: ) opinions to make it interesting....

Here's a hint: Bob is totally cheating with his phaser selection! :lol:

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PostPosted: Thu Oct 11, 2012 10:09 am 
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Morgan wrote:
Here's a hint: Bob is totally cheating with his phaser selection! :lol:

Au contraire, mon ami! I stated "...I recorded a very simple demo clip including this modified Phase Royal and three other phasers." And that's exactly what I did. Four different pedals, all of which conform to this definition of "phaser" from Wikipedia:

"A phaser is an audio signal processing technique used to filter a signal by creating a series of peaks and troughs in the frequency spectrum. The position of the peaks and troughs is typically modulated so that they vary over time, creating a sweeping effect. For this purpose, phasers usually include a low-frequency oscillator."

So who else is gonna give this a shot? :twisted:

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PostPosted: Thu Oct 11, 2012 10:59 am 
Yeah - I wondered what you meant by 'cheating' seemed like a pretty straight forward comparison of 4 different phaser pedals to me.

The thing I found most interesting was the extra low end on sample 3. Looking at the Phase Royal, while the 47nF output cap will result in a bit of roll off because it is loaded by the 150K pull down (odd value for a pull down resistor). C11 47nF could also do with being increased, particularly if my proposed 68K bias mod is used. Nothing else in the PR will affect the bass response.


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PostPosted: Thu Oct 11, 2012 11:09 am 
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Tark wrote:
The thing I found most interesting was the extra low end on sample 3. Looking at the Phase Royal, while the 47nF output cap will result in a bit of roll off because it is loaded by the 150K pull down (odd value for a pull down resistor). C11 47nF could also do with being increased, particularly if my proposed 68K bias mod is used. Nothing else in the PR will affect the bass response.

AAMOF, I pulled the 47n's out of C1, 11 and 12 last night, socketed those three spots, and tried a couple of higher cap values there. I've got 150n's running in all three positions now, and it fills out the bottom end quite nicely. (BTW, the fact that I did that ought to give you a strong hint on which of those 4 phaser clips is my Phase Royal, since I recorded it before changing the caps.)

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PostPosted: Thu Oct 11, 2012 12:18 pm 
Excellent! although C1 could stay at 47nF which should be plenty big enough running into 470K. I have added another mod point about the bass response to my original post.


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PostPosted: Thu Oct 11, 2012 3:02 pm 
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ok, at DVM's prodding, :lol: here's my guesses:

1. Dunlop Rotovibe
2. MXR 45
3. Skillet
4. BYOC Royale in 6-stage mode

And my fave? probably #2, but only because it had the least phasing to my ear. I like a lighter effect most of the time, not those thick, chewy tones from #3 for instance.

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PostPosted: Wed Oct 17, 2012 10:06 am 
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Here are the 4 phasers used in the above clip, in their order of use:

#1 - Danelectro Chicken Salad. Really quite good sounding for a $29 UniVibe wannabe. It even uses the 'Vibe's "lighthouse" technique with the flashing lightbulb and LDR's. How they can do this at this price is pretty astonishing.

#2 - Soaring Skillet. The depth is set down around 9 o'clock, and it's still the lushest phase in this group, to my ear. My personal favorite phaser.

#3 - Voodoo Lab Micro Vibe. Another UniVibe LDR-based phaser. Has quite good bass response, but I also hear quite a bit of distortion in this one and the bottom end gets muddy quite easily. Listen to that final chord and you'll hear a bit of what I mean.

#4 - My Phase Royal, with the resonance control disconnected and the output tranny rebiased. On the 6-stage setting with depth at about 2 o'clock. Hearing it with the others, you can pick up a bit of that typical Phase 90 distortion (though I never really notice it when just playing through the pedal), and the loss of bass is quite noticeable vs. the Skillet or the Micro Vibe. I subsequently changed C1, C11 and C12 from the stock 47n's to 150n's, and the bottom end has filled in nicely.

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PostPosted: Sat Dec 15, 2012 1:03 pm 
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Hi people. I'm new to the board and BYOC. What a great forum! I have just completed a Wah, Flanger, ESV Fuzz, and Phase Royal this week. The Wah and ESV Fuzz are fantastic. The Flanger is great (I may try an OPA2134 upgrade). The Phase Royal in stock form is so-so. My MXR phase 90 reissue (with R28 mod) is much better. So I thought I'd try Tark's suggestions.

Working through the mods one at a time I ended up with 1, 2, 4, 5 and just the feedback disconnected rather than a full mod 3.
The PR was much better and very close to my phase 90 but still not quite as rich. So full blown mod 3 it is. Much better. With the resonance at minimum it has the stock phase 90 sound but even richer. Using the mix and/or depth one can dial back the effect to equal the phase 90. The resonance is now very effective as well although it jumps into oscillation past noon.

I did make a few changes to the mods.

For mod 3, I left the feedback source from the stage 4 output and left the 47nF cap instead of a 39nF.
For mod 4, the suggested bias configuration didn't work for me. I ended up with an 82K resistor to vref, a 100K pot in the middle, and a 68K resistor to ground.

Next I think I'll work on the resonance a bit to avoid the oscillation.

Thanks Tark for all the work and simulation and thanks everyone for a great forum.


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PostPosted: Sat Dec 15, 2012 5:51 pm 
I'm not sure why you are getting oscillation, it doesn't happen in the simulation. That 39nF value is a vestige of a previous experiment where I was trying to get the original resonance feedback circuit to do something useful. I actually have a 10uF in there now, although in practice putting an electrolytic in there is a bit awkward because there isn't much in the way of a polarising voltage across it. Maybe increasing that cap value will stop the oscillation.

Just tried changing the cap value in the simulation - going from 10uF to 39nF makes a noticeable difference, really as you would expect because any cap n the feedback is going to introduce its own phase shift. With a 39nF the positive resonance peaks are more prominent, so yes the circuit is less stable. I notice the Soaring Skillet has a 0.22uF in that position. So just try making that cap substantially bigger.

With all that phase shifting going on there is always the chance that the feedback will turn positive with enough gain for it to oscillate. You could also try reducing the gain of the feedback op-amp by reducing the 100K shown as R43 on my modified schematic.

Anyway great to hear you found the suggestions useful.


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PostPosted: Sat Dec 15, 2012 8:37 pm 
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Thanks Tark. I tried a .68uF. It changed the frequency of the oscillation but not the point in the sweep where it occurs. The effect is already very intense by that point though and I can't imagine requiring anything more than that. I'm going to try reducing the gain a bit.

Have you tried running the simulation taking the source of the feedback from the output of the fourth stage? That is what I am using.


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PostPosted: Sat Dec 15, 2012 9:15 pm 
Yes one of the things I wanted to do with the simulation was to check the effect of switching the feedback point and the number of phase stages around. Switching the feedback input and the pedal output between 2, 4 and 6 stages works just fine. You can even do weird stuff like a split stereo output where 2 stages provide a single notch for one side and 4 stages a double notch for the other side (needs its own mix with the original signal and output buffer). No idea what that would sound like. Listening to it would probably either induce instant enlightenment or a nasty case of double vision.

Simulation is so useful for tinkering around with this stuff. Way faster and more informative than building stuff for real.

Tweaking that 100K down to perhaps 47K or 33K for a bit less gain sounds like the way to go. I must get around to breadboarding up one of these with all the mods I can think of.


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