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UPDATE: I did manage to get the attached circuit (essentially a 2-stage P90 with the passive P45's output mixer) working on a breadboard. However, it's not *there* yet.

A few observations:

1. The circuit started working after I removed C4. This cap seems to exist in schematics for both the Smoothie (C7) and NomNom (C7), but was screwing things up for me. This cap also appears to be omitted from the DOD 201 version of the P45 circuit.

2. C6 changes the LFO behavior: lower values make the max rate faster, larger values make it slower. I gather that this is because larger values have a bigger "tank" and take longer to "fill up" or "drain" and turn the opamp output into a triangle wave, if I understand correctly how it works (source).

3. The RATE pot (500kC) effects are only perceptible in the last 1/3 of the rotation or so. It's as if the first 2/3 of the turn is so slow as to be imperceptible. I tried a 100kC pot and lower/higher values for R19, which tweaked the response a little but didn't seem to fix this problem. This is weird, since you usually see this problem with linear/audio tapers, not reverse-log. 

4. The MODE switch is the "vibe mod," which cancels out the dry signal to make a faux vibrato effect. While the summed effect is a very characteristic phaser sound — and also fairly strong — the wet signal alone is not very perceptible... basically only for low notes at the higher LFO rates.

5. The whole thing seems quieter than my P45, which is surprising because there are fewer (and smaller) resistor values in the signal path.

Any guidance on where to go from here is welcome!

UPDATE: So last night, I breadboarded up the proposed circuit (essentially a 2-stage Phase 90, but with a 10uF cap in the LFO and a passive Phase 45 output mixer) — and nothing. The "wet" path only does a little tone shaping regardless of where I set the trimmer (tried both 200k and 500k pots, and removed the dry path from the circuit). I know the JFETs are matched because they're leftover from my previous by-the-numbers Smoothie project (tried 3 different matched pairs).

Any ideas?

[The 45 has a 10k R in the input buffer opamp loop.]

I'm trying to learn more about how phase/vibe type effects work (in particular), so my current project is to hack together a "minimal parts count" 2-stage phaser for a 1590a. (I built a MBP Smoothie about a year ago but would like something a tad grittier, hence this project.) Anyway, as a starting point, I've been studying the MXR and DOD circuits, and extrapolated this Phase 90 analysis by Electrosmash to the Phase 45, as well, so I can see the "diff" in these two similar circuits.

I'm attaching screenshots of these "diffs" — blue highlights are minor value changes, green are things added from the P90 to make a P45, and red are things removed from the P90 to make a P45. The main differences seem to be (aside from the obvious 4 vs. 2 stages)...

The 45 has a 10k R in the input buffer opamp loop.
What does this do? (And on 2nd thought, perhaps the other 22k R to REF voltage should really be part of the output mixer section?)

The 45 uses a 4.7v Zener as opposed to a 5.1v.
Does this contribute to the "smoothness" by running things at a slightly lower voltage?

The 45 LFO adds a 150k resistor to ground.
Does this further half the LFO voltage to 2.4v or somesuch, and temper the LFO?

Several LFO cap/resistor values are different.
What do these changes do?

The 45 adds an G/S feedback circuit to the JFETs, with a cap and resistor.
I gather from this post by RG Keen that this loop cuts down on distortion added by the JFETs. Is this correct? If so, how does that work?

The 90 has a PNP buffer in the output mixer
What purpose does this serve? Is there something particular "lost" in the 45 by its absence, or is this responsible for some of the "grit" in the 90 circuit compared to the 45?

More generally, is there a reason these extra components exist in the Phase 45 circuit other than to "smooth it out" relative to the 90? In other words, could you get away mixing-and-matching the "simplest" of these sections, which would essentially be a 2-stage Phase 90 without the PNP buffer in the output mixer?

Bonus question: if one were to experiment with a sine LFO (or some other waveform), how would one do that while keeping the parts count low.

Thanks!

A while back I posted a few pics here of the first few pedals I designed myself, in order to learn the ropes of breadboarding, modding, EAGLE layouts, etc.

I just posted a more detailed writeup on my blog in case you're interested: Designing the JANUS Overdrive

This is mostly a data science blog, but I've been posting more about DIY guitar electronics projects lately (such as transistor clustering for phaser matching and rehabbing an old amp).

Thanks and enjoy!

Thanks everyone!!

Since — nearly a month ago — chordball asked for soundclips, I got around to making a video tonight for the JANUS overdrive and posted it to YouTube.

I look forward to learning more and making more designs — in particular for the 1590a. I find it to be a neat and intuitive form factor. So far I've been working on a tremolo, chorus, booster, and envelope filter...

Thanks, Jon. This is all very helpful.

Thanks! I'll see if I can make any vids and/or audio clips this weekend. That's an obvious omission...

The ringer is more of a weirdo toy, but the two dirt boxes sound great and I've been using them with my band. Our bass player even commissioned me to build him a JANUS.

[JANUS overdrive]

After a few kits and builds from others' PCBs, I decided to start learning EAGLE and hacking together my own projects a few months back. This forum has been a great resource for learning my way around, and special thanks to jkokura, m-Kresol, and markeprice and others for answering my EAGLE and OSHPark questions!!

Last weekend I finally finished my first couple of builds on this journey. They're all are simple circuits based on DIY classics plus some extra mods, designed for 1590a boxes.

Images below link to my project docs, which I'm hosting on Github. EAGLE files are included, if you want to fab your own for personal use. I also have some extra boards from my OSH Park order, in case anyone's interested in a buy/trade!

JANUS overdrive
A mashup of the Dan Armstrong Blue Clipper, MXR Dist+, and DOD 250 with switchable clipping diodes.



GAIUS ringer
Dan Armstrong Green Ringer with a "NULL" pot that varies the intensity of the effect.



CYCLOPS fuzz
This is a snarly one-knob beast. Based on the runoffgroove 22/7 (CMOS attempt at the EHX Big Muff), but with the tone stack stripped out for a flat frequency response, and the sustain pot brought inside as a trimmer. Just a volume knob!

Follow-up.

So here's another very simple PT2399 delay, the Dirty Punk Repeater, which I drew based on the perf/PCB layout here. Schematic attached, again meant to be "parallel" to the previous two.

If I understand correctly, the input goes straight into LPF2 (inverted) and then split into (1) the Q1 output buffer and (2) LPF1, where it gets delayed out of pin 12 and looped back to (1) the Q1 buffer and (2) LPF1. Super simple.

My questions about this relative to the previous two are:

• Is each repeat out of phase with the previous one? If so, is that (probably) on purpose?
• Compared to CD and Rebote, there are very few res/caps on the analog side of the PT2399. What are these doing?

OK, I think I learned a couple things here.

THING ONE: I guess I don't understand how buffers work. I haven't been able to google out super-useful information, but this page suggests that if the output comes from the emitter of a BJT transistor, the signal is not inverted; if it comes from the collector (aka "transistor switch"), the signal is inverted. I assume that FETs have the same properties, such that G/S/D correspond to B/E/C?

If so, from the CaveDweller design, it makes sense how we're using both buffer signals. (1) The inverted (drain) signal is the "dry" one, and gets inverted by LPF2 (pins 13-14 in the original CaveDweller schematic but not my redraw) and mixed with the delays via the MIX pot for output. (2) The non-inverted (source) signal gets fed into LPF1 (16) which is delayed, inverted, spit out of OP2 (pin 12), then mixed with the already-inverted dry signal into LPF2... but also fed back into the LP1 (16) via the FBK pot for more repeats.

THING TWO: I made a mistake in my redraw of the CaveDweller schematic. I assumed LPF1 and LPF2 were interchangeable opamps based just on the PT data sheet, so I swapped them in the drawing to look more parallel with the Rebote schematic. However... that ValveWizard doc you pointed to suggests that the input of LPF1 (pin 16) gets inverted for output (15) **AND** is what goes into the delay circuit and output on OP2 (12)

If that's the case, then Rebote is a little harder to follow, but what I think is happening is that only the inverted (drain) signal ever gets used, both for "dry" and repeats. The "dry" signal gets fed into LPF1 (16) and deinverted for output (15)... while its delayed signal comes out of OP2 already deinverted (12), so it has to be dampened and re-inverted by OP1 (13-14), and finally merged with the "dry" signal back into LPF1, etc. (The rat's nest is confusing to me, and while I suspect the polarized caps have something to do with keeping the inverted/deinverted signals going in the right direction, I'm still confused.)

Anway... is this any closer?

For anyone following along at home, the docs Jon mentioned are here:

• Hamlet Delay build doc
• ValveWizard's PT2399 notes
• DIYStompbox thread on PT2399

PS: Hey Jon, I see that you've been a fawmer since 2013! I'm Burr, the guy who started FAWM. Pedal-building is a recent development... thanks so much for the help!

PPS: I actually investigated your Hamlet circuit a little bit over the weekend as well, but after scratching my head too many times decided to focus on Rebote & CaveDweller since they are simpler.

After about 15 builds from kits and PCBs, I'm branching out into hacking/modding/designing my own circuits, and learning more about the basic building blocks. AMZ and ElectroSmash have been good resources, and I've also appreciated the circuit analysis in some MBP projects (like the Slow Loris).

Anyway, right now I'm trying to understand simple PT2399 delays. I started by redrawing two low-parts-count delay schematics to have similar layouts: MBP's 2015 Cave Dweller and Tonepad's Rebote v1 (which appears to just be the manufacturer's example circuit, plus the voltage regular and input buffer?). My plan was to compare and learn from the "diff" between these two schematics.

However, I'm kinda lost.

My drawings of the Cave Dweller and Rebote schematics are attached. Can you fine folks help dissect these circuits for a relative n00b like myself?

Specific questions:

• I'm guessing the JFETs are input buffers to retain tone. However, neither of them look like Jack Orman's example JFET buffer here. How do these two buffers work?
• My understanding is that the PT2399 is notorious for distorting repeats at long delay times (I build a Zero Point Micro II and its delays are too crunchy for my taste). So you have to do a lot of filtering. Where are the filters in these circuits? Is the LED on pin 7 in the Cave Dweller meant to help with the distortion?
• Most everything coming off pins 9-16 on the PT2399 looks wildly different between the two circuits to me. So are they using the LPFs and OAs on the chip differently?
• How does the mix pot in the Cave Dweller work? Is there a corresponding (fixed) portion of the Rebote?

I'd appreciate any help understanding these things. Thanks!

Thanks, Torgoslayer — I knew it would function the same, but wondered if there were any particular risks to shorting out, etc. bending the pins might help...

Do they exist?

I can't seem to find this hardware combination at any of the usual sources (smallbear, BLMS, PPP, mammoth, mouser).

Alternatively, I do have some knurled 16mm PCB MOUNT (right-angle) pots that are log-tapered. Any advice on using them as a soldered pot in a one-knob 1590a build?

Cool, thanks for the input. When you say "feedback control," do you mean a toggle, or a pot that controls the amount of feedback?

As I've ruminated on this idea more, I've thought of doing that and also adding a mix knob (speed, mix, and feedback pots, plus a 2/4 stage switch). As a fan of simplicity, I worry that that's over-tweaking. But it has occurred to me.

OK, I guess I'll just try prototyping a Phase 90 with 2-stage bypass and see how that sounds. Thanks!