The Veil, another PT2399 delay design

[snk]

Hello,
After building a DIY PT2399 delay and studiying some PT2399 designs, I started designing a custom circuit.
- My first goal is to have a PT2399 with long delay times and a brighter sound than the usual PT2399 designs (most PT2399 use heavy lowpass filtering in the feedback path to get rid of the chip noise, leading to a very damped delay sound).
- I wanted to try to get a "vintage" sound, but with some lowcut (to remove mud) and less damping than some other designs.
- I have also included a crude limiter to avoid PT2399 clipping, as well as another limiter in the feedback path, because the "core" clipping of the chip can be quite harsh.
- I have included a KillDry switch, and I plan to wire it with Tails (instead of TrueBypass).

I haven't built it yet (but i plan to do so tomorrow), so maybe there are some mistakes and flaws are still to be expected 
This is a first sketch of the layout, and some resistors values might need to be adjusted (filtering might lower feedback, and i want to be able to get runaway delays).
If anyone has comments, hints, tips or advices, they are all welcome !

[ElectricDruid]

Do you have a schematic? I can't really see the circuit from the layout alone, although I believe some people who use them all the time are pretty good at it...

Thanks!

Tom

[snk]

Hi, Tom
Sorry, I don't have a schematic : I am not very good at converting whole schematics into veroboards and vice-versa...

[snk]

I have attached the schematics of the 4 circuits i borrowed documentation from (but i based my search on veroboards : My way of working is not very scientific , it's a mix of studying various layouts, browsing forums and quite a lot of trial and error ).

The idea is the following :
- I built the EQD Disaster Transport Jr. It sounds good, but it's too dark for my needs. It features a clever "diode limiter" in the feedback path, which i borrowed.
- I made some searches around the Hamlet Delay, which features a very good documentation about the circuit design, and helped me fine-tuning the filtering of the signal path. It also features a gree led to avoid clipping the PT2399.
- The Wampler Faux Echo / Madbean Leviathan also have quite a lot of useful tips and documentation, so it also helped me making custom mods.
- The Deprofundis Delay is a simple layout, so i used it as the core for my "own/custom" layout. I used the "long delay time" mod from it.
I added diodes in the feedback path, a green led for the PT2399, and adjusted capacitors and resistors values in the whole feedback path to (hopefully) make it brighter, and with a slight hipass (to get rid of the "mud").


- I understand a bit the way the feedback path work : the capacitor will adjust the cutoff frequency, but if you change one cap value, you'll need to change the resistor accordingly (else you may loose some volume and some feedback). I think the values I used are quite "safe" (in the "ballpark" of other PT2399 designs), but i cannot socket every single caps and resistors to try each single combination, so i would welcome any advice telling me if i might indeed get the results I am aiming for ( = brighter tone than EQD Disaster Transport -roughly aiming for a 3kHz cutoff point- and being able to reach self-oscillation).

- On the other hand, I know next to nothing about the way the input section works, and i see big differences in the components values (resistors, capacitors) between the DeProfundis and the Wampler for instance... :scratch:

- Also, I would like to build it with tails (no true bypass 3PDT), but with this layout i am not sure where i should put my in/out wires 


EQD Disaster transport schematic : https://www.pedalpcb.com/docs/Cataclysm.pdf

Deprofundis :


Madbean Leviathan schematic :

[ElectricDruid]

Woah, lot to digest here. Give me time to have a dig through it all.

You should really get into the habit of keeping an up to date schematic of your circuit, even if you do that by pasting together bits of other schematics (literally copy-and-paste in a graphic program and scribble a few lines on them). You'll find it eliminates a few of the errors in the trial-and-error part of the process. When you can see what you've got, all sorts of stuff starts to seem obvious.

HTH,
Tom

[snk]

Thank you for the advice (I think it's a wise one!)

In fact, I made some very sketches of some parts of the schematic (the feedback path, the part between the opamp output and the pt2399), but it is a total mess (and i'm using pen and paper)
I started from the principle that most PT2399 designs mainly differ from the filtering they use in the feedback path (i may be wrong, but i have read this often). So, I started to compare the similarities and differences, but it took me some time to begin to understand which component does what (and what is the downside of an attempt to improve something by changing a component's value ).

By the way, i also considered the Multiplex Jr, when comparing schematics, but avoiding the modulation part.
http://1776effects.com/wp-content/uploads/2015/03/Multiplex-Jr-BOM.pdf

[snk]

Hello,
A kind fellow made a schematic out of the layout i provided him.



- I have read that C3 (5pF on my layout, C2 on the schematic, with a 100pF value) is here to filter out some noise from the chip/circuit, but it might also "dull" the dry signal (cutting of some highs) : is that true ? As I don't plan to have it true bypass, I wouldn't want my dry signal changed. On several other PT2399 layouts, the value of this capacitor is 100pF, but i have read that a lower value is more prone to avoid filtering out the dry signal high frequency content. If i put a high value (around100pF), would it put a lowpass filter on my dry signal ? And if i reduce the value to 5pF, will it still be filtering out the circuit noise ?


- I have read a couple times that Tantalum caps should be used for decoupling. Am I correct assuming that they are the 47µF and 100µF electrolytics caps at row 1 and 5 of my layout ? Should i try tantalum caps instead ?


- Am I right assuming that i may have a cutoff point around 3kHz for the tone lowpass filter, with the cap/resistor values i picked ?


- If I was going to wire it to have tails enabled, where on the circuit should i put the switch ?

[ElectricDruid]

- I have read that C3 (5pF on my layout, C2 on the schematic, with a 100pF value) is here to filter out some noise from the chip/circuit, but it might also "dull" the dry signal (cutting of some highs) : is that true ?

Yes, it looks like it is. Sticking the values (R3/C2) in a filter calculator gives a cutoff at 4.8KHz, which is definitely audible treble loss. I'd reduce to 33pf (14.6KHz) or 22pF (21.9KHz) which should give you a much more realistic direct signal.

As I don't plan to have it true bypass, I wouldn't want my dry signal changed. On several other PT2399 layouts, the value of this capacitor is 100pF, but i have read that a lower value is more prone to avoid filtering out the dry signal high frequency content. If i put a high value (around100pF), would it put a lowpass filter on my dry signal ? And if i reduce the value to 5pF, will it still be filtering out the circuit noise?

The general rule is that you only amplify the frequencies you're interested in. So if you're designing an audio amplifier (even if it's just a stage inside a bigger pedal design) then it needs to *not* amplify DC (so add a highpass somewhere down below the lowest frequency of interest - say 30-40Hz or so?) and it needs to not amplify ultrasonics and cause high frequency instability (so add a lowpass up around the top of audio, 20KHz or so, or maybe a bit less for guitar).
100pF is attenuating the level of the top couple of octaves of audio, but 5pF lets the amplifier work out to 100KHz, which is overkill and could make the amp unstable. A median value around 22-33pF is better.

- I have read a couple times that Tantalum caps should be used for decoupling. Am I correct assuming that they are the 47µF and 100µF electrolytics caps at row 1 and 5 of my layout ? Should i try tantalum caps instead ?

I don't know about this, but I suspect (I'm pretty sure, actually) that you won't hear any difference over standard electrolytic caps. The equivalent series resistance of Tantalum caps used to be much lower than electrolytics, and this was sometimes important, but nowadays it's not as true as it once was, so it matters a lot less. If you're really concerned (and you probably don't need to be - don't try and fix problems you haven't got) then you can add a ceramic 100nF cap in parallel with the electrolytic. This provides a easy route to ground for really high frequency signals, while the electrolytic cap deals with the rest of it.

- Am I right assuming that i may have a cutoff point around 3kHz for the tone lowpass filter, with the cap/resistor values i picked ?

I'm not sure. Those look to be bits tacked onto a multiple-feedback-filter stage, so it's going to get complicated. I'd use LTspice to work out what that looks like.

- If I was going to wire it to have tails enabled, where on the circuit should i put the switch ?

Before the PT2399 delay. You want to continue to hear the echoes coming out of the delay, but you don't want any more signal going into it. Best would probably be a SPDT switch that switches the PT delay input between the input or to ground when it's not required.

BIG IMPORTANT NOTE: That schematic isn't right yet though! There's no connection to the PT delay's input, so there won't be any delay. There's quite a lot else that's weird about it too. It has a certain frankencircuit feeling, which makes sense given it's origins...

HTH,
Tom

[pinkjimiphoton]

nice concept!

makin popcorn.

a guy who's crazy enough to start with veros and keep tinkering with it til it works like i one did?

#RESPECT.

you're either genius or mad, i can't decide which yet. i hate muddy delays. tape echoes get duller, not muddier. so this looks intriguing.

popcorn made, subscrib'd, carry on!

[snk]

nice concept!

makin popcorn.

a guy who's crazy enough to start with veros and keep tinkering with it til it works like i one did?

#RESPECT.

you're either genius or mad, i can't decide which yet. i hate muddy delays. tape echoes get duller, not muddier. so this looks intriguing.

popcorn made, subscrib'd, carry on!

Ah ah !
Yes, it seems that most PT2399 "analog sounding" delays are darker than actual BBD or tape delays. Also, i have a couple analog delays with a tilt eq, and I like them when the bass are filtered out.
So i wanted to try to tune some circuits a little bit... but i definitively do not fit in the "genius" category 
Veroboard seemed to me a nice way to tweak a circuit, as i it easy to add or remove a row. A PCB is more a "closed " thing (when everything is perfectly setup and tuned), while a veroboard can be a perpetual "work i progress" imho.

[snk]

Thank you very much for your knowledge, Electric Druid, that is very helpful !

Sticking the values (R3/C2) in a filter calculator gives a cutoff at 4.8KHz, which is definitely audible treble loss. I'd reduce to 33pf (14.6KHz) or 22pF (21.9KHz) which should give you a much more realistic direct signal.

I didn't realize that the 5pF capacitor was involved into a lowpass. I will use 22pF then, so it will be full spectrum and won't "eat" hi frequencies (i want to use the delay with synths, drum machines, etc).

I'm not sure. Those look to be bits tacked onto a multiple-feedback-filter stage, so it's going to get complicated.

Yes, from what i know it is indeed a multiple-feedback-filter stage. I used this calculator : http://sim.okawa-denshi.jp/en/OPtazyuLowkeisan.htm

BIG IMPORTANT NOTE: That schematic isn't right yet though! There's no connection to the PT delay's input, so there won't be any delay. There's quite a lot else that's weird about it too.

I need to go back to my layout and check what is missing... On the schematic, isn't it that R8 is not connected to R9/R10/R11 ?

It has a certain frankencircuit feeling, which makes sense given it's origins...

Ah ah ! Yes, Frankencircuit, that's quite right indeed

[pinkjimiphoton]

hahaha, yep, still lurking, munching popcorn....

yeah ALL pt2399 circuits to me sound unrealistic.  way too little high end, but most of i think happens IN the chip. i've tried simple ones, complex ones, companded ones, and they all sound dark and sterile.

analog delays DID sound muddy, so the pt pretty much nails that digitally, but they don't sound like tape echo AT ALL. well, no more than anything else does that seems to be out there.

the old ibanez em5 circuit is about the best, but it uses an actual digital memory chip, not a 2399. if ya modulate the power supply on that circuit slightly you can actually nab  a good percentage of the actual tape echp mojo.

fresh tape is bright and kinda brassy. really old worn out tape sounds like ... a 2399. sorta.

ot for a second.. and the echoplex preamp bullshit all over the net?

i sold my others and only kept one of my 3, but every echoplex i owned did not ever "boost" the signal. ever. the "boost" they sell people on is actually the gain amplifier for the record head molested into a preamp.

the actual echoplexes are at or more usually slightly below unity gain, and they have some treble loss from not being quite unity and not being true bypassed. if they WERE true bypassed, the volume would drop when engaged. so they eat up some treble, make the overall tone "warmer" but not louder.

cranking up the record head level does two things.. makes the echos louder than the dry signal, and makes it hissier and noisier, and that's it.. on the wet signal, i mean.

so yeah, i wanna see ya nail this suckah!

ot rant mode over!

[snk]

I agree with you about the Echoplex hype, it's nothing incredible imho.
The most impressive mojo box i know is the Dynacord/Klemt Echolette : the tube preamp is great, and the tone knob is an incredible sound shaper. The echo path is also very colored, and while you are stuck with preset speeds, it is a very nice tape echo.

Not all BBD delays are created equals, by the way, and they don't all sound the same. I guess it depends on the amount of BBD chips used, and the delay lenght they allow. It also depends on the companding circuits, and if they feature a tone knob (most "tone" knob on PT2399 delays are lowpass, while most "tone" knobs on BBD delays are tilt eq).

By the way, here is a new version of the schematic, with some fixes.

[pinkjimiphoton]

I agree with you about the Echoplex hype, it's nothing incredible imho.

well, i didn't say that, i said the "echoplex boost" is bullshit

Not all BBD delays are created equals, by the way, and they don't all sound the same.

yeah, in the end, they all pretty much sound the same most of the time. some are clearer, some slightly darker, but the overall sound is fairly similar.

I guess it depends on the amount of BBD chips used, and the delay lenght they allow.

bingo

It also depends on the companding circuits, and if they feature a tone knob (most "tone" knob on PT2399 delays are lowpass, while most "tone" knobs on BBD delays are tilt eq).

well, at least in MY day, most analog delays didn't feature tone knobs at all, usually time, repeats, mix. echoplexes didn't have them, the jap standalones didn't have them, the oil can echoes didn't have them. nobody really wanted to make them DARKER.
we used to run sends to them, then bring them back thru channels on the mixer to be able to try and get some tonal variation out of them. it helped, but was only so much you could do.
some folks used to use a treble booster in front, and then cut the highs afterwards. sorta like a primitive dolby. it worked fairly well, but everything still lacked the crisp transparent sound to come with digital, which didn't have any "tone" to speak of. it did, but wasn't supposed to. then everyone missed the warm analog brown saturated silky smooth tape echo sound, and they began adding tone control to delays sometimes, often they'd just add a cap so each repeat got darker. not a great solution, but in a live environment close enough for rocknroll.

By the way, here is a new version of the schematic, with some fixes.

suhweeeeeeeet, thankee kine'ly!



[/quote]

[Kipper4]

Just an observation but the kill switch.
It might be worth changing it so that the now open connection goes to Vr.
Just to keep the dc similar.

[snk]

Hi,
I should be able to build it next week... in the meantime i am gathering last advices
As the schematic and layout look like now, should it work and behave as expected (i mean : delaying the sound, going into self-oscillation, sounding not too dark) ?

[BetterOffShred]

I prefer 2n on the 15/16 and 13/14 caps.  It darkens the sound but trims quite a bit of noise in my experiences.  Others may have better science for what I'm saying, but even on delays that call for 1n i usually go 1.8 or 2.2 

[ElectricDruid]

By the way, here is a new version of the schematic, with some fixes.

Entering the values for the initial pre-delay MFB filter into the Okawa-Denshi filter tool, I get "invalid parameter" messages, with either 6n8 or 68n values. What's going on? Is there a mistake or am I doing something wrong?
Note I only tried the dry path (e.g. R1 = 22K) not the effect feedback path (R1 = 10K).

HTH,
Tom

[snk]

Hi, Electric Druid,
I can not find back my notes where i calculated the feedback filter frequencies....
From what i remember, i gathered informations from the Wampler Faux Echo from here (and i see you contributed to it) : https://www.diystompboxes.com/smfforum/index.php?topic=117744.msg1094127#msg1094127
(obviously, R12, C11 all refer to the Wampler schematic from the quoted thread)
I remember reading this thread, looking at the values chosen for the Wampler, and then comparing to the ones from the EQD Disaster Jr and the DeProfundis, and then trying different values for the multiple-feedback filter in the website mentioned below.

Changing R12 from 47K to 100K probably won't help it oscillate. If I'm reading it right, R10, R11, R12 and C11, C14 are a multiple-feedback filter. Sticking the values into this tool tells us what it's up to: http://sim.okawa-denshi.jp/en/OPtazyuLowkeisan.htm

- With R12=47K, this gives me 2.5KHz, Q=0.85, a reasonable cutoff and a bit of a hi-mids peak.
- With R12=100K, the cutoff comes down to 1.76KHz, a bit darker, and the Q drops to 0.63, a much softer roll-off. Since it'll reduce the overall level more than 47K does, it won't make it oscillate more easily.

If you're having trouble getting it to oscillate, rather than messing with the input filter (which is what R12 is part of) which has all sorts of other side-effects, I'd have a play with R22. Reducing the value of that (18K? 15K? or a 22K trim) should boost the level of the repeats signal going back into the input of the delay.
That'll let you put the 47K back for R12 and avoid the dirt.

[ElectricDruid]

There's no problem with the values in that Wampler thread. Here's the schematic:



If you stick these into the calculator (12K, 12K, 12K, 47K, 10n, 10n, 680n) you get a reasonable result. Note that it's a three-pole filter because of the extra passive stage stuck on the front:

http://sim.okawa-denshi.jp/en/Multiple3tool.php

It looks from the thread like I ignored the first RC stage and just looked at the active 2-pole part after it (e.g 12K, 12K, 47K, 10n, 680p) - a bit sloppy, since there will be an interaction, but good enough for a first pass.

But this is all ancient history at this point! What we're trying to work out is whether *your* circuit is good!

So...have you tried that filter on the breadboard? does it work? can you get signal through it? Does it sound as you'd expect? If the Okawa tool has a problem with it, I'd expect it to "blow up" in some way - oscillate horribly or something. I've used that tool a lot and it hasn't let me down, so I'm inclined to trust it.

Tom