Adding an FX loop for wet signal in a PT2399 delay

[RBDP92]

Hi everyone,

I am learning, I do my best to be clear, please forgive my lack of skills there, I really hope to progress.

I am currently "building" a PT2399 delay kit. By "building" I mean soldering components to a PCB without knowing or understanding what I am doing. I have assembled quite a few kits before but have no knowledge whatsoever and really wish to make this change.

I would like to add further effects to only affect the wet signal in my delay pedal. I would like to restrain as much as possible pedalboard space, therefore I do not want to build a looper/mixer in a second enclosure.

Is it as easy as adding a jack IN and jack OUT between two holes in the PCB ? Or do I need to build a buffer for each input/ouput ?
I do not want yet to add a Volume pot for the FX Loop. I am, for the moment, just looking to add a vibrato/chorus/octaver/overdrive or anything funny to the wet signal.

Many thanks for your time and help


The kit is a Bluelay from Musikding.

[RBDP92]

I have found this pretty detailed explanation online but can not wrap my head around the technical details.



- How do one understands where is the "delay line" on the schematics ? (I know, I know....please do not laugh...)
- What is the "inverting pin of the op amp" ?
- What is a "dual op amp" ? Is it a way to efficiently build two buffers with few parts/leads as he suggests to do ?
- What is a "Vb" ?
- How can I make sure that, after building two buffers, my signal has remained the same phase ?

Many thanks guys...! I hope one day I will be more independant...

[Keeb]

- How do one understands where is the "delay line" on the schematics ? (I know, I know....please do not laugh...)

Great question, no one is laughing. The simple answer is that you look at the schematic. What part of the signal branches off to the delay IC? Where do they meet? Following the schematic the signal splits after IC1a, the delayed signal goes onward through C3 while the non delayed signal goes through R4. They meet at the mix pot.

- What is the "inverting pin of the op amp" ?

The input with the minus on it. It inverts the signal, this will phase shift your signal.

- What is a "dual op amp" ? Is it a way to efficiently build two buffers with few parts/leads as he suggests to do ?

A dual opamp is exactly what it sounds like, two op amps in one package. An opamp can be used as an amplifier, a buffer, an LFO etc. They are commonly used in guitar pedals. Yes, it's common to use an opamp if two buffers are needed.

(On the schematic you linked, IC1a and IC1b are the same physical part. They function as individual opamps though and are drawn as such on the schematic. They have the same number, IC1, but are separated by "a" and "b".)

- What is a "Vb" ?

Look at the top right corner of your schematic. That's the power section, you see how the 9V goes through R7 and the to R20? Notice how R21 connects to ground and has the same value as R20? That's a voltage divider. The voltage at R20 is 9V and the voltage at ground is zero. Since R20 and R21 have the same value it means that the voltage is split in half, ie 9/2=4,5V. See how there's a Vb notation connected to the junction between R20 and R21? Vb is used to bias the opamps. This is very common in single supply circuits. Look up voltage divider, unipolar power supply, rail to rail opamps.

- How can I make sure that, after building two buffers, my signal has remained the same phase ?

If you use the same type of buffers the signal will be the same as it was going into the first buffer. A buffer either shifts the phase or it doesn't. If you phase shift the signal twice it shifts the first time and the second time it will shift again and be back to the original phase. If you don't shift the signal then you don't and that's that.

Other people on this forum will be able to answer your questions more clearly. What really helped me was reading R.G.'s Technology of ... articles at http://www.geofex.com/. The technology of the tube screamer brings up a bit about opamps and buffers. http://www.geofex.com/Article_Folders/TStech/tsxfram.htm.

[RBDP92]

Hello Keeb,

Many thanks for taking the time to write all these informations here. I am really thankful and will now take some time to process all this.
It seems pretty clear to me and I will gladly grab more knowledge though the links you provided. I definitely need to learn a lot but I feel I am now getting a headstart  by knowing what to look for.

I will come back here if I need more clarifications about something. If I manage to make this work I will  happily describe the path I followed to help some future users.

Many thanks again for all these details.

[ElectricDruid]

- How do one understands where is the "delay line" on the schematics ? (I know, I know....please do not laugh...)

The "delay line" in the context of midway fair's comment is the path back from the PT2399 delay chip.

It's worth taking a quick look at where the delayed signal comes from and goes to. It starts out at C3 after the input buffer IC1a, and then goes down to a lowpass filter based around pins 15 and 16 of the PT2399. Then the PT2399 does its stuff to it and the delayed signal comes back out of the chip at pin 12. From there it goes through another lowpass filter based around pins 13 and 14, and then heads right through R10 and C5. From there it splits and goes two ways. One way it goes to the Feedback pot and is then fed back to the first lowpass filter via C13 and R15 (and therefore back into the delay). This part generates the repeats. The other way it goes to the Mix pot and is then fed into IC1b which acts as a final mixer and combines the undelayed "dry" signal from R4 with the delayed "wet" signal via C14 and R17.

You could tap into this delay path in a couple of places, and you'd get different results. You could take out C5 and connect the loop there. That's *before* the feedback pot, so any effects you put in the loop would also affect the feedback, and repeating signals would get more and more heavily affected as they go through the loop multiple times. That's the good news. The bad news is that it might well be quite hard to control the feedback, because the effect in the loop might alter the volume of the signal.
The other place is to remove C14, which is what midway fair was proposing. This is probably more sensible and would give a more reliable result since you're just adding effects once to the delayed signal, rather than adding them inside the feedback loop.

- What is a "Vb" ?

It's a bias voltage (hence "Vb"). It's sometimes also called Vref, and sometimes referred as a "virtual ground". It's all the same thing, so don't let different terminology confuse you. Keeb explained where it comes from, so I'll just add that it serves to keep the signal in the middle of the power supply range half way between 0V and 9V so that we've got the most headroom we can have before our signal starts clipping.

- How can I make sure that, after building two buffers, my signal has remained the same phase ?

You have to add up the number of times it gets inverted by the various stages. If it's an even number, you're fine - you flipped it upside down, but then later you flipped it back. If it's an odd number, you've got one extra flip, so the signal comes out upside down. This requires being able to recognise different circuit building blocks (different amplifier and filter designs, for example) and knowing whether those blocks are inverting or non-inverting (whether they flip the signal upside down or not).

To be honest, in this particular situation with a delay I don't think it matters very much because the signal is shifted in time significantly by the delay itself, a phase shift caused by an inversion is irrelevant.

That said, the easiest buffers to build are non-inverting op-amp buffers like midwayfair suggested (one 8-pin dual op-amp chip will make two buffers, so that's perfect) so you wouldn't even have any inversions to count. It makes it simple and you don't have to worry about it.

HTH,
Tom

[RBDP92]

Hello Tom !

Many thanks for your enlightening !

Things start to become clearer to me ! This makes me very happy but as some questions starts to get answered, some other interrogations are coming up ...!

The simple answer is that you look at the schematic.

That's the power section

I start to quite understand how to follow a path on the schematic thanks to your explanations. This helps a lot.
I start to understand it now. Not every single detail happening here but at least the broad idea of these different sections of the scheme and that makes me happy.

They meet at the mix pot

The other place is to remove C14, which is what midway fair was proposing. This is probably more sensible and would give a more reliable result since you're just adding effects once to the delayed signal, rather than adding them inside the feedback loop.

Awesome, this sounds great ! Thanks !
So it should look like this if I am not mistaken :
- From C14 "positive" --> buffer --> external effect pedal
- From external effect pedal --> buffer --> to C14 "negative"




But here is my new situation...I do not fully understand yet how to "buffer" the signal.
I have spend quite some time on the internet looking for buffers but I keep reading about big fat "buffer pedals" (like a TC Electronic Bonafide I guess).

A dual opamp is exactly what it sounds like, two op amps in one package [...] If you use the same type of buffers the signal will be the same as it was going into the first buffer.

So this sounds good and this I what I have tried to look for. But I do not know yet how to make a conscious choice of a dual opamp for this project.

the easiest buffers to build are non-inverting op-amp buffers like midwayfair suggested (one 8-pin dual op-amp chip will make two buffers, so that's perfect) so you wouldn't even have any inversions to count.

I do not know (yet) how to build a dual op-amp buffer neither how to choose a dual op-amp. Here is what I have came up with for the moment :
According to what I think I understand, the TL072 is a popular dual op-amp. But I have also found LM358N, LM358P, LF353...and do not know what makes these different from each other. On Pin 4, some mentions "ground" and other "v-" . I have read again what you explained sooner about power supply, but I do not know if grounds are common to the whole circuit or "v-" has to be linked to a "negative" part of the regulated power supply instead of a common ground.
For "V+", Google seems to tell me that I can plug anything up to +18V, am I right ?

Should it work like this ?



When looking for buffer schematics I have only found single buffer circuits like these two :
I notice some additional components (capacitors and resistors) but my skills do not allow me to understand their purpose.
From Muzike.com


Or this PCB for a footswitch that includes a buffer :
From Guitar PCB



What do you guys think ? What is the best way to assemble a good and reliable dual buffer for this project ?

[ElectricDruid]

The buffer you need is on this page: http://www.muzique.com/lab/buffers.htm
(Since there are so many, that's almost always true! It's a great resource)

Specifically, I'd recommend this one:


This shows a single op-amp version using a TL071. You need two, so use a TL072 dual op-amp. The "Vr" mentioned in the diagram is "Vb" in your circuit., They called it "V reference" instead of "V bias" - see, I told you the terminology was going to get messy!

Sorry, I couldn't find a stripboard/veroboard layout for this, but it shouldn't be impossible. Your diagram taking C14 out and putting the buffers and the FX loop in is basically good, but it looks to me like the "+" side is the return, not the send. Swap the Send/Return over. White=+, so that's the Return coming back to the wet/dry mixer. Black=-, that's the Send from the Mix pot.
https://postlmg.cc/H8j0PHmf

Good luck!

[cab42]

Yesterday I built a Madbean Cave Dweller on breadboard, my first PT2399 effect.  I built it mostly to test the ic's  I bought at Tayda a while ago. To my big surprise it worked almost instantly, only one misplaced resistor. And all my ic's worked 

I plan to build a Rebote 2.5 and I would like to add an effects loop, so I have been reading this thread with interest.

I just did a quick vero layout of a dual buffer using the Vr from the main board.

However just after finishing, I found this other vero layout of a dual buffer: https://www.diystompboxes.com/smfforum/index.php?topic=101982.0

It does not use Vr, but is biased with a voltage divider (it's also in the AMZ article Tom referenced, just below the one showed in the previous post). But isn't it an advantage to use Vr if you have it present, even though the buffers are on a daughter board?

If anyone are interested, I'll be happy to share my layout.

[RBDP92]

Your diagram taking C14 out and putting the buffers and the FX loop in is basically good, but it looks to me like the "+" side is the return, not the send. Swap the Send/Return over. White=+, so that's the Return coming back to the wet/dry mixer. Black=-, that's the Send from the Mix pot.

Yes, sure, you are right ! Thanks !
Here is the updated diagram :

The buffer you need is on this page: http://www.muzique.com/lab/buffers.htm
(Since there are so many, that's almost always true! It's a great resource)

Great ! Thanks, I'll keep that page in my tabs from now on !

They called it "V reference" instead of "V bias" - see, I told you the terminology was going to get messy!

Haha, I get it now ! My mind starts to get just a tiny little bit flexible !

Specifically, I'd recommend this one:


This shows a single op-amp version using a TL071. You need two, so use a TL072 dual op-amp. [...]

Sorry, I couldn't find a stripboard/veroboard layout for this, but it shouldn't be impossible.

Thank you very much. I have spend a lot of time trying to wrap my mind around this to translate that schematic to a strip board layout. But I am very frustrated to not be able to adapt a scheme that is even that simple... Modifying it from a single buffer TL071 to a double buffer with a TL072 has lost me...as frustrating as it can be...
I have downloaded a software called DIYLC that is very well made and I have really tried hard to make the TL072 conversion from the scheme you shared but I am completely failing.
Even with a good tool and all the informations I am supposed to need, I can't complete it...

[RBDP92]

Yesterday I built a Madbean Cave Dweller on breadboard, my first PT2399 effect.  I built it mostly to test the ic's  I bought at Tayda a while ago. To my big surprise it worked almost instantly, only one misplaced resistor. And all my ic's worked 

I plan to build a Rebote 2.5 and I would like to add an effects loop, so I have been reading this thread with interest.

Great ! Congrats for your build ! And welcome aboard here !

However just after finishing, I found this other vero layout of a dual buffer: https://www.diystompboxes.com/smfforum/index.php?topic=101982.0

That's wizardry, I have been looking for this for hours now...!

Here it is embedded for further references :


Original link

Are the blue round components the same as the 2.2M resistor but displayed vertically ?
Are "2M2" values a typo or is there a difference between a 2.2M and a 2M2 ?

It does not use Vr, but is biased with a voltage divider (it's also in the AMZ article Tom referenced, just below the one showed in the previous post). But isn't it an advantage to use Vr if you have it present, even though the buffers are on a daughter board?

I am interested in that answer too.

If anyone are interested, I'll be happy to share my layout.

Sure !

[patrick398]

Yes the resistors are 2M2 but 'upright' to save space. 2M2 is not a typo, that also saves space. Instead of typing 2.2M it's easier to replace the decimal point with the multiplier. So 4.7K is 4K7, 2.2nf is 2n2 etc.

It does not use Vr, but is biased with a voltage divider (it's also in the AMZ article Tom referenced, just below the one showed in the previous post). But isn't it an advantage to use Vr if you have it present, even though the buffers are on a daughter board?

Let me just say that i haven't followed this thread from the beginning and i got lost in the quotes within quotes quagmire, so disregard if this does not apply. But in this instance Vr is the same as the voltage divider. The voltage divider uses equal value resistors between 9v and ground. This means that the mid point is half of the supply (ohms law), 4.5v, which is used to bias the op am so it sits in the middle of our supply and allows the signal to swing both positive and negative with maximum available headroom. This is called a voltage reference, or Vr, or Vbias.

[ElectricDruid]

It does not use Vr, but is biased with a voltage divider (it's also in the AMZ article Tom referenced, just below the one showed in the previous post). But isn't it an advantage to use Vr if you have it present, even though the buffers are on a daughter board?

I am interested in that answer too.

It shouldn't really make any difference. Practically, it makes sense to use an existing Vr if you've got one rather than waste a few more components replicating it again somewhere else (but it's only a couple of resistors and a cap, so it's not a big deal). There's certainly no fantastic tonal or noise reason to use one global Vr rather than two separate ones, assuming they're reasonably well designed for the job at hand.

[cab42]

There's certainly no fantastic tonal or noise reason to use one global Vr rather than two separate ones, assuming they're reasonably well designed for the job at hand.

I was mostly thinking about noise when I asked, but if that's not an issue, I think I'll use the one posted above. You may need a couple of extra components, but you will save a wire for the Vr.

BTW, I think it is possible to shave a column off that layout by moving the two links in column 4 and 5 to column 7 (under the ic), move the 100uF from column 7 to column 4 and move the 2.2M in column 5 to column 2. That should free up column 5