Advice for a beginner/ PT2399 echo pedal

[PraiseIommi]

Hello everyone,

     I recently began doing some research about guitar pedals and how they work, and I have found it to be very intriguing. I am currently an undergrad engineering student and musician, so I have some experience with circuit analysis and signal processing, but I would like to know more about the ins and outs and theory behind how/why effect circuits behave the way they do. I have a specific desire to build a fairly simple echo/delay effect pedal for learning purposes. So far, I have seen multiple examples online that have implemented the PT2399, and this seems like it may be a good option for me to start with, since its datasheet also provides an example circuit for an echo effect which seems less complex than other designs I've seen. http://www.diyaudiocircuits.com/wp-content/uploads/2012/09/2399.pdf.

     I have many questions about implementing this design. First of all, If I were to fabricate the exact echo circuit from the datasheet on a breadboard and hook up the correct voltage source, a 3pdt footswitch for power, and two 1/4" mono open-circuit instrument jacks as the input and output, would the circuit function as is with a guitar and amplifier without any problems like input/output impedance or harming any internal components, and how would I be able to know this ahead of time by analyzing the diagram?. Secondly, I know that a guitars pickups produces an AC signal, but what kind of voltage and amperage can it produce, and does this factor into mathematical equations when analyzing? This circuit has two potentiometers which are from what I can tell, controls for feedback and echo rate, but I would like to also add a volume control knob, how might this be done with this circuit? Beyond a volume knob, I would also like my circuit to feature a true bypass associated with the 3pdt footswitch so that it will still allow the direct signal from the guitar to flow through the pedal when it is unenergized, how can this be done? Where can I find information about the internal circuitry of the pt2399, there is some information on the data sheet, but some information is left out. For example, the Vcc pin has no internal circuitry listed, though I have read that Vcc stands for Voltage common collector and is implemented internally by a transistor whose base and collector voltages are supplied by the Vcc pin, which I'm assuming since the chip requires a +5v dc signal then this directs current to the transistors emitter and further powers the rest of the ic, is this a correct assumption? This design has separate grounds for analog and digital signals, how do you ground both of these different grounds safely and correctly, do they need to be separated entirely? Would it be better to use a voltage regulator instead of a resistor or some other kind of component to create a 4v drop between the 9v power adapter and the +5v Vcc pin of the PT2399 and why? I noticed the PT2399 also has an unused CLK_O (System Clock Output) pin, if an LED were connected to this pin would it blink on and off according to the clock cycle, does this reflect the echo rate so you can see a flash in sync with echoes, and if you did hook up an LED to this pin would it need to then go to digital ground? Also an in depth explanation as to how this specific circuit functions as is would be nice.

If anyone has any questions for me feel free to ask. Any and all information that might be helpful to me would be greatly appreciated, I thank anyone who takes the time to read this and offer their expertise. Thank you and I hope everyone has a nice day.

[EBK]

Welcome!
You've come to the right place.
You have also asked a lot of questions in one post.  It will take some time to get all the answers.  I'll see if I can pick off an easy answer or two before someone else comes along.

The internal circuitry of the PT2399 is a bit of a mystery, but there are quite a few things folks have figured out.  Here is a sample from Electric Druid:
https://electricdruid.net/useful-design-equations-for-the-pt2399/


Regarding Vcc, this is a supply rail label often used when the circuit contains BJTs, but you don't really need to concern yourself with what it goes to inside the chip.  The important thing is that you supply +5VDC, preferably from a 5-volt regulator like a 7805.

[patrick398]

Lots of good info about the 2399 on the electrosmash page:

https://www.electrosmash.com/pt2399-analysis

The circuit from the datasheet would work but there are lots of schematics of circuits which optimize it for use with guitars without too much additional circuitry. Often an external op amp is used rather than using the one onboard the 2399.

Adding a volume control is easy, simply a pot wired as a variable resistor on the output like this: https://cdn.sparkfun.com/assets/6/3/e/5/e/511ac8f5ce395f5846000000.png
That would simply lower the effect of both the delay and the dry signal, can't see that being much use.
More useful is a 'mix' knob which controls how much for the delay is mixed with the clean signal. The Small Time delay is a simple PT2399 delay which implements this:
http://www.valvewizard.co.uk/smalltime.html

For 3PDT bypass wiring there are few different ways to do it, here is my favourite:
http://www.generalguitargadgets.com/pdf/ggg_sw_3pdt_tb_gi_dcj.pdf

[EBK]

Also, there are a couple FAQs linked at the top of this page.  They are worth a read through, even if you have prior circuits knowledge.

[Kipper4]

Welcome to the board mate.

For things to work well with guitar your going to need a high input impedance.

Rebote 2 schematic is a good example of a lo fi delay for guitar.

It's high z input buffer is simple too.

Take a look at some guitar buffer circuits it's an integral part of most pedal building.

I'm messing with some pt2399 chips and delays and pitch shifting kinda mess.
Great fun. Beware there was a fake batch at one time and if the resistor to gnd from pin 6 is less than around 1k the chip can lock up. The smaller that resistor is the faster the delay time .down into the reverb ranges.

Have fun
Rich

[PraiseIommi]

Thank you for your replies,

I like the idea for using a mix pot instead of a volume pot. Would the same method of volume control work at the input stage of the circuit instead of the output though, or is this really just superfluous seeing as how the guitar will have its own volume control? As for the 3pdt wiring diagram I really appreciate that, very informative. If I were to not use a 9v battery in my design and just a power adapter, would I then have to just use a 2pin DC jack for power and two 2pin open-circuit mono jacks with only tip and sleeve, the tip going to the input of the effect and to the tip of the output jack through the 3pdt, and the sleeve of the input going to the sleeve of the output and to ground?

[EBK]

Would the same method of volume control work at the input stage of the circuit instead of the output though, or is this really just superfluous seeing as how the guitar will have its own volume control?

You could put a volume control at the input of a circuit, but it generally isn't a good idea.*  One reason is noise.  Pretty much any circuit is going to add some noise to the signal before it reaches the output (there are many different sources of noise--I'm keeping things simple in this explanation).  If the volume control is at the end, then turning down the volume also turns down the noise. Set the volume control to 0 and the output is effectively muted.  If the volume control is at the beginning, turning down the volume control won't turn down the noise as well, and you'll likely still have something audible even with the control set at 0.

*There are always exceptions, I suppose. 

As for the 3pdt wiring diagram I really appreciate that, very informative. If I were to not use a 9v battery in my design and just a power adapter, would I then have to just use a 2pin DC jack for power and two 2pin open-circuit mono jacks with only tip and sleeve, the tip going to the input of the effect and to the tip of the output jack through the 3pdt, and the sleeve of the input going to the sleeve of the output and to ground?

What you are describing sounds right. You could still use a 3-pin DC jack and ignore the "battery +" pin, of course.

[duck_arse]

also welcome.

use an "l" low power three terminal regulator for the +5V - they are cheap enough to not think about, they provide the same 5V no matter what is going on around them, and they provide some isolation for the external audio section from the internal digital hash.

in general, we short the two ground pins together, and don't ask questions unless the circuit fails. as for the clock out pin - the human eye can't detect flashings much above 60Hz [?] or so, and the internal clock runs at megahertz rates. as your delay time decreases, your clock speed increases to a max listed 10MHz.

of more importance is the anti-lockup circuit additions and preventions - they are worth searching for.

[EBK]

As for what that clock output can be used for, here is a cool looking project that uses it in a microcontroller-based control system to precisely adjust the delay time:
http://blog.dzl.dk/2018/03/20/midi-synchronized-echo-with-pt2399/
I may have to try this.  For what, I have no idea yet. 

[PraiseIommi]

I'm very interested in the theory and circuit analysis of guitar effect circuits and I would love to have a deeper understanding of how they work in order to explain why they work and understand how they can be altered or improved. I'm familiar with some more basic circuit analysis techniques that I've learned in school so far, but I was never really exposed to circuits as complex as some of these PT2399 designs seem from their schematics. I feel there has to be a way to look at an effect schematic and determine ok this component does this and these components make up this which does that, calculate voltages and currents at different places in this circuit, and be able to say ok then this is going to sound like this because, etc. It would be a great learning experience to see someone step through every component of a specific pt2399 based delay and explain these sorts of details.
If anyone knows where to find more information about this sort of thing that would be awesome, otherwise any advice regarding this subject would be greatly appreciated as well. Thank you for all your responses so far, it has been very educational for me. Hope everyone has a nice day. 

[EBK]

A PT2399 circuit design walkthrough:
https://sound-au.com/project26a.htm



Not PT2399 stuff, but here are some more excellent resources to help you understand the building blocks of guitar pedals:


https://www.electrosmash.com/
In the menu, pick Pedals, then you will find some great analyses of specific pedal circuits. 

Lots of great pedal analysis here too:
http://www.geofex.com/fxtech.htm

[duck_arse]

that's a fascinating first post [welcome to the forum] Arthurnottheking, but one small point - it is MOST impolite to misquote another members post, especially when you include a spamming link within. we're onto those tricks.

[patrick398]

that's a fascinating first post [welcome to the forum] Arthurnottheking, but one small point - it is MOST impolite to misquote another members post, especially when you include a spamming link within. we're onto those tricks.

I enjoyed that perfunctory welcome. Polite, even to spammers.
Gotta love this place

[ElectricDruid]

As for what that clock output can be used for, here is a cool looking project that uses it in a microcontroller-based control system to precisely adjust the delay time:
http://blog.dzl.dk/2018/03/20/midi-synchronized-echo-with-pt2399/
I may have to try this.  For what, I have no idea yet. 

That's a terrible way to control the VCO of the PT2399. Just sayin'. It's controlled by a current to ground, so there are a lot of current sink circuits you can use. That on/off voltage-based thing doesn't even come close. No wonder they say "the voltage to frequency is very non linear". Well, quite! Perhaps if they did something a *little* bit more like the chip was expecting, it might behave in a manner more like they were expecting!