"Little Angel" - Super Simple PT2399 Mini Chorus

[garcho]

I was a bit disappointed because I thought it would be analog

why disappointed? other than it's not what you thought you were buying (gear rarely is...)

Ι am guessing it is a little angel circuit , it has depth, level and speed controls.

DIY gifts like FCUK's Little Angel have been commercialized before, it's true, but any chip that does delay can probably work as a chorus, too, it doesn't make it a nefarious rip-off. Depth and speed are controls basically on any pedal with an LFO. Anyway, most of those cheapo delays and chorus pedals are PT2399 based, and all of them, including the Little Angel are more or less based on the app circuit from the datasheet. One more reason to build your own pedals!

BBD pedals might not have DACs but plenty of digital delays have filtering on them now. All things being equal I can't hear the difference between a good digi delay and an analog delay, especially when it's on a guitar playing in a group. That doesn't mean PT2399s are awesome; there are "real" digi delay ICs (and micro computers) out there.

For what it's worth, I built a Little Angel years ago and still use it live all the time, I love it. I added a 'wet/dry' tilt control, which is key to its usefulness for me.

[eatfeta]

well I was looking at this circuit
https://i.warosu.org/data/diy/img/0007/98/1429543934493.png

is the the signal completely converted to digital?
or does it go through the 10k resistor on top and the 0.1 mikrofarad capacitor and blend with what comes from the pt2399?

[garcho]

through the 10k resistor on top and the 0.1 mikrofarad capacitor and blend

yes, that's it, and that's the resistor you can replace with a rheostat/potentiometer if you want.

welcome to the forum!

i eat feta almost everyday

[rankot]

I swapped few PT2399 chips and they all sound the same. Like I have some hi-freqency distortion or buzz. What could be the cause?

[chuckd666]

Check your power supply doesn't make the noise with other similar circuits?

[garcho]

and double check capacitor values, some of the noise is filtered externally

[rankot]

My power supply is very well filtered, using toroidal transformer and is very capable - up to 3A. Other pedals work fine with it. I have another one with less power, but this pedal has the same noise with both of them.

I first built it using "birthday edition" schematic, which used 100n between pins 9 and 10, 100n for 11/12, 10n for 13/14 and 3n3 for 15/16. It was noisy as described. Then I found schematic called 2.0, and replaced capacitors as shown there: 10n for 9/10, 10n fo 11/12, 1n for 13/14 and 1n for 15/16, but noise is the same as before.



I have even tried to add some more filtering - 100n between pin 1 and ground an 1u between pin 2 and ground (as proposed somewhere), but it didn't change a thing.

The noise I hear is very similar to the sound of snare resonance when playing guitar loud in the same room, but I don't have a snare around and when pedal is bypassed, the sound is pristine. I have even tried what PRR proposed in another thread, to reduce input buffer's amplification, but it didn't change a damn thing...

[rankot]

Could it be some kind of layout problem? This is my PCB:

[garcho]

why are you using a 5532?

[rankot]

I had one at hand. Why not?

[garcho]

try a different op amp, like TL072. op amps are not all the same, different input impedance, slew rates, headroom, etc.

[rankot]

I know they are not the same, I used this one because it has the smallest noise rate, and the circuit is a standard op amp buffer, no reasons why it shouldn't work. I will try to see what is going on at op amp output, if there is distortion at this point, then 5532 to blame, but if signal is clean, then problem is somewhere else.

[garcho]

the circuit is a standard op amp buffer, no reasons why it shouldn't work

it's not a voltage follower, it's an inverting amplifier with 5VDC bias, and that's only one of the op amps. the other half of the dual package is an op amp LFO

[rankot]

I know that. But there's nothing special with both of them, and they both work. But I will try to replace 5532 and put TL072 and see what happens.

[garcho]

The noise I hear is very similar to the sound of snare resonance when playing guitar loud in the same room

something becoming microphonic? cracked solder joint, ceramic capacitor, something like that? could the 78L05 whine for some reason?

[rankot]

I have checked all joints, they're fine. I will try to check LM78L05 with scope, but no, it is not microphonic, just some tiny resonance like distortion. It is faint, but it is there. I have multilayer ceramic capacitors, but they are used for DC filtering only; all audio path caps are polyester.

I will try to bypass 1M feedback resistor at op amp with 100p, maybe it will help.

[rankot]

I have finally found enough time to mess with this one, so I found the problem. This is complete report on what I tried, hoping it will help someone in the future. 

I built it a long time ago, so I forgot that I didn't have 1n capacitors at hand then, and used 680p for filter caps instead. I tried to change a lots of things - including LFO resistors (changed all to 220k as proposed by Jack Orman, it seems to make it even worse), I have added different filtering caps on PT2399's pin 2, swapped different PT's, cut PCB traces to separate AGND and DGND (and it didn't work at all after that so I had to short them again), but the noise was still there and it was actually dictated by LFO. Finally I have found a comment somewhere on Internet mentioning putting 3n3 instead of 1n between PT2399 pins 15&16, and then I found that I have 680p instead. So I removed both 680p caps, put 1n between 13&14 and 3n3 between 15&16 and, voila, almost all the noise was gone. I also left 1u capacitor between PT2399 pin 2 and ground, cause it makes LFO a little bit smoother.

Now I was left with one more issue - some kind of LFO thumping when depth pot is at minimum, and rotating it I found that it stops after approx 8k. Since I have used 9k1 (I don't know why I didn't use original 33k, perhaps found this value on some of later schematics') between pot and pin 2 of PT2399, I replaced it with 20k and now thumping is gone even when pot is at zero.

I have also found out that depth pot sounds almost the same above 250k, so I used A250k instead of 470k, and I used A100k for speed - it seems better than original B100k to me.

So, I will most probably leave it like this, but there is also a suggestion to use 10k for voltage splitter at LFO with 47k for LFO feedback resistors, so I will maybe try that too, before I box it for good

At the end, I have one question for experienced Little Angel builders: I've built it with NPN anti-lock circuit. There is a resistor going from pin 6 of PT2399 to ground. I've used 2k2, but I see 4k7 on some schematics. It is even absent at Jack Orman's version (which uses JFET). Is this value important or not?

[Kipper4]

The R from pin6 to find sets the delay time.  Bigger is longer time until it gets into delay territory.

[rankot]

Hm, interesting! But it is only 100 ohm on original schematic and on v2; 4k7 on one labelled v2.7 and 2k2 on V3 (or "Birthday Edition"). And it is completely missing at Jack Orman's schematic. Now I am totally confused with this. 

[Kipper4]

Look at the pt2339 data sheet it has a guide to delay times v resistors