"Little Angel" - Super Simple PT2399 Mini Chorus

[Seven64]

i dunno about the birthday version, but i did a vero of the original and just finished troubleshooting it last night and am loving it!!!! great sounding pedal!!!!!

[frequencycentral]

So is there an updated schematic with these fixes...?

If so I might do an Onboard Pot 1590B with room for battery PCB...

Schematic for the Birthday Edition:

[merlinb]

R2 and R6 don't seem to be doing anything... did you mean to add the extra shunt capacitors to make second order filters?

[Brymus]

^^^

Well I was gonna save this as a surprise for LA's 1st birthday in August, but........




I wanted to keep it 1590A-able, but also add good things like the Pin 6 Fix and a seperate vref for the audio path. Here's the lowdown:

• Pin 6 Fix: D1/C16/Q2/R21/R22
  I did add a little Pin 6 Fix subcircuit to a PT2399 that I know won't work without. It still has issues, but I think raising R22 to 100R and C16 to 47uF will sort it
• Audio vref: R23/R24/C17 - suggested by merlinb as a way to reduce noise dumped into the older version's 5v vref supply by the PT2399
• C4 changed to 3.3nF - might be a bit darker but will be less hissy

 
As it's planned to be 1590A-able, hopefully all the electro caps should be able to be bend over 90⁰ to lie flat against the PCB. C16 might be the trickiest.

I PM'ed this layout last week to a couple of people who expressed interest in verifying it, so you never know, it could be verified within a day or so. The layout does need a little tidy up, as I notice the pad sizes are not all the same size.

I am a little confused Rick .
Is R1 just bypassing D1 ? and why ? and why isnt it on the schematic ?
Shouldnt R1 be in series with D1 ?

That last sound clip that was posted by Jvanort sounded really good,thanks for the demo.
I hope to finally get mine built next week.

[Brymus]

OK I see it on the schematic now,doh.
So the diode is just polarity protection for the gate of the npn,and the 68k limits the current,is that about right ?
I remember now some people were having trouble with the 100r ,so thats a work around to keep the shortest delay time possible I'm guessing.

[merlinb]

So the diode is just polarity protection for the gate of the npn,and the 68k limits the current,is that about right ?

The 68k+47u causes the transistor to switch on slowly at power up, which prevents the PT2399 from going into latch up.
The diode allows the cap to discharge quickly at power down, so the circuit is immediately ready for the next power up (otherwise the PT might latch up is someone switched the power on/off too quickly)

[Brymus]

Thanks for the explanation Merlin !

I breadboarded this,well 3 times actually double checking all my connections.
And I could not get any chorus,any warble to be accurate.
Turning the rate pot would yield a wah like effect while turning it,but turning the speed knob gave no change.
I tried different IC's,my PT2399's are a year or two old at least,I can get delay from it by increasing the 100R just no chorus.
Its like my LFO isnt oscillating I tried TL072, RC4558 and (NE5532 the only one to yield the wah sound) the rest had no modulation.
I get 2.5V at pin 2 but its stable doesnt wiggle at all.

Any suggestions ? 4 hours of this and I put everything back in the bins ,I will try again this weekend.

[Brymus]

I tried again same result,used all metal film resistors this time and a different layout on the breadboard.
I noticed the schematic says 2n3904 the PCB /layout says 2N3906
I tried with just the 100r to ground and actually had some modulation it was slight but the speed knob did affect the speed and the depth made the effect more or less noticable.
Absolutely no where near as modulated as the sound clips that are posted though.
With 2n3904 the 4148 got really hot,so tried 2n3906 the 4148 wasnt hot but still no modulation just a very slight reverb and a couple db of boost.
I seriously have tripple checked the values and connections ,just cant get it working.

[merlinb]

I seriously have tripple checked the values and connections ,just cant get it working.

It should be 2N3904 (or any NPN). Sounds like your diode is backwards.

You could also add a 10k resistor + LED from the output of the LFO (pin2) to ground, that way you get a visual check that the LFO is working (cuz is flashes...).

[Pigyboy]

I finally just got back to this after getting it to work the other day with Rick's new layout.  I built it with a pnp as per the pcb layout and it did fire up so I changed the transistor and it is still working the same but like Brymus mentioned it is not like the others I have here. There is no depth or lushness though all the knobs work. My PT is getting a little hot but not too bad. Gonna look at the two pedals side by side and see if I can get anywhere.

[Brymus]

It should be 2N3904 (or any NPN). Sounds like your diode is backwards.

You could also add a 10k resistor + LED from the output of the LFO (pin2) to ground, that way you get a visual check that the LFO is working (cuz is flashes...).

thanks again Merlin ,thats a great idea.
I will try it later when I can.
I left it on the breadboard this 4th time,   

[Brymus]

OK the 10k isnt needed since its at 2.5 v ,still no flash just constant dim light.
Ive tried 4 different op amps ,two types of NE 5532,no LFO 
Its wired right to.
Any ideas ? anyone,maybe a diferent LFO set up IDK ?

[Earthscum]

Any ideas ? anyone,maybe a diferent LFO set up IDK ?

Try different pots... you may have fried a lug? I have a non-functioning LFO section I gotta troubleshoot tonight, and probably upgrade with the new parts.

[Brymus]

I built a new seperate LFO with a TL072.
Got it modulating between 2V and 3V,but when I connect it to pin 2 it just stabilizes to 2.5V
There is a slight modulated effect but still not anything like it should be.
Is there an internal cap on the 2399 that stabilizes the Vref ?
Everything is exact to the schematic .
Any ideas ?

[Earthscum]

I wonder... try firing it up with a 2k or larger pot (set above 2 k) with a 100R in series to ground. Then after a second or 2, turn the pot down. See if that works. I wonder if the transistor isn't trying to compensate for the warble for some reason? That's the last place I can think of.

[Brymus]

I wonder... try firing it up with a 2k or larger pot (set above 2 k) with a 100R in series to ground. Then after a second or 2, turn the pot down. See if that works. I wonder if the transistor isn't trying to compensate for the warble for some reason? That's the last place I can think of.

I used a 100k pot,that gave me a nice adjustable delay/reverb but still pin 2 stays right at 2.5 v,it only moves a couple hundreths of a volt when turning the speed or depth knobs.

[Brymus]

If I connect my LFO to pin 6 I get a weird pitch shift tune/detune effect on the repeats though

[merlinb]

OK the 10k isnt needed since its at 2.5 v ,still no flash just constant dim light.

You do need the resistor! If you just hook an LED up to the output of the LFO it will stop it oscillating!

[Earthscum]

Straight from Merlin's Pdf:
"A significant source of noise is due to the internal clock modulating the power supply rail.
The following advice may be used to minimise this noise:

-----
• Leave the digital ground (pin 4) unconnected; it is already connected to the analog
ground internally via a ~10Ω resistance."

2 thoughts here:

First, for the people still getting noise, maybe try cutting the trace between pin 3 and 4. If it does bad things (I doubt it), then you can always solder bridge it back together. I think that ground may be for connecting to the other digital grounded circuits, such as whatever you are using the clock out from Pin 5 to drive, so that it acts as line noise canceling circuit. Another thought on that is maybe playing with a cap from Pin 4 to ground or another pin, maybe even + supply? Anyways...

Secondly, "A significant source of noise is due to the internal clock modulating the power supply rail." I wonder if the weak signal isn't coming from the supply compensating through such a large cap to ground. Maybe use a smaller cap to ground and see if the LFO signal improves? Maybe a 10uF Tant if you have one.

ETA: Just had a head-slapper. If anyone has this breadboarded, maybe try connecting the delay resistance to pin 4, leaving pin 4 unconnected from ground. Looking at the suggested circuits on the datasheet, those are the only 2 pins that are labeled as eventually terminating at a digital ground. Since there's no other circuitry that requires the digital ground's rail, we can leave it unconnected from anything else in the circuit. Thus, the digital ground and the delay resistance will be coupled together, and isolated from Analog ground by 10 ohms. It makes sense to me in a wierd way.

[Brymus]

OK can someone check their chips with a capacitance meter?

With no power I get 3.74 uf between pin 2 and ground,definitely enough to kill the chorus effect.
As noted on the first page, 1uf will stop the modulation.
WEIRD PART:
With power on it measures 374 uf exactly 100 times more,and it varies a bit too.(the Vref might be messing with my meter IDK)

All (4) of my PT2399 are 2-3 years old from Tayda,stamped with the PTC logo.

EDIT: This was measured with nothing but the meter + IC connected at pin 2,and the rest of the circuit still set up.