PT2399 noise problem. Have you encountered it?

[merlinb]

great job!is the delay now super clean?is the maximum delay less time?
as for the noise problem did you try  increasing the 560pf cap to 3.3nf as i told you?

Well, it's pretty clean. It's on PCB so I can't make as many changes to the pre/post filtering as I would like; I will do that next time I make a PCB.
Yes I increased the NFB cap on the post-filter (I think that's what you mean by the 560pF cap), which helped a lot by reducing the cut-off frequency to about 1kHz.

[slacker]

Glad to hear you got it working, sounds like that was quite a debug.

(Do other pedals like the Echobase or LittleAngle suffer this same hissing problem, since they use the 5V rail as Vref?)

Yes the orignal Echo Base suffers from this. When I first built it I didn't notice it or at least I didn't think it was a problem, this was probably because the audio section was always in the signal path so the noise was always there, so there was no increase in noise when engaging the effect. Had I of made it true bypass, I'd probably have noticed the increase in noise straight away.
A couple of people mentioned that it was quite noisy so I took a look at it and discovered the same as you, that even with decent filtering on the 5 volt line there was still some noise from the PT2399. I solved it by separating the Vref from the regulator using a series resistor and cap to ground. If I did it again for the sake of an extra resistor I'd just use a separate 4.5 volt reference for the audio side.

[merlinb]

I solved it by separating the Vref from the regulator using a series resistor and cap to ground.

That's precisely what I had to do on mine!

[frequencycentral]

Do other pedals like the Echobase or Little Angel suffer this same hissing problem, since they use the 5V rail as Vref?

Yeah, there's hiss there. It seemed kinda logical to use the 5v rail for the vref, as I was trying for a minimal parts count - it didn't occur to me that it would introduce hiss into the opamp buffer. Certainly for the addition of a resistor and a cap it would be worth having a proper vref.

[tasos]

great job!is the delay now super clean?is the maximum delay less time?
as for the noise problem did you try  increasing the 560pf cap to 3.3nf as i told you?

Well, it's pretty clean. It's on PCB so I can't make as many changes to the pre/post filtering as I would like; I will do that next time I make a PCB.
Yes I increased the NFB cap on the post-filter (I think that's what you mean by the 560pF cap), which helped a lot by reducing the cut-off frequency to about 1kHz.

excellent!
can you make a post of all the changes you made to the circuit?

[merlinb]

can you make a post of all the changes you made to the circuit?

Sure. Here is the schem (or click on page 2 of this thread, since it is updated there also). I've included the LFO and other gubbins on the schem, now that it is working properly. The dry audio section isn't shown, but it's standard stuff.
http://i81.photobucket.com/albums/j207/merlinblencowe/Time%20Piece/DelaySchem.jpg

[slacker]

Looks interesting Merlin, I like the control voltage generator section, very neat.

[tasos]

very interesting work!
i have already populated the pcb for the rebote delay 2.5 [original from tonepad]....are there any parts substisutions to make it better-not farting??

[merlinb]

i have already populated the pcb for the rebote delay 2.5 [original from tonepad]....are there any parts substisutions to make it better-not farting??

It farts?

[tasos]

i mean the distorted delays at long delay time!

[merlinb]

i mean the distorted delays at long delay time!

The rebote has a lot more filtering than most, so I think it will be OK as it is.

[tasos]

i mean the distorted delays at long delay time!

The rebote has a lot more filtering than most, so I think it will be OK as it is.

ok...anyway!i will probably make the delay pot around 30k!i want it to be total clean!

[~arph]

Is there a reason why you have the digital ground (pin 4) disconnected on your schematic?

[merlinb]

Is there a reason why you have the digital ground (pin 4) disconnected on your schematic?

Not really. It doesn't actually need to be grounded as it is already grounded via a 10R resistance internally. I thought it might help the noise if the digital ground remained separated from analogue ground by 10 ohms, although its probably negligible.

[~arph]

Ok, yes it makes sense. I had not tried it yet. I was thinking of using a separate filtered ground for DGND, but I missed the 10R connection. No need then, no way of disconnecting that  . Keeping pin 4  disconnected sounds like the best option.

[~arph]

Shame though.. Wouldn't it be better if we could hook up AGND to 4.5V vref and flip the pin2/pin3 electrolyte.
And then hook up DGND to the actual GND.. (EDIT: Probably not such a good plan with a 5V supply)

Btw, where did you find the 10R resistance value? did you measure that? It's not in the datasheets or the techincal notes from the valve wizard (you?)

[merlinb]

Shame though.. Wouldn't it be better if we could hook up AGND to 4.5V vref and flip the pin2/pin3 electrolyte.
And then hook up DGND to the actual GND.. (EDIT: Probably not such a good plan with a 5V supply)

Yeah, really separate grounds probably wouldn't be worth it unless you also had really separate V+ supplies too.
One thing I have considered ,though, is to drive pin 2 with 2.5Vdc from an opamp buffer in an attempt to supply a super clean internal Vref. I intend to do some extensive experiments on the PT in a few weeks to characterise it, and make a better data sheet for it.

Btw, where did you find the 10R resistance value? did you measure that?

Yeah just measure between pins 3+4; it comes out at a stable 10-15 ohms every time.

[kdowqo]

where there any difference between the known good chip vs the ones you had?

[Morocotopo]

 Merlinb, I made an Echobase and to get rid of noise, used a 2 resistor and a cap VR for the audio power, the 5v regulator only powers the 2399. And replaced the first opamp buffer with a FET. Still, it makes a bit of hiss at the output, but not too much, just what a typical analog pedal would do.

I also made a ping pong delay, from BYOC (designed my own layout). That uses two 2399´s, switchable from series to parallell,  and I don´t get any heterodyning...

It will be nice to see your test´s result on this thing.

Oh, and I also made a Rebote, with a switch to add a 50K in series with the delay pot. When in the longest settings (around 1 sec.) you can hear the thing purring in the background.