A different approach to the PT2399 delay

[rring]

Its always 100x the corner freq. In my case, I use the internal oscillator with a 247 pF cap and I get a oscillator frequency of about 170KHz

[Mark Hammer]

So there's no risk of the PT2399 and MAX7401 clocks heterodyning and producing audible sideband products?

[rring]

Good question.... I looked at this possibility - of course based on the audio sample - you can hear the answer is currently no it does not create beats. However is there any circumstance where this could be a problem? I don't think so. The two clk frequencies would have to be close in frequency to each other  and have phase shift between them to create audio beats. This will never happen based on the ocsillator frequency range of the filter IC. Also if the clk for the filter is derived from the PT2399, the two clks will be completely in sync. If there is no phase difference between the two clk signals no hetrodyning occurs in this sort of filter.

[nordine]

sounds really great, congrats
but i had problems upping the pt2399 8-9 pins cap to .47uf, i did it, and i got nasty feedbacking, any ideas?

[rring]

" i had problems upping the pt2399 8-9 pins cap to .47uf, i did it, and i got nasty feedbacking, any ideas?"


Just to clarify: it is pins 9 and 10 not 8 and 9. I assume that was just a typo - if not- that may be your problem. Not sure - I would have to see your circuit. This mod increased the output for me and so in the echo return and the output of the mixer, I had to attenuate the signal. I was curious how high I could go and at about 1uF I got distortion from high gain but not feedback. Perhaps the tolerances on the chips vary and this is not reliably repeatable? Does your circuit work with the traditional .1uF - if so double it and see what happens.
I would be interested to hear more about you experiences with this or anyone else. I tried it with 3 different ICs and got the same result. They were all from a batch of 10, I bought on ebay recently.

[rring]

I thought of something else... my input filter on pins 15, 16 is unity gain. Many other designs have gain in this filter and this may be overloading the part. The cap value change is probably is not a "drop in" sort of mod.

[culturejam]

Great concept!

I looked into using the multi-pole filter ICs as a way to shave highs off the PT2399's output a few months back. But I put off testing because of the cost of the ICs.

[rring]

yes - not cheap but very easy to use..... from my perspective - an extra five bucks well spent.

[slacker]

That's sounds really good, nice work. Like the electronic "tails" switching as well.

[rring]

Yes I was looking at true bypass, etc, but its quiet enough now that I can just disable the input to the delay and it sounds fine - unless you set it for endless repeats in which case you end up with sound of an exploding garbage can in about 10 seconds!

[Morocotopo]

Rring, very nice work. I´m currently (slowly) prototyping a dual PT2399 delay, I´ll try the cap value change to see if it improves s/n ratio in my build.
Regarding the filter chip, let me ask you a question: functionally, it´s the same as eight R-C (1st order) lowpass filters in series? Or is there another advantage in it besides the size/number of parts/smaller layout?

[rring]

Yes it is as you have described except for two other features: one, it has unity gain (for passband) where as the passive filters will attenuate significantly as they are cascaded, two, for passive filters the driving impedance must be low and the loading impedance has to be very high in order to perform well. If you just cascade them one after another - you will not meet this requirement and performance will suffer.

[Morocotopo]

Thanks rring. Most delays with 2399´s use multiple feedback filters, that are, hmmm, second order if I´m not mistaken. But the problem with those, since the filters "slope" is not that steep, when you try to remove the hiss/aliasing/etc totally you end up reducing too much treble. An eight order filter allows to have a better filtering of the highs without affecting the useful signal, right?

[rring]

Yes you are correct - The sharper the filter - the closer you can get to the "edge of the noise" also with lots of repeats any noise you introduce get recycled only making things worse - so the filter is  a good place to invest some effort!

I was orignally going to use two in my design one for the input filter and one for the output. Then you could build a highly tweakable delay with even less parts. I didn't because I figured I would be banned from the DIY community for life because of the price!

[nordine]

I will try your low gain configuration, i think then i wont be getting crazy feedback

BTW, is there any way to come close to the filter youre using.. but without the chip (because its a rare chip, we dont have it down here)? ...a discrete, simplified solution perhaps? a 6 pole filter could be made using a 4069, without too much circuitry i think

[rring]

Yes, I posted an alternate filter on www.circuitsalad.com. If you can't get the opamp I specified you can use others it just needs to work at 5v and be rail to rail output or you can add a coupling cap  and 1/2 supply ref and use a 9v supply and a more generic op amp. Also you can use discrete transistors to create the same filter I show with the op amps. I think this is done in the PT80 delay. The 4069 could work, but biasing all those cmos inverters will suck current. My current design only draws 20ma so I can use a battery  -  which I like.

[nordine]

then that filter could fairly replace the MAX chip? thats cool, ill try it
what would be common rail to rail opamps? by common i mean, available at smallbear
edit: will a 5532 work ok with this?

nice page btw, ill be looking forward for your updates  

[rring]

Just got PC boards back for this project. Going to build this weekend. Will post pictures, any schematic tweaks and some more sound clips to demo the tails switching, infinite repeat, etc at www.circuitsalad.com.....hopefully I got the board artwork right - I rushed through it earlier this week.

Try these  from Small Bear:

TLC2262CP
this is a dual op amp that will work with 4 -16V supply and is rail to rail out - low noise as well.

V4580L
this is another that would work but the output swing is not quite as good - for 9v supplies this looks like it would be a good choice for input buffers, etc.

[rring]

Posted  a simple demo of infinite repeat and tails on bypass, using my new PCB circuit board. The board built circuit performs just as well as the bread boarded prototype which helps confirm my results were not a fluke.

picture of board:
http://circuitsaladdotcom.files.wordpress.com/2012/09/img_4882.jpg

video demo:
http://www.youtube.com/watch?v=rj_SBWmYGN4&feature=player_detailpage

[artifus]

nice video!