A different approach to the PT2399 delay

[Marcvv]

Again really nice.

[mth5044]

That was beautiful! I'm hoping the alternate filter works just as well, as I'd like to try that one.

EDIT:So if this where to be turned into a true bypass set up, Q2 and all it's supporting circuitry could be taken away, as well as all the circuitry on the LP2 mixer? And then FOUT2 would essentially become the DOUT,  That leaves pins 13 and 14 for something else...

EDIT2: That makes no sense, there would be no dry signal. Hmm.

[rring]

Thanks

I am moist as spring with anticipation to see if someone else can reproduce my results with this - let me know how it works for you.

I think the analog filter will do well. I experimented with uping that cap between pins 9,10 even higher(.68uF or so) with good results. So there is some potential to squeeze even more out of one of these "karaoke specials". If one were to do this you would probably need to optimize resistors R2,R11,R12(reducing gain a little). I built a PCB board but you can definitely perf this if you are careful. I found that having the 5 volt regulator  as close as possible to the PT2399 is important. I say this because when I was breadboarding this thing - I had all kinds of trouble with noise, stability, etc ...until I basically soldered the regulator output pin to pin 1 of the PT2399 - creating electronic conjoined twins!

[mth5044]

Thanks rring.

I edited my initial post with a true bypass idea, do you think it will work? I just want to make sure I'm following your schematic correctly.

[Skreddy]

Super nice ideas and execution rring!

I, too, use a larger cap than the app notes specify between pins 9 and 10; I use a .12uf.  I experimented with bigger, too, but it begins to low-pass a bit much for my taste.  As for multipole filters, I'm using some combination of LC, RC, and all the available op-amps on the chip to do mine (and of course I use another op-amp for input and output) to get as sharp a low-pass curve as close to the upper HF audio limit as possible.  Great work!

[garcho]

A little OT but... I've been messing with the resistors and caps in the filter too; lots of fun! What a great IC to noodle with. LDR in series with 'delay time' is infinitely entertaining. Same with feedback loops within the circuit. In the filter section I put an electrolytic cap between pins 10 and 11 (op 1 and 2 ins), and made the repeats sound more like tape echo; increasing a band-pass sort of filtering effect with each repeat - somewhere between subtle and immediately noticeable. No idea why that would work, but I don't understand electronics anyway, so moot point.     I also put an electrolytic on a switch between 9 and 10 (op 1 in/out), which creates an instant short scale looper, almost a like a chunky S&H or granular synth, in which the 'repeats' or 'echoes' (I'm referring to the datasheet application) pot affects the mix of the 'loop' and dry signals. Good times.

[rring]

Thats very interesting about the low pass as the cap is increased. Looking at the data sheet and how I thought this cap would function - I expected what you described happening - for me though I measured a increase in gain but not much high freq roll off behavior - that why I thought it might really help to increase it. In my circuit, I just did not find there to be roll off?  

I can say that the thing is so quiet now I don't have true bypass to cut out noise being introduced by the PT2399 just sitting there and I can do that tails schtick where the output is always connected.

Hopefully someone can try experimenting with this and give some feedback.

thanks for the info

[rring]

To do true bypass I would just put one switch across EIn and the the input jack and one across Eout and the output jack - you could then just remove the jfet(Q2) in series with DIn as you described. You still need to use the spare op amp as the mixer as you were concluding in your earlier post.

[Skreddy]

Thats very interesting about the low pass as the cap is increased. Looking at the data sheet and how I thought this cap would function - I expected what you described happening - for me though I measured a increase in gain but not much high freq roll off behavior - that why I thought it might really help to increase it. In my circuit, I just did not find there to be roll off?  

I can say that the thing is so quiet now I don't have true bypass to cut out noise being introduced by the PT2399 just sitting there and I can do that tails schtick where the output is always connected.

Hopefully someone can try experimenting with this and give some feedback.

thanks for the info

Cool.  Well it's been a number of years since I put mine together, and I do recall the increase in gain, too.  When I do get around to re-visiting this circuit from the ground up, I'll likely toss all my old assumptions.  Don't really have time to get into a new version right now, but there are a few things I'm keeping on the back burn for when that time comes.

[Jdansti]

rring,

I noticed that the photo of your circuit shows that you used SMD components. Are you using the standard SMD ceramic chip capacitors?  Just curious because I want to do some SMD work and the ceramic chip style is inexpensive and easy to find. My concern is the effect they might have on the sound.

[~arph]

Super nice ideas and execution rring!

I, too, use a larger cap than the app notes specify between pins 9 and 10; I use a .12uf.  I experimented with bigger, too, but it begins to low-pass a bit much for my taste.  As for multipole filters, I'm using some combination of LC, RC, and all the available op-amps on the chip to do mine (and of course I use another op-amp for input and output) to get as sharp a low-pass curve as close to the upper HF audio limit as possible.  Great work!

I've noticed the same roll off if the cap gets too large. It is the ratio between the pin 9, 10 and 11,12 caps that produce the difference in volume. Lowering the cap on 11,12 would also have increased volume.

[Marcvv]

I have been playing with quite some pt2399 delays. My preference untill now has been the pt80. For me working with humbucking pickups, and thus a slightly hotter signal, the compander in the pt80 keeps the signal cleaner with less initial distortion. Even with the solutions for the other designs like the echobase and rebote like delays, the pt80 keeps the signal cleaner to my experience.

Now what would interest me with your approach, with the bigger cap and the resulting higher output, if it is possible to reduce the input signal and in that way keep the signal cleaner with a hotter input signal. Or maybe it is not an issue at all....
I guess I will have to start breadboarding this.

Would be nice to pair this with a taptation.

[rring]

Yes I have built a circuit similar to the PT80 with companding. Careful gain manegement is important..... and in this design I have not found any issues with overload. I play with hot pickups and agree the PT2399 is not tolerant of big swing. In this design, the source follower input is almost unity(slightly less) and input filter is at unity(slightly less really, except for the lowest frequencies). I am getting some gain in the delta moudulator conversion process (with that increased integrator cap). The echo return is less than unity as well.

The irony of the PT80(a great sounding delay) is it uses TL072 op amps which really don't perform well @ 9V and single supply.  They have lousy output swing and waste current.
The compander eats some current also.  My total current is at 20mA - which is tolerable for a decent 9V

I love this info about the ratio of the CAPs..I did adjust some other values and basically just felt my current config sounded right and left good enough alone. It sounds like a more methodical and detailed set of tests and comaprisons may be in order to squeeze every last possible drop of performance out of the PT2399. One thing I know is that the data sheetis not reliable---- thanks for this Jdansti

[Marcvv]

I could not really be sure from the clip what kind of pickups you were using. Also in the clip I do not hear any louder chords. That's were most delays show there headroom in my experience.

I have the TL 072 (which I replaced by a BB 2604) and the compander in the PT 80 running at 12 volt by the way. That gives a slightly bette headroom. I think that was part of the original design.
You are probably right about the current waste, but i run all my pedals from a power supply....

[garcho]

One thing I know is that the data sheet is not reliable

How so? Actual errors, or just lack of information?
Is it common for 'special' ICs like this to have such a crappy datasheet (as opposed to say, a TL0xx datasheet)? Their schematics are annoying to read, too. Some of Princeton Technology's other products have more in depth datasheets, like their preset EQ chip (2389). Apparently, the PT2399 came out in 1997, and they list it as an early 'milestone' in their company's history - perhaps they learned as they grew and didn't bother to revisit old datasheets? Or do they just give the 'real' datasheet to karaoke-machine manufacturers? Or does it not really matter and I'm just a dumbass?

an aside - karaoke means 'empty orchestra'. fitting!

[rring]

To an earlier post... I use ceramics for decoupling and for filters but not for coupling caps - you can get small smd electrolytics and metal film caps not to expensive.
The main thing for me is the ceramics are microphonic sometimes

To an earlier post.... Yes you are corrcet the TL072's will work fine at 12 Volts - I just need to be picky - so I can feel better about my dreary life - so I come up with criteria to make everything difficult - just ignore me and I'll go away.  I will do cook up a short demo where I really beat out some loud chords.



With regard to the data sheet
The data sheet shows component values for filters that make no sense (10Khz rolloff) - the noise spec is suspect, not enough detail to really understand the part. I just figure other application circuit values might not be optimum. My main point is just wouldn't assume it represents optimum design applications

[roseblood11]

This is really a great project!

I think, you should sell those boards (...but I'd prefer a version without pcb-mounted jacks), together with the MAX7401, as it's hard to find.
Or maybe in cooperation with Taylor/musicpcb.com?

But a version without such an exotic part would really be preferable. Does the version with rail-to-tail opamp sound identical?

Uwe from uk-electronic.de (known as "lmkv15" here) suggested to do the filtering with the LT1063, a 5th order butterworth lowpass filter:
http://www.datasheetcatalog.org/datasheet/lineartechnology/lt1063.pdf

[Ronan]

I have no objection to a $7 or $10 part, (re MAX7401), if it makes an improvement to the sound, its well and truly justified IMO. It is less than the cost of a set of guitar strings, only it doesn't need to be changed ever again.

[rring]

Originally I used the MAX7401 for a 10Hz filter for a vibration sensor I had to design for my day job - so I had some laying around to play with. I realize everyone doesn't have a lab with hundreds of specialized parts floating around - so I  think its good to design around some basic "beef stew" sort of parts people have or can get easily. I must say though it is really cool to be able to adjust the filter with one cap change. You can get them through digikey which only has a $25.000 minimum.

If  enough people are interested I could spin some boards and figure some way to sell them(at cost). I guess it would be best to do a through hole layout? Use the DIP PT2399 foot print, No PCB Jacks?  I think I can do a through hole layout in  a 2"square and not be that crowded. I alway try to design to fit into a 1590B box.

[roseblood11]

If  enough people are interested I could spin some boards and figure some way to sell them(at cost). I guess it would be best to do a through hole layout? Use the DIP PT2399 foot print, No PCB Jacks?  I think I can do a through hole layout in  a 2"square and not be that crowded. I alway try to design to fit into a 1590B box.

Sounds great!
I would definitely take a few...