A different approach to the PT2399 delay

[Jdansti]

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.

Thanks for the info on the caps.

Thanks again for a great project and for considering a run of boards!  If you do this, you owe yourself some profit for your time, effort, and great idea. Don't sell yourself short!  I think 99% of the folks on the forum do through hole and not SMD. Also, most of us don't use PCB jacks, although many do use PCB mount pots. Either way, if you have components mounted to the board, the builder can always run wires for off board mounting. 1590B would be fine.

[~arph]

Yeah looks like I can get that max IC at 6,44 at digikey here in the Netherlands. DIP format too. No minimum order either. Wonder what the shipping is. Never dealt with digikey

[Michael Allen]

just breadboarded this. I like it a lot! It doesn't have any of the hash i'm accustomed to with the pt2399. Excellent design!

I used J201 for the buffer, with 10k source to get appropriate voltage to the second FET. I used a PN4393 there since I had one. Originally had both fets as J201 and it wasn't opening the switch. I popped the PN4393 in since it has higher Idss, then discovered I wasn't getting enough voltage from the first FET to set the Vbias state for the switching FET. I bet it will work with the J201 as the switch now.

I'm going to add a second, highly filtered feedback to add some really dark and low ambience.

Great design, thanks for sharing!

[Ronan]

Just wondering Michael, did you use the MAX7401 as a filter or the dual op-amp filters?

[Michael Allen]

I did use the max7401, with 220pF corner cap

[Ronan]

Thanks for that, I am interested in that IC, but will try the alternate dual opamp filter in the meantime.

I've been wondering if that MAX7401 (or some other equivalent) might be useful in a lot of other effects that generate noise outside the guitar frequency range.

[R O Tiree]

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!

I've learned to take datasheets with a fairly large pinch of salt... Take the NE570/1, for example.  On the vast majority of datasheets for this family of companders, the gain formula is incorrect for the expander.  Even "Making Music With The NE570 Compander", which set out to de-mystify this chip and its applications, simply copies the same error blindly.  I set up a spreadsheet with a graphics "front end" to enable me (and, if it worked, anyone else who wanted it) to click a button, set a resistor value here and there to see what would happen to the gain curves and the output DC bias values.  The sums just did not add up.  So, I breadboarded a bog-standard compressor linked directly to an expander as per the datasheets' test circuits.  Unsurprisingly, I found that the test signal out of the expander matched the input signal to the compressor within a couple of percent.  That is, after all, what the thing was designed to do in the first place.  However, comparing the signal going into the expander and that coming out, I just could not reconcile it with my spreadsheet.

There's a square-root in the gain equation for the compressor, and the equation works perfectly in real-life.  Have a look at this excerpt taken from the relevant page of the OnSemi datasheet:



On most datasheets, however, you'll see a square term for the expander section, as above.  The truth is, that square term should not be there.  One might think that symmetry would dictate that it should be and, clearly, so did the person who originally made this error, which most people have copied verbatim.  The equation is correct on the Philips datasheet for the SA570, by the way, dated 2003 on the version I have squirrelled away on my hard drive, and the 2 chips are direct drop-in replacements for each other.

In order for the sums to work correctly, Vin(Avg) is defined as 0.9*Vrms, for a sine wave test signal, and (for those who need it) Vrms is V/SQRT(2), where V is the amplitude, or ½Vp-p.  The resistor values in the equations are those internal to the chip, values as follows: R1 = 10k, R2 and R3 = 20k.  So, work out Vin(Avg), do the sum for the compressor gain and then multiply by Vin again to get the output amplitude.  That's your "new" Vin.  Now do the sum again for the expander gain (work out Vin(Avg) based on "new" Vin), multiply by "new" Vin and you should end up with the same as the original.

Bottom line... See that test circuit on the datasheet?  How about building it and proving that it works?  99+% of the time, it will be exactly as advertised, but at least you proved you haven't got a duff chip.  If it doesn't work, try another chip and then start investigating further if your results are consistently different to the datasheet's.

Lastly, a big "Thank You!" to all those who have devoted a great many hours messing around with the PT2399 to find out how to wring every last drop of performance from this thing

[rring]

I am very happy someone else has built this thing and verfied my basic performance.

Speaking of bad data sheets - I just recently was working with a data sheet for a high speed optoisolator evaluation circuit that had mistakes and I spent an hour figuring out what was going on, finally I got it working.
It is wise to be skeptical!

I am going to post a a PC board soon on my blog: www.circuitsalad.com. I am going to create it using the expresspcb.com  tool(free) so anyone can have it produced directly from expresspcb.com or mod the layout, etc and make it their own. Going to do it with only a couple of SMT parts and will provide a part  list.

[Mark Hammer]

I finally listened to the Youtube soundclips.

Don't let me know where you live, because I will come to your home and steal that gorgeous archtop! 

[rring]

Dude - that guitar is so very nice. I got it off of ebay for $700.00 and change. Its is all solid wood (no plastic anywhere) and it is some weird guitar made for some promotional trade show - with serial number 00001. The company never got going - so go figure.
Normally, I like beaters beause I experiment with them and busk alot but I had to grab that sucker!

[Mark Hammer]

Well, without wishing to appear a brown-noser, you do it justice.  Normally, we hear designs or pedals demoed with the same tired blues licks (and I would be a card-carrying member of that club if I made more videos).  How sweet to hear some lovely comping and interesting chord changes!

I hope that axe has a long and productive life in your hands.

[rring]

well thanks
you will be happy to know I am popular with 60 and older housewives. Which of course is everyone's performing fantasy....

[Jdansti]

^^+1 Very nice! 

You might have mentioned it earlier in the thread, but which battery powered amp are you playing through?

[rring]

Yes that amp is home brew - the schematic and design info is on my blog.

[Jdansti]

Yes that amp is home brew - the schematic and design info is on my blog.

Thanks-It sounds great.  I'll check it out!

[Michael Allen]

Can anyone enlighten me to the purpose of R9? It is a 10k going to the inverting input, within the negative feedback loop. I've never seen this before so I'm curious. Is this common, and can someone tell me what to google for to read up on it?

[R O Tiree]

That opamp is wired up as a Multiple Feedback Path 2nd Order Low Pass Filter.  It's got a gentle roll-off, starting at about 200Hz (where you first notice the gain dropping) and it's quite severe from about 1000Hz upwards.

If you haven't got it, download "Opamps For Everyone" edited by Ron Mancini (for Texas Instruments in 2002).  It's a mine of useful info and circuit fragments, how-tos, how-not-tos, all the equations you will ever need... well worth a read and the first edition is absolutely free.  The second and third editions will cost you some moolah, but the fundamentals won't have changed.

[John Lyons]

Thanks for the link RO. 

[Michael Allen]

Excellent! Thank you so much!

[Jdansti]

My thanks as well!!!  Not to go OT (but I guess I will ). Does anyone know if the forum supports something like a library where we could post links to reference documents? It would be a shame to have valuable references like this buried in thousands of threads.