A different approach to the PT2399 delay

[rring]

Hey there
I am new to the forum but I design alot of electronics so I thought this may be of some use to other builders.

Like many, I have struggled with getting a decent sounding delay from a PT2399. I have been working on very simple design that really performs well and only has 1 PT2399, 1 JFET and 1 MAX7401 in the signal chain.
I want to post the schematic here but not sure how? It can be seen at my personal circuit blog: www.circuitsalad.com. There is also a picture of breadboarded prototype and an audio sample from this prototype.
What is different about my approach is that I make the cap between pins 9,10 much larger than normal. This helped with my siganl to noise ratio significantly. I also use a 8 pole switch cap filter (MAX7401) as the output filter. It works great and only requires two decoupling caps and a cap to set the corner frequency (1Hz to 5Khz total range). Finally I have a leftover opamp on board the PT2399, which I use for the wet/dry mixer. So this design uses no other op amp IC's. The part count is small and I could not be happier with how it sounds.

[Kesh]

press the image button and include the picture's location inside it

like this (if you press quote you will see how my post included your pic)

[artifus]

the space station called, they said the pic looks good!  

yes like that but with smaller images and a link to the the larger image should it be required.

*edit* anyone know the board code to resize images without having to do it manually in another prog?

[rring]

DUH ----thanks!

[Kesh]

the space station called, they said the pic looks good!  

yes like that but with smaller images and a link to the the larger image should it be required.

*edit* anyone know the board code to resize images without having to do it manually in another prog?

i sometime forget i have a giant monitor

[Pyr0]

Hey rring, that audio sample of your delay sounds very nice.
I also like your portable 20W guitar amp circuit.

[PRR]

> anyone know the board code to resize images without having to do it manually in another prog?

See the button at the top of the forum? This leads to forum-software Help.

It takes a little digging, but SMF Bulletin Board Code answers your question.

[Jdansti]

Welcome to the forum!  Great reverb and other stuff on your site!  Looking forward to seeing you more here on the forum.

[Marcvv]

Very interesting approach and it sounds very good! Worth getting my breadboard out.
Thanks for putting this up here.

[mth5044]

It is a bit of a map to read, with all the labels, but the sound clip sounded great! Also, those MAX7401's aren't cheap!

[Ronan]

Those Max7401 might also be useful for filtering in other effects, specifically, wah pedals and formant pedals.

Nice to see some new ideas and component choices.

[~arph]

pricey little bugger though that max ic. and not available at our favourite FX stores.

[rring]

Yes the MAX7401 is not cheap @ $7.00 - $8.00, but it is very easy to use and adjust filter response - gets the part count down! One thing you could do, that would be interesting, is to use the clock from the PT23999 to drive the MAX7401. As the clock slows on the PT2399 it will lower the corner freq of the filter  allowing  the filter to dynamically "track" the noise.

Having said all that, you can easily throw in a dual op amp set up as a four pole filter and I bet it will work great. You would not need a coupling cap either from the PT2399 to the opamp filter because it will provide the 1/2 supply reference for the opamp when direct coupled. A great choice for such an op amp would be the OP1652 low noise fet op amp - it works  fine @ 5 Volts has rail to rail output and extremely low noise. I will post an alternate 4 pole analog filter design.

Someone mentioned all the labels in the schematic made it difficult to read. Sorry about that - I use all those labels because it helps me with PCB layout and avoid showing crossed wires that may be mistakenly seen as connections. I can chill on the labels in future postings!

[~arph]

Yes the MAX7401 is not cheap @ $7.00 - $8.00, but it is very easy to use and adjust filter response - gets the part count down! One thing you could do, that would be interesting, is to use the clock from the PT23999 to drive the MAX7401. As the clock slows on the PT2399 it will lower the corner freq of the filter  allowing  the filter to dynamically "track" the noise.

That occurred to me too when I checked the datasheet, but I think we need to divide the clock as i think I read that the MAX has a clock freq range of well below 1Mhz.

[rring]

Yes you are correct the max 7401 clock is always 100X the filter corner freq - so it will require some flops!

Here is an alternate filter:
http://circuitsaladdotcom.files.wordpress.com/2012/09/alternate-delay-filter.gif

I posted some more detail and a filter plot at:  www.circuitsalad.com

[~arph]

You do have a 'vref' mismatch of 0.7 volts, so the signal stays between 0.0V and 3.6V. So that rail-to-rail opamp really is important.

[rring]

Yes  your point is important... it could be referenced to 4.5V and use the 9 volt supply. I thought it might help to share the vref on the Pt2399 and the supply, for the sake of reducing ground bounce digital noise, etc - it also eliminates a coupling cap and a resistor voltage divider.

[cpm]

you can divide the clock with a PIC, only one 8dip chip..., should be around 32 or 64 factor

[rring]

Thats a great idea - infact scrap the PT2399 clk and use the PIC to make and adjustable tone control by changing clk speed or heck just use a 555 timer with limited freq range adjustment to drive the clock. The MIC1557 is a small version of a 555 which always produces a 50% duty cycle output. It might be a good choice as would a little PIC. I wonder what it would sound like to modulate the filter a little bit as the repeats echo? Perhaps it would sound like a haggard old tape echo!

[Mark Hammer]

Yes the MAX7401 is not cheap @ $7.00 - $8.00, but it is very easy to use and adjust filter response - gets the part count down! One thing you could do, that would be interesting, is to use the clock from the PT23999 to drive the MAX7401. As the clock slows on the PT2399 it will lower the corner freq of the filter  allowing  the filter to dynamically "track" the noise.

Having said all that, you can easily throw in a dual op amp set up as a four pole filter and I bet it will work great. You would not need a coupling cap either from the PT2399 to the opamp filter because it will provide the 1/2 supply reference for the opamp when direct coupled. A great choice for such an op amp would be the OP1652 low noise fet op amp - it works  fine @ 5 Volts has rail to rail output and extremely low noise. I will post an alternate 4 pole analog filter design.

Someone mentioned all the labels in the schematic made it difficult to read. Sorry about that - I use all those labels because it helps me with PCB layout and avoid showing crossed wires that may be mistakenly seen as connections. I can chill on the labels in future postings!

1) Nice work.  An auspicious debut! 
2) What sort of clock frequency is the MAX7401 running at?
3) Your suggestion to harness the clocking of the MAX7401 to the clock of the PT2399 remnds me of the old green MXR Analog Delay, that also used CMOS-switched resistors to tune the filter corner frequency to the delay time.  The master HF clock would step the BBD through its paces, and a series of flip-flops would divide down that HF clock to switch the 8 poles of lowpass filtering to achieve maximum bandwidth for the delay time.