Help needed: AD-3208 delay effect loop

[cathexis]

Hi!
I just finished building Scott Swartz' superbly sweet-sounding analog delay, the AD-3208. I used the schematic from General Guitar Gadgets, and I built it on vero (yes -it's HUGE ).

*****************
Note to aspiring builders: check your pinouts for the 3208 chips - the pin numbering on the schematic refers to the 14pin-sized chip, and the pins on that one does NOT correspond to the 8pin 3208! This has been noted in several other posts, but worth mentioning again. I spent hours reworking my board to correct this.
*****************

I really like the sound of this delay - I've built several of the 2399-based digital delays (I loved Slacker's Echo Base), but I still found it worthwile to build this one. The way the repeats tend to lose highs AND lows on long feedback settings is very sweet!

I'm ready to box this one up now, which brings me to my question:

I really like to be able to process the delayed signal without affecting the original signal - I did the effects loop mod on my Rebote, I really dig the modulation part of the Echo Base, soft vibrato on the returns is great.
Where do I put send and return jacks on the AD-3208? I've searched the forum and found the referral to Moosapotamus mod for the PT-80, but I couldn't figure out how to implement that on my AD-3208 (and I don't need to be able to switch between the straight and the delayed signal anyway - just an effects loop for the returns would be fine). I suppose the send/return jacks should be placed somewhere close to the delay level pot, I tried to put them just after the 1uF cap coming from pin 6 and 7 on the SA571 chip, but that didn't sound right...

I'd appreciate some guidance here

And if anyone is interested in one of the most convoluted and afterthought-ridden vero layouts ever - just let me know...



Ain't she pretty??

LARS

[sean k]

Heys Lars, I'm going to make one of those so this is what I'd do. For starters I'd remove the 10k between the two opamps, point A, and put in a 100k pot to help define the level of the original signal with whatever you bring in. Then I'd do this.

[cathexis]

Hi Sean,

Thank you for your swift reply. I'll need to spend some time with your schematic to figure out what it all does. First out after pin 12 of the 571 is a 3PDT switching between the original signal path and the send/return path - I need to switch this on in order to engage the loop. The send jack has a variable resistor for level adjustment before hitting the external effect... I think I'm with you this far, but then there's a SPDT, RC networks and a FET... What's going on there? I've got a 4PDT lying around - could I use that for a neat solutin? Please help me understand this, if you have the time. Of course, I could just breadboard your circuit and try it out.
Intriguing as your solution is, I was really hoping for a simpler setup where I would not have to add any extra controls apart from the send/return jacks á la the effects loop that can be used for the Rebote (or the Skyripper, for that matter), but maybe that's not doable with this circuit...

LARS

[cathexis]

Amazingly, there was some interest in this veroboard layout - probably one of the nastiest I've produced, but I'll post it anyway. I've made accommodations for the huge 1uF TF caps that i had lying around (the design calls for several 1uF NP caps). The layout got extra complicated since I had to rework it after finding out about the different pinouts for the 3208 chips...
Seriously friends - buy the board from GGG or etch your own,  this layout is for vero masochists only It works though!

Any other takers on my effects loop question?

LARS

[mth5044]

o.
m.
f.
g.

That is beautiful.

[sean k]

Hey Lars, The return goes into the 2N5457 and the switch allows you to reverse phase by either taking the drain signal or the source. The two pots I suggested are just to make things adjustable but I suppose you could use the volumes in the effect loop so they don't need to be there. You could get away without the 3PDT switch and just send out a parallel signal but when it came time to mix it back in it would need to be summed with the original or switched in and the original switched out. Like I said, thats what I'd do because I like the control and once I'd done it it might have issues and I'd be here asking questions myself.

The fet might need a resistor or two to stabilise it but I think its a viable way to switch phase easily and you'll see it implemented in octave schemo's.
So the schematic pin numbers are not for the 8pin BL3208?

[idiot savant]

i did a simpler loop based on charlie's(moosapotamus) pt-80 loop for my ad3208 build

[cathexis]

Sean: Thanks for your explanation - so it's a phase reversal switch, neat idea! And yes, the schematic pin designations are for the 14-pin chip (just like the GGG PCB), and if you build it like that using the 8-pin chip you won't have a delay, you'll have a noisemaker on par with the Atari Punk Console. Trust me, I know . If not for this forum, i'd still be debugging in vain... My layout above is for the 8-pin chips, obviously.

Mr Savant: (I'll stay away from calling you by your first name ) What pin on the 571 chip does Moosapotamus schematic refer to? Is it pin 12 as well? Did your loop work without issues?

[sean k]

Okay, so the chip is just reversed. No awkwardness there. Good Golly, I've never ever done a veroboard 'cause I started straight into etching so following your layout was quite an education. I thought designing for etching was sometimes difficult but you veroboard guys are definitely into extremely puzzling options. It reminds me of a friends desciption of a one dimensional universe where you can only go one way... but then again its also like building with four by twos; the curves have to be made up by building different sizes squares.

I'm kinda glad I haven't even seen veroboard for sale! I can understand, though, that the challenge is definitely there and it would be a good mental exercise but I'll leave it to you guys   

[cathexis]

Here's the difference in pinout between the two chip types.


And yes - vero board is a challenge. I make many of my layouts on paper, partly because large designs tend to choke Bancika's DIY layout creator memorywise, but also because it's like a big crossword puzzle that you can bring along. The AD-3208 layout is drawn in my summer house, partly by candlelight, since there is no electricity there. I hate crossword puzzles, sudoku etc, seems so pointless. This is way more rock 'n roll.  

[sean k]

Thankyou Lars, thats a great help. So the ground and power are in the same corners but everything else has beed rotated 180 degrees. Now I suppose I'll get to designing my own DCB for it (drawn circuit board)

When I first looked at the schematic, after your question, my initial idea was to cut in and out at the input of the first 3208  with a rational that the 571 had compressed it then the filter around the 5088 has bandpass'd it so why not send it out and return it there?

There are potentially lots of places to send a signal out and then return it and the big question is mostly about what you'd want to add, or even subtract, with an effects chain. I mean you could even send it out after the 3208's and return it before them. Something like that might work if your into endless repeats and put an equaliser in the chain because, after all, it would just be a repeat type circuit. Okay, maybe thats a bit far fetched but I think knowing what you'd have in the send return chain and what you'd have it there for would allow you to put a send and return in the best place so you could acheive what you wanted to acheive.

Because you've got your board up and running why not just get a bunch of jumper leads and lift a trace or two and experiment a bit?

[cathexis]

Sean,
Thanks a lot for your suggestions! All I really wanted with the effects loop was to be able to add a little modulation to the returns, like vibrato or phasing. The more I play with this pedal, the more I like it as it is. I admit to having at least three other delays with that option - guess I was going to mod for the sake of modding, really . Maybe my AD3208 will be "stock"...
There's a sound clip in my brand-new gallery:

http://www.aronnelson.com/gallery/main.php/v/cathexis/

LARS

[mth5044]

Ah thats a great clip, as well as the rest of your album. Very impressive stuff. Its surprisingly clean too for an analoge delay. Great job.