New Looper Design

[Jaicen_solo]

Ok ladies and gentlemen, i'm ready now to unleash my looper on the world at large
Below is the PCB and parts layout for my deluxe looper. Since this is quite far removed from the Mobius, i've re-named it Lazarus for reasons I'd think obvious
I'll draw up the schem tomorrow, but i'm fairly sure that it's 100% correct. I've been over the layout a number of times and fixed a couple of errors.
I do need to re-arrange the rectifier on the Mobius Lite i've already posted, as the pinout is wrong. The top two pins need to be reversed, and there's an error on the biasing of the TL074.
The PCB needs to be printed at 300dpi, should be sized to 3.9"x2.7" (97.5x67.5mm for you europeans and modern englishmen like myself )
As always, thoughts and suggestions welcomed.

edit: New revisions posted below.

[robbiemcm]

Alright, I think that makes sense. So the wiper of the Play pot goes to a capacitor and then to an output jack. And that's the loop play. The other signal will be coming from the output which already has a capcitor there? And when I recombine the signals later, do I need any special buffers or something?

[Jaicen_solo]

Yeah, you probably will need to recombine the signals using some sort of buffer. There's no way to know how different pedal combinations are going to react when passively combined. Particularly if one or more is bypassed.
To be honest, I think you might be better off building the Lazarus, as it it has everything you seem to be needing all on one board. It's not that much harder to build either, though the wiring is slightly more complex. I've uploaded a wiring diagram for the Lazarus, have you seen it??

[robbiemcm]

Hmmm.. I still like the idea of the Mobius Lite for my ideas. It would allow me to change the sound of both the loop and the straight up guitar signal. For example, if I wanted distortion on the guitar signal I wouldn't have to have it on the loop aswell. It can't be that hard to build some form of special circuit with a buffer purely for recombining the signals can it? Wouldn't I use the same principles as you except I would just be doing mine in a seperate box.

And just wondering this..

So the green wire off the pot is the loop out and what was originally the combined out is now straight guitar out?

[Jaicen_solo]

So the green wire off the pot is the loop out and what was originally the combined out is now straight guitar out?

Yeah, you got it now. Like I say, you'll need some sort of buffer to recombine them. I see where you're coming from now. I think R.G. has schematics for a splitter/buffer. There's certainly one at GGG.
One thing on a similar vein i'd like to note is that i've inserted trim pots onto my lazarus board, to fine tune the send/return volumes of the effects loop. You might want to try something similar.

[robbiemcm]

Ahhh... isn't that what the play volume pot is for, you know.. the one I'm taking the loop output from? As for straight guitar I suppose it would be easy enough to chuck a volume pot onto that output.

One thing I've always wondered with volume pots. Is it okay to have that final cap before the volume pot, or is it better to have it between the volume pot output and the output jack?

[Jaicen_solo]

Yeah, you can use the play pot for a master volume control, but you might find that the signal coming out of your effects varies in strength, so I added the option of decreasing the gain somewhat at both the send and return. That way i've covered all the bases so to speak.
Usually with volume controls, you want the cap on the wiper, that is going back into the circuit. That will protect the output from any scratchy DC which might be getting onto the pot. I'm not sure how much difference it makes putting it before the pot, but that's the way I try to do things.

Anyway, here are the layouts and PCB's for both versions of my Looper. The schem is also shown, which is the same for both versions, except for the effects loop.
I'm going to say they're complete for now, unless anybody spots any errors, I'd appreciate them being checked.

MOBIUS LITE:


MOBIUS LITE PCB:


LAZARUS LAYOUT:


LAZARUS PCB:


LAZARUS SCHEMATIC:


DIGITAL SCHEM:

[robbiemcm]

Well done on getting those complete, Jaicen. It must have taken alot of hard work, so thankyou. I was looking for the TL074 and LM317 and I seem to have found myself a few different types. Are they all pretty much the same? Some seem to be double the price of others? Maybe you can help distinguish between them..

TL074: Search for it on www.futurlec.com it has two possibilities

LM317: http://www.altronics.com.au/index.asp?area=item&id=Y0926
http://www.altronics.com.au/index.asp?area=item&id=Z0545

I can get all the other components easily, but I'm just not sure which of those are suitable.

Thanks once again, hopefully I'll be building this beast some time over the summer holidays (chances are it'll have to wait til after I get back from England).

[Jaicen_solo]

It wasn't really all that much work, but thanks!

You can get an LM317 from Futurelec for $0.45, that should work fine. The second of the accutronics rectifiers should work fine too. Don't spend too much either way. 100mA is more than enough for this project.

The TL074 you can use just about any 14pin DIP package. At futurelec you want the TL074CN, which is the one I use. The CD is surface mount, which would make life pretty difficult for you
I'm getting all the components I need to build mine tomorrow hopefully. I'll keep you updated.

[Herr Masel]

Thanks for your work. I would like to build a looper but I'm having trouble finding the ICs for the pt80 delay so I don't know if it will be possible, unless I have them shipped from far away..

[Jaicen_solo]

Well Herr Masel, i'm not sure I fully understand your question. The PT80 is not used in this project, it's a delay chip. Going by your name, i'd guess you are German, or at least european. In that case, look up your local RS-Components, who will have an ISD25xx in stock. This is the chip you'll need for this looper project.

[mathflan]

fantastatic work jaicen!
glad to see your looper's works finished.

I have  a question about your FX loops schematics..
But How is it works without Effect in the FX loops, Do the Play pots fuction??
On the dirtysam, it uses Jack with CLosed circuit...

[Jaicen_solo]

Thanks mathflan!

Yeah, the FX loop works with a break jack on the send just like the Dirtysam. basically, Send is connected to Return unless a jack is inserted into the send socket. When this happens, the connection is broken, and the signal goes out to the external loop, and back into the FX return via the Return socket.
I guess you could add a DPDT switch to toggle the FX loop on and off too. Play pot controls the loop volume wether an effect is in the loop or not.

[mathflan]

ok, but I'am asking if is it good that the signal pass trough two Opamp ?? when you don't plug jack into FX loop...
For Me I thought that when you don't plug jack in FX loops,  the center of the PLAY and the Resistor 33k are connected directly??

[domenico]

hi ,
some months ago I've tried those schematics from isd's web site and I've found that if you don't use a circuit similar to http://humano.ya.com/guruzeta/Guitarra/dirtysam/digital.html you will have huge pops when pressing record/play switchs
domenico

[robbiemcm]

Tell us how it goes Jaicen, it will be interesting to see if these pops are present like domenico suspects.

[Jaicen_solo]

Hey domenico, thanks for your input. Would you care to share the schematics and/or designs you were working with?
To be honest I don't see why there would be pops, since the CMOS circuit is doing pretty much the same job as mechanical switches.
There was some discussion between Zvex and a few others about the Power Down feature causing thumps when the chip is turned on again. Apparently it has something to do with the input impedance of the amp they're connected to. I'm not too sure, but we'll soon see if it's a problem!
Mathflan, that was something that didn't occur to me actually! I guess it would be easy enough to do, just a case of wiring things up differently. I don't suppose it makes a heap of difference to have the signal buffered again, it's non inverting and low gain, but I'll give it some thought.

[domenico]

I don't know why,  but the isd25xx series make pops when switching from record to play or when resetting the chip using mechanical switch.

as I can see the pop is a common problem on isd devices , read this
http://www.winbond.com/c-wINBONDHTm/partner/PDFresult.asp?Pname=593 ,
http://www.laserforums.com/forum/archive/index.php/t-313.html ,
http://www.winbond.com.tw/e-winbondhtm/partner/b_2_a_4_d.htm
or search on google : isd2532+pops

The only thing I know is that I've solved the problem using a digital circuit similar to dirtysam's one , try it !

[robbiemcm]

Hmmm.. I got this off the laser website that Domenico linked us to..



That seems to be an incredibly simplified version, or is that one not at all suitable for what we intend ours for? I read that the better a circuit is designed, the less audible the pops will be. And considering that abrely has any caps I'm going to assume you'd get some big ones, also I've read about not having caps at the output lets DC in to your amp or something.

[Jaicen_solo]

That is a pretty simplified version of what i've built.
The schematic shown there will not loop, only playback once and go back to the start.
I can assure you i've waded through all those pages and more. I read all the technical articles at winbond too, including those for the other chip series' such as the 3300 etc.
Like i've said before, the popping sound us usually caused by the chip coming out of power down mode, and there's not really much that can be done about that if you want the looper to start from the beginnine each time it's bypassed. Even the Lo-Fi loop junky has this problem with certain amps. I'm driving the output of the chip differentially, so hopefully that will help somewhat. I've also applied loads of filtering to the power supply!