Beating a dead horse named expandora...

[jayp5150]

I know it's been discussed, and I've searched (repeatedly), but no one seems to have posted anything about a successful Expandora build, or if they've even found an ample sub for the Sharp pc419 (does Mouser have a "hen's teeth" section?).

So, has anyone ever successfully conquered the Exp, or should I just start saving my change? lol

I'm sorry, I've just had a bad case of Gibbons tone-envy lately 

[soulsonic]

I have an original one.... I'd be willing to dissect or trace it or whatever you'd be looking for... I never use it (it disappoints me...).
I could pull the Sharp opto out of it and attempt to measure it to so we can figure a good substitute.

[Paul Perry (Frostwave)]

http://www.datasheetcatalog.com/datasheets_pdf/P/C/4/1/PC419.shtml
looks pretty standard, I imagine a Fairchild H11F1 or H11F2 would do (check the pin outs though!)

[WGTP]

Does anyone know if the new ones use the Sharp and/or the dual 308's and 4558? 

[Mark Hammer]

Looking at the schematic, it would seem you can use a lot of things in there.

The role of the Sharp photo-FET is essentially to provide a bit of dynamic expansion to the signal during intial pick attack.  The specific amount of expansion would depend on the combined parallel resistance of the FET and the switch-selectable feedback resistors.  When either of the parallel feedback resistors is selected, the action of the optoisolator is significantly reduced.  This occurs because the combined parallel resistance is always governed by the lesser of the two (or three) resistances).

When both dipswitches are closed, the feedback resistance will generally be around 370R.  With ground-leg resistances of 560 and 47R, there won't be much gain from that stage.  Of course, with the preceding stage providing a gain of up to 10x, adding a gain of 7.8 (for the high end) in the subsequent stage still provides some opportunity for clipping to occur.

In theory, at least, a person could mimic the properties of the Expandora using a different optical element (LDR, photo-FET), though the actual gain produced, or rather aimed for, would require adjustment of the various other passive component values.  You'll need to have your calculator by your side, but feasible, though.

The one aspect that might be a little tricky is the response time.  This drawing - http://home.hawaii.rr.com/stompbox/BIXEXP.JPG - shows the PC419 as a photo-FET, and not an LDR-type optoisolator.  The photo-FET will most certainly have a faster response time, with near instantaneous attack.  If that is essential to the tonal character of the Expandora, then you'd need to be picky about LDRs.  On the other hand, perhaps this is an occasion to use the H11F1/2/3 units you might have kicking around.

[BRingoC]

I have an original one, I tried opeing it once however the footswitch is wired directly to the board, when I tried to unscrew the bolt on the front I heard a cracking noise, that was the end of that.  Be careful if you are attempting to take it apart, its nearly impossible to brace the switch soulsonic.  Like soulsonic, I too havn't used mine once since I started making pedals, sounds just like a tubescreamer, the expander is subtle at best, and the 'extra' setting is not as good as some of the diy fuzzes.

[jayp5150]

Thanks for the replies. I'll dig into it a bit more, thanks for the datasheet link.

I think right now my head is just swirling from hype, and someone I know on another forum bought one and raved about it.

I'll study up a bit more and go from there. It would help if I could find one in a store around here, then I'd actually know if I wanted it or not lol.

Thanks again.

[tcobretti]

If memory serves, there was a layout floating around a while back that had two LM308s and a 4558 as the opamps.

[soulsonic]

For something that supposedly has an expander in it, it sure sounds compressed. I've been messing with mine a bit this week to see if I can get a good sound from it, and I'm now convinced that it only sounds good with single note stuff - with chords it sounds like total mush no matter how you have it set. The freak out setting can be fun to play around with, but I'd be hard pressed to find it useful in an actual song and like BRingoC said, most there's alot of fuzzes that sound better. The tone is way too shrill with most amps too. It's a weird pedal - with solos it can sound really sweet, but with any kind of rhythm playing it just sounds like any other cheap junky distortion box. Overall, I'd say its less interesting or usable than my modded Rat. It works okay as a boost with conservative settings - I'm going to use it at a gig tomorrow as a solo boost to see how well it cuts through... cutting through a dense mix seems to be one of its strengths.

[Gus]

The schematic section has more
http://www.diystompboxes.com/pedals/exp2.gif

IMO The adjustment of the gain is different than just turn it up, because the side chain and distortion sections are driven by the same signal with the only finer balance adjustment  being the gain switches in the distortion section

Look close the side chain(led driver section) is very simple it is driven by the first gain stage and it has no gain control of its own.

The rat section has some gain control via the switches

SO IIRC ideas posted before at this and maybe other forums

Add a gain section after the first opamp and before the side chain (extra external  pot)

Make the switched resistors into a pot and Fixed min value resistor (extra external  pot)

change the .1uf cap or 100K  after the diode in the side chain to a different value attack decay

[jayp5150]

^ I hadn't seen that schem before. Thanks.

Didn't the revised one have an external pot instead of the switches (that's what the "drive" pot was, I believe)?

I think this would be an excellent opportunity for me to dig a bit deeper into actually understanding circuits. I've been on here for a couple of years, but I'm still at that stage where I can follow a schem or layout, and understand basic mods, but I need to fully understand what actually happens to the signal through a circuit.

I'm back to the studying level I guess lol.

I appreciate all the feedback guys.

[Mark Hammer]

For something that supposedly has an expander in it, it sure sounds compressed.

What the initial expansion of the pre-clipping signal does, in theory, is produce more clipping at the outset of the note by presenting a higher amplitude signal to the diodes for the expanded initial attack.  Although this will affect the tone of the signal, the amplitude expansion of the entire signal will be wiped clean by the clipping diodes.

[soulsonic]

Such an odd circuit - a Rat with the weird expansion tacked on. In the "Freakout" mode you only have the resistance of the optofet in the feedback loop, so it suggests that it's certainly rather high because of all the splattery gain. It splats in a way that's similar to a stock Rat. I wonder why they use the additional 308 for the input amp? I could think of many other ways that could be done...
After studying and playing with both an Expandora and a Rat, I've come to the conclusion that you could potentially do a cool fuzz circuit with a 308... the weirdness of the overdriven 308 has an almost germanium quality to it when coupled with the hard diode clipping. Before this, I've tried to tame the weirdness of the 308 to get the Rat style circuit to have a smoother overdrive/distortion, but now I'm wondering what kind of results I could get if I exploited the strange qualities of the 308 to get it to be more of a fuzz. The freakout mode of the Expandora hints at some interesting possibilities for something that has a very old school fuzz sort of quality. The 308 begins to get a nice soulful feel to it when it gets pushed too far.

[WGTP]

The second version replaced the switches with a pot, but lost the "Forbidden" setting.  It also had more bass.

http://www.bixonic.com/

The third version has the original 2 switches, now external, plus a 3rd switch to toggle between the bass settings of the previous 2 versions.

http://www.godlyke.com/shopping/pgm-more_information.php?id=29&thiscat=10&frompage=Online_Store&page_num=1&=SID

Most the standard Rat mods should apply. 

I think it may all be about the box. 

[jayp5150]

I think it may all be about the box. 

LOL, and now we have Mick Farlow for that