# So this is where I am with my Pandora...



## HamishR

I've read a few things here and there about ways of adding some beef to the Pandora circuit.  I am surprised at how much I like this pedal but have always felt it lacked some grunt in the low end. So with my latest version I have increased C13 to 470nF (from 100nF) to let some more beefy goodness through, added a 470nF cap in place of C4 (22nF) and swapped C14 from 1µF to 2µ2.

My goal is to add some punch overall and help the "forbidden" setting have a little more Big Muff style sludge.  I'm not quite there yet but I really like what I have now.  So I intend to keep this version as it is now and build another with even more low-end. I think the sound as it is now is a big improvement for my tastes and also still very usable in a band context.  Obviously too much low end can be a bad thing!

Adding some lows makes all the settings more useful to me.  Even the lowest gain setting is more useful with some extra grunt - it's a surprisingly good OD.  I guess my "ideal" Pandora would have a bass pot.


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## phi1

Nice. For the next one, a classic way to add a bass knob (like the Timmy and many others) would be to replace R6 with a pot and resistor, maybe 47R fixed resister and 1k pot. If you do that, you may want to adjust C8 since the bass cutoff frequency of that branch will go up when you decrease resistance (f=1/(2piRC)). You could also try increasing C9 to let some more low mids thought that branch (currently cuts off around 1.5khz).


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## DGWVI

You could increase C8 to 22u (lowers to cutoff to 12hz), and c9 to 3.3u (lowers the cutoff to 1029hz) , as found in the GCI Brutalist Jr. 1k trimpots in place of R6 and R7 probably wouldn't be a bad idea, if you did increase the caps. 
The bass boost from the version of the Expandora that had it, just swapped the output cap (C14) to 10u


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## Chuck D. Bones

Here's what I did with mine...
I connected a C5K *CONTOUR *pot + 47uF cap + 22R resistor in series and then put them in parallel with C9.  With the pot fully CCW, it's the normal Expandora bass rolloff.  As the pot is rotated clockwise, more and more bass comes in until at full CW the response is basically flat from 50Hz on up.





I also added a CLIP switch, similar to what reubenreub did here.

BTW, all of those cap changes you made affected the frequency response below 4Hz.  C14 might help a little if you're driving a real low impedance pedal.  The bass response is really all about C8, C9, R6 & R7.


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## DGWVI

Chuck D. Bones said:


> Here's what I did with mine...
> I connected a C5K *CONTOUR *pot + 47uF cap + 22R resistor in series and then put them in parallel with C9.  With the pot fully CCW, it's the normal Expandora bass rolloff.  As the pot is rotated clockwise. more and more bass comes in until at full CW the response is basically flat from 50Hz on up.
> 
> View attachment 2114


This is an idea I'm going to have to give a try. Love it


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## HamishR

Thanks guys!  I will try your suggestions in my next installment!  That bass pot sounds exactly like what I am chasing.


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## Chuck D. Bones

I should have mentioned that this mod also works on the RAT.  Compare the Expandora & RAT schematics and you'll see why.


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## HamishR

I just tried the bass pot mod that Chuck has detailed here - oh wow, it's awesome!  It's EXACTLY what I wanted.  So far the only issue has been that if you use the forbidden setting with gain up past noon it has started a weird oscillation where it behaves like an on/off setting - like a gated tremolo.  Very odd.  But apart from that it's amazing.

It really makes the pedal much more useful.  Brilliant.  Thanks Chuck!  I feel like building a Rat now just so I can use this mod.


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## DGWVI

HamishR said:


> So far the only issue has been that if you use the forbidden setting with gain up past noon it has started a weird oscillation where it behaves like an on/off setting - like a gated tremolo.  Very odd.


That's what the forbidden setting is supposed to do. It's basically forcing the op-amp to put out much more gain than it can handle. It happens with 308 and OP07 equipped RATs with the gain cranked and high output pickups as well


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## Chuck D. Bones

In the _Forbidden Mode_, the opamp runs "open-loop." That means there is no feedback to limit the gain or set the operating point of the opamp. The open-loop gain on an LM308 is typically in excess of 100,000. When the guitar signal is large enough to open the noise gate, the opamp slams back and forth from rail-to-rail, producing the characteristic _Forbidden Tone_.

I didn't go as far as the _Forbidden Mode_ in my Expandora, but I did replace R8, R9 and the two toggle switches with an A10K (DRIVE) pot in series with a 240R resistor. The Expandora 2001 does the same thing, except with a B2K pot. Higher setting of GAIN and DRIVE get the Expandora well into the range of RAT tones.

That got me up to 5 knobs on the front panel, but I wasn't done screwing around with the circuit...
The Expandora's noise gate function is simple and subtle.  But with a little tweak, it's capable of producing splattery, gated, ripping tones.  I replaced R15 with a B5K (GATE) pot and replaced R16 with 470R.  I wired the GATE pot to be maximum resistance when full CCW.  When GATE is turned all the way down, the box behaves like a normal Expandora.  At higher GATE settings, notes decay into dying amp crossover-distortion and then finally a quiet clean tone.  The GAIN knob controls when the gating effect happens, the GATE and DRIVE knobs control how nasty it gets.

Here's a pic.  The left toggle switch selects clipping diodes.  The right toggle switch puts the pedal in RAT mode by disabling the noise gate.


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## HamishR

Well I'm happy with my Expandora now.  It didn't do the weird tremolo thing before - it just gated like crazy, which I love.  It seems that if I can keep the gain knob high enough to get some sound but low enough I can avoid the tremolo thing and just get lovely gated, crackly dirt.

The tremolo thing is strange - it's like a gate opening and closing but it's not random at all - it's a pulse.  My pedal didn't do it before.


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## Chuck D. Bones

It's gonna react differently if you crank up the bass.


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## HamishR

Yeah I worked that out!      Seriously though I really appreciate your knowledge. You have helped take my pedal from something I would only use as a novelty to something I can use on my pedalboard.  Fab.


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## Chuck D. Bones

I'm happy to help.  I was thinking the "tremolo" action might be related to the increased bass response.  Running wide open like that, the opamp can be pretty unpredictable.  Minuscule bits of sound can drive it to saturation, even subsonic stuff.


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## HamishR

So I did build a Rat today to try out the bass pot mod.  It really works, and sounds killer!  While I was in there I swapped from a 3n3 cap to a 6n8 cap to roll off a little more fizz and I really like what that did too.  So now I have a very usable Rat style OD/distortion.  I never really liked Rats before.

Chuck you are a treasure chest of info.


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## Chuck D. Bones

Good idea!  I bumped R7 up to 6.8K in mine, but I like your solution better.  I'm still fiddling with the diodes to find the combination I like.


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## HamishR

Yeah I was wondering about that also...My fall back diode selection is a pair of 1N4001s and a pair of 1N4148s.  So each opposing side has one of each diode.  I might have to see what that sounds like with the Rat.

Also might try a C100K pot for the filter and turn it around to work like a normal tone pot.  Otherwise I'm sure I'l forget...


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## HamishR

And then I compared the sound of the Rat to the Expandora - hmmm...  They are quite similar aren't they?  Now I have the bass pot I am even more impressed with the Expandora.


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## Chuck D. Bones

HamishR said:


> Yeah I was wondering about that also...My fall back diode selection is a pair of 1N4001s and a pair of 1N4148s.  So each opposing side has one of each diode.  I might have to see what that sounds like with the Rat.
> 
> Also might try a C100K pot for the filter and turn it around to work like a normal tone pot.  Otherwise I'm sure I'l forget...



I did that with the tone pot on mine.  I'll have to try out that diode combo.  I like LEDs and I like asymmetric diodes.


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## HamishR

It's all the more interesting when you realise that a fair few BJF pedals like the Honey Bee are based on the Rat circuit...  He uses various coloured LEDs as clippers, as well as 1N400x diodes.

Now I want to try a lower gain Rat - a lot lower because there is way more gain than I can use there!  I'm guessing a 10-25K gain pot would help for a start.


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## phi1

I love the Honey Bee as a low gain overdrive, but if you expect it to sound like a rat at low gain, you might be disappointed.  The Honey Bee has soft clipping LEDs, a different corner frequency for bass roll off, a different tone control configuration, the extra feedback loops... basically everything that gives the honey bee its characteristic sound (frequency response, clipping character, dynamics) is different than the rat.  That said, I think both have their place as great effects.


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## HamishR

Oh I agree!  I've built and played the HB a lot and appreciate that there is a lot of difference between them!  There are things I like and don't like about both.  One thing I like about my modded Rat is that the low strings are quite clear at low gain, and that's something I really like.  So if I can extend the low gain range a bit more - ie lose some of the really high gain area - that would be worthwhile for me.  As it is I doubt I would ever use it with gain above 10.00!


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## HamishR

Well, after getting my Expandora to sound amazing (better than I'd hoped) I have now also built myself a Rat which I really like.  This is what I love about forums like this - one thing leads to another and you never know where you'll end up.

So today I built a Rat with the bass pot mod, used a 5n6 cap instead of the 3n3 on the tone pot, a 47nF input cap and an A10K gain pot.  I also used a 3u3 cap from the 47R resistor on pin 2 to ground - not because I felt it tonally superior but because I ran out of 2u2 caps.  

And it sounds great!  With my Gibsons it's a simple, great sounding OD with a very tight but full low-end.  I would like to increase the lower mids a little but it's a very usable overdrive as it is.

And all of this because of an Expandora.


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## HamishR

So a small confession - the Expandora I built for this thread was on a board from Fuzz Dog.  I have now built another with a Pedal PCB board and an exterior bass pot with what I have learnt from Chuck and it's a better pedal - again, thanks to Chuck.  It seems that the Pedal PCB board is better designed because I have no issues with weird tremolo effects - in this case that is a good thing!


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## HamishR

So the changes from stock were the bass pot mod - which I can show a vero layout for - and I added a 330nF cap to C13 to make it a virtual 430nF cap. It does make a difference - it helps my Strat sound fatter and squawkier.  If I was to build it again I would just use a 470nF cap for C13. This was from an idea from Fuzz Dog.

Here is where I added the cap - the board looks terrible in this pic.  I think it's reflections because it doesn't look this bad in real life.


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## HamishR

And here is everything bolted down.  You can see the bass pot mod very floating around there - don't worry, I'll be putting something around it to keep it safe.





And here is the incredibly simple bass pot mod as described to me by Chuck. I grounded it at the output jack alongside the board ground. The connection to the board is underneath at the + connection of C9.  I'm surprised at how much I like this pedal. It's actually a great sounding overdrive/distortion/fuzz pedal. It's a lot of fun and I am so grateful to Chuck to helping me get the most out of it. For me it is so much more useful than stock now. Not only can I get great Strat bridge pickup sounds from it but the neck pickup on the Strat is fantastic through this - really good!


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## Chuck D. Bones

I don't see how making C13 bigger has any audible effect.  C13 (stock value=100nF) and R19 (1M) have a 100ms time constant.  That equates to a bass rollof freq of 1.6Hz.  Adding 330nF in parallel with C13 extends the freq response at that point in the circuit down to 0.4Hz. Either you hearing is  extraordinary or...  

Nice build though.


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## HamishR

Yeah I don't know either!  But it definitely made a difference, or else I'm crazy.  Could be either. I am used to making changes to things which shouldn't work according to people far wiser than myself and finding I prefer it. I have been told that Sozo caps in amps don't sound any different from Orange Drops but I can hear it.  I can't hear the difference with caps in guitars though when the tone knob is on full - and I have told that I should. I don't know enough to know when I should hear things or not but in general I trust my ears because they're the one thing I have no choice with.    

I have tried to understand more about electronics but I get to a point where my head hurts and no more knowledge can fit. I don't believe in half the crap that mojo-guys try to tell me but sometimes I try something and it just sounds good to me.  Maybe I nudged something else when trying out my pedal after installing the cap - entirely possible.  All I know for sure is that I like this pedal a lot now and you're the one I need to thank for most of that - so thanks! It's great to have someone such as yourself help out and make my tinkering more worthwhile.


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## Chuck D. Bones

Listening is a subjective experience, which makes it next to impossible to be objective about it.  Suppose we built two identical amplifiers, that 10 of us agreed sounded the same.  Then you left for an hour and when you got back I told you I changed the caps in one of them from Orange Drop to SoZo.  What do you think would happen when you listened to them after that? 

The other mod I like for the Expandora, one that Bixonic did themselves on later models, it to replace the switches with a pot.


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## Boba7

Chuck D. Bones said:


> Here's what I did with mine...
> I connected a C5K *CONTOUR *pot + 47uF cap + 22R resistor in series and then put them in parallel with C9.  With the pot fully CCW, it's the normal Expandora bass rolloff.  As the pot is rotated clockwise, more and more bass comes in until at full CW the response is basically flat from 50Hz on up.
> 
> View attachment 2114
> 
> I also added a CLIP switch, similar to what reubenreub did here.
> 
> BTW, all of those cap changes you made affected the frequency response below 4Hz.  C14 might help a little if you're driving a real low impedance pedal.  The bass response is really all about C8, C9, R6 & R7.


Just came accross your post, thanks for sharing! I imagine it sounds about the same as the Ruetz mod (adding a 1k pot between R7 and C9), am I right?


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## Chuck D. Bones

Both mods flatten the freq response as you turn the BASS knob up.  The Ruetz mod lowers gain for the mids and treble frequencies. My mod raises the gain for the bass frequencies.  If you use lower DISTORTION settings, then the Ruetz mod will work for you.  If you prefer high DISTORTION settings, then my mod is the way to go.


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## Boba7

Chuck D. Bones said:


> Both mods flatten the freq response as you turn the BASS knob up.  The Ruetz mod lowers gain for the mids and treble frequencies. My mod raises the gain for the bass frequencies.  If you use lower DISTORTION settings, then the Ruetz mod will work for you.  If you prefer high DISTORTION settings, then my mod is the way to go.



thanks a lot, I’ll keep using the Ruetz mod then as I really like the lower drive tones it offers. Thanks again !


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## linjor

Just saw this and figured I would try it out on my Pandora's Box. Currently, it seems like nothing happens until the knob is basically all the way up. At that point, I do hear some increased bass response. The rest of the sweep has no effect on the bass.

I went over everything multiple times and couldn't find anything obvious. Any ideas on what might be going on?

I'm using a C5k pot and grounded it to the output jack.


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## Chuck D. Bones

You should hear the bass sweep across the entire control range.  You're connected to pins 2 and 3 of the C5K pot, right?

The effect is much more pronounced with the Rat because the Rat has more gain in the 2nd stage.  Try it in Forbidden Mode.


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## DGWVI

I've used it on various Rat based circuits, including the Expandora, and it's always worked worked as described


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## Chuck D. Bones

If you want to troubleshoot your build, then post in the Troubleshooting forum and include detailed pix.


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## Barry

Me thinks we need a Boneyard Pandora!


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## DGWVI

Barry said:


> Me thinks we need a Boneyard Pandora!


I posted @Chuck D. Bones Pandora schem in a recent Requests thread. I'd be more than down to build a few of em


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## Chuck D. Bones

I think the GATE control is totally optional.  It's useful if you want sputter, but is otherwise not needed.  Having two gain controls definitely adds flavor.


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## HamishR

After hearing how Linjor's build sounds I went and tried my Pandora's box with the mod - and I think I hear what he is saying. There is a gradual rise in bassiness but most of it _is_ at the end of most bassiness. And that's with a C-taper pot. I don't mind - I get what I want.

So I had a listen to a Rat I had built with a bass pot done in a similar way and which now sports a metal-can LM308n (!) and it behaves in a similar way. Again, it doesn't bother me. I get what I want.

So then, because _I always have to know_, I swapped out the 47µF cap for a 100µF. Ha! Now it's fun! Crazy FUN! Extra doom. And lots of varieties of doom along the way. Actually I can see it turning into a godawful mess with powerful HB pickups but with a Tele it's hilarious. Recommended.


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