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.

 

[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).

 

[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

 

[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.

 

[DGWVI]

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

 

[HamishR]

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

 

[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.

 

[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.

 

[DGWVI]

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

 

[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.

 

[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.

 

[Chuck D. Bones]

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

 

[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.

 

[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.

 

[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.

 

[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.

 

[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...

 

[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.

 

[Chuck D. Bones]

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.

 

[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.