Red Llama BASS mod question

[snk]

Hello,
I have built a Red Llama with a couple tweaks.
My goal is to make a very "clean" and transparent overdrive, with low gain and deep basses(i will be using it on a bass synth, not a guitar).
I have seen quite a lot of mods suggestions on various places of the internet, but I find the distorsion sound of my build still too audible and lacking some bass.
So, as i have already changed several component values (and some of them did not make any audible changes), i'm asking for suggestions for the best tweak to make to get more bass (maybe more "clean bass"?) and less distorsion...

Here is one schematic for reference :

http://beavisaudio.com/beavisboard/projects/bbp_RedLlama.pdf

This is what i have done so far :
- change the GAIN pot for a 500K.
- lowering R2 to 33K to get less minimum gain.
- Increase the input cap C2 for a 220nF. I have put 2 caps on a switch (68/220nF), but i don't hear any differences...
- Increase the first feedback cap C3 from 51pF to 68pF.
- use a switch for C5 to switch from 47pF to 100pF. I don't hear any difference at all.
- Increase the 33nF coupling cap (C4) to 68nF.


This is what i am considering doing :
- lowering the 1K resistor (the Anderton tube sound fuzz use a 100R), and i have read that lower values could lead to more bass and more headroom...
- Lowering R2 some more (from33K to 10K ?), to get even less distorsion at minimum gain.
- Increasing the coupling cap ?
- Increasing C5 (second feedback loop) even more (up to 200pF ?), to get a slight trebble roll-off ?
- Lowering the gain pot value even more (down to 250K), so i get even less gain from the pedal...


One question : Can i "tailor" the distorsion character with the feedback loop ? Maybe i could allow more "clean bass" to pass through by increasing C2, BUT decreasing the amount of bass triggering the distorsion (like a sidechain circuit on a compressor) ? So the distorsion would be less sensitive to the bass range... How should i do that (if that is possible at all) ?

I do not care if I have a bit less output volume (the pedal can be loud !) and slightly less trebble, but i wish i could get a bit more deep bass, and less distorsion if possible.
Is there any suggestion ?

Thank you in advance

[bushidov]

I haven't tried this with my Whipped Llama, which is based off that circuit, but I don't see why a blend circuit would help resolve your issue. That is typically the route taken with a lot of "Bass" effects: Guitar effect + Blend knob. That way you can blend in the dry, clean bass signal to the wet signal.

[DIY Bass]

I have a copy of a Darkglass vintage microtubes.  It is a light distortion for bass that has a blend control and based on digital inverter distortion similar to the Red Llama.  The schematic will probably give you some ideas about component values that will work well for bass.

[Mark Hammer]

I use 100nf in the C2 and C4 positions, 120pf in the C3 and C5 positions, and 680k for R5.  I find it yields a nice "grunt" or Keith Richard "chug".  Nothing I would consider clean or transparent, but a modest overdrive that can push an amp nicely.  Keep in mind that gain is multiplicative, so that total gain can be altered via the first and/OR second stage of the circuit.

Personally, I like to keep the gain of the invertors modest, with a 100k feedback resistor and 330pf feedback cap, and push the invertors with an op-amp input/gain stage, but the simpler two-invertor arrangement works well enough.

[Fancy Lime]

Hi snk,

the gain of the inverter stages is set by the series resistor and the feedback resistor. For the first stage it would be (R4+VR1)/R2. So if you want less gain you do NOT want to LOWER R2. Keeping it at 100k should be fine. I would also keep R4 and VR1 at stock values. Instead I suggest lowering the gain of the second stage. As you can see, that one has no series resistor and a big (1M) feedback resistor (R5). That means that this stage runs "full tilt" (more professionally speaking: at open loop gain, aka. maximum possible gain that the inverter can deliver, highly dependent on the individual device) all the time, no matter what the gain setting on stage one. That is what is responsible for the high gain of the overall circuit, mostly. What I did when I tried to get the exact same result that you are describing (for bass guitar instead of synth but same difference), is to make R5 100k and exchange the "coupling cap" C4 with a 100k resistor as well. In fact, C4 is not really a coupling cap in this position because both inverters automatically run at the same DC bias point (V/2), due to the feedback bias set by (R4 + VR1) and R5, respectively. So DC decoupling between the inverters is unnecessary. All that C4 does, is cut bass. That is a deliberate part of the tone shaping in red llama type circuits but it is precisely what you do not want. With a 33n cap, however, the cutoff is below the audio band, son increasing it will not change anything. Having a big cap here instead of no cap has the advantage of cutting out subsonic rumble, which makes the whole thing run more stably with less-then-perfect power supplies. So instead of just swapping C4 for a 100k resistor, you might want to replace it with a 100n cap and put a 100k resistor after that cap. Increasing C4 to 100n is necessary in that case because you lower the input impedance of the second inverter. I would also change C5 to 330p, so you get treble roll off above 4800Hz or 220p for 7200Hz.

The result of the changes above would be a gain of 1 on the second inverter and a gain of 1-11 on the first. That should provide a good gain range from perfectly clean to quite audibly distorted. The second inverter still influences the sound because it clips in parallel with the first inverter, making the sound "less linear" or more "saturate" right at the edge of first clipping. I like that setup. A lot. If you want less gain still (because your instrument's output is very hot), lower R4 or replace it with a wire bridge. If you want more gain, lower the resistor that you added at the C4 position. For more variability, add a switch that allows you to bypass that resistor for a gain boost.

Hope that helps,
Andy

[snk]

Wonderful ! So much informations, thank you 

[snk]

Again, thank you so much to all of you : I have been trying the suggested mods, and it really gets me into the ballpark of what i am aiming at 
I need to further tweak some parts a little bit, and then i will let you know which values i kept.
But for now, my custom Red Llama is muuuch more bass friendly, and also more transparent (less overdriven, less gain).

[Mark Hammer]

Gneral question to the more knowledgeable:  what happens to the tone of such invertor-based overdrives as one increases the supply voltage from 9v to 15V?

[snk]

Hi, Some more feedback : the pedal is boxed and working
- About the input caps, i decided to go with 15nF/150nF on a switch. 150-220nF is bassy, and 15-22nF allows for a lot of bass to pass through, but it cleans up the low end a bit before hitting the distorsion, so it changes the distorsion behaviour and i like that.
- About the cap for the 2nd  feedback loop : i didn't had any 330pF cap, so i tried with 470pF, which was too dark. Then, i tried with 220pF : it's perfect
- I replaced the 1M resistor with a 220K resistor, and it is ok.
- For the first feedback loop, i went for 100pF (i didn't had any 120pF cap), and it sounds great.

Overall, I really like the sound i have now : quite transparent, but getting thicker and beefier with more harmonics while increasing the gain. There is no bass loss, it's all fine.

Thank you all for your suggestions and technical knowledge : it sounds very different than the previous version i built, just like what i had in mind

[duck_arse]

now, snk, give it a name, draw out your schem, show the world and save the next guy all the work. how about it?

[snk]

Yes, i need to do that.
I have already written above the mods i did. Now i have to publish a layout with all the tweaks. I'm not much a schematic guy, i'm using veroboards, but the Red Llama schematic is very easy to read, so this shouldn't be too hard to do.
I will also post some pictures of the build once it will be into a proper enclosure.

[Fancy Lime]

Good to hear that you are happy with the result!

A question to all: does anyone know a good EASY TO USE schematics editor? I'm very happy with KiCad right now but I wouldn't necessarily recommend it to anyone who is just starting out drawing schematics. The drawing of schematics is superbly easy with KiCad but the component libraries are massive and confusing unless you know exactly what you need and where to look for it. Something like bancika's DIY Layout Creator but for schematics would be great for beginners. Is there any such thing?

Gneral question to the more knowledgeable:  what happens to the tone of such invertor-based overdrives as one increases the supply voltage from 9v to 15V?

Despite not fitting that description, I'll answer anyway. Open loop gain and gain bandwidth go DOWN with increasing voltage, which I always find counter intuitive. If you have a circuit that (1) contains only the inverters, (2) has gain and bandwidth fixed to something well below the device's capabilities at all voltages you're gonna throw at it, and (3) feed it with input voltages that are scaled proportionally to the changed supply voltages, then the difference should be minimal. But of course, none of that is the case in most circuits where we use CMOS inverters for distortion. I never tried more than 9V but I did try a Llama type circuit (don't remember the specifics, was just some breadboard experimentation that went nowhere) with 6V to 9V. I really didn't like the lowered voltage but that may have been because I first optimized the sound at 9V. So I would encourage trying out higher voltages but my suspicion is that something optimized at 9V is going to sound worse at 15V. After optimization at 15V, however, it may sound better than the original 9V circuit. Although my suspicion is that the difference would not be all that dramatic.

Andy

[Mark Hammer]

Thanks for your reply, Andy.  Useful info.

I guess one should probably distinguish between what might happen to a 4040 with a higher supply voltage, when used on its own, versus what would happen when there is an op-amp input stage driving the invertors, and the invertors themselves are set for lower gain.

[Fancy Lime]

Thanks for your reply, Andy.  Useful info.

I guess one should probably distinguish between what might happen to a 4040 with a higher supply voltage, when used on its own, versus what would happen when there is an op-amp input stage driving the invertors, and the invertors themselves are set for lower gain.

4040 or 4049? You're not trying to coerce a ripple counter into linear operation for use as an overdrive, are you? But otherwise, yes. Raising the supply voltage raises the "clipping ceiling", so all other things equal, you just get more headroom / less overdrive. But that is easily adjusted for with a little thingamajig I like to call "gain knob" I have no idea how the clipping character changes, though. Since gain bandwidth drops with increased voltage, raising the voltage might lead to even gentler clipping. I would wager that the biggest difference is going to be heard around the "juuust starting to clip a little" point. Which is where I like CMOS drives best anyway. If you test it, be sure to let us know, I'd be really interested in the results of a practical test.

Cheers,
Andy

p.s. Sorry snk, did not mean to hijack your thread.

[snk]

p.s. Sorry snk, did not mean to hijack your thread.

No problem, please be my guest
It's not even "my" thread : just a discussion open to everyone to share information and give help, so that's fine, ahah !


By the way, as far as i understand, this CMOS chip features several "modules" (by lack of better words) which aren't used with that circuit, right ? So, if i understand correctly, some people managed to stack several "layers" to give the effect more distorsion ("double D", "thrice", etc), but am i right assuming we could build an easy stereo module out of it, by using only one single chip ?

[Fancy Lime]

...

By the way, as far as i understand, this CMOS chip features several "modules" (by lack of better words) which aren't used with that circuit, right ? So, if i understand correctly, some people managed to stack several "layers" to give the effect more distorsion ("double D", "thrice", etc), but am i right assuming we could build an easy stereo module out of it, by using only one single chip ?

Yes, the CD4049 and CD4069 are so called "hex inverters", meaning the chip houses 6 individual inverter circuits. There are lots of things you can do with the "leftover" inverters. Doubling your 2-inverter overdrive to make it stereo is a good option, if you think that might come in handy. Just don't fall into the trap of thinking "I need to use them for something, can't let 'em go to waste". Sometimes 2 Inverters are all you need and putting more into the circuit makes the result worse rater than better. Also remember that you need to tie the inputs of all unused inverters to ground in order to stabilize them. Not doing so can cause unexpected behavior, oscillations etc. in the other inverters.

Andy

[Mark Hammer]

Thanks for your reply, Andy.  Useful info.

I guess one should probably distinguish between what might happen to a 4040 with a higher supply voltage, when used on its own, versus what would happen when there is an op-amp input stage driving the invertors, and the invertors themselves are set for lower gain.

4040 or 4049? You're not trying to coerce a ripple counter into linear operation for use as an overdrive, are you? But otherwise, yes. Raising the supply voltage raises the "clipping ceiling", so all other things equal, you just get more headroom / less overdrive. But that is easily adjusted for with a little thingamajig I like to call "gain knob" I have no idea how the clipping character changes, though. Since gain bandwidth drops with increased voltage, raising the voltage might lead to even gentler clipping. I would wager that the biggest difference is going to be heard around the "juuust starting to clip a little" point. Which is where I like CMOS drives best anyway. If you test it, be sure to let us know, I'd be really interested in the results of a practical test.

Cheers,
Andy

p.s. Sorry snk, did not mean to hijack your thread.

Gah!  Tiny characters on a small tablet screen + aging eyes = no visible difference between 0 and 9

I asked my question because I have yet to see a gain/bandwidth product graph in a datasheet for a 4049.  In general open-loop gain does decline with bandwidth for op-amps, and I assume for other devices that one might wish to use like op-amps.  However, any decline with frequency may have little to no relevance for guitar.  If a device shows a maximum gain of 70db at 50khz with a 9v supply and 70db at only 20khz with 15V supply, that has precious little relevence to guitar signals.

[snk]

There are lots of things you can do with the "leftover" inverters. Doubling your 2-inverter overdrive to make it stereo is a good option, if you think that might come in handy. Just don't fall into the trap of thinking "I need to use them for something, can't let 'em go to waste". Sometimes 2 Inverters are all you need and putting more into the circuit makes the result worse rater than better.

I'm a synth player, and i use a lot of stereo effects, drum machines... So, for a while, i have in mind a true stereo effect. I am still trying various designs, and knowing that one single 4049 chip could allow for creating a stereo effect is quite interesting.

[PRR]

> I have yet to see a gain/bandwidth product graph in a datasheet for a 4049

General curve for CMOS inverters:



http://bitsavers.trailing-edge.com/components/national/_appNotes/AN-0088.pdf

[Mark Hammer]

Thanks, Paul.  I always look for datasheets, but tend not to look for appnotes.  I guess I should start doing so!

Extrapolating from what's shown in the graph, it doesn't seem like bumping the supply voltage up from 9 to 15V would do anything of any particular use for the invertor chip itself.  If the invertors are driven by an op-amp or discrete JFET stage (like Frank Clarke's old designs from 20 years ago), changing the headroom on the input stage may have some tonal consequences.  Whether those wash out after two or more invertor stages is a whole other ball of wax.