Low Gain, neutral dist/OD

[Mark Hammer]

That "Rat trick" (2nd ground leg) is handy, innit?

[Fancy Lime]

That "Rat trick" (2nd ground leg) is handy, innit?

Yepp, sure is, especially with one of the legs being variable.

BTW, here is what I went on about yesterday:
https://www.diystompboxes.com/smfforum/index.php?topic=123516.0

Cheers,
Andy

[Killthepopular]

I think I'll put my 2 favourite circuits together in a box so I can have one or the other or stack them. Currently my 2 favorite circuits are the red llama and the electra. The reason is that the main premise for this project was to make an amp like drive, one that didn't cut bass and had a nice natural (not crackly) low gain sound, and those 2 circuits seem to fulfill those requirements better than the other circuits I've tried. I recently built (breadboard) the son of screamer, rat, OCD and the timmy. I liked all of them but I found that they all cut a lot of bass in that typical overdrive way and didn't sound as full as the electra or red llama. I also don't really like the clean blend sound you get from these tube screamer style pedals. I was going to do the barber Ltd next. Is it worth bothering with or will I not like it if I didn't like the tube screamer and timmy?

[roseblood11]

Suggestion for the bypass switching:
https://forum.pedalpcb.com/threads/two-fx-toggle-flex-relay-switching.499/

PS: I had this idea of putting my "two favourite" overdrive/distortion pedals in one box many years ago. It NEVER ends. You'll always find another two circuits that look promising. 

[Killthepopular]

I quite like Joe Davisson's EZ-250.  Only 15 parts (+ pots).  Links for: thread, schematic, my layout.

Seeing as my fave circuit so far is a transistor boost followed by diodes clipping to ground I thought I ought to try this one out. J201, 2n5088, bs170, 2n3904, 2n2222, vp3203. Those are my transistors. Which should I use? I might also try modding an lpb1 as I like the sound of it.

[Killthepopular]

OK, just built the ez 250 using a 5k gain pot and 2n5088s. I like it. What's the difference between this and the electra? I find it still sounds pretty good when I max the gain and hit it with a boosted signal whereas the electra would sound like crap, just a totally farty, saggy transistor sound. What's the difference there? Is it the different transistor i'm using (2n3904 on the electra)? Is it something in the design of the circuit (two transistors rather than just one)? Or did i perhaps just not set up my electra correctly?

[Elijah-Baley]

I didn't find any video or audio demo of the EZ 250. (I'm searching a tiny distortion circuit to fit in a 1590A box).

[bluebunny]

I didn't find any video or audio demo of the EZ 250. (I'm searching a tiny distortion circuit to fit in a 1590A box).

I can't recall Joe doing a demo.  It certainly fits in a 1590A.

[Killthepopular]

I didn't find any video or audio demo of the EZ 250. (I'm searching a tiny distortion circuit to fit in a 1590A box).

My demo of the EZ-250: https://youtu.be/7SJKp7w4u28

This was the 13th dirt circuit I've breadboarded for this project so far. It's not got as much low end and top end as the llama and the electra but it is also not as mid-focussed as the tubescreamer, timmy, OCD or rat. It sounds good at low gain to my ears. I used 2n5088 and 1n34a and 1n4001 diodes.

I think it has bumped the llama off the #2 spot. It doesn't sound as badass as the electra but I think it is probably a classier sound, smoother but still well balanced. Llama is very good but it lets through some harsh tones sometimes and the lack of high gain is disappointing. Electra is still my fave and this is #2, so currently my finished pedal would be electra into ez-250. I'd probably also add a diode lift to both circuits so i could use either one as a boost too.

I ought to have a look at more transistor with diodes clipping to ground type circuits. Transistor fuzz/overdrive sounds too bright and crackly to me. Opamp distortions and overdrives all seem to cut too much bass or sound no good at low gain. Transistor plus diode distortions seem to be some of the rare circuits that have a fairly natural sounding grit and don't cut bass.

[Elijah-Baley]

I like it. I could try it, it could be my overdrive 1590A.
I hope it's not too harsh, nor poor low end, and it will sound nice at high gain setting. Actually I prefer something a bit more more aggressive.
Maybe is not the right place, but how I can make it more gainy?

[Fancy Lime]

@Killthepopular

You aren't going to be satisfied until you tested *all* the overdrives, are you? May I suggest you call the pedal that will hopefully someday be the result of your labors "The Stickler"? I mean that in the most endearing sense of the word.
One thing that strikes me about your recent posts in this thread is, that you seem to know pretty well by now what you like sound-wise but not really how a schematic translates to sound. For example, you say that the Son of Screamer, OCD, Timmy, and Rat all cut bass and that that is what you disliked about them. You can change that bass cut (which is actually a mid or treble boost, but same difference) very easily by changing just a few component values. I think it is time you started designing your very own overdrive, starting at one of those that you like *pretty much* and changing the aspects that are not 100% on the nose for you. I think this may ultimately get you closer to the sound in your head than trying lots of existing designs, many of which are, fundamentally, very similar. Tell us what you want as precisely as you can in relation to the designs you have tried, and we'll be glad to help you figure out what to tinker with. If I understand you correctly, you want:
1. Flat frequency response
2. Hard clipping with smooth transition into clipping
3. Low to medium gain
Right? Anything else? What designs tick which boxes for you?

I like it. I could try it, it could be my overdrive 1590A.
I hope it's not too harsh, nor poor low end, and it will sound nice at high gain setting. Actually I prefer something a bit more more aggressive.
Maybe is not the right place, but how I can make it more gainy?

More gainy? Bootstrapping! Inspired by this thread, I started tinkering with a dual-boostrapped version of the EZ-250. The first transistor is bootstrapped for higher input impedance and the second one for higher gain. I'm still polishing the component values but so far I'm pretty happy with where it is going, namely a higher gain EZ-250. One could of course just properly redesign a DOD 250 with a modern opamp instead and get even more gain, lower noise, less parts... But where's the fun in that? Discreet transistor designs still have that certain je ne sais quoi, don't they?

Cheers,
Andy

[Fancy Lime]

Here you go, a higher gain EZ-250:

https://www.diystompboxes.com/smfforum/index.php?topic=123610.0

Andy

[Killthepopular]

1. Flat frequency response
2. Hard clipping with smooth transition into clipping
3. Low to medium gain
Right? Anything else? What designs tick which boxes for you?

Thanks for all the help Andy. I was going to start trying variations on the electra (folk driver, english gent, clock of tone, ghetto stomp) and the red llama (tube sound fuzz, Double D) but your approach is probably the more correct one.

1. Flat frequency response: Yes. However, I also probably want a switch or pot to roll off the low end when I want to i.e. playing with a bassist. I would also add "consistency" to this requirement. I'd like a circuit that sounds well balanced at all gain settings or at least has a good tone control that can counter any imbalances. The variation in tone in the dod 250 (really dull at minimum gain, no low end at max gain) makes it unuseable. Most circuits have consistency although some transistor based things seem to get a bit too edgy at high gain. Many circuits don't have a flat response. I've tried letting more bass into the tube screamer design and this just seems to make things too muddy but still quite thin.
2. Hard clipping: Maybe. Probably. Smooth transition into clipping: Yes. Electra style circuits seem to be quite smooth. Tube screamer things are good but they have too much clean signal mixed in. Llama is very good. Overloaded opamps are cool but not good at smooth clipping. Vox and dod were bad at low gain. Rat and OCD were pretty good but still not as good as llama and electra. I find that asymmetrical clipping diodes help with this as the sweet spot is easier to find when you are only clipping one side of the wave instead of both.
3. Low to medium gain: Yes, although a circuit that can get into high gain too would be a big plus. Llama is the only circuit that had a really narrow gain range.

Those are probably the main things but there's a few more things I've picked up:

4. I like a circuit that's not too transparent. For me the Llama and tube screamer seem to emphasise or let through the natural twanginess of my high strings which sounds ugly. Circuits like the electra and the rat seem to turn the signal to mush, even at low gain, in a way that's pleasing to the ear.
5. I just find some circuits more exciting than others. The llama ticks lots of boxes but feels a bit boring. It was the only circuit which stood out as feeling a bit boring. The dod 250, rat and vox distortion tick hardly any boxes but they feel more exciting than most other circuits.
6. Warmth. That thing where people describe transistors as warm, buttery and musical and opamps as cold and sterile, I can hear that a little bit. The clipping and the overall tone of transistor circuits can sometimes be ugly but there does seem to be something in the feel that's a bit more sweet and glassy somehow. Opamp circuits can have their own nice tone but transistor circuits almost seem to make my guitar sound like a better guitar. I guess it's like, the sound of the distortion is one thing, but when it comes to being fairly clean, transistors seem to improve the overall tone.
7. It might be nice for it to take a really hot input. I might want to boost into it some time for a really high gain sound. It often seems to me that transistors don't like being hit with too much gain but opamps can handle it better. That might just be my limited experience.

Maybe I could have 3 sections, each with a bypass switch. A: transistor boost with gain and volume pots. B: Opamp Boost with gain and volume. C: Diodes with master volume and low cut switch. A and B could be footswitchable so I could use one as always on and one as a boost for more gain. Section C could just have a finger switch as its not the sort of section you would turn on and off during a song. Having bypassable diodes means I could just use the rest of the pedal as a boost or fuzz. Having a separate gain and vol for each section means I could control exactly how much clipping each section is doing.

[marcelomd]

Hi,

Suggestion: Try the Blackstone Appliances MOSFET overdrive. At its core it's a CMOS inverter overdrive, like the llama with some tone shaping. Looks like a quick build.

Schematic: http://fuzzypedals.blogspot.com/2015/06/blackstone-mosfet-overdrive.html

There are lots of demos, this is just the first one:

I think you are going to end with something like a Tech 21 character series. Cascaded gain/clipping stages with heavy filtering/shaping in between but with lots more knobs.

[Fancy Lime]

Hi Killthepopular,

from your points 4 and 6, I take it that you want a bit of high cut before clipping. Ponit 5 indicates that you seem to like some higher order harmonics. Let me explain:

The Tube Screamer and Llama have very high input impedance (hundrets of kiloohms), the transistor circuits have very low input impedance (single digit kiloohms). When placed first in your signal chain after the guitar (with no buffers in between), then the inductance of the guitar pickups and the input impedance of the distortion form a low pass filter together. With the high input impedances of opamp or CMOS devices, the cutoff is usually above the audio band but with the low impedances of the transistor circuits, you can hear it This loss of highs can be quite pleasing and is often described as "warmth". Interestingly, when you turn down the volume on your guitar, the extra resistance in line with the inductance of the pickup shifts the cutoff up, so that you get more sparkle when you roll down the volume, which is something many people like. The Fuzz Face is famous for doing that extremely well. You can get the high frequency rolloff before clipping with high impedance inputs as well by simply adding an appropriately designed low pass filter in front of it or increasing the cap in the feedback loop. That does not interact with the guitar volume, though.

The Llama has a rather soft clipping knee, meaning you get a lot of 3rd order harmonics, much less 5th order and hardly anything above that  when you clip it just a bit. This hardly even sounds like distortion at first. If you go to higher gain, you get progressively more of the higher order harmonics. With diodes to ground clippers, you get a lot more of the higher orders early on and if you make it asymmetrical, you also get even order harmonics. I suspect that this is what sounds more exciting to you than the Llama behavior.

If you want it to be able to take really hot input without unpleasant distortion but will have diodes to ground clipping anyway, I suggest to put an additional pair of such diodes to ground before the input coupling cap. This clips any excess signal before it can cause the amplifying element to distort. Whether that should be 0.7V Silicon diodes or red LEDs depends on what stage comes then.

The "3 Sections" plan sounds like a good idea. There is a reason why many people use this exact kind of setup I would suggest to make the first section such that you can either have a full range or a treble boost. If this is only supposed to be an additional clean boost stage in front of the "actual" distortion, then you can probably get away with just a gain pot. You can make it so that gain is minimal with the pot on 12 o'clock and turning it left increases the full range boost, while turning it right from noon increases the treble boost. One knob, two sounds. If that is what you want. This can be done with a few alterations to an Electra-style amplifier stage.

Cheers,
Andy

[Killthepopular]

from your points 4 and 6, I take it that you want a bit of high cut before clipping.

The Tube Screamer and Llama have very high input impedance (hundrets of kiloohms), the transistor circuits have very low input impedance (single digit kiloohms). When placed first in your signal chain after the guitar (with no buffers in between), then the inductance of the guitar pickups and the input impedance of the distortion form a low pass filter together. With the high input impedances of opamp or CMOS devices, the cutoff is usually above the audio band but with the low impedances of the transistor circuits, you can hear it. This loss of highs can be quite pleasing and is often described as "warmth". Interestingly, when you turn down the volume on your guitar, the extra resistance in line with the inductance of the pickup shifts the cutoff up, so that you get more sparkle when you roll down the volume, which is something many people like. The Fuzz Face is famous for doing that extremely well. You can get the high frequency rolloff before clipping with high impedance inputs as well by simply adding an appropriately designed low pass filter in front of it or increasing the cap in the feedback loop. That does not interact with the guitar volume, though.

I think you're probably right about point 4. If i don't like a certain twanginess to the llama which I don't hear in the electra then it sounds correct to me that there is a LPF in the pre clipping stage. That makes sense. I'm not sure I agree that this also applies to point 6. There's a certain sweet, musical quality that I'm hearing when I compare transistors and opamps. The difference sounds like more than just a difference in the amount of high frequencies, although it's hard to say exactly what it is. The sound or feel is "sweeter" but not necessarily brighter or darker. Opamps seem to sound a bit flat by comparison. If anything the transistor circuits seem to sound a bit brighter, like the sound is sort of being subtly focussed in a midrange area that sounds really flattering to the guitar, whereas opamps seem to mostly sound like a very flat response which is less pleasing to the ear. It feels as if the transistor is "hearing" my guitar in a more intelligent way, whereas the opamp doesn't understand what it's hearing and processes the signal less sypathetically. Maybe the input impedance is part of what I'm hearing when I talk about "warmth" but I'm not convinced that it's the whole story.

I'm not sure if I like the idea of a pedal that changes the tone with the volume knob. Presumably I could set up the transistor in such a way that it has a high input impedance (e.g. the input buffers of a ts9 or DS1) and then use an RC filter to manually set an LPF rather than sort of "leaving it to chance" and relying on the input impedance to give me my pre-clipping high end rolloff?

Point 5 indicates that you seem to like some higher order harmonics.

The Llama has a rather soft clipping knee, meaning you get a lot of 3rd order harmonics, much less 5th order and hardly anything above that  when you clip it just a bit. This hardly even sounds like distortion at first. If you go to higher gain, you get progressively more of the higher order harmonics. With diodes to ground clippers, you get a lot more of the higher orders early on and if you make it asymmetrical, you also get even order harmonics. I suspect that this is what sounds more exciting to you than the Llama behavior.

This sounds right to me. Diodes to ground has almost always been the most "exciting" sound for me so I guess it's the way to go. The only concern will be to construct the circuit in such a way that I can still get smooth low gain clipping.

If you want it to be able to take really hot input without unpleasant distortion but will have diodes to ground clipping anyway, I suggest to put an additional pair of such diodes to ground before the input coupling cap. This clips any excess signal before it can cause the amplifying element to distort. Whether that should be 0.7V Silicon diodes or red LEDs depends on what stage comes then.

Sounds like an excellent Idea. I know the ROG Ginger uses Red Leds to ground BEFORE each of its transistor boost stages and that circuit sounds really good to me.

You can make it so that gain is minimal with the pot on 12 o'clock and turning it left increases the full range boost, while turning it right from noon increases the treble boost. One knob, two sounds. If that is what you want. This can be done with a few alterations to an Electra-style amplifier stage.

Like this?

In this thread https://www.diystompboxes.com/smfforum/index.php?topic=122356.msg1153524#msg1153524 I was trying out a few different transistor boost circuits. I was thinking I might figure out what the difference is between some of them and figure out what I like and then stick some diodes clipping to ground in there. Start building my circuit that way. Currently fiddling about with the LPB1 circuit.

[Killthepopular]

When I listen to my recorded clips of the circuits I've built I seem to hear something nice and "musical" in the transistor based circuits whereas the opamp circuits sound sort of flat and stiff by comparison. I wanted to test this observation a bit further so I breadboarded the lpb1 and the mxr micro amp and compared them. I found the same thing with these two circuits. The most obvious difference is that the micro amp has way more high end which makes it sound harsh but there's more to it than that. The low end also feels stiffer somehow. It's like the frequency response is too flat whereas the response of the lpb1 subtly emphasises some flattering frequencies. I can't really articulate exactly what the difference is other than to say that (besides the obvious difference of input impedance) the transistor sounds sweeter and more musical somehow.

So if i want to mainly use a transistor boost section but I want my clipping to come from the diodes, not from the transistors (which often sound crackly or farty or gargly to me) then it would appear that I want a transistor based boost with as much output and as much clean headroom as I can manage.

How would I go about making a transistor boost that gives me the most gain possible without distorting too much?

[Fancy Lime]

OK, I think I get what you mean by "sweetness" of transistors as opposed to opamps. The thing is, opamps are a lot better at reproducing the original signal in amplified form than simple transistor circuits. Transistor circuits always distort the waveform slightly*, although not enough for us to perceive that as "distortion". Instead, we only notice that it seems "harmonically enhanced" or"sweetened". This is partly because human ears are pretty bad at Fourier analysis of waveforms and partly because we are feeding these circuits with a guitar signal, which has a lot of harmonics to begin with, so a little more harmonics don't make it seem obviously distorted. What many simple transistor designs do really well, is add some rater soft asymmetrical distortion. The Dallas Range Master is famous for that. A good analysis is found here:
https://www.electrosmash.com/dallas-rangemaster
So if this subtle sweetness is important to you, going with discreet transistor designs is probably a good idea.

...

How would I go about making a transistor boost that gives me the most gain possible without distorting too much?

Funny you should ask... This is precisely what I tried to achieve with the 250 Biturbo:
https://www.diystompboxes.com/smfforum/index.php?topic=123610.0
The initial idea for it came from your thread that we are talking on right now, btw so thanks for that! For your specific wants and needs, you may want to modify some things like so:



This design allows to get very high gain from a discreet design (but gain range starts clean) without ever getting the transistors themselves to clip. The trick is that the clipping diodes limit the bootstrapping, thus limiting the gain of the circuit dynamically. I'm sure someone else has done that sort of thing back in the 70's or so but I am not aware of any other designs using that trick in this way.

R10 needs to be adjusted to get ~4.5V at the collector of Q2. Q1 and Q2 should be low noise, high gain NPN transistors. I used 2N5088. 2N5089 or MPSA18 will work fine and have even better noise specs (otherwise they are essentially the same transistor), BC549C or BC550C should also be fine. Q3 can be 2V4403, 2N2907A, 2N5087, BC560 or any other small signal PNP. It is possible to modify the design to use an NPN in the Q3 position if you don't have any PNP's around. Let me know if that applies and I'll try to find a minute to draw that schematic. I only used a PNP because it allows to draw a neater schematic without crossing wires and the layout for the PCB is easier.

The Boost pot is at minimum at 12 o'clock and lets you dial in a treble boost going counterclockwise or full range boost going clockwise. This is what Imeant in my earlier post (so not really like the Folk Driver but the ideas are related). The Boost switch can be a foot switch. That way you have two different gain levels and gain structures on tap. The maximum gain on this thing is substantial. I used it with back to back 2V7 Zeners (clipping threshold ~3.4V) and it still gets very distorted on max. You can use any clipping diode arrangement of you choice in place of D2 and D3. Considering the gain range, you may want to go for higher thresholds, something like 2xSi in one direction, red LED in the other. With germaniums it may get too gainy so that most of the gain pot travel will not be very usable. Experimentation encouraged, as always.

If you want the thing to sound a bit darker, try increasing C8. You could try 1n and see if you need to go even higher (meaning darker) or take it back down again. R12 and C12 make a fixed low pass filter at ~7kHz as a sort of absolute minimalist tone control. These could be replaced with a proper variable high cut tone control. I'll try to design that in the days to come if I find the time. Been meaning to do that anyway.

Hope that helps,
Andy

P.s.: If you build this thing, I'd be much obliged to read your thoughts and maybe hear some sound samples.

* EDIT: I should probably have written here that "transistor circuits typically have stronger non-linearity than opamps" instead of calling it "distortion". The slight non-linearity in a (theoretically) clean amplification circuit *is* a kind of distortion but it's not the thing that we usually mean when we discuss methods of distortion. So "non-linearity" would have been the less confusing term to use here.

[Killthepopular]

OK, I think I get what you mean by "sweetness" of transistors as opposed to opamps. The thing is, opamps are a lot better at reproducing the original signal in amplified form than simple transistor circuits. Transistor circuits always distort the waveform slightly*, although not enough for us to perceive that as "distortion". Instead, we only notice that it seems "harmonically enhanced" or"sweetened".
The slight non-linearity in a (theoretically) clean amplification circuit *is* a kind of distortion but it's not the thing that we usually mean when we discuss methods of distortion. So "non-linearity" would have been the less confusing term to use here.

Yeah this makes sense. I hear a lot of people saying that transistors distort like tubes, when we all know that overdriven transistors tend to sound like old fuzz pedals, not like tubes. Transistors sound tubey to me so long as you don't get them to actually properly distort. I wanted to say that transistors sounded more "compressed" but I thought that didn't make sense, but maybe they are slightly more compressed somehow, as part of their subtle, always slightly distorted sound.

I've ordered some 2n5087s so I might try your circuit at some point. That won't be for a while though. For now I'm going back and trying those transistor boosts again, the ones from my other thread, and adding clipping diodes to them to see what kind of distortion circuits they make. So far I like the LPB1 more than the EP3 or the electra circuits. I like it both as a subtle tone colour boost and also a distortion (with clipping diodes). I'm working on the Zvex SHO now.

Something odd i noticed. I'm sticking some diodes in after the last capacitor (in the boost circuit) and then sticking a volume pot after the diodes. Sometimes it sounds like the signal gets less distorted as it gets quieter and more distorted as it gets louder. It's like the volume pot is affecting the gain before the diodes even though I'm placing it after. I added another 0.1uf cap between the diodes and the volume pot and this seems to have stopped the effect, so now the distortion level stays the same even when i turn down the volume. The 0.1uf cap seemed to cut the bass very slightly. I switched to a 0.22uf cap and didn't hear the bass cut effect any more.

[Killthepopular]

I'm having trouble with these circuits. I've been adding diodes after the circuit and then following them with a volume pot. I find that the volume pot seems to lower the gain even though I'm placing it after the diodes, so that the circuit, when I turn down the volume pot, gets quieter and cleaner. With the lpb and the SHO I put a capacitor between the diodes and the pot and I found that this reduced the effect. With the stratoblaster I found that the cap didn't seem to help. I replaced the cap with a 10k resistor and this helped a lot. Then I tried a 100k and this was even better but it seemed to reduce the output level too. I have my amp set very clean and the output level of these circuits doesn't seem that High so I assume the amount of gain in my amp is not the issue and that it is something going on within the circuit itself.

Anyone know what the nature of the problem is here? And what would be the standard solution?