My DIY distorsion

[Basstyra]

Ok, so after reading a lot, studiing lots of schematics, and thinking a lot about all this, here I come, here it is, my first DIY distorsion !!!

Really simple, in fact. A neutral op amp stage. 3 resistors, 2 caps, 1 opamp, 2 leds. Believe it or not, it sounds better to my ears than my Jacques Tubeblower. I found MY sound, the sound I was looking for.

I don't really understand why all the OD are that complex. Why they all have a tone control after the amp stage, not before. Before is better to control OD tone, I find.

Anyway, exit my Tubeblower, welcome to my first DIY pedal on my pedalboard !! Not finished yet, but that's details.I found the tone, the rest is details.

[JimRayden]

Congrats, mate! Any sound clips of your circuit, or the Tubeblower?


---------
Jimbo

[Basstyra]

http://perso.wanadoo.fr/jumping-jack/autre/madisto.mp3

Bass alone.
Then, max gain, open tone on the bass.
30s, I cut off the trebles on the bass.
58s, I boost in the low with a preamp put before.
1min24, I reopen the tone.
1min35, pickups are dephased.
1min56, less gain.
2min20, bridge pickup.

[Marcos - Munky]

Cool. I'm not a bass player, but liked the sound a lot. Schematic?

[Basstyra]

Not yet, it isn't finished. But there isn't really something special. Just a preamp stage that I overdrive. The simpliest way to o an overdrive, in fact.

I'll add some stuff around, i'll post a schematic when it's in the box, tested and approved !  

[kusi]

hi basstrya

do you saw my P.M.?

greetings, kusi

[MartyMart]

Congratulations !!
Finding "your sound" is what this is all about and I have to say that so far
I like "simple" circuits for OD/Distortion over "complex" every time !!

http://aronnelson.com/gallery/Martys-layouts-and-photos/MrDrive

Here's mine ...

Marty.

[Basstyra]

Thanks !  

What are the two 1k resistors for ?

[Basstyra]

Up for Marty !

By the way, I switched to 1n4148 for the clipping diodes. The LEDs leave too much headroom, and I can't boost again my signal, or the op amp will be overdriven. I'll try 1n4001.

I tried Ge diodes, but no. Too much noise, not my kind of sound.

I got my final schematic, I think. I just have to test some values for caps. I think there will be a clean boost first, then a overdrive stage, with a controllable filter in the feedback loop (the traditional cap, but with a pot in serie, to cut the cap if needed. This filter is much more efficient than a simple tone select after the stage. And without any filter (pot at max), it sounds just like I love, really really hard. On an open E, it's heaven...) And volume.

Boost, Gain, Tone, Volume. Maybe a rotary switch to select some cap value, I'm not sure of that yet. Maybe the diodes will be switchable. Anyway, it remains simple.

I connected the - input to bias instead of ground throught a cap, I save a component.  :wink:

[MartyMart]

Thanks !  

What are the two 1k resistors for ?

Sorry Basstyra, I'm not getting notified of many "replies" at the 'mo !!
That's copied from the "Rat", just setting the level in and out of the op amp, I tried some different values and it got too "nice: sounding !
Its a very cool little overdrive, with a good range from clean to "NUTZ" !!
In the end I used two 100k log pots ....

Looking forward to a schem soon ..... ?

Marty.

[Basstyra]

My schematic is coming soon. it's just that I don't want to update it every hours when I change stuff on my test board.  :wink:

[Basstyra]

Here is my current schematic :



Where there is no value, I don't really know yet what I'll finally put. The in and out caps will be 0.1 or 1µ probably.

The drive stage (2nd opamp and diodes) allow a great range of gain, the boost stage (1st opamp) can really boost this gain.

The colour is a tone control, and allows to remove almost completely the cap, letting all the dirt pass trought. GREAT !!  

The first stage is a clean boost, so I'm wondering... Maybe I should put 2 footswitches, one for the 1st stage,  one for the 2nd, this pedal would be a clean boost, an overdrive, and a big nasty distorsion with the 2nd stage boosted with the 1st.

What do you think of this ? Or should I just make this an overdrive, letting the 1st stage be just a "more gain" setting ?

For those who wonder why 2 stages : because I had a tl072 to make the first tests, so why not using the 2nd half of it ?? And with 2 stages I found I could have more gain more easily. Finally, a clean boost in a pedal is allways usefull...    I made the first tests using another clean boost, I found usefull to keep one in there.

[Marcos - Munky]

Interesting. I have two suggestions: use 2 footswitches like you said, it would make it more versatile, and maybe I'm wrong, but I think you don't need that output resistor, since you have a volume pot.

Another thing, you drew your schematic as if it uses two single opamps. This isn't a problem, but as you said you used a dual opamp, you can confuse some people with the pinouts for the ICs.

[Basstyra]

Yep, the schematic has been cleaned ! Reload the page if not.

The output resistor simply allow a first volume down.

[Mark Hammer]

Shorting the output and inverting input pins of the 2nd stage make it a unity gain stage.  Remember that the gain of the noninverting op-amp is a function of the feedback resistance plus ground leg resistance (1k in your diagram), divided by the ground leg resistance.  If the feedback resistance is zero (produced by simply shorting the two pins with a jumper or SPST footswitch), it doesn't matter how small or large the ground leg resistance is.  The stage's gain will be 1.  Don't credit me for this.  You can see it used on the Anderton/PAiA Rocktave Divider (although Anderton uses this to drop the gain of an inverting stage down to zero).

So, if the 2nd stage gain is dropped to 1 from wherever it is, the diodes should have little or no effect since the gain of the 1st stage is not likely to push the signal above the clipping threshold unless you use GE diodes.

Alternatively, you could stick the footswitch in the first stage and use it to kick in a preset degree of boost on top of what the 2nd stage provides.

Here's another idea.  You have the Drive and Colour controls working separately. How about a single pot which varies drive and colour simultaneously.  So, imagine you have a 250k pot (linear is probably in the direction of ideal but not necessarily), with the wiper where you currently have the Drive and Colour pots tied.  We'll call one leg the "drive" leg, and the other side of the pot the "colour" leg.  Rotate in one direction and the Rdrive goes down as Rcolour goes up.  This produces less gain but reduces the impact of the feedback cap.  Rotate in the other direction and the gain goes up but the cap has a more pronounced effect on the high end.  Given that the diodes will increase the apparent high end, trimming back on high end in the 2nd gain stage usually is a sensible idea.

Note that the colour leg does not have to be used on its own.  You could easily stick a resistor in parallel with the colour leg so that rotating the pot produces much bigger resistance changes on one side of the wiper than it does on the other.

Here's an example.

If the pot were 250k (plus 2.2k series resistor), the max gain from the stage could be 253.2k/1k = 253.  Factoring in the additional gain of 11 from stage 1, that gives a total gain of 2783, which is juuuuuuuuuust enough to cause clipping ( :lol:  ).  With the pot rotated to yield 0 ohms on the colour leg, a 390pf feedback cap gives a rolloff around 1600hz, which is warm.  Not mute but warm.  Roll the pot in the other direction so that the colour leg resistance increases, and as the gain is reduced, the control eases up on the treble cut.  

Note that the effect of doing this is actually greater than one would normally get simply by reducing the feedback resistance.  If you simply dropped the entire feedback resistance to, say, 50k, the gain would be 51 from that stage (561 in all, with stage 1 maxed), and the high end rolloff (using a 390pf cap) would be 8160hz.  Feeding that much bandwidth, at that gain, into the diodes, without any post-diode treble cut, would likely get you a LOT of annoying fizz.  Sticking the colour leg of the pot in series with that same cap would get you even LESS treble reduction pre-clip.  Yeccch

So, perhaps the smart thing to do is to place a medium value resistor in parallel with the drive leg of the pot.  This would allow you to set a max drive, while still being able to apply the full resistance of the pot to reducing the effect of the feedback cap.  

Let's say the parallel resistor was 100k.  Using a 250k pot, this would set a max feedback resistance of 71.4, which would give a max stage gain of 74.6 with the other components involved and an overall stage 1 times stage 2 gain of 820.  Hot enough to do lots of squaring at the diodes.

With the pot set so that the drive leg was at 50k, the combined parallel fixed+drive resistance would be 33k, which provides a gain of (33k+2.2k+1k)/1k = about 36 for stage 2 alone, and about 400 with the first stage maxed.  With the other 200k on the colour leg of the pot, there would be little high-end trimmed off.  So, hot, hot hot, with all sizzle preserved.  Given the reduction in maximum feedback resistance, let's up the feedback cap to 1200pf.  That will give max rolloff at max stage 2 gain of around 1800hz.  A 1500pf cap will net you a rolloff around 1450hz.

Stick another 1500pf cap in parallel with the diodes just to tame any residual fizz, and you should have a fairly flexible unit, using just 3 controls.  The nice thing about the two gain stages and the variable gain in the first is that you can achieve everything from gently boosted sparkle to serious shriek and bite to "wooly" tones.

Make sense?

[Basstyra]

It does make sens !!  

I was just thinking of this kind of bypass, just bypassing the feedback resistor. At least for one stage. I used 1n4148, not GE, partially because the GE didn't left enough headroom.

Alternatively, you could stick the footswitch in the first stage and use it to kick in a preset degree of boost on top of what the 2nd stage provides.

Yes, this is a way I meant to use it.  :wink:

For the drive/colour pot, this is a great idea, really !    As I want this cap only for high gain. I'll test it. I'm not really sure I'll choose to do this, because  it could bother me for just some settings, but it's worth a try !

For the versatility, yep, it's a versatile unit. The 2nd stage goes from almost no overdrive to good distorsion, the 1st can make the 2nd opamp clip, mixing diode and opamp clipping. Pretty agressive. Almost a little fuzz, depending on what you put in.

[Mark Hammer]

Good!  

If you don't have a 250k pot, use a 100k, and try a 220k fixed resistor in parallel with the drive leg.

[Basstyra]

Just one question : how do you calculate the roll off frequency, in this case ? I know this is R*C, but what is R exactly, here ?

Wait, while I write, I got the answer. It's Rcolour+1k (from - to bias). Isn't it ?

But remember I sometimes roll off trebles before this overdrive, using my tone control on my bass. So some of your considerations on roll off frequency may be not as you meant.

[Basstyra]

EDIT : no, it's even simplier finally. Just a manual switch to cut the overdrive stage. I also may cut the diodes wire, to assure a perfect clean boost. But the price difference between spdt and dpdt is not really huge, for manual switches.

The switch will probably be a relay with spdt. The led is in // on the self of the relay. Or simplier, a 3pdt, I don't know yet.

[Basstyra]

Ok, so I tested your solution, Mark Hammer. Well, it's not really great, because it creates a mid point where the tone changes suddenly.

I think I'll go for a switch, maybe labelled "control those f**king trebles !!", which will just put the feedback cap on or off the circuit.