án Overdrive circuit

[JohnH]

Who’s interested in home-brewed overdrive boxes?

I’ve build a few using JFETs, and the latest one is the simplest of all and I'm quite pleased with it so I thought I’d share.

This is the basic circuit:


I added a volume control and another JFET as a buffer after that, and a toggle switch to cut out C2 or C2 and C4, which gives three levels of gain, from a slightly dirty volume boost, a medium crunch, or a medium crunch with extra mid and treble. There’s no seperate gain control – the guitar volume works for that, and no attempt to EQ the signal after the gain. Instead, I tried to bias the transistors to get the best balance of tone.

Here is a sound sample, it is recorded by micing a cab, driven by my PowerBlock amp, set to a very clean position so it is adding no extra crunch. No effects added. It starts on clean using a bridge pickup , then at 9seconds I engage the circuit. The second uses the neck pickup, and the third has some light finger picking, to show how it adds harmonics to the sound.

OD box sample

I'm happy to post more info if interested.

cheers

John

[flateric]

sounds sweet John, I have a couple of spare 5457's looking for a pcb. Would you be able to post the pcb or vero layout for this circuit? I'd like to give it a try.

[JohnH]

Yay! thanks, I have a vero layout that Ill post soon. I was about to lose heart and conclude that we had no 'active' minds (ie, as in active electronics) on GN2.

cheers

John

[sumgai]

John,

That would be incorrect - I am actively active! ;D However, my aims are a little different from yours, so I desisted.

But I too would like to see more of these things, the small, simple, easy to understand and build, and most importantly, cheap! circuits that Nutz like us can use to baffle the so-called cognoscenti.




sumgai

[JohnH]

Here is an updated schematic, vero layout and photos of the board and finished prototype:



Ive added the buffer circuit, being a third JFET. It actually works fine without the buffer however, provided it is run into a high impedance input such as a guitar amp, stomp-box or modeller, rather than a line-in. If the buffer is deleted, then output can come from the pot at point "X" on the circuit, and the pot would preferably be reduced to 250K

The vero layout is shown from above, as if transparent, and the copper strips are underneath. the green circle is a cut in the track, the mauve ellipse is a bridge between tracks. The references for the R's and C;s etc should match the schematic, and I've referenced them at the edges of the vero layout. The yellow rectangles are caps, white rectangles or small white circles are resistors (horizontal or vertical), the black disc is an electrolytic cap.

Some of the components are off the board, being the source bypass R8, C2 and C4. These are where the gain and tone is determined, and I have them on an on/off/on switch, as described before. Also the power smoothing R14 and C4, and the LED, switched jacks etc.

I might do a diagram to show all those in context.

What do you think?, does the layout look followable?





I started playing with JFETs after reading www.runnoffgroove.com. On that site, there are many such designs. Mostly they used J201s, but I think 2N5457s hit the sweet spot for overdrive tone in a 9V circuit. More recently, I see that they have started to used them more too. Other key elements of setting these up right for a nice warm drive tone is to bias them so that the drain (the upper wire on the schematic) is biased to a bit above 1/2 the supply, say 5.5V for a 9V supply. Another thing I have found is that the more current is allowed to run through the JFET the softer the clipping is, so the values chosen are where I have found the best sound to be. The values of source bypass are also key, and the higher the gain, the more the gain has to be focussed on the mids and not the bass, to control bass flub.

One thing about JFETs though is that they are not very consistent critters. The best value of R2 and R6 (15k shown here) could easily vary by 20%. some builders use presets here so they can be tweaked to suit. I tend to breadboard it first to match values with the actual JFETs to be used.

The three settiings on the toggle switch give cleanish boost, and two flavours of drive. I'm find this device quite useful, both into my amps to give them a boost, and also before my small Zoom G2 modeller, to which it provides a nice analog crunch.

cheers

John

[flateric]

That's great - thanks John, if you get the time can you post up the entire layout with LED, off-board comps, mode switch and footswitch just to ensure I don't get it wrong! I'm ok with a soldering iron but electronics is a steep learning curve for me...
I've been tending to use the Beavis Electronics wirings for my stompboxes to give true bypass, power , led, etc.
www.beavisaudio.com/techpages/StompboxWiring/

[JohnH]

Here is the overall hook-up:



Its really the same in principle as the beavisaudio diagram, with the addition of the power smoothing cap and resistor R14 and C8, and the off- board switch and components to control the gain and tone, C2, C4 & R8.

These parts could go on board, but I find that with items where I'm likely to want to tweak them later, it easier to wire them off board, even though its less neat. The values of C2 and R8 have much scope to adjust. For example, changing C2 to about 490nF and omitting R8 will add further gain, with about the same tone - but I picked it the way I liked it. Reducing C2 to about 39nF cuts some mid boost, leaving a high end sparkle. I had thought of using a more complicated switching arrangement to play with more variations, but in the end I stayed simple.

Here's my build, which is almost the same as the diagram:



cheers

John

[flateric]

Great - thanks for posting, I'm ordering a couple more enclosures and will have a go at building this.


- how about rounding it off with a little tone control?

[JohnH]

Jan 23, 2010 4:43:26 GMT -5 flateric said:

Great - thanks for posting, I'm ordering a couple more enclosures and will have a go at building this.


- how about rounding it off with a little tone control?

Well, it could have one. I suggest to keep it simple, just a cap and pot in series, from the left volume pot lug to ground. Suggested values would be 47nF and a 100k audio pot, but allow for trying a few values. An alternative would be another on/off/on switch, to give two fixed treble cut options plus off=max. This could involve different cap values, and would need experimentation.

As you can see from my photo, my version is getting quite full inside the small box, so to pursue these thoughts might lead to the need for a larger enclosure.

The other usual thing that this design intentionally omits is a gain knob. This could be done either by inserting a 'volume' control after the first stage, or in the form of a 10k linear pot in series with capacitor C4.

My thought with is design was to build something absolutely minimal and working on optimising the core tones of it, then see what can be done with it, using the old-school approach of controlling gain with the guitar, and tone at the amp. The two overdrive settings already have a nice contrast in tone, being thicker/thinner.

cheers

John

[JohnH]

Ooo..just found an easter egg.

This box can feed straight into my Marshall effects return, and has plenty of output grunt to drive it. On my amp, the overdrive tone stack and master volume are after the FX return, so I get these working too, but it seems to be an exceptionally juicy crunch that is available this way.

John