BlueJuice - an Overdrive using JFETs

[JohnH]

This is re-posted from the effects discussion section, and describes a nice crunchy overdrive using JFETs

This is the circuit and veroboard layout:



The first two JFETs set the drive, followed by a volume control. There’s no separate 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.

After the drive, I 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 modeler, 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.

Here is the overall hook-up:



The power smoothing cap and resistor is R14 and C8, and the switch and components to control the gain and tone are C2, C4 & R8. these are all off-board.

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.





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



Background

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 focused 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.

Sounds
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.


www.soundclick.com/music/songInfo.cfm?songID=8640126

[JohnH]

A year later, this is still my best overdrive. Ive been experimenting with some new settings, and by varying C2 R8 and C4 a wide range of gain and tone can be wrought from this circuit. Currently, I have three switch positions, with the first cutting out these parts for a slightly dirty clean, second, C2=660nF and R1=1k, with C4 disconnected for a sparkling light crunch, then third with C2 switched to 270nF and C4 engaged for maximum drive and a singing sustain.

I like this little box.....
J

[sbgodofmetal]

where'd you get your fx box at john

[JohnH]

Mar 27, 2011 14:51:53 GMT -5 sbgodofmetal said:

where'd you get your fx box at john

I get all my parts at www.jaycar.com , which is a chain of electronics shops down here in Oz. The box is nothing special though, should be available from other outlets like mouser too.

[JohnH]

I recently had another go at this circuit, to give it a continuous gain control, in place of the switched selections as before:

Schematic



The gain is now controlled by the two variable resistors, labelled R16 and R17 above. These are on a 100k dual-ganged pot. As the pot resistance is reduced, the source resistors get increasingly bypassed, and with less negative feedback, gain increases. The bypass is via capacitors, and in the second stage, this has a fairly large value so effects the full tonal range, while that of the first stage has a smaller capacitance. This means that as gain rises, the extra gain is increasingly added to the high frequencies. So with low gain, it does a full range boost, while high gain tends towards treble boost.

Response

Here is the gain v frequency plots, with the values of R16 and R17 stepped in 10% increments.



You can see the full range for most of the traces, then the last 10% jumping up to 50db above 1kHz.

Set at minimum, there is still about 20db of gain available, controlled by the volume pot. That will stay clean for low signals but will start to crunch with heavy attack. For a fully clean boost, probably need to reduce guitar volume first, set minimum gain, then adjust the volume control to push the amp as desired.

Since most of the action is in the last 10%, a log pot is best. Here is the same trace as above, in steps from 90% to 100%, representing the last half of the pot turn:



Actually, a normal log pot needs to be wired so max gain is anticlockwise to get smooth gain build up. Its not really a big deal and you get used to it, but an anti log pot would be perfect.

Here is the shape of the signal waveform, as gain increases:
]

You can see how the clipping builds up with nice rounded corners, somewhat asymetrically, up towards squarer waves at higher gain. The ability to move through that range of soft to hard clipping, dependent on dynamics and signal level, is the whole secret of why these circuits sound good for overdrive boxes.

Wiring

This is the revised wire up. The basic board construction is unchanged:



Results

The tones are similar to before, but can be dialled in much easier now. Its very dynamic and becomes part of the amp, responding to picking attack and control settings. I have not found need for a tone control. It tends to find a tone suitable for the level of gain (more gain = less bass), and this can be adjusted with amp and guitar controls.

More sound clips to follow.

John

[reTrEaD]

Dec 26, 2011 6:54:08 GMT -5 JohnH said:

Response

Here is the gain v frequency plots, with the values of R16 and R17 stepped in 10% increments.



You can see the full range for most of the traces, then the last 10% jumping up to 50db above 1kHz.

An antilog pot (or log pot with max gain at ccw) would go a long way to making this more controllable. But I think there might be another step that could lead to even more control.

If you used a 25k pot with a 75k resistor from pot to ground, that would put the rotation more in the "useable range". Your baseline gain would be around 22dB instead of 20dB. But what's a few dB among friends?

Worth considering?

[ashcatlt]

Dec 26, 2011 6:54:08 GMT -5 JohnH said:

]

How'd you do that? I haven't found this feature in 5spice yet.

[JohnH]

Dec 26, 2011 11:24:49 GMT -5 reTrEaD said:

If you used a 25k pot with a 75k resistor from pot to ground, that would put the rotation more in the "useable range". Your baseline gain would be around 22dB instead of 20dB. But what's a few dB among friends?

Worth considering?

Cant see it. The range of resistance values where most of the action takes place is 0 to 10k, whereas if I'm understanding correctly, what you suggest would put the pot operating in the 75k to 100k range, where gain changes only slightly.

Although the 0 to 10k range is the most entertaining for gain, the rest of the turn is important for adding a few extra db of headroom. For use as a clean booster, this type of circuit should really only have one gain stage active (about 10db of gain) or else you get crunch. So with 20db minimum, it definitely happens but it can be controlled at the guitar. Its a range I'm finding works well and I can set a neck single coil rhythm, slightly reduced to be clean with a few extra harmonics, then flick to full bridge Hb for Angus crunch.

The next version will have a second foot switch, to engage a change from clean to dirty or a volume/gain boost - not sure yet.

Actually its fine with the anticlockwise gain increase, I got used it it straight away. If one was building this commercially, or even just for somebody else, and if 100k 'C' type anti-log pots were not forthcoming, I'd add some big arrow-type motifs to each pot to show which way to turn them.

How'd you do that? I haven't found this feature in 5spice yet

Ash - that is a Transient analysis in 5Spice. The parameters on the analysis menu are the times at the start and end of the graph (0 to 0.01 sec in this case), and the time step (I used 0.00001s). You also have to set parameters for the signal source on the schematic, under the Tran/FFT option. I set the signal to a sine-wave, with 0.1V amplitude and 500Hz.

John

[reTrEaD]

Dec 26, 2011 14:37:02 GMT -5 JohnH said:

Cant see it. The range of resistance values where most of the action takes place is 0 to 10k, whereas if I'm understanding correctly, what you suggest would put the pot operating in the 75k to 100k range, where gain changes only slightly.

I suppose I should have added a few more words in my description.

The wiper would remain connected to ground, but a 75k resistor is placed between the 25k potentiometer element and ground. So the pot would be operating in the 0 to 25k range. Between that and a log taper on the element, it should offer a little finer control than just a log taper alone.

[JohnH]

But is that different from just using a 25k pot, with no added resistor? since the lower half of the pot and 75k resistor sounds like they would be bypassed?

BTW, there is another way of adding variable gain using bypass of the source resistor:



In this case, the pot replaces the source resistor, and the cap partly bypasses the pot, depending where it is set. The pot would be 5k in this case, and the drain resistors increased in proportion. This arrngement has the advantage that gain increase is linear-ish, and linear pots could be used. This design is used in at least a couple of comercial FET based ocerdrives that i know of (ZVex 'Box of Rock' and 'Liquid Sunshine' by Subdecay.

The trouble with it is that it crackles as you turn the pot, and theres nothing you can do about it (never run dc through a pot!), These makers have to point this out to their customers or else they would get many unhappy returns. ZVex even writes 'Crackle OK!' on the box graphics. Well I say its not OK. My arrangement does not crackle!

John

[reTrEaD]

Dec 26, 2011 14:56:28 GMT -5 JohnH said:

But is that different from just using a 25k pot, with no added resistor? since the lower half of the pot and 75k resistor sounds like they would be bypassed?

lol. Now that you mention it, I realize how silly the 75k resistor is. Yeah, I think just a 25k pot would be the ticket.

[asmith]

I'm in pre-prod for this new circuit, and have the necessary parts sat upstairs in a box.

Dec 26, 2011 15:14:33 GMT -5 reTrEaD said:

lol. Now that you mention it, I realize how silly the 75k resistor is. Yeah, I think just a 25k pot would be the ticket.

Yes - although John specified that he needed the extra range for 'headroom.' Voila:

Dec 26, 2011 14:37:02 GMT -5 JohnH said:

Although the 0 to 10k range is the most entertaining for gain, the rest of the turn is important for adding a few extra db of headroom. For use as a clean booster, this type of circuit should really only have one gain stage active (about 10db of gain) or else you get crunch. So with 20db minimum, it definitely happens but it can be controlled at the guitar. Its a range I'm finding works well and I can set a neck single coil rhythm, slightly reduced to be clean with a few extra harmonics, then flick to full bridge Hb for Angus crunch.

I'm in both camps. While the clean/dirty switch sounds great - and let's not forget I lurve more switches - perhaps we could placate both parties for now by adding some resistors between wipers and lugs to morph the 'log' of the sweep into an extreme?

[EDIT]

Dec 26, 2011 6:54:08 GMT -5 JohnH said:

Here is the same trace as above, in steps from 90% to 100%, representing the last half of the pot turn:

Image

Oh, forget I said anything.

My question still stands though:

[/EDIT]

I'd like to do some 5spice experimentation on this, but it's screaming at me for JFET models. John, where did you get your 2JN5457 subcircuits from?

Cheers.

[JohnH]

I've just had the same problem with JFETs in 5Spice. The version I was running up until last week had quite a few components modelled, but then I decided to format my computer (it really needed it), and i got the newer version of 5Spice, with only 1 JFET in it. So Im hunting for models. Ive found plenty of normal BJT transistors on suppliers sites but have not been sucessful yet in getting the JFETs. I might try an older 5Spice download, and copy the library from there.

Im using 5Spice version 1.65, which do you have?

John

[JohnH]

I found some info. There are quite a few variations in the parameters that people use for modelling JFETs, maybe because the devices themselves are not very consistent. But this thread on DIYstompboxes seems to have a considered view, and covers the three that I’ve tried which are MPF102, J201 and 2n5457.

www.diystompboxes.com/smfforum/index.php?topic=71466.0

So what you do is open up notepad and paste in this text:

* J201 with VGSoff=-0.8V and IDSS=0.6mA
.MODEL J201 NJF (VTO=-0.8 BETA=0.94M LAMBDA=2M IS=114.5F RD=1 RS=1
+ CGD=4.667P CGS=2.992P M=.2271 PB=.5 FC=.5 VTOTC=-2.5M BETATCE=-.5
+ KF=604.2E-18)

* 2N5457 with VGSoff=-1.6V and IDSS=3.3mA
.MODEL 2N5457 NJF (VTO=-1.6 BETA=1.29M LAMBDA=2M RD=1 RS=1 CGD=6E-12
+ CGS=2.25E-12 KF=6.5E-17 AF=0.5)

* MPF102 with VGSoff=-2.5V and IDSS=6mA
.MODEL MPF102 NJF (VTO=-2.5 BETA=0.96M LAMBDA=5M RD=1 RS=1 CGD=1.54248P
+ CGS=2.567P PB=1.49 KF=7.90591F AF=499.953M)

Save it as a file called JFET.lib, in this directory:
C:\ProgramData\5Spice Analysis\Library

Then in 5Spice, go to Tools and rebuild/update 5spice library, and hit the button.

And off you go. I’ll try it and see if I get the same results for my circuit
John

[asmith]

Bingo. I'm using v1.64. A bit of rearranging with the file location later, and that all works. Thanks John.


I'll just leave this note here below for future ref - oftentimes I Google for solutions, and the top five is filled with GN2 results.

If you're reading this thread and are experiencing the same problem as John and I did, the "JFET.lib" file needs to go in the designated 'Library' folder in "Tools -> Rebuild / Update Spice Library..." You can either move the file, or move the library, your call. Hope this helps.

[JohnH]

Great! I had another look at he directories in the version 1.65 that I'm on, and the .lib files for transistors and JFETs etc go in this folder here:

C:\ProgramData\5Spice Analysis\Library\Diode_BJT_FET

And I ran my OD circuit again, and it works fine but slightly different numbers on the plots. Its well near enough to be a useful tool in developing a design, but I think before building, it is essential to work out the right ratios of the source and drain resistors, to bias the drain at the intended level (ideal is about 5V), based on your actual components. That means either using a trim pot for the drain resistor, or making a simple breadboard rig for each one to choose the best fixed values. Eg, for Q1, plug in a JFET with R1, R2 and R3 and try diffeernt values of R2 until you get the best. Its not super critical, but in place of 15k for R2, you might well find that a particular JFET wants 18k or 12k. the higher the value used there, the more gain will result.

In the case of the output buffer Q3, its much less critical but the bias is set by R12 and R13 if you wish to tweak.

Hunting around on the net, I came across a couple more versions of models for 2N5457. Its interesting to try them, because you can see what variations might do. If you paste these into your file, and rebuild, they will be identified as 2n5457-2 and 2n5457-3.

.MODEL 2N5457-2 NJF(IS=1N VT0=-1.5 BETA=1.125M LAMBDA=2.3M CGD=4PF CGS=5PF)

.MODEL 2N5457-3 NJF(VTO=-1.8 BETA=0.00135 LAMBDA=0.001 RD=35 RS=31.5
+ CGS=2.25E-12 CGD=6E-12 KF=6.5E-17 AF=0.5 )

The first came from a science assignment that was online. The second was from a guy who said he had this model but he didnt know where it came from. Actually, it seems the nearest of the three to what I was using before. But I think the first model posted yesterday is giving results that are nearest to the JFETs in my spares box. In all of them however, I find that if I work things out using a slightly higher battery voltage, 10V instead of 9V, the resulting component values match better with what I build.

John

[asmith]

That works a treat.

As for the 'anti-log pot' and '100k / or less' discussion. This morning I had a brainwave. I had been rooting around in the band's guitarist's cable bag last night to plug in, and came across two homebrew stompbox enclosures, pre-used. He'd filled them years ago with just a volume control connected to a true bypass, and an A-B/Y-Z setup. Therefore I've got enclosures and 3PDT stompswitches - I'm just a little limited with what holes I can use and drill, etc.

With that in mind, 4real's potential use of wheels on pots put me in mind for something like this:



This solves both problems. You move the wheel 'up' for more gain, but this motion actually turns the pot down, so there's no need to sniff about for a dual anti-log pot. Alternatively, swap the side that the wheel's on, and thumb it down for more gain.

Also, because there's no visual 'reference' for the 'ends' of the pot, it doesn't feel like you're 'wasting' half the control. Maybe I'm the only one who feels like that here, maybe not. Either way, now you've got a 'clean boost' sweep fading into a 'gain' sweep.

When I build this I'm going to hand-paint the wheel, so that the 'clean boost' is a sweep of green and the 'crunch' section is a sweep of red.



I'd like to get this built very soon. The new Linux installation now fully works, my bass has been painted with the shielding paint... that checklist is getting pleasantly smaller.

[JohnH]

Thats a good idea. But Id suggest to make sure you have plenty of space. If I built mine again, Id use a bigger box.

I was playing with mine yesterday, and I found that the 100k dual pot is giving a significant gain boost right from mimimum. It may not be much in theory, but the first 1/4 turn has a marked effect as it starts to crunch, so I still like the100k pots.

John

[lefty19]

JohnH, new member lefty here. I am writing a note, because this design looks like an interesting pedal. Do you have any kind of retail buisiness ? Im just trying to find out, do you make amps or pedals for sale ?  Let me know, I would be interested in at least a pcb for this Bluejuice, that is if you didnt mind. 

please let me know,  Lefty

[JohnH]

Hi lefty, welcome to GN2 and thanks for your interest in this circuit.

I'm just a hobbiest and work more than full time in other fields. So I'm sorry but I don't do anything commercially with my circuit designs.

But, to make this one, you just need some veroboard or similar, and once the track indicated by the green circle is cut, then you effectively have a pcb ready to wire from the diagram.

cheers
John