Jfet/Mosfet ovedrive

Here’s a tinny circuit that I did few weeks ago . The idea was to use only a limited number of parts and make a really simple circuit.  I don’t like op-amp saturation but really like jfet/mosfet saturation . The drawback is that they need quite often trimmering . And I hate that , it’s annoying and require multi-tour trim to set things properly .  So, I though about an hybrid circuit with the use of input/ouput op-amp and the JFET transitor sandwitched in between. (the transistor can be any other type, germanium, mosfet , BJT )

jfet drive

As you can see, it’s pretty simple. The input op amp must be high input impendance  (not that shitty 4558 ). The key  is to keep this stage as clean as possible , the op amp must not saturate it should only give boost ! then a JFET (or any other transistor) which will saturate with the drive pot. A bipolar transistor wired as a buffer to isolate the jfet from the tone section (that way the JFET gain ins’t affected by the loading and stay high). A tone section , a simple low pass filter and another op amp buffer to offer a low output impedance

the bias network should be calculated to give Vcc/2 at the transistor’s emetter (don’t forget the diode voltage drop at the junction of base-emmetter (0.7V))

That’s it ! I tried this with a mosfet , worked really well ! a massive distortion with a lespaul. Indeed you can put any transistor you’d like . Mosfet, JFET, bipolar silicon transistor even germanium ! Of course, the bias need to be set properly for each of them (hence the drain trimmer)

However, some limitations appear. The op-amp saturation will depend on the input signal’s amplitude . A telecaster pickup doesn’t have the same amplitude than a beefy lespaul.

To avoid that  you can select a good rail-to-rail op amp , I know that some french overdrive pedal use a LM358 for their overdrive and sound quite good (once again , we don’t give a shit about “hi quality” in guitar espacially if you create a distortion ,otherwise you don’t use a 4558 or a germanium transistor which are the shit )  Two choises, live with the slight overdriven op-amp ( note that some op amp are made exclusively of mosfet , tech21 use them in their pedal , and could be a nice feature !)  or set a lower gain for a perfect balance between instruments (a switch for hi/low gain could be also wired). The input amplitude will also determine if the circuit will give a light drive boost or a massive saturation.

Some tweaks can be done as well to improve the circuit. More distortion ? reduce the voltage coming into the drain voltage , it will reduce the headroom of the jfet and therefore can make it saturate more easily. The source resistor can be tweaked as welll to create a more buzzy or fuzzy circuit in starving the current.

To avoid the saturation of the op amp the power supply can be increased to 18V (keep in mind the maximun value of the op-amp used) . But the JFET will be more hard to saturate as well (bigger headroom) so it can be kept at 9V.

for the all mosfet rail-to-rail op amp , here for instance the TCL2262 used in tech21 drive pedal

http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slos177d&fileType=pdf

I’d really like to make some drive circuit with this but it is quite pricey . Around 2.50euros/piece I think.

thanks for reading

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5 Comments

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5 responses to “Jfet/Mosfet ovedrive

  1. Micromegas

    Hi,

    I just found this blog on the net and wanted to thank you for all the interesting info presented here.

    I came across this post when searching on the timeline and would like to test this circuit on breadboard. Is it possible to have any known values/working ICs for the schematic? I know that the polarization networks for the opamps depend on the kind used, but it should be nice to have a starting point.

    I’m also interested in the hi/low gain switch implementation, so any further light throwed on it may be helpfull too.

    Maybe I’m asking too much…. but I see a lot of potential for this topology.

    Well, anyway, kudos for the blog.

    Cheers,
    Adán

    • thanks Adán for your interest. Really nice to know that some people find interest of my little things !

      Actually it’s pretty basic circuit topology.

      The op-amp can be anything from a 4558 to a TL72 or any general purpose op-amps. ((R9+R8)/R6) + 1) gives the gain of the stage. There’s no typical value however values higher than few Kohm is required to avoid too much current to be used. This will really depends on the gain you want, but let’s say R8 = 10K, R9 = 50K or 100k and R6 = 1K.

      The key is to have the maximun gain available and the less possible distortion of the op-amp (can be tricky). We only want the jfet saturation. R7/R3 forms a divider for the op-amps bias (generally Vcc/2) but the diode in the Base-Emetter path of the transistor eats 0.6V so the voltage at the base should be ((Vcc/2)+0.6) to have the proper bias at the emetter. This is if you want to be really correct, but as you don’t saturate the op-amp, we can deal with the 0.6 error, it won’t do much. R4 can be 10K, there’s no need for much current in here.

      The hi/low gain switch is simply an idea for playing with the op-amp gain and access high gain for low output guitar and beefy ones. A lespaul will require a lower gain to avoid op-amp saturation but this low gain will only give a sligh drive with a telecaster. The switch will accomdate the circuit for both, offering high gain for both if needed. This switch will modify R6 value. It could be also a potentiometer but it will become quite redondant and odd to set. Note that can transform this circuit into a nice clean boost as well.

      C5 allows the AC to pass but retreive the DC voltage from the op-amps. Value will depend on your taste, high value = all the signal (no filter) but more distortion or even blocking distortion. You can put a low valued cap to reduce a bit basses. You can start from 22n and go lower (the frequency corner will depend of the cap value and R10 value).

      R10 is the bias resistor for the Jfet gate, usually it can be 1M (but can be lower considering the low output impedance of the op-amp). R11/C6 is simply a low pass filter to tame high ends (it’s optional and depends on your tastes). R14 can be 10K (really traditional bipolar transistor buffer).

      R12 and R13 set the bias of the jfet. R13 set the current and R12 needs to be set to have Vcc/2 at the drain of Q1 (and base of T2)

      C8/C9/R15 is a low pass filter. You can try 25K for the pot, 10nF for C9 and 22n for C8. But experiment as well ! C10 is a coupling cap, it can any values you like. Larger will allow more frequency to pass (1uF is good in general). R17 allows the cap to discharge when the circuit is off (pop-noise remover if you will) and R18 can be 470R or 1K. This resistor simply protects the op-amp’s output if a short occurs.

      R16 is the volume and set the bias of the “+” input of the second op-amp.

      If you want more infos on that, I may write additional infos or a new post with oscillscope shots and stuff. Let’s me know how it works for you.

      thanks for reading !

  2. micromegas

    Wow! That was a heck of an explanation

    Thank you fuzzo, you managed to cover all the bases with it and also made it easy to understand.

    I’m about to put an order to my supplier and will add some extra parts too to breadboard this circuit.

    I really dig the apparent simplicity of it and the idea of wrapping the fet stage between opamps.

    Mosfet &/or fet distortion it’s way more musical than that one produced by clipping the opamp, and is nice to see something that deviates from the increasingly common “fet preamp emulation” technique (not that I don’t like it either..).

    As I’ve said before, I’ll breadboard it. I can get out of my mind the idea of replacing the gain stage with a mu-amp topology in order to eliminate the trimmering completely…

    By the way, if you love mosfet clipping as much as I do, you should really take a look at this circuit (not my design, the credit goes to Robert Abernathy):

    http://rock.it-frog.com/Downloads/Docs/Scramjet_V11.pdf

    It uses mosfets wired as hard clippers and two parallel gain structures that exploit the clean capabilities of the opamps to boost different frequency regions. A different approach that the one you took for this design, but it shares the main idea of avoiding opamp clipping in order to get the transistors to saturate the signal.

    I’ll let you know how my breadboarding ends.

    Thank you.

    All the best,
    Adán

    • I find also that mosfet/JFET are way better sounding circuit than regular saturated op-amp. Another chips to use are CD4007 and CD4069UBE which are JFET array and hex inverter. You can easily create a double overdrive with the CD4069UBE. And they sound really well.

      I think op-amps/diodes clippers are used in mass produced stuff because it’s easier to get distortion that way (few commun componenents, just copy or improve slightly old design, ect…)

      The trimmer is indeed a bit of a pain but for a one shot circuit is acceptable. However, If I had to produce this in quantity i’ll do different. The mu-amp is a good idea !

      I encourage you to try other transistor in place of the JFET as well. As the Gate is AC coupled the bias of the overdriven transistor is totally independant.

      I made a similar topology (actually it’s a really classic topology of distortion, a gain stage driving another one to saturation) with a mosfet driving a Germanium transistor. Possibilities are endless.

      I didn’t know the project you mention. Really interesting and very well documented! In this approche I remember a circuit made by Graig Anderton called “quadrafuzz” (if my memory is right) which is 4 separated EQ band with distortion circuit for each. A complex beast for a pedal (I think that was a rackmounted circuit by the way). I think that PAIA Electronic sold kits for this.

      anyway, thanks for reading !

      Julien.

  3. Pingback: Mosfet Driver – Return of the Mosfet | Smoking tip

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