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 )
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
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