In reading the post about Runoffgroove and Jake Nagy’s Blue Magic and seeing that no less than 8 people wanting a schematic that was taken down, I feel an opportunity to issue a challenge. Before going further, I want to say that I in no way mean to diminish or demean the great work that Jake (whose designs you should always check out, Jake has a golden ear and a remarkable sense for designing good tone) and Runoffgroove have done and I feel that you should ALWAYS credit those who helped or inspired you to create something (you want people to do that to you don't you? ) . But my challenge stems from the fact that we know something good can come from a MOSFET driving a JFET with filtering in there. So, I challenge those willing to try their hand at going beyond building and getting into the whole DIY FX “roll your own” experience.
When I first got into this hobby I guess five years ago on Aron’s original site, there were a ton of new designs coming out. Jack released the minibooster, Aron’s released the Shaka stuff, Doug and Ed built on the Mini-Tubes and Aron’s work, Gus was slamming out simple home runs constantly, R.G., GFR. Ed. R. etc. There was a lot of work going on, everyone bouncing ideas off each other; the emphasis seemed to be more on creating than building old designs. Sure, everyone started out modding something, building a FF or cloning something else, but eventually most went to designing, and nearly all of what came out was better than what you could buy commercially.
So, I am using this post to try to help those who want to try to get into designing their own by using the Blue Magic and PepperMill’s basic topology as a base. Again, without belittling what Jake and Runoff have done.
Well where do we start? First off, we know that a MOSFET into a JFET with some filtering can sound good, so we will use that. Now of course this leaves a TON of information we need out but if we do a little work we can get something drawn up that will at least work well enough to form a base on a breadboard that we can experiment with to build a great effect.
So, we start with a blank sheet of paper and start drawing up a rough schematic of the circuit. First thing to do is the housekeeping. All effects need some common things to work but the most basic are: a power supply and click prevention. So, we know to put pull down resistors on caps on the input and output and we have to create a power supply.
We start with our “Vin”, put it over or through a reverse polarity protection diode (everything here is a choice, from resistor values to cap types, here we choose whether we sacrifice a little bit of safety or voltage with our choice of diode arrangement), then through a small resistor say 100 ohm, then over a large cap, 10uf to 470uf will do, to filter and clean the voltage and then we have a “V+”. Now we know we won’t need a specific bias voltage as would be the case with an op amp in the circuit, but if we did, put V+ through two 10ks, take the voltage from the junction of the resistors, filter it with another cap and then we would have a “Vbias”.
OK, we have a power supply and reverse polarity protection so we are ready to start with the actual circuit. We start with a high value resistor, say 1 Meg, to pull down excess voltage, into a DC blocking/AC coupling cap to bring our guitar signal into our circuit without it’s own DC bias voltage there to mess up all of our soon to be hard work. We do not know the value of this cap specifically yet so we will pick one before we go to the breadboard. But that is later.
The next step is to figure out how we are going to bias our first MOSFET stage. Glancing through any EE textbook should get you at least three different ways to bias any transistor, looking at the internal schematics of commercial opamps will blow your mind as to how many ways there are to bias a transistor stage. What we need right now is to come up with something that works, is fairly easy, and allows for adjustment of the bias of the stage because we are experimenting and need control over bias.
I would choose what is called constant voltage biasing to start with, then if later on I wanted to do work on tightening up the sound or changing specific characteristics of the circuit’s distortion, I can go back and mess with the bias arrangement of the stages. For now we will put a pot with one outside lug to V+ the other outside lug to ground and the wiper going to a high value bias resistor, 1 Meg ought to cut it, high enough to not load down the previous stage and not too high to induce noise. This resistor goes from the wiper of the bias trim pot to the junction of the input cap and the gate of the MOSFET. Also we will put a high value cap from the wiper to ground to further filter the bias voltage from any noise. The value of the pot can be arbitrary but with a little thought we can save ourselves some power. A MOSFETs base draws no current, so we can use a higher value trim pot as we only need voltage, not current, say 100k or 470k; the cap off the wiper should filter out any noise that the higher value trim pot induces.
Now we have some more questions to ask ourselves: Do we want high gain or a specific lower amount? It is a distortion so we will want high gain. Do we want to run in the high or low current section of the devices transfer curve? For now this is arbitrary but we need to choose one to get it to work off the bat so I will choose high current, which mean lower drain and resistor values (opposite for low current). Next, do we want the gain to be stable or a little unstable using the devices internal inconsistencies to add character to the distortion? If we want it stable we use a source resistor for its negative feedback and a cap to bypass it to jump up the gain. If we want a little unstable we leave resistor off the source and go straight to ground. For this experiment I will pick stable high gain using high current. So we pick some values that fall into this range. I would say 20k for the drain resistor, 1k for the source resistor and a 10uF cap to bypass the source resistor and allow the whole frequency range to be boosted. In may be worth noting that in modifying the frequency shaping of your circuit, you are most likely better off starting out experimenting with the values of the caps and then fine tuning with the resistor values.
Ok, so we have a MOSFET stage with a tune-able bias with high current and high gain, good enough for rock and roll right now, remember we just want to get this thing up and running, we can dicker with the tone to our heart’s content after that.
Next we need to put in another DC blocking/AC coupling cap at the drain of the MOSFET and then from there to the drive stage which will allow us to adjust how hard we hit the JFET stage with our now amplified signal. For now we will want the whole range, so we can find a sweet spot if there is one, and add tapering and scaling resistors later to limit the Drive pot’s influence to that range if we want to. Again, getting it going is paramount right now, so we will leave any filtering networks out for now and go back to that once it is on the breadboard. From the coupling cap, we send the signal to an outer lug of the Drive pot, the other outer lug to ground and the wiper to another DC blocking/AC coupling cap to ensure that the Drive pot doesn’t interfere with the biasing of the next stage. As for values for these parts, again, the coupling caps will be changed later almost assuredly so their value we will wait on, the pot however is a bit tricky. We do not want to load down the previous stage and rob ourselves of the gain we worked so hard to get and we do not want it TOO high so that it makes for a hard source and is loaded down by the next stage. I would pick 100k for now, the 20k output impedance of the MOSFET stage won’t be loaded down too much and it won’t be too high for what will most likely be a high, 1 Meg, input impedance of the following JFET stage.
Ok so now we are at the JFET stage. All of the questions I put forth earlier about the MOSFET stage apply here too, but the biasing is a little different as the JFET functions different than the MOSFET. A JFET needs its gate to be LOWER than the source where as a MOSFET needs it to be HIGHER. This pretty much ensures that I will be using a source resistor (as I am using a single supply V+) on the JFET stage. No biggie really. In fact I am going to use the same set up to be able to control HOW MUCH lower the gate is than the source, as well as being able to adjust the bias voltage at the drain without messing with the gain structure by using a trim as a resistor on the drain (though there is no reason you can’t do both).
So I pick a stable, high gain, high current stage, keeping in mind that JFETs have significantly less gain available than MOSFETs, so I pick a 5k drain resistor, 470 ohm source resistor and the same 10uF source bypass cap. Again, these values are arbitrary mostly, I just want to get the thing up and running, I can come back and mess with them later and almost certainly, I will. Add another DC blocking/AC coupling cap at the drain and that stage is finished.
Now we come to an important point, the tone control, not that we couldn’t have put one earlier, and when it is on the breadboard whose to say we wont? No one. The question lies in the fact that if we put one in now we will not hear the distortion from the basic circuit itself unadulterated, it will always be filtered through the tone control. So, for now I would skip it and come back later once I like the overall sound the distortion is giving me.
SO to finish the circuit on paper, I put in a simple Level pot (audio taper of course), again 100k for the same reasons, take the output from the wiper, no need for a pull down resistor in this case and BINGO! We can go to the breadboard.
So we start by putting the circuit on the breadboard, power supply first, using our multimeter to test to ensure we are getting the right voltage out. Next, we put in the circuit, all we do not know are the coupling cap values but we do know the input impedances of the stages and knowing that we can figure that anything in the range of .001uF to .1uF will work. Once we complete the circuit, we twiddle the bias trims until we get the drains to read ½ V+ (and checking and fixing things if we can’t), then we plug in our guitar, turn the level pot all the way down, plug into our amp, increase the level pot and check out our initial creation!
Now I have no idea what the above circuit I sketched out will sound like but I am 99% sure you will get something out of it worth messing with. The important thing here is to start taking notes and going through the range of all of the pots and trims noting what sounds good overall and also under specific conditions, what don’t you like about the tone? The distortion? Etc.
Once we have our notes, we have a base to start messing with parts. Is the sound woofy and bassy? Try lowering the coupling cap values and the source bypass cap values. Is there too mush high-end hash? Try putting in small caps in different places; across drain resistors, at the gates, off the drive pot wiper. Does the sound get muddy as you turn down the drive? Try putting a small cap from the “hot” lug of the Drive pot to the wiper to keep highs strong as the pot is turned down. Now is the time to start trying filter networks in places, noting “sweet spots” if any in the Drive pot’s travel, the effect of different bias points on the tone etc. Then you can decide if you want to add a tone control, if you want to buffer the tone control or buffer the tone control’s output, or maybe even the Level pots output (I like this myself, tend to lose less highs as the level is turned down). If you want to try and tighten up the over all distortion you can mess with the drain and source resistor values, you can try adding negative feedback or crack open that textbook and find a different way to bias the stages. Not to be trite but the possibilities are literally endless.
Basically, you are now ready to create your own distortion EXACTLY how you like it, you may get lucky, you may stumble on your tone quickly, or it may take you forever with complicated filter networks and biasing arrangements. It is usually when you are perfecting something that you come up with a new “building block” that other people can grab and use in their designs. Then when you are done, perf it up, throw it in a box, make sure it works, and then draw up the schematic, give credit to those you helped or inspired you to create what you did and post it if you want. Hopefully you will teach someone else and maybe even get paid the ultimate compliment by having someone use your design on stage in front of people somewhere.
I hope that this little write up will help some of you make the jump into creating new stuff on your own and keep this hobby growing and to maintain our communities ability to come up with circuits that are miles ahead of what is available commercially.