Guitar / Bass Overdrive

It goes without saying that if you are in to DIY audio electronics, you have made a fuzzbox for your guitar. Distortion is a very personal thing. Some like it warm, and subtle, while others need something aggressive and over the top. Here’s my take.

I often use a Boss DS-1 and SD-1 combo. It does everything I need. I can get really crunchy clipping from the DS-1 and that classic, fuzzy sound out of the SD-1, because it’s really just a Tube Screamer clone. That said, those are simple, three-knob pedals that give you standard performance with predictably bad noise levels, and somewhat generic sounds.

The biggest problem with a typical Boss distortion pedal is that is is limited by what is marketable, and what is easy for the end user to understand and use. DIY isn’t constrained in that way. DIY is perfectly happy to do things that are unconventional, and never need to be marketed or turn a profit. DIY begs to be messed with, and is happy to accept a distortion pedal with five knobs.

I don’t offer this up as a real project to be built by numbers, like a birdhouse, but rather to show some ideas you might want to use when cooking your own distortion, as the number of controls, and their functions, makes using it a bit awkward.

GT-11

Explanation

First and foremost, be aware that this schematic has completely random component numbers, so don’t try keeping track. The design is meant to use 100K linear pots throughout, so don’t go using audio taper for the gain knob, because it will not work very well.

For clarity, power connections are not shown. Use your favorite stompbox power supply setup. Because I have an extra opamp section available, mine looks like this:

9V power supply

The input section is a pretty basic buffer setup, but the pre-gain tone controls are a bit odd. When reading about DIY distortion, you will often see claims that pre-gain tone can give you a huge variety of sounds, but then you see design after design without any sort of pre-gain tone control. This design does incorporate controls that can give you a huge variety of sounds.

GT-11 input

First, the feedback network of the first opamp section sets up a variable active midrange boost, controlled with VR5. This is followed by a passive tone stack, controlled by VR6, which is a variation on the Big Muff Pi tone controls, and has a notch in the middle. When both controls are set to the middle, the result is a wavy, not-very-flat response of about ±2dB in the midrange.

Pre-gain tone control

Remember that this is a device that intentionally adds distortion to your signal, so flatness isn’t as important as what other results you can get with these controls. For example:

Pre-gain midrange control

This setup is very flexible, and can yield some really cool tones. I really like playing with the midrange boost to get some nice crunchy mids with relatively clear lows and highs. The tone knob is not tuned very well. It gets very muddy when turned down, and I may come back and modify that later so that it cuts more low frequencies through its full range.

The clipping and gain stage is also unconventional, but mostly because it uses a linear pot for controlling the gain. Clipping is provided by diodes between the output and inverting input, just like a Tube Screamer or SD-1. This typically provides softer clipping than diodes that short to ground. The clipping is also asymmetrical  to bring out some even-order harmonics.

GT-11 gain and clipping

The gain pot, VR2, and surrounding resistors set the gain range for this stage from 6 to 36 dB, and tracks a true logarithmic scale fairly well. That means that you can turn the gain down far enough to get a really clean sound, that will clip if driven hard, or that will only cause clipping in ranges that have been boosted by the preceding tone controls. 36dB is plenty of gain for me, but if you like the sound of a guitar driving a CMOS inverter (which is horrifyingly common), you would need to decrease to value of R8, at the expense of making the gain knob really touchy at high gain settings.

Finally, we have the master tone control, VR1, and output buffer.

GT-11 tone and output

Again, I use the Big Muff Pi tone stack, because it gives you more control than the more common low pass filter. This one actually gives a flat response when centered. The master volume pot, VR4, is also linear, and is placed outside of the AC coupling capacitor, C5. This means that there is a relatively low DC resistance between the output jack and chassis ground, and it is easy to incorporate an LED indicator and true bypass switching with a DPDT switch, á la the Proco Rat. I have used R.G. Keen’s Millenium 1 with this arrangement in the past, and it works really well.

If I were any good with a guitar I would include some sound clips, but I’m not, so I won’t. If you like playing with different sounds, and have been thinking about building your own franken-fuzzbox, then breadboard this up, and give it a try. Have fun.

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  1. Pingback: Distortion pedal design added to the projects page | Loudifier

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