#1
Lately, I've been looking at trying to build a few simple pedals. Unfortunately, I'm the kind of guy who can't follow a schematic without needing to know what each and every part does

Does anyone have any good resources I can take a look at? I understand what the various parts do in the context of a circuit (resistors and capacitors and stuff) but I don't understand how they affect the tone. The only part I really understand is the basic 3 band tone stack.

I'm particularly interested in the parts that affect the voicing of, say, a distortion pedal (the gain levels and frequency response). Thanks to anyone who can help me out.
#4
^That link looks great, thank you!

Quote by Invader Jim
This is literally the most vague question one could possibly ask about
electronics. Which pedal do you want to know about, for example?


Sorry, I guess I just don't know the right questions to ask. I'm mostly looking at overdrive/distortion, and I don't really understand how the sound is shaped by different circuits. What I'm trying to understand is if I want to modify a pedal to change its sound, how would I know what I need to change specifically. I'm doing a very bad job of explaining this, sorry.
Last edited by ibanez1997 at Feb 14, 2014,
#5
It depends on the circuit but each circuit type has certain common characteristics. In other words, most op-amp circuits (like Tubescreamers) are designed around the same topology (the way each circuit type's components are wired up), most transistor circuits (at least the common-emitter circuits [like the Electra distortion]) use another topology. While the basic topologies are the same, extra stuff can be added to improve the performance or shape the sound. The best way to learn is to read articles (geofex [linked above] is one of the best places to learn about effects and how they work).

As a general rule, these points can guide you. They're just random things that come to mind and this is by no means complete or comprehensive.

A good way to shape the overall tone of an effect is to mess with the values of the input and output capacitors. Higher values let in more lows.

The gain of an op-amp stage is determined by two resistors. One across the output and inverting input and another across the inverting input and ground. Make either resistor value larger to increase the gain of the stage. High gains lead to distortion (which may or may not sound good).

The emitter bypass caps of transistor gain stages can be made arbitrarily large to maximize the gain or can be made fairly small to give a treble-boosted tone. I've almost never see this done in pedals or guitar amps as a tone-shaping technique. Most people just slap a big cap across the emitter resistor and call it done.

Putting a very small cap across the collector and base of a transistor will shave off some highs and can even be used to simulate a germanium transistor's tone when using a silicon type.

Changing the tone of an op-amp based distortion can even be as simple as using a different op-amp. Op-amps have a certain slew rate (how fast the output can swing from one voltage level to another) depending on the type of op-amp. Slew rate is given in volts per microsecond and measured at unity gain (gain of 1; no amplification is occurring) to portray the thing in the best light. Generally, slower op-amps make better distortions because they tend not to be so harsh. For example, the 741 is an abysmal audio part with its 0.5v/uS slew rate, but imho it sounds pretty darn good in an MXR Distortion+. The 4558 used in Tubescreamers and countless others has a slew rate of 1v/uS, twice as fast as the 741. To put that in perspective, a higher-quality op-amp like the TL072 has a slew rate of 16v/uS. TL072s are very common in higher-quality practice amps. Lower-quality practice amps usually use 4558s. The 741s and 4558s are definitely not "high-performance" parts.

Well that's it for now. I'm tired of typing on this damn phone.
#6
I think Invader Jim gave a lot of great tips above. I would recommend separating your project into two pieces. 1) something that amplifies and clips (causing distortion) and looking up reference designs using a couple of transistors or an op-amp will solve that. 2) adjusting the tone of that.

For the tone, that basic starting circuit will have limited abilities to tailor the sound. By adjusting the caps, you can make it more or less treble-y, but you probably won't be able to simply adjust at what frequency the treble is changing. And you probably won't be able to boost the bass, for example. If you're happy with a simple treble adjustment (like some pedals give you), then you might be able to just swap caps, or add a variable resistor in series with an appropriate cap.

If you're looking for something fancier with regard to tone control (and I'm assuming you want the adjust the tone after you distort), then it amounts to running your distortion circuit into some type of tone/EQ control. You can do something like that with another op-amp. It depends on how much time you want to invest - it's not hard and EQ is about as easy as overdrive.

Is this just a messing around project to learn about electronics, or you're trying to save money on pedals, or you need some fancy sound that regular pedals don't offer? I'm just curious...
#7
Thanks, Invader Jim, lots of good stuff in there. I appreciate the detailed response.

timbo, I'm really just trying to give this a go because this sort of thing fascinates me in general. I'm not looking to save money or create something unique, I just like knowing how stuff works. I see what you're saying about breaking a dirt box down into two parts, makes a lot of sense. Thanks

EDIT: I've been looking at some circuit diagrams, and I understand most of the components except one. In some places I'm seeing a wire to a point that has a voltage next to it. What do those points refer to? You can see what I'm talking about in the diagrams on the page TJHague linked to.
Last edited by ibanez1997 at Feb 15, 2014,
#8
I actually made the distortion/EQ combo a long time ago using schematics from a Craig Anderton book and it came out really well. I used the state-variable EQ, which you don't see around (but easy to make with a couple op-amps and I really like it).

In that schematic in the link, there's a voltage divider in top-left of the schematic (the complete one at the end). They're taking the 9V in and using resistors to divide it in half. That gives you 4.5V. I didn't really study this, but I'm assuming that it might give you a lower voltage section where you clip it and then the ability to amplify it in the end. So none of the caps in the voltage supply section will do anything for you tone-wise. Normally you wouldn't use a voltage divider like that to create a local supply. The reason is that as the 4.5V circuits draw current, they increase the current through that 10K resistor, which lowers the 4.5V. But they have a big 47uF cap on there to handle transient current and the 4.5V circuits look to be pretty low power, so it's probably fine (of course the pedal makers want to minimize component expense).

If you're looking to have fun, you might be able to get a "SPICE" circuit simulator, input the schematic, and then do things like sweep the input frequency and see what you get by changing components (although if it's simple, you may as well build it and just listen!).
#9
Those wire ends with voltages next to them are voltage nodes. All nodes with the same voltage are connected together. There are supposed to be little circles on the wire ends to denote a node but R.G. draws his diagrams in a rather piss-poor way.

Anyways that 4.5v level is a bias supply for the op-amp and transistors. These devices must be biased into a certain operating range in order to work properly.

Check this out:
http://williamson-labs.com/480_xtor.htm

Timbo is right about this type of voltage divider having poor regulation, but in pedals there is almost always very little current drawn from the bias supply that you can basically ignore those effects.