Circuit Basics

Some our module descriptions, particularly our descriptions of various options, often contain references to the circuits or portions of the circuits.  We try to avoid getting overly technical, but the nature of what we're discussing sometimes strays into the tech details a bit.  In this article, a number of basic circuit concepts are explained, but without going into great detail.  The purpose is to give a little background info to help understand some of the circuit details and options in the module descriptions.  This material is not definitive, complete, or rigorous in any way, and will probably raise eyebrows of those with an electronics background.  But it should be "good enough" to help make good configuration decisions for our modules.

Stages

You'll often see references to a particular "stage" in an effect circuit.  What is meant by that?  Very generally, your audio signal passes through some number of steps, where each step is designed to have some specific effect on the signal.  Often, these steps are just a single sequential path from the input of the circuit to the output, but sometimes the signal gets split into multiple paths, paths are re-combined, or there are feedback loops where some of the signal gets routed back to an earlier step to be processed again.  Most of the time, multiple components such as capacitors, resistors, diodes, transistors, and op amps are required in each step of signal processing to achieve the goal of that step.  These collections of components are often called "stages" in the circuit. 

These stages aren't always independent of each other, so depending on the purpose of the discussion, people may look at the schematic and identify stages a little bit differently.  Some may lump some parts together into a single stage, or choose to identify them separately to highlight their function.  Not all components in a circuit fall into a stage.  Some components are meant to divide or connect stages, for instance.  They may or may not get included in one of the stages, identified as their own little stage, mentioned only in passing, or ignored completely.  Usually, each stage identified will contain a "main" component and some supporting components that are performing a particular function or group of very closely related functions.  These main components tend to be tubes, transistors, or op amps, but may also be a collection of passive components performing a specific function where the main component may be a potentiometer, some diodes, or some other component.

The point is that discussions of stages of the same circuit may treat the circuit and stages in different ways.

 

 

The diagram above is a schematic of the basic Big Muff Pi circuit.  To get a feel for what "stages" are, we can discuss how this circuit might be divided for discussion purposes.  Many refer to the Big Muff circuit as having four "transistor stages" (the transistors are labeled Q1 - Q4 in the schematic), or perhaps four "transistor gain stages".  That's true.  It does have four transistor stages (well, almost always).  We might identify the second transistor stage as everything between R7 (resistor 7) and C7 (capacitor 7).  All those components are working with transistor Q2 to perform some specific related functions to change the audio signal as it passes through them. 

What should be included in the third transistor stage?  Many would agree that it begins after R12.  And probably most would also agree that it ends at the junction of C10 and R17, where the path splits.  It turns out that components C10, R17, C11, R18, and TONE (a potentiometer) are really a stage of their own between the third and fourth transistor stages.  Their stage is called a "tone stack", and provides the circuitry that allows you to control the frequency content of your signal with the TONE knob.  So in addition to the four transistor stages, there is also a tone stack stage.  After the fourth transistor stage, which ends at C13, there is a tiny circuit that allows you to control the VOLUME.  That may be considered another stage in the circuit or it might get lumped in with the fourth transistor stage since it is somewhat related to it.  If you look at the very beginning of the circuit, you'll see R1 and C1, which come ahead of the first transistor stage.  Sometimes they are lumped in with the first transistor stage, or they might be identified as an "input stage", which makes some changes to the incoming signal before it even gets to the first transistor stage.  There are also a couple of components, the SUS pot and R6 that provide the SUSTAIN control for the circuit.  And there are some miscellaneous parts like C4, C7, C12, R7, and R12 that connect other stages and perform some small, but important functions of their own.  Those wouldn't normally be considered stages of their own, and as you'll see below they have another name.

As you can see, the "four transistor stages" label is accurate, since there are four transistor stages.  But depending on your discussion, you might also identify four additional stages, more or less.  Even the four transistor stages may be given different names for the discussion.  For example, the second transistor stage might well be called the first clipping stage, since it is the first of two stages with clipping diodes (D1 and D2).  Or the fourth transistor stage might be called a "gain recovery" stage since its function is to add gain to make up for volume lost in the tone stack.  In both cases, instead of identifying the stage by their main components (transistors), they are identified by their function/position - "first clipping", "gain recovery".

There are other pieces of circuitry that may not be identified as stages, even though they contain multiple components acting together.  Usually they get some other name depending on that function they perform, and the audio signal usually doesn't pass through them.  An example is the power supply section, which isn't even shown in the schematic above.  There may be some capacitors, resistors, diodes, perhaps some potentiometers, or maybe even some chips in the power section.  But since the audio signal doesn't pass through there, it isn't a "stage" in the signal path.

Yes, it's a little loosey-goosey.  The good news is that it isn't usually critical to know everything about the stages, or even to know all the components in them.  The context of the discussion will usually clue you in to what is important.  Typically stages are used to identify a section of the circuit, and the discussion will be focused on some obvious components in that section.  For instance, if we were discussing the "first clipping stage" of a Big Muff circuit, we're probably discussing the clipping diodes in that stage, D1 and D2.  Or if we were discussing transistor options, you would know that we might be talking about all four of the transistors in those four transistor stages.

High Pass and Low Pass Filters

One of the most common transformations to an audio signal is to send it through a "filter".  There are different types of filters, but we'll focus on only two of the simplest and most common here.  One is called a "high pass" filter.  The other is called a "low pass filter".  They perform a similar function.  We'll keep it simple and just consider the case where the filter is made of one resistor plus one capacitor.  In a high pass filter, your signal goes through the resistor.  As it comes out the other side, the capacitor connects the signal to ground as the signal "goes by".  In a low pass filter, the configuration is the opposite.  Your signal passes through the capacitor and is then connected by the resistor to ground as it passes by.

In both cases, the values of the resistor/capacitor combination define a "cutoff frequency".  In the case of a high pass filter, the parts of your signal that have a higher frequency than the cutoff frequency will pass right by the capacitor to ground and continue through your circuit.  The portion of your signal that has a lower frequency than the cutoff frequency will instead go through that capacitor to ground, and do not continue through the circuit.  Those frequencies aren't 100% removed, but they are greatly reduced, making it sound like your lows have been chopped down, which they have.  The low pass filter works the same way, except it is the frequencies lower than the cutoff that continue through your circuit, and the higher frequencies that get chopped and sent to ground.

Filtering like that happens in multiple ways, often multiple times, in a single effect circuit.  There are also numerous variations of the basic principles.  For example, a very common variation is to replace the resistor with a potentiometer, which is a resistor with a varying resistance.  By adjusting the potentiometer, you adjust the resistance value, which then raises or lowers the cutoff frequency, which changes the tone of your signal to be higher or lower.  Another similar function is to use just the capacitor to allow different frequencies of your signal to pass through (or be blocked from) some portion of your circuit.

One of the most popular types of circuit modifications is to adjust the values of the resistors or capacitors in a filter to change the tone of a circuit by changing the cutoff frequency of the filter.  Another very popular type of modification is to replace a fixed value resistor with a potentiometer so that you have a new control over the tone in a specific part of the circuit.  Sometimes, a filter is completely removed or a new filter that was not a part of the original circuit is added.  As you can imagine, tweaking these filters can have a major impact on the tone.  Some effects such as Tube Screamers and Big Muffs are famous for filters that have the effect of enhancing or reducing their mids, for instance.  You can make those circuits sound very different by adjusting the filters responsible for the adjustments of their mids.  In nearly every case, if there is a capacitor in the signal path, it is somehow changing the frequency content of the signal.  All the capacitors in the Big Muff diagram above have an effect on the tone, although in some versions of the Big Muff some of their values are such that the effect is minimized.

Limiting Resistors

In some sense, all resistors are limiting resistors since their primary function is to resist the flow of electricity.  Sometimes that is a particularly critical function.  For example, if the effect is trying to keep a clean signal, there may be limiting resistors put in place to make sure that a voltage isn't high enough that it would cause clipping or distortion of the signal.  Or in the case of distortion effects, there may be limiting resistors to control the level of distortion as the signal goes from one stage to another, to help achieve a characteristic distortion sound.  In another case, some effects are intended to leave the volume the same, even though it may be increased inside the circuit.  A resistor at the end may be used to cut the signal back down to unity. 

Resistors can also help divide and cause a portion of a signal to go through an alternate path in the circuit.  In the Big Muff circuit, resistors R1, R7, and R12 are all selected to help limit the signal going into three of the transistor stages where the signal is amplified.  They contribute to the overall level of transistor distortion in the circuit.  However, as is often the case, components contribute to more than one function, and adjusting them for one purpose may lead to unwanted adjustments in some other function they perform.  Limiting resistors often serve other functions, so options related to changing them must be carefully designed or they may (by choice) come with some additional side effects.

Active vs Passive

"Active" portions of a circuit usually refer to sub-circuits that require power.  "Passive" ones wouldn't require additional power.  Some types of components require power, such as tubes, transistors, and integrated circuits.  Others, like resistors, capacitors, diodes, and potentiometers don't require power.  Almost all circuits will have at least one active stage, requiring power.  When some discussion turns to "active" vs "passive" circuitry, there may be some discussion of a need to change the power requirements of the circuit or add/remove some power feeds in the circuit.

More likely in effects circuits, the active vs passive discussion will be related to a circuit type that could be used for some control in the circuit, commonly a tone control.  A passive tone control usually works by routing some frequencies to ground.  The Big Muff tone stack is a passive tone control.  As you "increase the treble", what you are actually doing is routing more of the low frequencies to ground, making it appear that there is more treble.  There isn't more treble.  There is just less low frequency content.  As you adjust the Tone control, you are really adjusting the balance of your signal between a high pass filter and a low pass filter.  You can see the two filter connections to ground (signified by arrows pointing down) after R18 and C11.  You'll also see that there is no power connection in the tone stack (signified by the arrows pointing up to +9V), as are seen in other sections of the circuit.  One side effect of the passive tone stack is that you are always cutting some of the signal, so you lose volume.  Immediately after the tone stack in the Big Muff circuit, you see a connection to power and an active transistor stage that are there to boost the signal back up and regain lost volume.

There are also active tone stacks.  An active tone stack can add power to the affected frequencies, instead of taking it away from the opposite frequencies.  That can result in a volume boost.  In either tone stack, you'll get the desired effect of tone control, but the way it is done may have other effects on the circuit, such as changes in volume, and may give you different amounts of control to those changes.

Pull Down Resistors, Output Resistors, and Impedance

These are discussed in more detail in Buffering and Impedance so we won't go into a lot of detail here.  Most, but not all effects, have "pull down resistors" (PDR) and "output resistors" to help manage the input and output impedance of the effect circuit.  Managing impedance is important to making effects work well with other effects, particularly vintage effects that didn't properly manage impedance.

As described in the other article, managing impedance has an audible side effect.  Depending on whether you are raising or lowering impedance, you also manage the amount of high frequency content that is allowed into your effect circuit.  In the case of all but some of the older vintage effects, impedance is usually managed carefully and "done properly".  You wouldn't normally have to worry about PDRs, as they already have them.  Your audio signal is kept intact upon entry to the effect circuit.  But some of those vintage circuits, particularly vintage fuzz circuits, did not manage impedance correctly.  That's why they often sound muffled and bassy.  Since they were always that way, that has become an expected, often loved, aspect of their tone.  If you add a PDR, suddenly you allow a lot more highs into the circuit and it sounds very different - unless you do something to compensate for that, like perhaps turn down the Tone control.

In a sense, it is really unfortunate that some pedals, notably some wah's, Fuzz Faces, Tonebenders, and Big Muffs never managed their input impedance "properly", which is, of course, a matter of opinion.  It makes configuring signal chains a little tricky when you use those pedals.  But perhaps those pedals would never have been so popular had they not lost the highs.  Woulda, coulda, shoulda.  A number of GT modules let you choose to change the impedance, usually by the addition of a PDR or by increasing the value of an under-sized PDR.  There's no right or wrong to the matter, but we tend to find ourselves adding PDRs to those old fuzz circuits.  It makes them easier to move around in your signal chain without problems.  Of course, they sound a bit different, but in most cases you can still use a Tone control on the module, or on some upstream module, or your guitar, to cut the highs down and get wooly. 

Coupling Capacitors, DC Blocking Capacitors

We've touched on these, but didn't give them their proper names or mention one of their important functions.  Coupling capacitors connect stages or circuit portions to each other.  They are almost always used to also filter some frequencies in that connection, as noted previously.  Often they perform a second function.  Capacitors block DC currents.  In active stages of a circuit, DC power is often added to the audio signal.  DC blocking capacitors prevent that DC power from going into other parts of the circuit and causing unwanted problems.  Coupling capacitors are usually also DC blocking capacitors, keeping DC power confined to the stages where it is needed.  You won't likely be given an option to add or remove a capacitor that would change where DC power could flow.  That is a basic circuit design issue that must be managed to enable the circuit to work properly.  But you will definitely have the option to change the value of coupling capacitors to sculpt the frequencies flowing from one stage to another.

Input and Output Capacitors

These capacitors are really just coupling and DC blocking capacitors at the beginning and end of the circuit.  The "next stage" or "previous stage" would be other effects, guitars, or amps, where you definitely wouldn't want extra DC power to go.  Preventing DC power from leaking out of a circuit (or into it) is a key reason to have capacitors in those locations.  Often the value of the output capacitor is made high enough that it doesn't have any impact on the frequencies coming out of the circuit, but it can still block DC voltages.  In other cases, the output cap value may very well be selected to affect the output frequencies, often cutting bass or working with other components to reduce noise.  Input capacitors can have a major impact on the overall tone of the circuit, since they can restrict the level of selected frequencies that are allowed into the circuit.  Changing input and coupling capacitor values is a common option, allowing an overall change in the tone of the circuit.  Increasing these capacitors' values will allow more bass into a circuit.  Sometimes that can make a circuit "bass friendly" that previously wasn't.  Or it could allow in too much bass, resulting in muddiness, particularly in distortion effects.  Numerous configurations and options in our modules allow you to tweak these capacitors.

There's another interesting point about input capacitors.  In Switching, we discuss what causes switch popping.  One of the culprits can be the presence of unwanted DC (or AC) voltages on the connection you are switching to.  The input capacitor plays a big role in making sure no unwanted DC is flowing out of the main effect circuit to the circuit input that you connect to when you switch an effect on.  It is important to have that capacitor in place, sized properly, and functioning correctly to prevent switch popping.  If you have a circuit that is popping when you switch, this is one of the things you should check out to figure out what the problem might be.  A few old circuits don't have an input capacitor and may be more prone to switch popping if one is not added.  A value can be chosen so that it has no filtering effect on the tone.

In the Big Muff circuit above, C1 is the input capacitor and C13 is the output capacitor.

Noise Capacitors

Noise capacitors are used to filter out high frequency noise.  High gain circuits like fuzzes and distortions amplify noise in the signal and often add some as well.  The result is a noisy, hissy, or shrieking effect in some cases.  Capacitors with small values can be used to greatly reduce these unwanted high frequency noises.  Of course, if you have signal in those frequencies you don't consider noise, like maybe really high harmonics, those would also be reduced.

A number of fuzz, high gain, and even compressor module circuits have noise reduction capacitors in them.  Or we provide options to add them.  In either case, the values can be tweaked to change the cutoff frequency where the noise filtering kicks in.  In some circuits, there are multiple points where noise capacitors can be installed to provide multiple noise cuts through the circuit.  If a circuit has noise capacitor options or is a fuzz or other high gain effect, you should probably consider them, particularly if you play in an area where noise is a particular problem or you play high-gain music and setups.

Voltage Dividers

A voltage divider is a simple two-resistor circuit that is used to reduce a voltage to a lower level.  In some cases, one of the resistors may be a potentiometer, giving you control over the resulting voltage.  Voltage dividers are very common in effects circuits.  Much of the time there is no reason to change them.  They set voltages that are required by certain components to function properly.  You won't see options to "adjust a voltage divider" in our modules.  But there are some options that are basically doing just that.  We offer different operating voltages in some circuits.  Usually, in those cases using a higher voltage reduces distortion in the circuit and leads to a cleaner or at least less distorted sound.  Lower voltages may introduce distortion or run some components with a voltage that isn't really intended, either of which may cause just the sound you're looking for.  In addition to offering different voltage levels, we also offer a Voltage Sag option, which lets you dial down the voltage to simulate running the module with a dying battery.  Some people like that with some of the vintage fuzz circuits in particular.

Biasing Resistors

Biasing Resistors are usually a special case of a voltage divider or some other form of voltage control.  Some components such as transistors and op amps do their job "better" if the signal coming in to them is close to a certain value.  Bias resistors adjust the incoming signal to be near that value.  But you can change the bias to get different tone from those components.  Fuzzes are often a good example.  Biasing them out of their normal range can change the nature of the fuzz entirely.  You might get splatty, spitting fuzz or you might tighten it up to a velcro-like rip.  Some effects, such as the Big Muff, came in different versions.  The different Big Muff versions sound different, partly due to changes in the bias resistor values.  Resistors R3, R9, R14, R19, and R20 are all voltage divider or bias resistors that can are tweaked to different values in the many different variants of the Big Muff circuit.

You'll find options to take fixed bias resistors and make them potentiometers instead, so you can control the bias point.  This is also a good option for effects with germanium transistors.  Germanium transistors are sensitive to temperature changes and can sound quite different at different temperatures.  Having a bias control may let you adjust to minimize the tone changes related to temperature.  Either way, it is often a recommended option since it can have a significant impact on the sound of the effect.

Collector and Emitter Resistors

Transistors have three connections.  Two of them are called the collector and emitter.  They often connect the transistor to power and ground and effect how the transistor operates.  These perform functions somewhat similar to voltage dividers or biasing resistors, but specific to transistor operation.  By tweaking them, you change the gain produced by the transistor.  In the Big Muff circuit the collector resistors connect the four transistors to +9V.  The emitter resistors connect the transistors to ground.  They vary in value with the different versions of the Big Muff and can be tweaked to increase or decrease gain in the transistor stages.  Generally, lowering an emitter resistor value increases gain, while increasing the value decreases gain.  Collector resistors work just the opposite.  You will find configurations with different values for collector and emitter resistors, as well as some options to change their values.

Power Filter Capacitors

We've discussed power filtering capacitors in Power Filtering.  Most newer effects include some level of power filtering, and many older ones did not.  GT modules are powered in a different way than pedals.  Our modules contain power boards that perform several functions, including converting the incoming 18V DC power into the power needed in the module it powers, protecting from various types of bad power (eg wrong voltage, AC instead of DC, and polarity), and filtering and smoothing the power to reduce noise.  The incoming power gets filtered multiple times on the power board to provide good, clean power.  In addition, the power is further filtered when it gets to the various effects boards.  The result is that our power is more consistent and quiet than it can be in most pedals.  The power boards are a key part of our overall solution design, so they cannot really be changed up.  The power filtering on the effects boards is similar to, but in many cases, better than the power filtering of the original circuits they compare to.

We don't really offer options for changing this.  Good, clean power is essential, in our opinion, to good sound.  Some people know that some old vintage effects did not have any power filtering at all, and suggest that the lack of power filtering contributes to the "mojo".  Perhaps.  What is definitely true is that the absence of power filtering means that your power supply can then have a direct impact on the quality of the sound in the effect.  Perhaps some "good noise" might come in through that power supply and add to the mojo.  We don't rule that out.  After all, we like carbon comp resistors in some circuits and they are known to do exactly that - introduce some noise that some of us find pleasing.  However, we'll also point out another fact.  Many, if not all, of those old effects that lack power filtering also ran only on battery power.  Battery power is a direct source of DC.  It can be a very good, quiet source of pedal power.  If you have a good, quiet power source, then power filtering may not be necessary and you can save a few cents' worth of capacitors or a tiny bit of board space.  Just sayin'..

Voltage Limiting Diodes

One of the primary capabilities and uses for diodes is to limit the amount of voltage that passes through them.  They are sometimes used to protect other voltage-sensitive components from over-voltage damage.  Although it isn't especially common, they are used for that purpose in a small number of effects circuits and it is extremely rare for them to fail when used in this manner.

However, in many boutique or "improved" versions of effects circuits, that property of a diode is used, along with another property of only letting current flow in one direction, to "protect" effects circuits from power problems.  Many builders add "power protection" diodes to their boards in case you accidently plug in power with the wrong polarity or voltage.  These diodes would prevent damage to the effect circuit.  Maybe, and maybe depending on what you consider "damage" to be.  Common power protection diode usage will result in the diode "sacrificing itself" to protect the rest of the circuit.  With bad polarity or too high voltage, the diode will quickly overheat and fail if the bad power condition lasts very long.  When the diode fails, it will either fail "open" and allow no electricity to pass, or will fail "closed", forming a short circuit and allowing electricity to pass that maybe shouldn't, perhaps damaging the effect circuit.  Even if you don't end up with a damaged effect circuit, you end up with a dead diode.  In either case, your pedal may well be dead until it is repaired. 

We do have a tiny number of voltage limiting diode used to protect sensitive components.  But those diodes won't typically fail if they are used in that capacity.  The conditions shouldn't even be encountered.  But the accidental bad power scenarios are common, a leading contributor to damaged pedals.  We do not use diodes for power protection in the way described above.  Unlike in most pedals, we have space in our modules to implement more robust handling of power problems which will rarely lead to failed parts.  If it does, our power boards are easily and cheaply replaced.

Clipping Diodes

This is also a topic we've discussed elsewhere, in Clipping Configurations  Many effects have clipping diodes and one of our more common options for those effects is to offer alternative clipping configurations or even multiple clipping configurations you can switch between.  Clipping is often a defining characteristic of the tone of a circuit, so changes in clipping can change the tone of a circuit considerably.  If you are looking for an easy way to get an alternative sound from a pedal, definitely consider some clipping options if they are offered.  Options have to be selected carefully.  If you select an option that is close to another option, it can be nearly impossible to hear any difference at all.  This is addressed in the other article in much more detail

Op Amps

An op amp is a type of chip that can be used for a variety of purposes, such as increasing gain, creating a buffer, combining signals, and many others.  Most, but not all of the time, they are in the signal path.  You can't change the function of the op amp.  That is set by the circuit design.  But you can change the op amp in most cases, as long as you have another op amp in mind that is electrically compatible (pin-compatible).  Just like any other component your signal passes through, an op amp can have subtle (or in some cases not-so-subtle) impacts on the signal.  The impact may be a tiny change to the frequencies.  As a result, some op amps are considered "more transparent", or "warmer", or "brighter" than others and may be picked for precisely those reasons.  In other cases, the technology used to build the op amp and the various spec values it has can lead to other characteristics.  Some older op amps are not really all that good, and give the tone a "lo fi" or "dirty" character.  Some don't respond quickly to changes in their inputs, or their internal technology colors the tone in some way, resulting in other forms of signal mangling, which may be very desirable.  Or not.

We socket our op amps in our circuits so that they are not damaged by soldering.  That also makes it easy to try other op amps, as long as they are compatible.  So we offer op amp choices in almost every circuit that uses them.  Normally, swapping op amps won't cause a massive tone change.  It is often pretty subtle, but can be quite noticeable.  In some cases the op amp is a key part of the known sound of a circuit.  If you want a Rat to sound like a Rat or a Tube Screamer to sound like a Tube Screamer, you probably want to stick with their stock op amps.  But if you aren't wowed by some circuit, say a Tube Screamer, then changing to a different op amp may turn a "meh" circuit into The Bomb.  There probably wouldn't be that big of a difference in most cases, but it may be just enough to make the difference you'd like to have.  Check out our Op Amps page for compatibility information.