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Decoding Schematics
Lines, dots, squiggles, circles and arrows... What do they all mean?
By Paul Stamler

Recording regularly prints construction and modification projects. To do them you have to read the schematic. It’s also a useful skill when troubleshooting equipment; if you can open up a box and find a particular component is carbonized, a schematic may tell you which one it is, allowing you to replace it yourself and save a big repair bill. There’s another reason too—but I’ll save it for the end

A week ago I wrote an article about guitar amplifiers: how to clean them up for use in the studio and how to modify them for increased flexibility. In that article I described the process as “…relatively straightforward if you can handle a soldering iron and read a schematic diagram.” That article will appear in a couple of months; meanwhile, I’m here to show you how to read a circuit diagram and translate it into the real world. We’ll start with a real circuit.

Into the deep end

Take a look at Figure 1. This is the input section of a very good quality microphone preamp I designed a few years ago (a more detailed description should appear within a few months). I’ll take the bits one at a time, but first some geography.

In general, schematic diagrams read from left to right: the signal comes in somewhere at the left side of the page and goes out somewhere on the right side. (There are exceptions, of course, which can drive you round the bend when trying to trace circuits, but most authors and manufacturers abide by this convention.)

Many schematic diagrams, showing circuits that have a single (positive) power supply voltage, also obey the convention of showing the power supply rail at the top of the page and the ground connections at the bottom. Since this circuit uses both V+ and V- supplies, it doesn’t do that—but we’re getting ahead of ourselves.

I want to emphasize that a schematic is not a pictorial diagram showing where each part is mounted. Instead it’s a conceptual diagram showing how each part is electrically connected to the other. Later on I’ll show you how a schematic might translate into a physical layout.

We’ll start where the signal comes in: at the three terminals on the left side of the page, collectively labeled Mic Input Jack. The small white circles represent connection points; they’re labeled with numbers corresponding to the pins of an XLR jack—because this is an XLR jack, mounted in the back panel of the preamp.

The little triangular doodad connected to pin 1 is the symbol for ground; in other words, pin 1 is connected to the ground wire (or ground bus) of the preamp. (Ground is also sometimes symbolized by a plain triangle rather than the parallel line drawing shown here.)

There are often several ground systems in a piece of equipment, including signal ground, power supply ground, cable-shield ground, and chassis ground. Occasionally schematics will use different symbols to denote each of these grounds. For the moment we won’t worry about that.

The next devices we encounter are a pair of zig-zags. These are resistors, and most analog equipment uses more of these than anything else. They’re designated by the letter “R” and a number, which is useful in correlating the schematic diagram with a parts list or pictorial diagram.

Resistors are measured in ohms; 1,000 ohms = 1 kilohm, abbreviated “1K,” while 1,000,000 ohms = 1 megohm, abbreviated “1M” or sometimes “1 MEG.” These resistors are 5.62K, meaning their resistance is 5,260 ohms. The three digits indicate that these are high precision resistors, and in fact the parts list will tell you they’re 1% tolerance metal film units—very high quality. (Sometimes the tolerance is shown on the schematic; often, however, there will be a note saying something like “All resistors 5% 1/2W unless otherwise specified.”)

Resistors are also characterized by their wattage, a measure of how much power they can handle before they go up in smoke. Most resistors in audio equipment carry minuscule amounts of power, so their wattage rating isn’t specified.

In between the two resistors is another connection point labeled “+48V.” This is a DC voltage obtained from somewhere in the preamp’s power supply that provides phantom power for the microphone. The line running from this point to the junction between the two resistors indicates that the phantom supply is connected to those resistors; the black dot where they all meet indicates that there is, in fact, an electrical connection there. (Some diagrams omit the black dots and simply indicate junctions wherever lines intersect.)

Next comes T101. This is a transformer, a device that can step voltages and impedances up and down (relax, we won’t go near impedances today), and/or isolate one section of a circuit from another. A transformer in its simplest form comprises two coils of wire wrapped tightly around an iron core. When signal flows in one coil (termed the primary) it induces signal to flow in the other coil (the secondary). (Some transformers have several secondaries; we’ll get to those in a bit.)

If a transformer has more turns in its secondary than its primary, the voltage on the secondary will be correspondingly greater than the voltage on the primary. This is a step-up transformer, and T101 is an example. I’ve symbolized that by making the squiggle representing the secondary longer than the one representing the primary. In practice, transformers usually have wires coming out of them that are color-coded to indicate which is which; on the final version of this drawing I promise to label the wires by color.

Get on board

We now come to more connection points, labeled “T” and “U” in big square boxes. I use these boxes to indicate points where the signal enters or leaves the circuit board; the actual connection points on the board are labeled similarly. Lettered or numbered connection points can mean many things depending on the manufacturer. Usually you can figure out the meaning from the context.

Our next landmark is a triangular device labeled “IC101.” A triangle is the generic symbol for an amplifier of some sort, and the “-” and “+” signs at the left side indicate that this is an operational amplifier or opamp (sometimes hyphenated or written as two words, ‘op amp’). Most opamps are constructed as microchips, or integrated circuits (ICs), but they can sometimes be built of separate parts—so-called discrete opamps.

This one’s made from an integrated circuit, the Burr-Brown OPA-604, so the part number is included in the label. This amplifier could also be labeled “A101.” If it was one of several units in a single IC package, each amp might be designated “IC101a,” “IC101b,” etc.

An opamp typically has two inputs. The first passes the signal through without inverting its polarity; a positive-going input yields a positive-going output, perhaps of greater voltage. This noninverting input is symbolized by the “+” sign inside the triangle (not to be confused with the positive power supply terminal, labeled “V+”).

The second input flips the signal upside down so that a positive-going input yields a negative-going output, again perhaps of greater voltage. This inverting input is symbolized by the “-” sign inside the triangle. The output of an amplifier is always at the far end of the triangle, away from the inputs.

Most ICs in audio gear come in dual-inline packages, or DIPs. A few come as surface-mounted devices or SMDs. If your gear has these, you probably can’t work on them. SMDs require specialized equipment to install or remove.

If the manufacturer is nice to you, they will show which pins on the IC package correspond to which connections in the schematic diagram. I’ve done so, being a nice guy at heart. If a manufacturer doesn’t tell you, there are standard layouts that most DIP IC opamp packages in audio gear follow, shown in Table 1.

DIP ICs usually have a small indentation in the top of the package next to pin 1. The pin numbers go counterclockwise around the package, as seen from the top. Some ICs have a notch at one end instead of an indentation; if the notch is at the top end viewing the IC from the top, then pin 1 is directly to its left.

ICs need power, so there are connections to “V+” and “V-,” the positive and negative power supplies. Each power supply also has a small device attached to it, with two parallel lines in the middle connected to ground at the other end. These are capacitors, or caps for short, designated by the letter “C” and usually measured in “microfarads,” abbreviated F or (in some old gear) mF. (A microfarad is a millionth of a farad, which is a very, very large unit of capacitance—you’ll never see a cap measured in farads in the audio world, or anywhere else for that matter, unless you’re into lasers that can blow holes in sheet metal.)

These caps are .047F, or 47/1000ths of a microfarad. Extremely small caps are measured in picofarads (trillionths of a farad), or pF; 1pF = 1/1,000,000 of a F. (C103 at the top of the page is an example.) Older equipment, including some guitar amplifiers, may designate pF as micro-microfarads, or F, or sometimes mmF. And some equipment, especially European made gear, uses an intermediate measurement called nanofarads (billionths of a farad), or nF; 1nF = 1,000pF = .001F.

I’m going to ignore the funny-looking resistor under the opamp temporarily. Proceeding downwards and to the right, we find Q101. It has a flat bar with a line coming out at a right angle and two more lines at acute angles. This is a transistor, and why it’s abbreviated “Q” is beyond me. (Okay, “T” was already taken by transformers.)

In some schematics there’s a circle around the symbol. In others, like this one, it’s naked. The right angled line is the base of the transistor (“B”); the diagonal line with an arrowhead on it is the emitter (“E”), and the diagonal line without an arrowhead is the collector (“C”).

There are two types of standard (bipolar) transistors, called NPN and PNP. Never mind the difference; all you need to know here is that this transistor is NPN. If the arrowhead on the emitter pointed inward instead of outward it would be PNP. “MPS-A06” is the part number of the transistor.

Different transistors have different ways of placing their pins. Often a schematic diagram will include a small picture of the transistor showing which pins correspond to B, C, and E.

Starting from the transistor, travel an inch to the right and down. You’ll see two things that look like peculiar top hats. These are diodes, symbolized by “D.” The end with the “hat brim” is called the cathode, while the other end is the anode. Diodes only pass current in one direction: from the anode to the cathode.

Most solid-state diodes (like these) come in a small tubular case looking very like a resistor, and have a band printed around one end. This designates the cathode. The part number in this case is 1N4148. (A 1N prefix denotes a diode, while a 2N prefix denotes a transistor. However, many transistors and diodes use other naming conventions.)

Above the diodes you’ll notice that R107 connects to V+. This is the same V+ that the opamp is connected to. Although they appear in different places on the schematic, they’re connected to the same terminals or circuit-board traces in the real world.

Jump, switch, and fade

Above R107 is a little hillock labeled “J102.” This is a jumper, a small piece of wire soldered into the circuit board. Jumpers are versatile tools for the circuit designer; in the original version of this board, you could install J102 and connect the opamp directly to the output section or you could leave it off, install another jumper (elsewhere on the board), and insert an eq section.

Movable jumpers are common enough in computer hardware (you probably use them on your modem), but hard-wired ones can also be useful in letting you custom-configure a board. (Confusingly, jacks and jumpers share the abbreviation “J.”)

Follow the circuit up and to the right. There you’ll find a switch labeled “S101.” In this circuit the switch selects varying degrees of bass rolloff. The switch has three positions (designated by the three small circles on the left) and a terminal connected to the “swinger” of the switch (on the right—the swinger is sy mbolized by the arrow).

This is a “center-off” toggle switch; the middle terminal doesn’t actually exist, and the swinger doesn’t connect to anything when the switch is in the centered (“100Hz”) position. When the switch connects to the “30Hz” position, C107 is connected in parallel with C106, creating a composite capacitor that’s larger than C106 by itself. This gives a lower bass cutoff frequency. Connecting to the “Flat” position bypasses both capacitors, eliminating the bass rolloff altogether; the circuit is now flat down to DC, or zero Hz.

It’s important to know that miniature switches of the kind usually found on modern audio equipment have the “swinger” terminal in the middle and the other two terminals on either end. In these miniature switches, moving the lever upwards connects the swinger to the lower terminal, not the upper—the switch works backwards from what you’d expect. Remember this, or someday you’ll wind up installing a switch where up equals “off” and you’ll feel silly like I did.

Switches are typically designated by the number of poles they have (each pole comprises a swinger and a set of terminals) and the throw (how many terminals are connected to each pole). This switch has one swinger, so it’s single pole, and there are two terminals with a center “off” position, so it’s double throw, center-off. It would be abbreviated “SPDT-CO” for short. Double-pole switches are also common (“DPDT” for example) and rotary switches can have multiple poles and multiple contacts (“4P12T”).

Below the switch is the “Channel Fader,” VR103. This is in essence a resistor with a slider attached; the slider taps the resistor at any level from top to bottom. This device is a variable resistor, often called a fader or potentiometer (pot for short—don’t inhale). It’s usually designated “VR,” but sometimes is called “P” or even “RV.” It’s measured in the same units as resistors (ohms, 
kilohms, and megohms).

On a rotary pot, typically the clockwise position (as seen from the front) is shown at the top, the counterclockwise position at the bottom. On most pots the terminal for the slider is mounted between the two terminals at the ends of the resistor. (Extremely nice schematic makers label the ends of the pot “CW” and “CCW”; these designations always refer to the clockwise and counterclockwise positions as seen from the front.)

“Taper” is a specialized designation. Most of the pots sold in the world are linear taper; turn them halfway down and the voltage goes down halfway. That won’t work for most audio functions. If you use a linear taper pot for a volume control, you’ll find as you turn it up from silence that it comes on very quickly, then increases only slightly during the rest of its travel. This makes for inconvenience, so manufacturers invented audio taper or log taper (short for “logarithmic”) pots. Most volume controls in audio gear are audio taper, while some tone controls are linear taper.

Before we jump off this board, go back to the triangle in the middle, IC101. Look again at the resistor directly underneath it. You’ll now recognize this as a variable resistor, similar to VR103. There’s a small symbol to the left of the slider that looks like a screw head; this signifies that VR102 is a screwdriver-adjusting pot rather than one controlled by a knob on the front panel. These are known as trim pots, or trimmers; they’re typically used for an adjustment you’d make once when the device is new, then leave alone.

In this case,VR102 is connected to a couple of the IC’s pins and to V-. It serves to adjust the output of the amplifier to exactly zero volts when there’s no signal going through it. If there was a voltage present on the output when the bass rolloff switch was set to “Flat,” this voltage would appear across the fader, which would eventually get noisy and crackly.

More squiggles

Figure 2 shows a different type of schematic. This is a simplified diagram; you’ll see these when a manufacturer (or an author) wants to illustrate a few salient points about a circuit without including extraneous detail.

The drawing is a simplified schematic of the power supply from a guitar amplifier (you’ll be seeing it again with discussion in the guitar mods article). Notably absent are labels for most of the parts, along with such necessities as an on/off switch. They exist, obviously, but they weren’t relevant to the article I was writing.

Let’s look at the new bits. We start again at the left side with a pair of terminals. This being a power supply, these are probably intended to connect to a wall outlet (117V in the USA). Occasionally you’ll see this indicated by a small drawing of an electrical plug.

On the wires running from the input there are two small cylinders. These are ferrite beads, hollow pieces of magnetically permeable material designed to slip over wires. They can help filter out radio frequency garbage that’s riding the power line; you may also find them (or decide to install them) on signal inputs and connections from wall-warts. (See ‘Shut Up!’ part 2 in the 7/97 issue.)

Next comes a weird looking thing resembling an hourglass. This is one way of denoting a metal oxide varistor, or MOV. These devices clamp voltages to a specified level, preventing short-term overloads and spikes from making their way into audio circuits (or digital circuits, where they can play havoc with the data). Hanging a 150V MOV across the incoming AC line can help keep equipment from making popping noises during line surges (as when, for example, an air conditioning or refrigerator compressor turns on or off). MOVs are also denoted by the symbol shown just below this one (it’s not connected to anything).

Next is a transformer, familiar from the previous drawing, but this one has some new wrinkles. In the first place there are two secondary windings, one for the main voltage that feeds the amp (for this is the power supply of a tube amplifier), another for the filament of the first tube. (In fact, there’s a third winding for the filaments of the other tubes in the amplifier but it’s not shown here.) Most tube equipment has multiple secondaries; most solid-state gear doesn’t.

Another new feature is a dot in the middle of the high-voltage winding. This is a center-tap, a connection to the middle of the coil of wire that forms the secondary. Center-taps are usually grounded; the voltage in the secondary can be thought of as swinging across the stationary fulcrum of the center-tap, which is anchored at zero volts (ground). Transformers can have other taps too. A single tap off-center on the coil might connect to the bias circuits of a tube power amplifier. Or an output transformer (again on a tube power amp) might have taps for various speaker impedances—4, 8, and/or 16 ohms.

The next gadget is a circle containing two vertical lines on the left and an angled line on the right. This is a rectifier tube or vacuum diode. The two vertical lines are plates or anodes, the angled line is a filament that in this case also serves as the cathode.

A rectifier tube performs the same function as the solid-state diode we met before: it only passes current in one direction, from the plates to the cathode. Unlike a solid-state diode, however, it only passes current when its filament is hot (the heating is done by the special transformer winding mentioned earlier). An actual rectifier tube looks like a small electric lamp (and in fact the diode tube was developed from the light bulb).

Moving right along, we come to a capacitor, C1, with some new twists. There is a “+” sign at one end, and the other end has a curved line instead of a straight one. This is an electrolytic capacitor, one of a class of capacitors known as polarized capacitors. Polarized caps (which include aluminum electrolytics and tantalum capacitors) must be connected with the correct polarity: the “+” side must be hooked to a voltage that is more positive than the “-” side, or the cap will overheat and eventually explode, frightening the cat. In practice, polarized caps are ubiquitous in power supplies, with the “+” end hooked to the positive voltage and the “-” end to ground. (In a supply that includes negative voltages the “+” end connects to ground and the “-” end to V-.)

Polarized capacitors are also often used as coupling capacitors, connecting signal from one stage of an amplifying circuit to another without passing along any DC voltage that might be on the first stage. Although this isn’t the best possible design, it works reasonably well and keeps down both the cost and the bulk (electrolytics pack more capacitance into a given size of case than non-polarized capacitors).

However, many manufacturers also use polarized capacitors in places where there is no DC voltage across them, and that’s a major no-no; designs like this add distortion and muddiness to the signal. If you find capacitors misused in this way, you should replace them with non-polar electrolytics. Hrmph.

Incidentally, the curved line in the symbol for electrolytic caps denotes the outer shell of the capacitor (in earlier days, usually a metal can). Some manufacturers don’t use the curved line, denoting polarized capacitors with only the “+” symbol. Power supply capacitors, especially in old tube gear, often come in multiple-unit packages with up to four caps in a single case. Inevitably these have a single negative terminal (usually the case) and up to four positive terminals.

At the bottom of C1 is a standard ground symbol; directly to its right is something that looks like a letter “E” lying on its side. This is the chassis ground. In earlier days most electronic equipment made all of its ground connections to the metal chassis. That’s seldom done anymore (most modern equipment uses printed circuit boards to hold everything, including the grounding system) but most equipment still connects the ground system to the chassis somewhere, usually at a single point, and the schematic may indicate that with this symbol.

Above the chassis ground is the “Standby” switch, which we can recognize from before as a “single-pole single-throw” device (SPST). To its right is a lumpy gadget that runs to the top of C2. If you said it looks like half of a transformer, go to the head of the class: it’s a single coil of wire and it’s called an inductor or sometimes a choke.

Power supply chokes look like transformers, being wound on steel cores; smaller inductors are wound on plastic or ferrite cores and are used in the crossover networks of loudspeakers. You’ll also find small inductors in old-style equalizers such as Pultecs and their derivatives.

The Phantom strikes

Figure 3 shows a regulator for phantom power, creating a clean +48V supply to power solid-state condenser microphones. Most of the parts are familiar, but there are three new ones.

At the center of the diagram is a rectangular box labeled VR801. In a schematic diagram, rectangles denote functional blocks—black boxes that can perform almost any function. This one is an adjustable voltage regulator, the LM-317T, and it has three terminals: Input (I), Adjust (A), and Output (O). Non-adjustable regulators are similar, but substitute a Ground (G) terminal for “Adjust.”

A functional block can be almost anything: a digital device, a microprocessor, an analog processing device such as a compressor chip, or something that combines several functions. Usually functional blocks’ terminals are labeled with cryptic abbreviations like DCON, FNDOC, or KVTCH22; if the manufacturer is being really nice to you the pin numbers will be included. Don’t bet on it though.

When I drew this diagram, I labeled the regulator chip “VR801.” This is a logical abbreviation for a voltage regulator, but it’s one I regret because “VR” also denotes a variable resistor as found in figure 1. When I go back and revise this drawing for another article, I think I’d better change this designation to something like “REG801” or “IC801” for clarity’s sake.

Below the voltage regulator is a diode with a twisty line at the top, D806. This is a zener diode, used in circuits to establish a reference voltage. (My only reference says it was named after C. Zener, who elucidated the mechanism by which it works. Anyone out there know his full name? [Note from MM to MM: Get this name or hand in your Physicists’ Club membership card!]) Zener diodes are specified by voltage and wattage; this one’s a 47V 1W unit, which seems to have vanished from current catalogs. Uh-oh, time to revise the design with a 5-watter…

Finally, at the lower right-hand corner of the drawing we find a diode with a lightning bolt next to it. This is a light-emitting diode, or LED.

Bits and pieces

In Figure 4 I’ve shown some active devices you’re likely to encounter in audio gear. Junction Field-Effect Transistors (JFETs) are small signal, low-power devices that are often found in audio circuits. They have three terminals: Drain (D), Gate (G), and Source (S). (Confusingly, the drain and source terminals look the same on the schematic. If you’re lucky, they’ll be labeled on the diagram, either by terminal name or pin number.)

JFETs come in two flavors: N-channel and P-channel. The terminal with the arrow is the gate; an inward-pointing arrow signifies an n-channel FET, while an outward-pointing arrow means it’s P-channel.

Metal-Oxide-Silicon Field Effect Transistors (MOSFETs) are similar to JFETs, but in addition to small signal low-power units you can now get high-power and high-voltage MOSFETs, which are often used in the output stages of power amplifiers. Their terminals are the same as those of JFETs.

We’ve already looked at bipolar transistors.

Vacuum tubes come in a great range of designs, but there are three types (in addition to rectifier tubes) most often found in audio circuits. The triode has three elements, the Plate (P) or Anode, the Grid (G), and the Cathode (K). (The latter abbreviation, from the German “Kathode,” is intended to prevent confusion with capacitors.)

Like a diode, the triode won’t work unless the cathode is heated. In modern audio tubes this is done with a filament, not shown in most schematics. (Usually the drawing of the power supply will show the filaments of the various tubes hooked across the transformer secondary from which they are powered.)

In early years the filament itself served as a cathode (tubes like these are known as directly-heated); some early power triodes such as the 2A3 and the 300B have become popular again in low-powered, high priced audiophile amplifiers. Most modern triodes are lower powered devices, such as the 6SN7 and the 12AX7.

Incidentally, tubes with this type of designation tell you something about how they’re designed. The first number gives the filament voltage (usually 6.3V or 12.6V), while the second number tells how many elements the tube has. In the case of a 12AX7 there are two separate triodes in a single bulb, adding up to six elements—the seventh is a 12.6V filament.

When tubes incorporating multiple elements are used, they are usually drawn on the schematic with part of the circle dashed, and the designator will usually read something like “V102: 1/2 6SN7.”

If the elements of a triode look suspiciously like those of a diode or rectifier tube, you’re right. A triode is basically a diode with a piece of metal screening (the grid) between the cathode and the plate. Like a diode it can only pass current in one direction; the voltage on the grid controls the amount of current, turning the tube into an amplifying device.

The action of the tube is rather like that of a valve that controls the flow of a stream of water, and in fact the British call vacuum tubes “valves” for this reason. The “V” in the designator stands for “vacuum tube” or “valve,” making it suitable on both sides of the Atlantic. (The Russians call them “electronic lamps,” also apt.) On a nice day, manufacturers will tell you which tube pins connect to which elements. Tube pin designations always go clockwise, as viewed from below the socket.

Tetrodes are similar to triodes, but they have a second grid between the main grid and the plate, sometimes called the screen grid. A similar design is called a beam-power tube; the famous 6L6, used in millions of guitar amps worldwide, is a classic example. Pentodes add a third grid, the suppressor; power pentodes include the tall, thin EL-34, which put the muscle behind Marshall guitar amplifiers. Low-power, small-signal pentodes were common in years past but don’t show up much these days.

Figure 5 shows a few more passive parts that you might encounter. The fuse is familiar; these are normally rated in amperes, and you should never use a larger one than the manufacturer recommends. Slow blow fuses can withstand a momentary surge of current when the device is turned on, but will pop if the current is sustained.

A relay consists of a swinger like the one in a switch, and an electromagnet. When current passes through the coil of the electromagnet it flips the swinger. Most relays are double-throw with varying numbers of poles. And now you know what a neon lamp looks like.

Reality, and welcome to it

Schematic drawings only show the electrical connections between parts. They may translate into very different arrangements in a piece of equipment.

Check out Figure 6. This is a printed circuit board layout for the phantom power regulator we looked at in Figure 3, shown life-size. The location of the parts on the component side is quite different from their layout on the schematic (although I’ve managed to keep the left-to-right flow intact), and the shape of the traces on the foil side is different from their shape on the schematic.

But if you parse out the connections, keeping in mind that the foil side is flipped left-and-right, you’ll discover that the electrical circuit is the same. (Incidentally, if you don’t have a schematic, it’s often possible to generate your own by tracing out connections on the circuit board.)

Older equipment, especially tubed gear, was often built “point-to-point” with components soldered directly to tube sockets or terminal strips, which are small plastic strips with metal terminals mounted in a row. Figure 7 shows a typical layout, in this case the vacuum tube power supply shown in Figure 2.

The transformer and choke aren’t visible—they’re mounted above the chassis on bolts or studs, and their wires poke through holes in the chassis. (The holes typically include grommets: rubber rings shaped like doughnuts, which prevent the wires from abrading against the metal chassis.) At the top is a terminal strip with the MOV mounted on it and with wires leading off to the power switch and fuse (neither of which is shown on the schematic).

The same terminals connect to the two ends of the transformer primary. Power transformers are color-coded, and the codes for tube-type transformers have been standardized; I’ve listed the code in Table 3.

Most of the secondary windings shown in the diagram connect to the rectifier tube. This has an octal base, meaning eight pins in a 1-3/8" diameter base. (Similar bases can have from four to eleven pins.) Most rectifier and output tubes use octal bases, while smaller tubes usually have seven or nine pins in a 5/8" or 3/4" base.

Notice the hole in the center of the tube socket with a slot in one edge; this ensures that the tube is inserted properly. Looking at the tube from the bottom, pin #1 is the first pin you encounter as you proceed clockwise around the circle, beginning at the slot.

If you need to solder or desolder anything on a tube socket, take the tube out first.

The filter capacitor in this amp is a multiple unit with four caps sharing a single case and the outer shell serving as their common ground. The capacitor is grounded to the chassis by the simple expedient of inserting the tabs on its shell into slots cut in the chassis. Twisting the tabs makes the ground connection. (Some amps use a plastic mounting washer, and connect the cap to chassis ground with a separate wire.) The center-tap wire from the transformer is soldered directly to one of the tabs on the shell.

This arrangement is handy for the manufacturer but hell for the guy who has to replace the capacitor. Trying to desolder a wire from a tab while the entire chassis is conducting heat away from it is a pain. Sometimes it’s simpler just to cut the wire, then snap the tab off the capacitor to free it from the chassis.

Afterword

I hope that by this point you can decode a schematic diagram and begin to use its information to figure out what’s really happening inside those rack-mounted boxes. Too many studio people never learn. They trudge through the years connecting up one black box to another with no conception of what’s inside and how it works. Inevitably their results are diminished.

For several years I was studio manager in a college recording program. I resisted the urge to order every new toy that came down the pike (just as well, since we didn’t have the money to buy them all). Instead I deliberately kept the studio bare-bones and insisted that the students solve recording problems using ingenuity rather than fancy boxes.

Rather than giving the students detailed training on the controls of a stack of gadgets that would already be obsolescent by the time most of them got studio jobs, I tried to teach them the fundamentals of sound and recording. I’d describe the problems the boxes were intended to solve, and challenge them to duplicate the functions without the gear. Many times they succeeded beyond their own expectations.

Knowing how your gear is made isn’t just an abstract satisfaction—although it is satisfying to curious critters like humans. By knowing what lives in the boxes, a good engineer can come up with new and unauthorized ways to use them. By making new conceptual connections based on understanding, you find ways to make all of your gear stand up and do tricks.

And that, my friends, is when you’re flying. Enjoy!

Table 1 - IC Opamp Pin Assignments (DIP packages)

Pin Single Dual Quad

Package Package Package

(8 pins) (8 pins) (14 pins)

1 Offset Comp* Output 1 Output 1

2 -Input -Input 1 -Input 1

3 +Input +Input 1 +Input 1

4 V- V- V+

5 Offset Comp* +Input 2 +Input 2

6 Output -Input 2 -Input 2

7 V+ Output 2 Output 2

8 See below** V+ Output 3

9 -Input 3

10 +Input 3

11 V-

12 +Input 4

13 -Input 4

14 Output 4

*The "Offset Compensation" pins are used to adjust the amplifier for zero volts DC at the output, as shown in Figure 1.

**A few opamps need an external capacitor for proper operation; depending on the opamp, this is connected between pin 8 and either pin 1 or pin 5.

Table 2 - Units and Conversion Factors

Resistors:

Basic Working Unit = ohms

1,000 ohms = 1 kilohm (1K)

1,000,000 ohms = 1,000 kilohms = 1 megohm (1M) [alt. abbreviation "1 MEG"]

Capacitors:

Basic Working Unit = microfarads (F) [alt. abbrev. "1mF")

1/1,000,000 microfarad = 1 picofarad (pF) [alt. abbrevs. "1F" or "1mmF"]

1/1,000 microfarad = 1,000 picofarads = 1 nanofarad (nF)

Inductors:

Basic Working Unit = henries (H)

1/1,000 henry = 1 millihenry (mH)

Table 3 - Power Transformer Color Code (Tube Equipment)

Primary: Black (both wires)

Tapped Primary: One wire black, one wire black/red; the tap is black/yellow

Secondaries:

High-Voltage (Plate): Red (both wires); center-tap red/yellow

Rectifier Filament: Yellow (both wires); center-tap yellow/blue

Filament Winding #1: Green (both wires); center-tap green/yellow

Filament Winding #2: Brown (both wires); center-tap brown/yellow

Filament Winding #3: Slate (both wires); center-tap slate/yellow




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