Plugs

Here are two artifically coloured 1/4″ phone plugs, mono above and stereo below. The green part is called the sleeve and the red part is called the tip. The stereo plug has an additional part (silver in this figure) called the ring. The tip, ring, and sleeve are all insulated from each other and can be used for different purposes. Stereo plugs are sometimes labeled TRS, the initials of Tip, Ring, and Sleeve.

Guitar cables generally have mono plugs where the tip carries the audio signal and the sleeve connects to ground. Stereo plugs appear on headphone cables. If you put a mono plug into a stereo headphone jack, the silver part is replaced by the grounded sleeve and one channel is grounded and, hence, silent. Because the Ring usually carries the Right channel, the right channel is typically silenced.

Jacks

These colourful images represent three panel mount Switchcraft jacks:

• mono (Switchcraft no. 11 on the left),
• stereo (Switchcraft no. 12B in the middle), and
• mono with NC (or normally closed) switch (Switchcraft 12A on the right).

Following the colour scheme above, I have made the sleeve connection green and the tip connection red.

The role of the silver part in this figure depends on the jack. For the (middle) stereo jack, the ring connection is silver. For the (right-hand) mono with NC switch, the shunt for the switch is silver.

Each part of the jack has a solder lug where wires are usually connected. The tip lug is not always located in the same place. Other manufacturers than Switchcraft may use a completely different configuration of the solder lugs for each of these jacks. You can use the contintuity test of your DMM to figure out which lug goes with each plug connector. Alternatively, you can figure this out visually.

These jacks are constructed in layers, with a single piece of metal comprising the lugs and the plug connectors. I have never taken one apart, but there must be some sort of insulation that separates these metal layers from the barrel that contacts the sleeve. So you can just look from the side and see which lugs and connectors are paired.

Schematic symbols for these three jacks often look like these. Layout symbols are similar.

Connections

Often mono plugs are mated to mono jacks and stereo plugs are mated to stereo jacks as shown above. Notice the position of the tip connection. The “click” that you feel when you plug into your guitar, stompbox, or amp is the (red) metal tab that contacts the tip snapping into the groove around the tip. This holds the plug firmly in place until you pull it out. After repeated use, jacks must be replaced when metal fatigue occurs and the jack no longer grips the tip tightly.

Also, compare the mono and stereo arrangements and you will see how a separate connection is made by the ring of the stereo plug touching only the silver metal tab of the stereo jack. Below we will discuss a case where such a separate connection is not desired.

When inserting a 1/4″ phone plug into these jacks, the tip first contacts the grounded socket before reaching the tip connection at the end of its travel. The sleeve connection comes later. This initial contact between tip and ground without a grounded sleeve causes the pops and hum one hears as a cable connected to an on-line amp is plugged into a guitar.

The figure above shows how the mono jack with NC switch has a tip connection that is closed without a plug and open when a plug is inserted. This is useful in stompboxes like Sean MacLennan’s B Blender. His circuit provides an “effects loop” that is blended with the input signal. By using mono jacks with NC switches for the effects loop, a default connection is possible through the shunt lugs when nothing is plugged into the loop jacks.

This figure illustrates how a stereo jack is used as the input jack for a stompbox to switch the ground connection of a battery. The plug is, of course, a mono plug. The ring part of the jack touches the sleeve of the mono plug. If the sleeve part of the jack is grounded as usual, then the mono plug grounds the ring connection. Without a plug, the ring connection is floating.

So the battery’s negative terminal is wired to the ring solder lug of the stereo jack. When there is a mono plug in the jack, the negative battery terminal is grounded and electrons flow. Without the plug, the ground connection is broken. In this way, the battery is disconnected whenever the stompbox is not in use.

Other Jacks

There are more elaborate jacks than these three. There are jacks with more rings, with more NC switches, and also with NO (normally open) switches. For examples, see the Switchcraft link listed below. Although these exotic jacks do not usually find their way into stompboxes, you will certainly encounter them in guitar amplifiers.

The illustrations above picture “open frame” metal jacks.  There are also panel mount jacks that enclose the plug connections inside a plastic box. For example, see these Switchcraft jacks.  Such jacks are often used to insulate the sleeve connection from the enclosure. This can also be accomplished with open frame metal jacks using nylon washers.

PCB mount jacks offer another possibility. These are also enclosed and quite compact. For an example of their application, see the design described in A Nice Design for 1590B Enclosures.

References and Additional Resources

1. Neutrik Jacks
2. Switchcraft Phone Jacks
3. Wikipedia: TRS Connector

Wiring up the off-board components, particularly a 3PDT switch, can be confusing. There are many good explanations of how to do it (see for example geofex.com, tonepad.com, and generalguitargadgets.com) and this one adds to the pile by breaking down the logic of one of the more elegant layouts with a series of detailed figures. I walk through the case where the audio jacks are not insulated from a metal enclosure, there is an LED to show when the circuit is on, and when the circuit is off there is simple by-passing with the circuit input grounded.

It seems simplest to start with the wiring for so-called true by-pass, a straight connection from the input jack to the output jack. The input jack is on the left and the output jack is on the right. They will be reversed to the usual placement when the stompbox is closed up and turned over.

The input jack pictured here is a stereo Switchcraft jack called the 12B. The output jack is a mono Switchcraft called the 11. You can read a post about these jacks in 1/4″ Phone Jacks and Plugs. The switch is a 3PDT Taiwan Blue. The picture shows

1. a white wire from the tip lug of the stereo jack (on the right, for input) to the switch;
2. in one switch position, this will connect to the white wire that goes across the bottom of the face of the switch;
3. which will then connect to the white wire that runs from the switch to the tip lug of the mono jack (on the left, for output).

So we have a simple connection from one tip lug to the other: true by-pass.

In the other switch position, we want to connect the input lug to the input of the circuit and the output lug to the output of the circuit. These are added in the following image:

The blue wire should be connected to the input pad of the circuit board. The yellow wire should be connected to the output pad of the circuit board, or to the middle lug of a level pot if one completes the circuit. I am leaving the board out of these pictures for simplicity. Any unconnected wires in this tutorial are supposed to connect somewhere on a circuit board.

As it stands, we do not need the 3PDT switch because we are only using 2 poles, one for input switching and one for output switching. The third (middle) pole can do the switching for an LED that lights up when the circuit is engaged (or not by-passed). For that we will also need a power supply, which we will get from a DC voltage supply.

This image shows the LED wiring from another vantage point, with the wires for the guitar signal removed for clarity. The LED switching opens and closes the ground connection for the LED circuit. The ground for the circuit is the sleeve lug of the output jack.

• The DC jack has a direct wire to that lug: the green wire that runs all the way across the middle of the picture.
• The LED also connects to that grounded lug through a resistor (2.2K for limiting current) and two green wires, one that goes from the LED to the switch and another that goes from the switch to the lug. Note that these two wires are connected when the switch is in the “not by-passed” position.

Also note that in this particular setup, the sleeve lugs of both jacks are connected through the aluminum enclosure that holds them. The entire stompbox is grounded through the output cable. So the input sleeve lug is grounded by its connection to the output sleeve lug through the enclosure.

In setups with insulated jacks, one must make these connections with wiring. You should still ground the enclosure in those cases because this improves the ability of the enclosure to shield the circuit from outside radio frequency (RF) signals.

Here is a close-up view of the DC jack connections:

The top lug is the positive power supply connection and the angled lower lug is the ground connection. We will use the third middle lug later when we hook up a 9V battery as an alternative power supply.

The red wire is the positive power supply. This colour is consistent with the leads found on most 9V battery snaps: red is positive and black is negative (or ground). In these pictures, I am using green for ground because it shows up better.

I prefer to use the DC jacks that are fastened with a nut on the outside of the enclosure and that is what is pictured in these figures. DC jacks also come configured with the nut on the inside of the enclosure. I find this inconvenient because it requires me to install the jack in the enclosure before I solder the wires to it. As a result, if I want to remove the circuit from the enclosure then I must unsolder these wires. The input and output jacks, the 3PDT switch, and any pots all have their nuts on the outside. If the DC jack does also, then one can remove the nuts and the whole circuit lifts out of the enclosure completely connected.

Here is a close-up view of the output sleeve lug for ground connections:

At this point, two wires are supposed to soldered to this lug. One wire is coming from the DC jack (not shown) above. The other wire is “flying in” from the switch.

Here is an image of the switch wiring with all of the wires in place: input, output, and ground connections:

Note that there is an additional green wire. This is the short wire that connects the first (input) pole of the switch to the middle (ground) pole of the switch.

• When the switch is in the by-pass position, this short green wire connects the input of the PCB to ground.
• When the switch is in the engaged position, this short green wire does not connect to anything.

This additional wire feeds the stompbox circuit a quiet input signal when the circuit is by-passed. That is the trickiest part of the switch wiring, making a nice use of that otherwise unused lug on the switch.

Now we are ready for the circuit board. Besides the input and output connections, the board needs the positive and ground connections which come from the same places as for the LED circuit: the positive lug of the DC jack and the sleeve lug of the mono output jack.

Wires for those connections appear in the figure above. Generally, it is good to run your positive supply lines next to ground lines. This is true of PCB traces as well. So I am showing the positive board supply wire running next to the ground wire for the DC jack.

I am not quite finished. I still need to add the wiring for the battery. It will take a while to make a figure for that, but in the mean time it is easy to describe. The red battery snap wire connnects to the remaining free lug on the DC jack. The black battery snap wire connects to the ring lug of the (stereo) input jack. That’s the obvious one facing up to the right of the tip connector. This battery snap hookup accomplishes two things:

1. The battery negative terminal will be connected to ground only when there is a mono plug inserted into the input jack. In that event, the ring connector is in contact with the sleeve of the mono plug and a ground connection is made through the ground lug of the input jack.
2. The battery positive terminal will be disconnected to the LED and the board when there is DC plug inserted into the DC jack.

Because of these properties, the battery will supply power when there is no alternative DC power supply and there is an input for the stompbox. Otherwise, the battery is preserved.

If you want to etch a PCB with a layout in Eagle then you will need to create an image like this one to transfer your design. This brief tutorial explains how to set things up so that you can use the Eagle export command.

Let’s say you are starting with this layout. This is Joe Davisson’s Antiquity Fuzz from his Analog Alchemy site.

First, change the background to white by entering the command “set palette white;” and then the command “window;” (or press function key F2 to refresh to the screen and see the change. I get this:

Next, display only the traces and pads by entering the command “display none bottom pad via;” to get something like this:

If you have any ground pour, remember to run the “ratsnest;” command. For this layout, I ran the GND polygon all the way around the board so that I get

Finally, enter “export image pcb.png monochrome 600″ and you will create a .png format graphics file at 600DPI that looks like this

The file will have the name pcb.png and it will appear in the same subdirectory as your board file. You can print this file with MS Paint or insert it into MS Word and it will print to scale. This is true no matter what resolution you choose for your file. For example, 300DPI works well for PnP Blue transfers to copper clad boards.

[[Show as slideshow]] ]]> http://gaussmarkov.net/wordpress/parts/connectors/slide-switch-tutorial-2/feed/ 2 http://gaussmarkov.net/wordpress/parts/op-amps/op-amps-4-divided-negative-feedback/ http://gaussmarkov.net/wordpress/parts/op-amps/op-amps-4-divided-negative-feedback/#comments Tue, 08 Jan 2008 19:58:13 +0000 gaussmarkov http://gaussmarkov.net/wordpress/parts/op-amps/op-amps-4-divided-negative-feedback/ Simple negative feedback, connecting the output to the inverting input, makes an op-amp into a unity gain amplifier. In that setup, all of the output goes to the inverting input. If instead the amount of feedback is reduced, through a voltage divider, then the gain of the op-amp circuit becomes greater than one. This may be the most common way op-amps are used for amplification in stompbox circuits. In addition, by adding some capacitors to the voltage divider, the gain also gets some tonal character that is the foundation of dozens of famous distortion and overdrive pedals (think Boss, Fulltone, Ibanez, Marshall, Proco, Voodoo Labs, …).

Because it keeps the op-amp circuit simpler, let’s work with a bipolar supply. Single-supply versions require only straight-forward additions.

We are going to replace the negative feedback loop from the previous tutorial with a resistive voltage divider. This arrangement is called divided negative feedback. Here is the basic circuit:

Resistors R1 and R2 make up the voltage divider. They have equal resistances, both 10K, so that the voltage at their junction will be half the voltage across R1 and R2 in series to ground. That means that the inverting input will see the output signal with half of its amplitude. Starting with our usual source with amplitude 1V and frequency 800Hz, here is what we get:

The result is that the output signal is the input signal with twice its amplitude. Therefore, by reducing the amount of negative feedback (through the voltage divider), the op-amp amplifies the input signal by a factor greater than one (or unity).

In Op-Amps 2: Hitting the Rails, I describe how an op-amp with no feedback produces a square output signal because the output is on one supply rail or the other. In Op-Amps 3: Between the Rails, I describe how feeding the entire output signal back to the inverting input reduced the effective gain of the ciruit to unity. Divided negative feedback is between these two cases. Or said more carefully, divided negative feedback has both of these as special cases. If R1 has infinite resistance, then the signal will hit the rails. And if R1 has no resistance, then signal is buffered.

If R1 has enough resistance, this circuit will produce some output clipping because part of the output signal hits the supply rails. For example, if we increase R1 in the circuit above from 10K up to 100K then the output looks like this:

The peaks of the output sine wave are clipped, or trimmed, by the limits of the supply voltage. This clipping is heard as distortion and it is one of the sources of distortion in the stompboxes like the DOD Overdrive 250.

Analysis

Before talking more about actual stompboxes, here is an analytical description of how this circuit works. If the op-amp’s gain is 200,000 then for small voltage levels so that output is not hitting the rails,

where V_out is the output voltage, V_in+ is the non-inverting input voltage, and V_in− is the inverting input voltage. The voltage divider in the feedback loop makes

where R₁ and R₂ are the values of resistors R1 and R2, respectively. Putting these two equations together gives a relationship between the source signal and the output signal:

In typical applications, 1/200,000 is negligible compared to R₂/(R₁ + R₂). This means that we can treat the left-hand side as approximately zero, giving the approximate solution

so that V_out is a multiple bigger than one of V_in+. The larger the ratio R₁/R₂ is the greater the gain is.

We can place a pot, wired as a variable resistor, in place of either R1 or R2 to make the gain variable. Note that decreasing R2 increases the gain so that if you want gain to increase for a clockwise turn you should have resistance decrease in that direction. R1 would be wired in the opposite way. Also, because the effect of R2 is nonlinear you might use a reverse log pot to compensate.

When R2 is made variable, it is common practice to add a fixed stop resistor in series with the pot to prevent the total R2 resistance from reaching zero ohms. Why this is a good idea is left as an exercise for the reader.

Adding Caps to Shape Tone

Often, in stompbox circuits that use divided negative feedback, a capacitor appears in parallel with R1 and another capacitor appears in series with R2. These capacitors are shaping the tone of this amplifier circuit. Except for single instead of split supply, here is the circuit for Shaka Boost by Aron Nelson (owner and founder of diystompboxes.com):

This schematic is drawn the way you will usually see this circuit. It does not emphasize that R1 and C1 are both in the voltage divider in parallel, though that is clear once it is pointed out. Also, note that while C2 usually appears at the junction of the voltage divider, the order of R2 and C2 could be reversed. Finally, the presence of C2 allows us to replace the ground connection at one lead of R2 with either V+ or V−. C2 will block any DC differences between the feedback loop and the voltage to which R2 connects. What is critical is that the voltage be constant. Capacitors respond only to the rate of change of voltage and that must be zero.

Consider the effect of C1, placed in parallel with R1. Capacitors attenuate AC signals more as frequency falls. C1 has the effect of making the R1 half of the voltage divider have more resistance for low frequencies. According to the analysis above, such resistance means more amplification. Therefore, C1 has the effect of boosting lows relative to highs.

C2, placed in series with R2, increases the resistance in the R2 half of the voltage divider as frequency falls and has the opposite effect of C1. With C1 removed, C2 emphasizes low frequencies at the inverting input and, therefore, causes the op-amp to attenuate low frequencies relative to high frequencies.

Fortunately, the effects of C1 and C2 do not cancel each other out. Their nonlinearity causes the combined effect to be a mid-frequency hump. Extreme lows and highs are both attenuated relative to the frequencies in between. Here is a plot of each effect alone, showing the amplitude of the output for each frequency.

The green line plots amplitude when no C1 capacitor is present and the blue one when no C2 is present. Around a frequency of 1KHz, both amplitudes reach their peak at 11V. This is the gain we would expect from the circuit without capacitors. Using the formula for gain given above,

So we see that around 1KHz, neither capacitor is having much effect. But at low frequencies C2 attenuates and at high frequencies C1 attenuates. With both C1 and C2 present, we get the combined effect shown in red below.

This plot shows that at the extremes the combined effect equals the attenuation of one capacitor or the other and, of course, in the middle they are all the same. This combined effect is often called a mid-frequency hump. The range of frequencies near the maximum gain is called the passband.

In LTSpice Analysis and the DOD Overdrive 250 you can see similar results for that stompbox. There, you will also see that changing gain is linked to changing a corner frequency.

More Analysis

It is possible to work out formulas for some of the features of this circuit. Divided Negative Feedback and Capacitors provides a detailed description. To close this tutorial, here is a summary of the results.

First, the frequency response plotted above for the Shaka Booster shows several general properties:

• the response is symmetric in log-frequencies around one maximum and
• at the extremes the gain approaches unity.

In addition, if the corner frequencies are far enough apart, then

• the lower and upper corner frequencies are approximately f₂ = 1/(2πR₂C₂) and f₁ = 1/(2πR₁C₁) respectively and
• the maximum gain equals the no-capacitor gain 1 + R₁/R₂ and occurs at the frequency f₀= √(f₁f₂), the square root of the product of the two corner frequencies.

For low gain levels, more accurate formulas for the corner frequencies are f₂′ = af₂ and f₁′ = f₁/a where a² = 1 − 2/g² and g = 1 + R₁/R₂ > 2. The corner frequencies are “far enough apart” if

The range of frequencies between these corner frequencies is the passband.

Note that the dependence of the corner frequencies on the values of the resistors may also affect the decision where to place the variable resistor. Making R₂ variable makes the lower corner frequency rise with gain and making R₁ variable makes the upper corner frequency fall with gain. Aron chose to make R₁ variable in the Shaka Booster whereas DOD chose to make R₂ variable in the Overdrive 250.

Examples

In Aron’s Shaka Boost,

and if we set R₁ = 10K then

Notice that with g = 11, the differences between f₁ and f₁′ and between f₂ and f₂′ are ignorable. Also

so that the corner frequency formulas are accurate for this case.

If we increase the gain pot to 25K, then f₂ will not change and f₁ = 28.9KHz. I suspect that my hearing would not notice this large difference in the upper corner frequency because it is so high in both cases.

If we turn the gain pot down to 2K, then g = 3, f₁ = 362KHz and f₁′ = 410KHz. There is a big difference in these frequencies, but they are so far out of the range of human hearing that this does not matter. So theory predicts that Aron’s Shaka Boost leaves the tone of the signal effectively unchanged over its entire range of gain.

The DOD Overdrive 250 is a different story. For this circuit, the variable resistor is in the R2 position in series with a 4.7K stop resistor:

and if we set R₂ = 500K then we get the minimum gain and

The difference between f₁ and f₁′ is appreciable this time, so f₁′ is preferred. Also

so that the corner frequency formulas are still accurate for this case. A passband from 6Hz–7.2KHz represents an audible cut in the high frequencies.

Based on separate simulations, we find that the op-amp starts to clip around R₂ = 30K, or about a 2/3 rotation, so that is a sensible place to check what happens for higher gains. Beyond this point, clipping of the signal invalidates the formulas for amplitudes. Changing R₂ to 30K increases g to 34.3, leaves f₁ unchanged, and raises the lower corner frequency to 113Hz. Because the bass E string on a guitar has a frequency of 82Hz, this will also be clearly audible.

These figures are slightly misleading, but they tell the correct qualitative story. When we check

we just violate the requirement for accuracy. This implies that both capacitors have an influence in for the passband frequencies and that the maximum gain falls short of the no-capacitor gain by 2% which is still small. To obtain more accurate information, one can use SPICE simulations, as illustrated in LTSpice Analysis and the DOD Overdrive 250.

To summarize what these calculations show, the DOD Overdrive 250 cuts highs and cuts lows more as the gain is increased.

Photo credit: Mike Malak on WikiMedia Commons

The simplest application of feedback is shown below, where the inverting input is connected directly to the output, instead of a constant reference voltage like ground. This is called negative feedback because the negative of the output feeds back into the output itself. The effect is to attenuate output.

In a plot of the input and the output for this circuit, one can only see one of the two because they lie in exactly the same place. In other words, the gain of this circuit is almost exactly unity.

As mentioned above, the gain of an op-amp is at least 200,000. For the sake of argument, suppose the gain is exactly 200,000. In the absence of supply constraints, the behaviour of the op-amp is described completely by

where V_out is the output voltage, V_in+ is the non -inverting input voltage, and V_in- is the inverting input voltage. For small voltage levels, an op-amp will permit a voltage 200,000 times the difference in the two input voltages.

This allowed voltage may exceed the available power supply and then, as described in the previous post, the output will hit a power rail.

When there is negative feedback,

as well. Putting these two equations together gives a relationship between the source signal and the output signal:

or

so that V_out is almost exactly V_in+.

You might well ask what good this is. We start with V_in+ and we end up with V_in+. The answer lies in understanding input and output impedance. The inputs of an op-amp are very high impedance which essentially means that the rest of the circuit is unaffected by their presence. The output of an op-amp is very low impedance and that essentially means that its output is affected only through its inputs. Apart from this, the rest of the circuit is typically irrelevant.

These properties are not shared, for example, by guitar pickups. Whatever you plug your guitar cable into, even your cable itself, can change the signal received from the pickups. So the benefit of the op-amp with negative feedback is that it can take a vulnerable guitar pickup signal and transform it into something invincible. That should sound good.

Single Supply and Virtual Ground

A single-supply version of this unity-gain circuit places the virtual ground half-way between ground and the positive supply so that the audio signal can swing equally in either direction. Using the same approach as in the preceding post,

with the resulting plot

where once again the noninverting input is hidden behind the output signal.

Let’s look at some applications of this circuit in actual stompboxes.

This is a Buffer

This single-supply op-amp circuit (without the load resistor) appears four times in the B. Blender circuit designed by Sean MacLennan. It provides (voltage) buffers that keep two channels, one for an effects loop and one for the clean signal, separate until they are mixed together at the end of the circuit, where another buffer provides a low impedance output. Buffer is the name for amplifiers that have unity gain and that lower the output impedance of a signal path. Buffers are also called voltage followers because the output voltage is a copy of the input voltage.

In general, the components throughout a circuit influence the signal at any point. It is tempting to think of circuits as sequential, with the signal flowing in just one direction–from input to output–but this is often incorrect. To give a simple example, putting an additional resistor in series with a resistive voltage divider changes the voltage at the junction of the divider, whether you put the resistor before the divider or after.

Considered in isolation, this op-amp buffer circuit is an exception: what’s happening at the output does not affect the non-inverting input without a route for feedback. So for audio, the op-amp buffer works as a backflow valve and prevents the signal from “backing up.” Returning to the divider example, placing this buffer in series with a voltage divider does not change the voltage produced by the divider. Indeed, this is a property exploited by another stompbox application.

Decoupling Virtual Ground

This buffer also appears in stompbox power supply circuits. The 9V Electric Mistress uses it in place of a decoupling capacitor to hold the virtual ground in the power supply steady. See the op-amp after the pair of 200K resistors, R2 and R3, in the upper left-hand corner of the schem linked here. And the Blues Driver uses both the capacitor and the buffer. See the Vref op-amp version in the project pdf file with the op-amp after the R25 and R26, a pair of 10K resistors, or R4 and R5 in the schematic available at freeinfosociety.com:

The Faucet Analogy

An analogy between an op-amp and a water faucet seems helpful once again. First, the pressure created in a hose by turning a water faucet highlights the difference between the input and the output. They are synchronized but they are not the same thing. The output is a power supply controlled by the input. In addition, a relatively small input force, turning the faucet, can result in a huge output force depending on the water supply pressure. Those features coincide with the high gain, low impedance characteristics of the op amp output.

Second, whatever happens to the water in the hose attached to the faucet, the water pressure (output) does not feedback on the faucet (input) unless special arrangements are made.

This analogy does not capture the high impedance of the inputs. Indeed, a lot of water faucets are very hard to turn and make your hand tired and sore. So such faucets load down the source (your hand) which is quite the opposite of op-amp inputs. They are like faucets that turn with the slightest breeze.

But that’s the problem with many analogies; they aren’t a substitute for understanding the real thing, just an aid. If you have questions about all this, please comment. It will help improve what’s written here. And chances are, if you have the question then lots of other people do also.

An example of this idea is to tie the input with a minus sign, called the inverting input, to ground and to feed the input with a positive sign, called the non-inverting input, a sine wave. In the LTSpice simulation shown here, the non-inverting input is connected to an AC source with a 1V 800Hz sine wave. The op-amp is supplied by a 9V DC source with the positive terminal connected to the positive power pin and ground connected to the negative power pin.

Just to be sure it’s clear, this schematic is equivalent to the next one, with a connection running explicitly from the power supply to the op-amp.

The V+ label on both the 9V power supply V2 and the op-amp positive power pin take the place of running a connection. The ground symbols do the same thing. Such labels simplify a schematic by reducing the number lines that appear.

Here is a plot of the input in+ and the output out signals for this simulation:

As indicated, the green curve is the in+ sine wave with a 1V amplitude and an 800Hz frequency. The blue plot is the signal at out which jumps between 0V (ground) and 9V, the two polar values of the power supply to the op-amp. Whenever the sine wave voltage is negative, or below ground, the output is 0V, and whenever the sine wave voltage is positive then the output is 9V.

This occurs in part because an op-amp responds only to the difference in the voltages at the two inputs. The difference is calculated by subtracting the inverting input voltage from the non-inverting input voltage. In the present case the inverting input voltage is connected to ground so that the difference in voltages simplifies to be the non-inverting input voltage alone.

So if we reversed the input connections, hooking the sine wave to the inverting input and ground to the non-inverting input, the op-amp would be responding to the negative value of the sine wave and the square wave would be inverted: when the sine wave voltage is positive then the output voltage would be 0V and when the sine wave is negative then the output voltage would be 9V.

The other reason for this square-wave output is that the op-amp has an extremely sensitive output “faucet.” A very small positive difference in input voltages “turns the faucet” to the maximum voltage. And the opposite is also true: a very small negative difference in input voltages “turns the faucet” to the minimum voltage.

Dual Polarity Power Supply

In the first example the minimum is 0V but things are symmetric if we introduce a dual polarity (or bipolar or split) power supply:

When the minimum voltage available is -9V then a negative voltage difference on the inputs produces an output of -9V. So the analogy with a water faucet does not extend to this case. Turning the faucet in opposite directions does not produce opposite directions of water pressure.

Again, switching the inputs inverts the output:

Virtual Ground

It is possible to create a similar relationship with the original single-supply circuit. One creates a virtual ground between actual ground and the positive voltage supply at the inputs of the op-amp.

Vr is the virtual ground in this circuit. A voltage divider places Vr at half the supply voltage V+. A 10uF decoupling capacitor (C2) assists in keeping Vr steady. The capacitor C1 and the resistor R1 pull the signal up from alternating around ground to alternating around the virtual ground. C1 acts as a coupling capacitor and preserves any difference in DC levels between the signal source and the non-inverting input. R1 sets the DC voltage level approximately at Vr. Here are the results:

The source signal (blue) is cycling around ground as before but the non-inverting input (green) is cycling around Vr (red). The inverting input is connected to Vr instead of ground.

As a result, the voltage difference across the inputs is the same as the signal source. Now the output signal is on the positive power rail when the green sine wave is above the red line and on the ground power rail when the green sine wave is below the red line. The red line at 4.5V is playing the same role as ground in the bipolar supply circuit above.

This virtual ground will be useful in the next post, which discusses output that stays between the power rails.

In all of the circuits in this post, the output is on one power rail or the other. Such op-amp circuits are called comparators because the output indicates whether one input voltage is greater than the other. That is, one voltage is compared to another and the output is high for one outcome and low for the other.

Op-amps (operational amplifiers) come in an integrated circuit, or IC. The one pictured on the right is in a form called DIP-8, which is short for dual in-line package with 8 pins. “Dual in-line” refers to two lines of pins, in this case 4 on each side. Each pin has a special function and they are numbered from 1 to 8. Often, there is a circle on the top of the case to show the location of pin 1. Also the case is usually notched at the same end with half-circle cut-out. At least one of these two markings appear, but not necessarily both.

The manufacturer, its documentation, and the name of the chip are printed on the top. In this case, the leading symbol is the circular N logo of National Semiconductor and the name of the chip is LM741CN. The DOD Overdrive 250 and the MXR Distortion Plus are popular examples of stompboxes built around this chip.

The names of chips are often abbreviated to their numbers, like 741. Different manufacturers may use different prefixes and suffixes while keeping the same numbers. For example, JRC produces the NJM741D. Although generally they can be used as functional substitutes, chips with the same numbers but different names are not necessarily identical. One of the characteristics that is often flagged with the suffix is the package or case of the IC. For example, National Semiconductor also manufactures the LM741H which is the 741 in an 8-pin “metal can.”

The numbering of the pins is always the same. With the notch or pin 1 marking at the top, count sequentially from pin 1 down the left side and then up the right side of the DIP. In other words, count counter-clockwise (CCW).

Op-amps can be soldered directly to the circuit board but often op-amps are seated in sockets instead. The sockets are soldered to the board, avoiding the risks of destroying the op-amp with too much heat. Also, one can exchange op-amps easily to make a repair or to try an experiment (like on a breadboard). A DIP-8 machine-pin socket by Mill-Max and a DIP-8 dual-leaf socket by AMP are shown below.

These sockets also have notches like the op-amps so that even without the op-amp on the board its orientation is still clear.

Symbols

Basic op-amps are denoted on schematics by a simple triangle that usually points to the right.

They have two inputs on the left-hand side, denoted by positive and negative signs, and a single output at the point on the right. The inputs are often shown in the opposite order.

Above and below are power supply connections, with the positive supply above and the negative supply below.

These directions are associated with typical audio schematic layouts, where the input is on the left, the output is on the right, the positive power connections go up, and negative or ground connections go down. Often, when they are implicitly clear, the power connections are not shown (as on the right).

On circuit board layouts, one sees the outline of the IC package. The characteristic notch appears at the end where pin 1 is located. On gaussmarkov.net layouts, the pad for pin 1 is also square while all of the other pads are round.

Values

Op-amps do not have a single salient characteristic like resistors and capacitors do. Op-amps are designed with various special tasks in mind so that they are particularly good in specific ways. The design goals include gain, low noise, fast response, high input impedance, and low output impedance.

DIP-8 op-amp ICs may contain one or two op-amps. The 741 contains only one op-amp. The 4558 that is found in tubescreamers has two and is often called a dual op-amp. You will also run into quad op-amps that contain 4 op-amps in a DIP-14 package. The pin assignments (or pinout) for the 741 can be described by drawing the op-amp into a pin diagram
so that the two inputs are connected to pins 2 and 3, the output is connected to pin 6, the positive power supply to pin 7, and the negative power supply to pin 4. Pin 8 serves no purpose. We will discuss pins 1 and 5 in a later note.

Dual op-amps like the 4558 generally have the same pinouts so that their ICs can be interchanged in circuits that use them with sockets. This is a popular thing to try with stompboxes like the tubescreamer. Op-amp A has inputs on pins 2 and 3 and output on pin 1 on one side while op-amp B has inputs on pin 5 and 6 and output on pin 7 on the other side. The positive and negative power pins are 8 and 4, respectively. On a schematic these op-amps would be labeled IC1A and and IC1B and the power supply pins for one would serve both.

One of the moderators on Aron’s diystompboxes forum, Peter Snowberg, has repeatedly suggested using a “fab house,” or a PCB fabrication company, to make PCBs. There are lots of reasons for doing this, including convenience, accuracy, and less pollution. I have been wanting to give this a try and waiting for the right project to do it. And I was not sure which company to try. Finally, I took the plunge by sending olimex.com a smaller version of markusw’s 9V electric mistress project that I have been working on.

Several things came together to push me over the brink. First, markusw mentioned that he had seen an Olimex board made by another forumite and thought highly of it. Another factor was that my project fit nicely as three copies onto a single small board as manufactured by Olimex. Because Olimex will arrange the copies (they call it panelization) on the board for you and cut the board up into the individual boards (they call that de-panelization) all for free, I could get three boards for a fairly low price. The final reason was that I could easily go with a 2-sided board for not much more money and that helped to keep my version of markusw’s project small.

To keep things inexpensive, you have to design within the specifications of Olimex, but that is not difficult. The gaussmarkov Eagle libraries all use a 0.7mm drill which is one of the default sizes that Olimex uses, so meeting that requirement was trivial. The only extra work that I had to do was replace the off-board components often pictured in layouts with pads for each connection. I am not sure yet what Olimex does with components that are positioned outside the perimeter of the board.

One other detail, which will not be an issue next time, is that the text for the silkscreen printing of component placement on the component side needs to be 50mils high or the text may not show properly. Olimex requires a 10mil width on the lines in the silkscreen layer. So I just told them not to print the silkscreen layer and made that easy. For the future, I am gradually working my way through the gaussmarkov libraries and increasing the text size from its current 40mil size.

Once I had the board ready to go, I only had to send Olimex the Eagle .brd file. I emailed it and the next day I received an email reply saying that I was good to go. Olimex also checks to make sure your board meets their requirements. They gave me a final price, which had an extra $3.30 charge for drilling over 100 holes on a board. I did not count my holes ahead of time, but it was no surprise so that was fine. Then I faxed them my credit card to pay for it.

Olimex promises a 3 to 5 working-day turnaround and they were finished my board after 5 days and shipped it immediately. I chose airmail delivery because I was not in a hurry. From Bulgaria to California, delivery took 8 days. And part way through that time, I got another email telling me that a package was on its way.

As you can tell, communication was good. If you are chatty, don’t look for a conversation partner at Olimex. The email I got confirming shipping contained just one word: “shipped.” And be ready to learn that you made a mistake when you get your boards. I have a feeling they do exactly what you ask, even if you did not know that was what you were asking for.

I have not populated my boards yet. But I am satisfied with the whole process and I will probably go with Olimex again when I attempt a rendition of the Deluxe Memory Man, our next two-sided project. And I am thinking about including an Olimex-ready version of some projects in the Circuits section of this site.

So dig this: probably the smallest Big Muff you’ll ever see. Chris Gregory (stobiepole) decided to make an all carbon comp version using the Big Muff Pi (Triangle Version) layout. This is a nice example of putting the PCB over the pots. Look at Krister’s Pearl OC-07 Octaver Clone build for another example.  View the image in a new window to see a slightly larger version.