PNP "Positive Ground" Pedal Considerations
In this article, we will look at what exactly makes a pedal a "positive ground pedal", the power-supply constraints that come with these pedals, and how to design a typically positive ground pedal so that it can be daisy-chained with other pedals.
Powering "Positive Ground" Pedals
Most guitar pedals call for a standardized 9VDC, center-negative power supply with a 2.1mm jack center pin and 5.5mm plug diameter.
Most pedals that use this connection type can be daisy-chained off of one supply using something like M-PAS-CABLE-1 or M-PAS-CABLE-2. You may have come across a guitar pedal where the manufacturer states that it cannot be daisy-chained. This type of pedal needs to use an isolated power supply or battery if it is wired for one. This type of pedal is most likely configured as a "positive ground" pedal, which is a common configuration in vintage (particularly germanium) fuzz designs. Positive ground pedals sometimes only take a 9V battery for power (no power input jack). This prevents users from connecting the pedal to a daisy-chain even by accident.
What is a "Positive Ground" pedal?
You may have seen schematics for guitar pedals, like the one in Figure 1, where the pedal is powered by GND (0V) and -9V. "Positive ground" refers to the fact that the signal considered ground and 0V (and the one connected to the jack sleeves and sometimes case) is the "more positive" of the two power supply connections; i.e. the GND voltage at 0V is "more positive" than the -9V supply.
You may wonder how a pedal powered this way can be used with a standard 9V power supply. The first thing to know is that voltage is a relative measurement of the difference between two points. When you see +9V on a schematic, it is not referring to an absolute voltage. It only means that the point labeled +9V is 9 volts higher than the point labeled 0V (or by convention, GND) in that schematic. However, this does not mean that these values are "locked" to +9V and 0V; the power supply voltages can be considered to be any two values as long as they are 9 volts apart. Below are some examples of possible configurations in which the difference is the required 9 volts.
|Power Jack||Battery||Convention 1||Convention 2||Convention 3||Convention 4|
|Center pin||Negative contact||0V||-3V||-4.5V||-9V|
All conventions seen in Table 1 are valid, though in practice one of the two power supply rails is pretty much always considered to be 0V. 0V and 9V or -9V and 0V are the two configurations you are likely to see. "Mirrored" positive and negative voltages like -4.5V and 4.5V are sometimes used if the power supply is bipolar or if a ground point of 0V is being created with a voltage divider in the circuit (such as in the Earth Sound Research Graphic Fuzz shown in Figure 2).
Figure 2: -4.5V, 0V, +4.5V from a single 9V battery and a voltage divider in the Earth Sound Research Graphic Fuzz. Note that the 0V point is not common with either of the battery contacts.
The schematic seen previously in Figure 1 uses Convention 4 in Table 1 with the positive supply rail being 0V/GND. The schematic in Figure 1 could also be drawn as seen in Figure 3 (using Convention 1) and still be correct.
In Figure 3, you can see how a standard power supply would be connected. The negative contact would be connected to 0V, while the positive contact would be connected to +9V. This looks more like a standard "negative ground" pedal, just with +9V at the bottom of the schematic instead of the top. Figure 4 shows how the circuit would look if it was flipped so that +9V is at the top.
Though "positive ground" pedals are usually thought of in terms of -9V and 0V, you can see from Figures 3 and 4 that they can also be thought of in terms of 0V and +9V. You’ll notice in Figures 3 and 4 that lug 1 of the 500kA volume potentiometer and the jack sleeves are connected to +9V, which is the opposite of what you usually see (normally they are connected to 0V/GND). Though it is helpful to understand that a positive ground pedal can be drawn this way (with 0V and +9V), it is not recommended that you do so. To understand why, we have to look at the pedal circuit in context of the larger signal chain it is a part of (guitar -> pedals -> amplifier).
Figure 5 shows some of the important power connections made in a typical guitar amp. The jack sleeves are common to the chassis, which is common to earth ground (the chassis should have one secure connection point to earth ground). Earth ground is used as our 0V reference point for the amplifier. The amplifier jack sleeves are also at 0V.
Figure 6 shows a positive-ground pedal circuit with its output connected to the amplifier’s input. When this connection is made, the jack sleeves of the positive ground pedal are forced to 0V because they are connected to our 0V reference set by the amplifier. Depending on which battery contact is common with the jack sleeves, the other pole of the battery will either be -9V or +9V (relative to the 0V of our full system). With the positive contact of the battery connected to the jack sleeves (0V) as seen in Figure 6, the negative contact will be at -9V.
Previously, we saw a positive ground schematic with the jack sleeves connected to +9V in Figure 3. It is not incorrect to think of the circuit this way in isolation. However, once the jack sleeves are connected to the system’s 0V point via the audio cable connections, it would be incorrect to label the pedal’s jack sleeves as +9V in the larger system. The pedal’s sleeves are common to the amplifier’s jack sleeves, so there can be no voltage difference between them. It is for this reason that it is easier to always think of positive-ground pedals as 0V and -9V even when looking at them in isolation. In the context of a larger system, the jack sleeves and positive battery contact will typically be considered common with the system’s 0V point, which requires the negative battery terminal to be at -9V.
With the negative contact of the battery connected to the jack sleeves (0V) for a negative-ground pedal as seen in Figure 7, the positive battery contact will be at +9V. This is how negative-ground pedals are usually drawn, with 0V and +9V power supply voltages.
What is the problem?
Users run into problems with positive ground pedals when they try to daisy chain power supplies with standard negative ground pedals. Remember that in positive-ground pedals, the jack sleeves are connected to the positive rail. In negative-ground pedals, the jack sleeves are connected to the negative rail. When audio cables are connected between the two pedals, the jack sleeves connect to each other, shorting the positive rail (on the positive ground pedal’s sleeves) to the negative rail (on the negative ground pedal’s sleeves).
Figure 8: Short circuit when connecting a positive-ground pedal to a negative-ground pedal on the same power supply
When the positive rail of the power supply is shorted to the negative rail, an extremely low impedance path from +9V to 0V is created. This will draw as much current as the power supply is capable of providing, and will prevent any of the pedals on the daisy chain from functioning. It also has the potential to damage the power supply, and the large amount of current can damage components it flows through if they are not rated for that much current.
What is the solution?
There are a few ways to prevent this from happening.
1. Solution 1: Battery only
The earliest fuzz pedals were PNP positive-ground circuits that ran on battery power only. Those fuzz circuits draw very little current, especially with no LED indicator, and 9V batteries would last a very long time in them. Some users also prefer the sound of particular fuzz circuits when they are run on 9V battery power, so it is still a fairly common choice for positive-ground fuzz circuits. The reason why battery power does not cause problems is that the positive-ground pedal has its own power supply (the 9V battery). When the sleeve of a positive ground pedal jack is shorted to the sleeve of a negative ground pedal jack, the positive battery contact is not being shorted to its own negative contact, the way it is when daisy-chaining supplies. The positive battery contact is being connected to the negative battery contact of the other battery or power supply, putting the batteries in series, as seen in Figure 9.
Figure 9: Battery-only positive-ground effect connected to negative-ground effect; the battery becomes connected in series with the other pedal’s battery or power supply
2. Solution 2: Isolated Power Supplies
If a positive ground pedal is powered with an isolated power supply rather than daisy-chained with other pedals, the power supplies become connected in series when the jack sleeves are connected, much like they do when battery-only pedals are connected as seen in Figure 9.
Figure 10: Positive ground effect connected to negative ground effect, both running on an isolated power supply - power supplies are connected in series
Some pedalboard power supplies offer multiple power outputs that are isolated from each other. These can be used to power a negative and positive ground pedal in the same chain. They result in a series connection like that seen in Figure 10.
3. Solution 3: Wire a positive ground pedal as negative ground
The shorting problem occurs because the jack sleeves of a positive ground pedal are connected to the positive rail, while the jack sleeves of a negative ground pedal are connected to the negative rail. When the pedals are connected to each other, the positive rail is shorted to the negative rail via the jack sleeves. So can we wire a typically positive ground pedal the same way we do a negative ground pedal, with the negative rail connected to the jack sleeves? The answer is yes, but with some caveats. First, let’s look at a pedal where we have only changed the connection to the jack sleeves from the positive rail to the negative rail in Figure 11.
Remember that when this pedal is connected to an amplifier, its jack sleeves will be held at the system’s 0V. For this reason, we will redraw the circuit with 0V and 9V rather than -9V and 0V, with the jack sleeves at the system’s 0V. We will also redraw the schematic with 9V on top.
Daisy-chaining the pedal in Figure 12 with a negative-ground pedal will not cause any shorting issues. Both pedals’ jack sleeves are connected to the negative rail, so they are already common before they are connected to each other. The pedals will be connected in parallel, both powered by the same supply.
There are two problems with the schematic in Figure 12. The first problem is that lug 1 of the volume knob is connected to +9V, not the system’s 0V. When the volume knob is turned all the way down, the tip of the audio jack will be connected directly to +9V. This will eliminate the audio signal, but it will send a DC voltage to the output. If the volume knob is turned higher than 0%, less and less DC voltage will make it to the output, but there will still be some DC voltage passed to the next part of the signal chain, which is bad practice. The severity of the results depends on what is next in the chain. It may result in a persistent, loud hum and/or pops and clicks when a connection is made. More critically, it can cause damage to the equipment it is connected to if the next item in the chain cannot properly handle a DC offset. One example of this would be a pedal with a polarized input capacitor. Depending on its orientation, a DC voltage on the input can cause the capacitor to explode.
When the volume is turned all the way down, we want the output to be at our system’s 0V, not 9 volts above it. So we must also change the lug 1 connection to the power supply’s negative rail (the system’s 0V).
The other problem is that the polarized input capacitor is now facing the wrong direction. The signal seen at the input jack of any pedal should be centered around the system’s 0V. In a positive ground fuzz face, the signal on the other side of the 2u2 input capacitor is centered somewhere between the system’s 0V and -9V (lower than the 0V on the input jack). It is for this reason that the positive lead of the polarized input capacitor faces the input jack (the input jack sits at a higher voltage). In a negative ground pedal, the same signal is seen on the input jack, but the signal on the other side of the input capacitor is now centered between the system’s 0V and 9V (higher than the 0V on the input jack). Because the voltage on the input jack is the lower voltage of the two, the capacitor must be flipped so that the negative lead is connected to the input jack. If the input jack was not polarized, it would not require a change.
Figure 13: Figure 12 circuit redrawn with lug 1 of the volume potentiometer connected to 0V and the input capacitor flipped.
Figure 13 shows the full conversion of the positive-ground pedal seen in Figure 1 to a negative-ground pedal that can be daisy-chained with other negative ground pedals. Note that we did not change any of the connections to the 1K fuzz pot. The 2u2 and 10n input and output capacitors block DC voltage from entering or exiting the pedal via the input or output jacks. In other words, the portion of the circuit between the input and output capacitors is "AC coupled", meaning it only allows AC signals through.
For this reason, no connections in the AC-coupled portion of the circuit need to be changed. If there were a volume pot inside the AC-coupled part of the circuit, it could leave +9VDC on its output lug without any problems as the output capacitor will block the DC voltage from making it to the next part of the chain. For example, a Dallas Rangemaster would not need any connection changes to its volume pot, as it is inside the AC-coupled portion of its circuit (after the input capacitor but before the output capacitor).
In Figure 16, the positive and negative power supply connections were reversed and the schematic has been redrawn with +9V at the top; there are otherwise no changes to anything in the original circuit because the entire circuit is AC coupled (between the input and output 5n and 10n capacitors). However, the original Rangemaster circuit is not properly designed to be a standalone guitar pedal. The original Rangemaster was an amp-top unit. It had a slide switch and a volume control, but it was not true bypass and was usually left on (or off).
The optional 1M resistors help the Rangemaster circuit work well if it’s wired for modern true-bypass switching. The reason that they are needed is that no capacitor is perfect. All capacitors have some amount of leakage which lets DC current through. If the original Rangemaster circuit is wired for true bypass, the input and output are left floating when the pedal is disengaged. In this state, as more and more current leaks out of the capacitors, a DC voltage will become present on the input and output. When the pedal is re-engaged, this voltage is likely to be discharged through the connected stage, which can result in loud pops and clicks.
If the optional 1M resistors are included, any leakage current that flows out of the input and output capacitors will flow through the 1M resistor to 0V, and the previously-floating input and output will be held at 0V. There will be no voltage to discharge when the pedal is re-engaged. Note that although nothing changes in the original Rangemaster circuit when rewiring it for negative ground, the connection point for these optional resistors changes because they are outside of the AC-coupled portion of the circuit. These resistors must be connected to the system’s 0V in both a positive and negative ground configuration. If they are connected to the other power rail, a DC voltage will be present at the output at all times.
There exists some anecdotal evidence that suggests rewiring a positive ground pedal as a negative ground pedal can result in more interference, noise, and/or oscillation, which may be caused by feedback through the negative supply line. High gain circuits are more susceptible to this feedback. There are reports of PNP fuzz builds that are noisy when connected this way, but quiet when using the standard positive-ground configuration.
This configuration may work with no issues, but if you try a build like this keep in mind that if you are experiencing excessive noise issues, a positive-ground configuration might be a simple way to solve it. Proper AC coupling of the power supply, shielded signal wire, and not using the input jack for power switching may all help minimize noise in fuzz builds. Also keep in mind that fuzz circuits can be very high gain and temperamental - what works with no issues in one environment may cause problems in another. If the end user of a PNP fuzz build is someone who will use it in different environments, it might be safer to use a different solution unless you have thoroughly tested the results.
4. Solution 4: Switch to an NPN build
In the Figure 1 Fuzz Face schematic, PNP transistors are used as they were in the original germanium Fuzz Face. Many of the early fuzz pedals and modern germanium fuzzes use PNP transistors, as they were easier to make in the germanium era and are more plentiful (and sometimes less leaky). If you have NPN transistors that have desirable characteristics for a fuzz build, they can be used in a standard negative ground configuration. PNP transistors operate with current flow in the reverse direction of NPN transistors. To switch from PNP to NPN, besides swapping the transistors, the power supply connections should be reversed and any polarized components (like electrolytic capacitors) should be reversed to accommodate the opposite direction of current flow.
Figure 17: Original germanium Fuzz Face schematic with NPN transistors, power connections reversed, and electrolytics reversed
The circuit will function exactly the same as the PNP version (with the exception of any differences in transistor characteristics like gain, frequency response, etc.). The Figure 17 schematic is a regular negative-ground circuit with the negative rail connected to the jack sleeves, and it will work with daisy-chained power supplies and most other power options.
5. Solution 5: Generate -9V from +9V
Perhaps the most robust solution is to generate a negative voltage from the positive rail. This allows the pedal to use the system 0V for its positive rail and -9V for its negative rail while daisy-chaining with standard negative ground pedals. The -9V can be generated using DC-DC switching voltage converters, often called "voltage inverters", such as the MAX1044, TC7660S, LT1054, TC1044S, etc. The main downside is that there are non-trivial additions to the circuit. These voltage inverter chips operate at a very high efficiency (e.g. 98% typical power efficiency for the TC7660S), so there will be minimal power loss when using them. These chips can all vary slightly in a few different ways:
Max Input Voltage
MAX1044, which was used in the Klon Centaur, has a maximum input voltage of 10V. This should work with no issues with a well-regulated 9V supply. There are, however, reports of poorly-regulated 9V power supplies frying the MAX1044 in Klons, so a chip with a higher max input voltage or overvoltage protection on the MAX1044 offer a less failure-prone solution. The TC7660S, LT1054, and TC1044S all have a max input voltage of 12V or more.
Output Voltage vs. Current
The output voltage of these chips drops as current draw is increased. Some chips drop less voltage than others for a particular current draw, but the current draw of a fuzz pedal is small enough that it is not a major concern in this circuit.
These DC-DC converters use internal clocking to generate the inverted voltage. To minimize induced noise, the clock frequency should be high enough to stay out of the audio spectrum (>20kHz). MAX1044, TC7660S, and TC1044S all have a boost pin (Pin 1) which should be connected to +9V. The boost pin increases the clock frequency when connected to V+. For example, in the TC1044S the clock speed increases from a nominal 10kHz (in the audio spectrum) to a nominal 45kHz (out of the audio spectrum). Note that there are similar chips - TC7660 (non-S version), ICL7660 - which do not have the boost pin and should be avoided. The clock speed of these chips cannot be increased beyond a lower nominal speed of ~10kHz, which would be in the audio spectrum. LT1054 does not have a boost pin, but it has a nominal frequency of 25kHz which is just outside of the audio spectrum.
The boosted frequency is displayed for MAX1044, TC1044S, and TC7660S. LT1054, TC7660, and ICL7660 do not have a boost option.
In Figure 19, the jack sleeves are connected to 0V just as they were in Figure 1, only this time it’s the negative rail of the power supply input (rather than the positive rail), which you can see if you trace the 0V point back to the barrel jack. In the original PNP Fuzz Face circuit (Figure 1), the positive rail of the power supply input is connected to the jack sleeves (system 0V), and the negative rail was used for -9V. In Figure 18, the negative rail of the power supply input is used as 0V and connected to the jack sleeves, just as it would be in a standard negative ground pedal. If this pedal is connected to a standard negative-ground pedal on the same power supply, the jack sleeves will be carrying the same signal in both pedals and will not cause a short. The circuit seen in Figure 19 can be used with a battery, daisy-chained power supplies, isolated power, or separate supplies with no concerns about connecting it to other pedals.
We carry multiple DC-DC converter chips required for the voltage inversion. All of these operate outside of the audio range (if the boost pin is properly connected in the TC1044S and ICL7660S):
We also have two utility PCBs for handling all of the voltage conversion required for PNP fuzz builds.
- P-PC-INV-SCREW can be screw-mounted or mounted inside of a case using our adhesive standoffs.
- P-PC-INV-SWITCH can be mounted to a 3PDT footswitch and also doubles as a 3PDT breakout board.