Low Voltage Boosters

The 9 volt battery is ubiquitous. This little package of power is the source of energy to drive 99%+ of the stompbox pedals on the market today. It is convenient, inexpensive and provides enough voltage for most circuits to operate comfortably. However, there is an option that is often overlooked in pedal circuits, and that is using a lower supply voltage to power a pedal.

The trend in computer chips has been to lower supply voltages, at first from 5v down to 3.3v, but now even lower! A lower supply voltage can be important in large-scale-integrated-circuit chips where there are many thousands of transistors packed into a very tiny amount of silicon. Lower voltages will mean less heat generated, faster speeds and more energy conservative computers. None of these items are important in stompboxes but reducing the power supply can be interesting nonetheless.

This is a basic bipolar booster circuit similar to that used in the old Electro-Harmonix LPB-1, and currently the featured design of numerous boutique booster pedals -- I have seen this exact circuit in several custom-made pedals. The single transistor can provide over 25db of gain; it is quiet, inexpensive and performs well to increase signal strength.

The current requirements for the bipolar booster are quite low and a 9v battery will last many months when powering this circuit. The transistor output can swing almost 8.5 volts peak-to-peak before running into the limits of the power supply. This is a very powerful signal considering that the typical guitar pickup is only producing around 0.1 volt output.

So, if we take the 0.1v signal from the pickup, multiply it by the 25x gain of the booster, we have a 2.5 volt output, which is within the limits of the voltage swing of the booster circuit.
What happens if we reduce the power supply voltage? A pair of AAA batteries will yield 3 volts and fit in a holder not much larger than a 9v battery. This is a good choice for beginning experiments but merely substituting a pair of AAA for the power supply does not give good results since the bias voltage on the base of the transistor is a fixed fraction of the power supply voltage. When the supply voltage is lowered, the bias is changed enough to prevent the circuit from functioning properly.

It is necessary to tweak the values of the resistors used in the booster to get it back into correct operating balance. The schematic to the left provides the revised values for a bipolar booster using 3 volts. A pair of AAA or AA batteries have sufficient energy reserves to keep this booster running for a long time.

The output of the 3v booster can swing more than 2.5v pk-pk but signal gain has been reduced slightly to 16x - 18x. With the 0.1v signal from the guitar, the booster will provide a 1.6v or greater output voltage. This is plenty to drive any amplifier to maximum wattage!

However, it is not quite as perfect as it sounds. While the steady state signal from a guitar pickup is 0.1v, the initial pick attack can be 1 volt or larger! This means that with a gain of 16, the booster will try to reproduce an output of 16 volts and it cannot. Even with a 9v battery, the circuit is not able to swing to the extremes required and some of the signal is clipped momentarily on the initial pick attack. There is a more complete description of this distortion in the AMZ article, Boosters, Gain and Distortion.

If the booster is going to clip on the pick attack with 9v power, it is going to be even more noticeable at 3 volts. More distortion is not always a bad thing when you are dealing with guitar effects and can give the booster a slightly fatter sound. When the circuit was built on a breadboard, the transistor made a somewhat smooth transition into clipping with the edges of the waveform rounded instead of hard clipping. This usually indicates a more pleasing selection of harmonics and distortion artifacts.

Taking the idea a step further, we can modify the circuit to work from a single 1.5 volt battery. Once again, the resistor values have to be tweaked in order to correctly bias the transistor. Maximum gain drops further to about 12x or so, but that is still plenty for use as a booster. Headroom also decreases since this circuit can only swing a little over 1 volt pk-pk before clipping. The 1.5v booster will clip much sooner because of the limited output. If you are looking for an alternative to a "clean boost", this might be just the ticket.

With the sustained 0.1v guitar output, the 12x gain circuit should produce a 1.2v output but it cannot and the tips of the wave are soft-clipped. This booster acts as a mild overdrive; however, rolling back on the guitar's volume knob will clean it up nicely.

For exact biasing, the collector resistor value may have to be altered so that the dc voltage from the collector to ground is one-half the power supply voltage, that is, 1.5v for 3v power and 0.75v for a single battery.
It immediately comes to mind that if this works for a bipolar booster, low voltage might also work with a jfet transistor. It does but with greater limitations. The following circuits were constructed on a breadboard and tested. A J201 was used for these test circuits. The main problem is that because the channel resistance of the jfet becomes substantial at these voltages, the gain that is produced is much lower than with the bipolar boosters. Available gain is only 3x to 6x depending on the transistor and the voltage supply.

         

The next logical step is using the bipolar booster to drive the jfet circuit. The bipolar can produce plenty of gain that will overdrive the jfet and produce smooth distortion with musically pleasing harmonics... a 3 volt screamer!

The two stages generate a mild overdrive with rounded edges as the signal clips. The low gain and lack of bypass capacitor on the jfet contribute to the production of tube-like harmonics. Its simplicity is part of its attraction.

For max distortion, set the Drive control high and then adjust the volume with the output control. For a cleaner overdrive sound, set the output volume control to max and then slowly adjust the Drive control to the desired loudness. The circuit will also clean up as the guitar's volume control is rolled back.

A holder for a pair of AAA batteries is no larger than the space required for a 9 volt battery and they will power this overdrive for months of occasional use. Use high capacity alkaline cells for this circuit.

The output signal can be over 2.5v and additional distortion can be obtained by using this circuit to overdrive the input of a tube amp. Fat juicy sounds are guaranteed!

The same idea can be applied to the 1.5 volt version of the boosters. With a single cell powering the circuit, this overdrive will be quite dirty sounding. It is wide-range but not flabby sounding. Again, note that the value of the collector and drain resistors will most likely have to be tweaked to the proper values.

One issue with powering a circuit from a single 1.5 volt cell is that an LED indicator requires about 1.8 volts to light up! There is not enough supply voltage to overcome the forward voltage drop of the LED so it will not conduct. A voltage doubler could be used to produce enough power for an LED but that would unnecessarily complicate this simple circuit. It is possible that there may be an LED with a very low Vf that is suitable for this booster but I have yet to researched that idea.

An interesting alternative is to use a pair of AA size Ni-Mh batteries to power these circuits. I have about a dozen of them that I have picked up for use in my digital camera. They are rechargeable with 1800 - 2000 mAh capacity or more. This means that a 2 ma. circuit would run 900+ hours on these cells before they would need recharging. Each AA battery puts out 1.2v so a pair in series would give 2.4 volts -- enough even to light an LED indicator (use a 270 ohm current limiting resistor).

These low voltage boosters are not for making clean sounds but they excel in adding a bit of flavor. They will provide some additional tone colors for your musical palette, so build one today and try it out!

Coming soon: a pcb and parts layout for the 1.5v/3v Overdrive