One of the more common questions I see about the Confidence Boost (CB) is "what is this circuit a clone of?", or something to that effect. The CB is not really a clone of anything. Kieth (the head honcho, the big cheese, el numero uno of BYOC) initially put the circuit together back in 2007 as a way for new builders to get their feet wet in assembling a stomp box circuit without spending a bunch of cash, and possibly destroying everything in the process. Back then, the kit came free with your first kit order. This proved economically unfeasible over time and now the kit is offered at the lowest price practical. In the circuit, he included the most common parts builders will need to deal with when building most any stomp box circuit; namely resistors, nonpolar (or film) capacitors, electrolytic capacitors, diodes, jacks, op amps (operational amplifiers, also known as "chips" or "ICs" for intergrated circuits), transistors, and a pcb (printed circuit board). The circuit includes an op amp-based input buffer stage, a silicon transistor-based booster stage, and a small power filtering stage. We'll look more closely at each stage below.
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The Input Buffer Stage
The Input Buffer Stage consists of the following components:
- Capacitor C1
- Resistors R1 and R2
- a dual op amp
C1 is known as the input cap. This cap isolates the buffer circuit from the input jack by blocking DC voltage present at the op amp input and at the junction of R1 and R2. It performs a tiny bit of tone shaping at the input by blocking a small amount of low frequency signal to keep the booster from becoming too muddy.
Resistors R1 and R2 form a voltage divider which delivers bias voltage to the op amp. Any time you have two resistors in series, you have a voltage divider. In this case, R1 is connected to the voltage supply (your battery or an AC adapter, known as V+) and R2 is connected to ground. The point where those two resistors are connected will have a fraction of the supply voltage. When the two resistors are the same value, as they are in this case, that voltage fraction will be 1/2 the supply voltage (4.5 volts if you are using a 9 volt source). Here is a quick read on voltage dividers: http://en.wikipedia.org/wiki/Voltage_divider
The voltage fraction from this voltage divider (referred to as reference voltage or Vref) is used for a bias voltage for the op amp. What, why? All common active components you'll typically find in a stomp box circuit need 3 separate voltage references: supply voltage (V+), ground, and bias voltage (Vref). That's just how they work. Here is some reading on bias: http://en.wikipedia.org/wiki/Biasing
So, the signal travels through the input jack, through C1 and the junction of R1 and R2, and then enters pin 3 of the op amp. The op amp is configured as a unity gain buffer. A buffer circuit is a little hard to explain; it's kind of like a firewall, it will stabilize the circuit from any loading or interference by external sources. It also allows the pedal to operate independently of what is before or after it. It basically takes a high impedance signal and converts it to a lower impedance signal, thereby increasing the strength of the signal (not making it louder) making it less susceptible to interference (at least that's how I think of it). The op amp gets V+ at pin 8, references ground at pin 4, and gets Vref and input signal at pin 3. The signal output is at pins 1 and 2, which are tied together.The Coupling Cap
Capacitor C2 is a coupling cap. I didn't highlight it in the schematic because it's not really its own stage and it doesn't really belong to the input buffer or the booster stages. Its function here basically keeps the two stages separate by blocking DC voltage on pins 1 & 2 of the opamp from affecting the booster stage and vice versa. It can also be used for further tone shaping, but in this case, it is not really doing much in that department.The Boost Stage
The boost stage consists of the following components:
- Resistors R3 through R6
- Transistor Q1
- Capacitor C3
- Potentiometer VR1
The boost stage is a silicon transistor booster very similar to the Electro Hamonix Linear Power Booster (LPB). Again, resistors R3 and R4 form a voltage divider which sets the input bias (and impedance) for the base, or input, of transistor Q1. Resistor R5 sits between the emitter of Q1 and ground, and it's value helps set the voltage gain of the booster. R6 sits between the collector of Q1 and V+, and sets the output voltage of the booster. Signal enters Q1 through the base, gets amplified, and exits through the collector, through C3, through VR1, and then to the output jack and into your amp. C3 is the output cap and blocks DC voltage from leaving the circuit, and also performs a little more tone shaping. VR1 is a typical volume control, which adjusts the amount of signal allowed to leave the circuit by dumping some amount of signal to ground (how much is determined by where the control is set).
Dano at Beavis Audio has an excellent write up on the LPB circuit: http://www.beavisaudio.com/techpages/HIW/hiw1.gifThe Power Section
The power section consists of the following components:
- The battery or other power source
- Diode D1
- Electrolytic capacitor C4
The power section is made up of a few components that provide and filter power to the circuit. Obviously, the battery provides the voltage source. Positive voltage then travels through diode D1, which functions as both protection from reverse polarity and as a line noise filter. What a diode does is allows voltage to flow one way, but not another way. The 1N4001 diode used in this build allows any voltage over 1 volt DC to pass through the component from the anode to the cathode, but blocks any voltage below 50 volts from passing through backwards from the cathode to the anode. This basically prevents any of the components from being harmed should the power supply be hooked up backwards. It functions as a noise filter it drops about one volt from the power supply and slows the current flow down a little with a resistance of about 100 ohms, which tends to remove very high frequency power supply noise from the audio portion of the circuit. Capacitor C4 is known as the filter cap; it basically removes power supply "ripple" by providing a constant current source as the active components do their amplifying.
And that's it! We've covered the CB from "in" to "out". That's all for now...