guitar-pedalschematics
I found this schematic of the "MXR Distortion +" guitar pedal.
I am trying to rebuild it, but I have some doubts about this +4.5V in the bottom left corner.
What does it mean?
I mean, I have only 1 power source: a 9V battery, so how should I add a 4.5V?
So basically my question is what is this 4.5V, and how can I add it to my circuit?
And I don’t understand exactly the same thing in the power supply stage right.
Analog synths are built around a voltage-controlled oscillator. For compatibility between keyboards and oscillators and to allow hooking of synths together a standard control of 1 V per octave became standard. This gave a reasonable range of 8 V for an 8-octave keyboard. So, if, for example 3.732 V gave a certain frequency then 4.732 V would give one octave higher. The oscillators were tricky to design as they needed to behave linearly over a wide range and have excellent temperature stability. On the other hand it made the keyboards simple as the same resistor value was used between each pair of keys.
Keyboards aren't the only pitch controllers, however. Two other common methods are pitch-bend controllers (traditionally a wheel on the left end of the keyboard, sprung to centre) and a vibrato wheel (also on the left end of the keyboard) which introduced a low-frequency vibrato to the oscillator.
Now your circuit should become clearer.
Figure 1. VCO (voltage-controlled oscillator control inputs.)
IC2 is configured as an (inverting) summing amplifier. The output to the VCO will be the sum of signals 1, 2 and 3.
IC1A intentionally is set to have high gain and it's a non-inverting amplifier like this: -
R5 in the OP's circuit is equivalent to R1 above.
However, it has two extra features that work together. The diodes in the OP's circuit heavily clip the guitar signal and this produces "wanted" distortion that can be adjusted by VR1. Note that the signal is clipped asymmetrically to provide a different type of characteristic "sound" compared to symmetrical clipping. The asymmetry means that R5 must then be capacitively connected (via C3) to stop the natural DC offset produced by the asymmetry causing the circuit not to work correctly i.e. it stops the output developing a significant DC offset that would tend to make clipping more or less symmetrical.
Best Answer
To make the circuit work, connect together the two points labeled "+4.5V".
This circuit produces three types of distortion in various combinations, depending on the adjustments: crossover (inherent in the 741, producing the "opamp sound"), diode or clipping (caused by D1 and D2), and overdrive (caused by adjusting RV-Distortion so the output of the 741 is distorted even before the signal gets to the diodes).
Because the 741 is powered by 9 V and 0 V (as opposed to bipolar supplies like +/-9 V), the inputs must be biased such that the output rests at 4.5 V when there is no signal. This gives the chip an output "swing" range of around +/- 2.5 V about the 4.5 V centerline voltage. It is limited to only +/- 2.5 V because the 741 is not a "rail-to-rail" opamp; its output cannot cover the entire 9 V range.
R6 and R7 divide the battery voltage in half, producing the 4.5 V bias voltage. C6 is a decoupling capacitor that lowers the apparent impedance of the 4.5 V source so it does not change with the audio signal. This prevents oscillation.
For more stable operation, another 1.0 uF capacitor should be added to the circuit to decouple the 741. If the round-trip connection path from the battery terminals to the 741 pins 4 and 7 is more than a couple of inches, that tiny amount of wire inductance can affect the 741's operation. Add the cap directly across the 741's power pins, keeping the leads as short as possible.