Basic circuits

	Simple attenuator This is a simple passive attenuator (i.e. no power supply required). J1 is the input socket, J2 the output socket. A typical value for P2 is 10k...100k. A linear or logarithmic type can be used for P2 (logarithmic especially for audio applications as the loudness characteristics of the human ear is approx. logarithmic).
	Simple lowpass This is a simple passive lowpass with 6dB/octave slope. A non-inverting amplifier can be added at the output (and even at the input) to make the circuit independent of input/output impedance (i.e. the "loads" connected to J1 resp. J2). Replacing of R1 by a vactrol leads to simple voltage controlled lowpass filter. Replacing R1 by a potentiometer leads to a simple manually controlled lowpass filter Frequency of the lowpass: f = 1/(2 * Pi * R1 * C1) with Pi = 3.14 Example: R1 = 47kOhm, C1 = 10nF -> f ~ 340 Hz
	Simple highpass This is a simple passive highpass with 6dB/octave slope. A non-inverting amplifier can be added at the output (and even at the input) to make the circuit independent of input/output impedance (i.e. the "loads" connected to J1 resp. J2). Replacing of R1 by a vactrol leads to simple voltage controlled highpass filter. Replacing R1 by a potentiometer leads to a simple manually controlled highpass filter Frequency of the highpass: f = 1/(2 * Pi * R1 * C1) with Pi = 3.14 Example: R1 = 10k, C1 = 2,2n -> f ~ 7.2 kHz
	Non-inverting amplifier This is a simple non-inverting amplifier: The term "non inverting" means that the polarity of input and output signal are the same. In other words: a positive input signal applied to J1 will cause a positive output signal at J2 and a negative input signal applied to J1 will cause a negative output signal at J2. The amplification of this circuit is 1 + R1/R2. Example 1: R1 = 0 Ohm (connection), R2 omitted -> A = 1 Example 2: R1 = 47k, R2 = 47k -> A = 2 Example 3: R1 = 100k, R2 = 10k -> A = 11 If R1 or R2 is replaced by a potentiometer the amplification can be adjusted. If e.g. R1 in the last example is replaced by a 100k potentiometer the amplification is adjustable in the range 1...11. This circuit can be used to built an simple amplifier if the desired audio or CV signal is too small for a certain application. Attention ! The minimum amplification of this circuit is 1 (no real attenuation possible provided that no external attenuator is used).
	Inverting amplifier This is a simple inverting amplifier: The term "inverting" means that the polarity of input and output signal are opposite. In other words: a positive input signal applied to J1 will cause a negative output signal at J2 and a negative input signal applied to J1 will cause a positive output signal at J2. The amplification of this circuit is  - R2/R1 (" - " indicates the opposite polarity of input and output) Example 1: R1 = R2 = 47k -> A = - 1 Example 2: R1 = 10k, R2 = 100k -> A =  - 10 If R2 is replaced by a potentiometer the amplification can be adjusted. If e.g. R2 in the last example is replaced by a 100k potentiometer the amplification is adjustable in the range 0...-10. This circuit can be used to built an simple (inverting !) amplifier if the desired audio or CV signal is too small for a certain application. The minimum amplification of this circuit is zero (if R2  = 0). To obtain a non-inverted output another inverting amplifier with amplification - 1 has to be used. The inverting amplifier can be extended by adding more input sockets (J1) and corresponding input resistors (R1). The right terminals of all input resistors are connected to the inverting input (-) of the operational amplifier O1. The relation between the corresponding input resistor R1 and R2 (the same for all inputs) defines the sensitivity of the input in question. If all resistors have the same value (e.g. 100 kOhm) the amplification is "1" for all inputs. Lowering R1 (e.g. 47k or 22k) increases the sensitivity of the input in question. Increasing R2 (e.g. 220k or 1M) increases the amplification resp. sensitivity for all inputs simultaneously. The minus terminal of the operational amplifier is often called "virtual GND" in this circuit as the voltage measured at this point is very close to GND within a few millivolts - independent of the incoming voltages! The first circuit example (chapter 3: "CV mixer with offset function") shows a typical application of inverting amplifiers with several inputs.