The target of this project was the design of a small portable mixer supplied by a 9V PP3 battery, keeping high quality performance. The mixer is formed assembling three main modules that can be varied in number and/or disposition to suit everyone needs. The three main modules are:
Input Amplifier Module: a low noise circuit equipped with a variable voltage-gain (10 - 100) preset, primarily intended as high quality microphone input, also suitable for low-level line input.
Tone Control Module: a three-band (Bass, Middle, Treble) tone control circuit providing unity-gain when its controls are set to flat frequency response. It can be inserted after one or more Input Amplifier Modules and/or after the Main Mixer Amplifiers.
Main Mixer Amplifier Module: a stereo circuit incorporating two virtual-earth mixers and showing the connection of one Main Fader and one Pan-Pot.
The image below shows a Block diagram of the entire mixer featuring four Input Amplifier Modules followed by four in-out switchable Tone Control Modules, one stereo Line input, four mono Main Faders, one stereo dual-ganged Main Fader, four Pan-Pots, a stereo Main Mixer Amplifier Module and two further Tone Control Modules switchable in and out for each channel, inserted before the main Left and Right outputs.
Obviously this layout can be rearranged at everyone wish. An astonishing feature of this design lies in the fact that a complete stereo mixer as shown below in the Block diagram draws less than 6mA current!
Block diagram:
Input Amplifier Module
Parts:
R1 = 22K - 1/4W Resistor
R2 = 22K - 1/4W Resistor
R3 = 47K - 1/4W Resistor
R4 = 47K - 1/4W Resistor
R5 = 47K - 1/4W Resistor
R6 = 4K7 - 1/4W Resistor
R7 = 22K - 1/4W Resistor
R8 = 220R - 1/4W Resistor
R9 = 2K - 1/2W Trimmer Cermet (See Notes)
R10 = 470K - 1/4W Resistor
R11 = 560R - 1/4W Resistor
R12 = 100K - 1/4W Resistor
R13 = 220R - 1/4W Resistor
C1 = 470nF - 63V Polyester Capacitor
C2 = 100µF - 25V Electrolytic Capacitor
C3 = 2µ2 - 63V Electrolytic Capacitor
C4 = 2µ2 - 63V Electrolytic Capacitor
C5 = 2µ2 - 63V Electrolytic Capacitor
C6 = 47pF - 63V Ceramic Capacitor
C7 = 4µ7 - 63V Electrolytic Capacitor
C8 = 100µF - 25V Electrolytic Capacitor
Q1 = BC560C - 45V 100mA Low noise High gain PNP Transistor
Q2 = BC550C - 45V 100mA Low noise High gain NPN Transistor
IC1 = TL061 - Low current BIFET Op-Amp
Circuit Description:
The basic arrangement of this circuit is derived from the old Quad magnetic pick-up cartridge module. The circuit was rearranged to cope with microphone input and a single-rail low voltage supply. This low-noise, fully symmetrical, two-transistor head amplifier layout, allows the use of a normal FET input Op-Amp as the second gain stage, even for very sensitive microphone inputs. The voltage-gain of this amplifier can be varied by means of R9 from 10 to 100, i.e. 20 to 40dB.
Notes:
- R9 can be a trimmer, a linear potentiometer or a fixed-value resistor at will.
- When voltage-gain is set to 10, the amplifier can cope with 800mV peak-to-peak maximum Line levels.
- Current drawing for one Input Amplifier Module is 600µA.
- Frequency response is 20Hz to 20KHz - 0.5dB.
- Total Harmonic Distortion measured with voltage-gain set to 100: 2V RMS output = <0.02%>
- Total Harmonic Distortion measured with voltage-gain set to 10 & 33: 2V RMS output = <0.02%>
- THD is much lower @ 1V RMS output.
- Maximum undistorted output voltage: 2.8V RMS.
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