Portable Mixer

Block diagram: 
Mixer Block diagram


Input Amplifier Module

Input Amplifier


Parts:
R1,R2,R7_______22K   1/4W Resistors
R3,R4,R5_______47K   1/4W Resistors
R6______________4K7  1/4W Resistor
R8,R13________220R   1/4W Resistors
R9______________2K   1/2W Trimmer Cermet (See Notes)
R10___________470K   1/4W Resistor
R11___________560R   1/4W Resistor
R12___________100K   1/4W Resistor

C1____________470nF   63V Polyester Capacitor
C2,C8_________100µF   25V Electrolytic Capacitors
C3,C4,C5________2µ2   63V Electrolytic Capacitors
C6_____________47pF   63V Ceramic Capacitor
C7______________4µ7   63V 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% @ 1KHz; < 0.04% @ 10KHz.
  • Total Harmonic Distortion measured with voltage-gain set to 10 & 33: 2V RMS output = < 0.02% @ 1KHz & 10KHz.
  • THD is much lower @ 1V RMS output.
  • Maximum undistorted output voltage: 2.8V RMS.

Tone Control Module

Tone Control


Parts:
P1,P2_________100K   Linear Potentiometers
P3____________470K   Linear Potentiometer

R1,R2,R3_______12K   1/4W Resistors
R4,R5___________3K9  1/4W Resistors
R6,R7___________1K8  1/4W Resistors
R8,R9__________22K   1/4W Resistors
R10___________560R   1/4W Resistor
R11___________100K   1/4W Resistor
R12___________220R   1/4W Resistor

C1______________1µF   63V Polyester Capacitor
C2_____________47nF   63V Polyester Capacitor
C3,C5___________4n7   63V Polyester Capacitors
C4_____________22nF   63V Polyester Capacitor
C6,C8_________100µF   25V Electrolytic Capacitors
C7______________4µ7   63V Electrolytic Capacitor

IC1___________TL061   Low current BIFET Op-Amp

Circuit description:
This is a straightforward design using the Baxandall-type active circuitry slightly modified to obtain a three-band control. Total voltage gain of this module is 1 when controls are set in their center position.
Notes:
  • Current drawing for one Tone Control Module is 400µA.
  • Frequency response is 20Hz to 20KHz - 0.5dB, controls flat.
  • Tone control frequency range: ± 15dB @ 30Hz; ± 19dB @ 1KHz; ± 16dB @ 10KHz.
  • Total Harmonic Distortion measured @ 2V RMS output = < 0.012% @ 1KHz; < 0.03% @ 10KHz.
  • THD is below 0.01% @ 1V RMS output.
  • Maximum undistorted output voltage: 2.5V RMS.

Main Mixer Amplifier Module

Main Mixer


Parts:
P1,___________100K   Linear Potentiometer
P2_____________10K   Linear Potentiometer

R1,R2,_________15K   1/4W Resistors
R3,R4,R11,R12_100K   1/4W Resistors
R5,R6__________22K   1/4W Resistors
R7,R8_________390K   1/4W Resistors
R9,R10________560R   1/4W Resistors
R13___________220R   1/4W Resistor

C1,C2_________330nF   63V Polyester Capacitors
C3,C8_________100µF   25V Electrolytic Capacitors
C4,C5__________10pF   63V Ceramic Capacitors
C6,C7___________4µ7   63V Electrolytic Capacitors

IC1___________TL062   Low current BIFET Dual Op-Amp

Circuit description:
The schematic of this circuit is drawn as a stereo unit to better show the input Main Fader and Pan-Pot connections. The TL062 chip contains two TL061 in the same 8 pin case and is wired as two virtual-earth mixer amplifiers having a voltage gain of about 4, to compensate for losses introduced in the passive Pan-Pot circuitry. Therefore, total voltage-gain is 1.
Each channel added to the mixer must include the following additional parts:
P1, P2, R1, R2, R3, R4, C1 and C2.
These parts must be wired as shown in the above circuit diagram, connecting R3 and R4 to pin #2 and pin #6 of IC1 for Right and Left channel respectively. These IC1 pins are the "virtual-earth mixing points" and can sum together a great number of channels.
Notes:
  • Current drawing for one stereo Main Mixer Amplifier Module is 800µA.
  • Frequency response is 20Hz to 20KHz - 0.5dB.
  • Total Harmonic Distortion measured @ 2V RMS output = < 0.008% @ 1KHz; < 0.017% @ 10KHz.
  • THD is 0.005% @ 1V RMS output.
  • Maximum undistorted output voltage: 2.8V RMS.


Further Parts:
To parts listed above should be added: one Main on-off SPST switch, a LED used as pilot-light with its dropping 2K2 1/4W series-resistor, DPDT switches to enable or omit Tone Control Modules as shown in the Block diagram, input and output connectors of the type preferred, one stereo dual-gang 100K potentiometer to fade the Stereo Line Input as shown in the Block diagram, battery clip, PP3 9V battery, knobs etc.
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