Audio Mixer - Transistorized
                 by John Lundgren 12-8-95 Ver 951219a
                          A Basic Schematic

    Legend:  ) = No Connection
             + = Connection
           All resistors are 1/4 W 5%.
           All capacitors are 16V or more.


             J1            J2           J3           J4              
          Line 1 In     Line 2 In    Line 3 In       etc             
       +-----(O)----------(O)----------(O)-----------(O)             
       |      |  C1        |  C2        |  C3         |              
       |     --- .47 uF   --- .47 uF   --- .47 uF    ---             
       |     ---          ---          ---           ---             
       |      |            |            |             .              
       |      +------      +-----+      +-------+     .              
       |  R1 50K     |           |              |     .              
       | Audio Taper |           |              |                    
       +---/\/\/\----+           |              |                    
       |      ^        R2  50K   |              |                    
       |      |      Audio Taper |              |                    
       +------)----------/\/\/\--+              |                    
       |      |             ^                   |                    
       |      |             |          R3  50K  |                    
       |      |             |       Audio Taper |                    
       +------)-------------)----------/\/\/\---+                    
       |      |             |            ^                           
       |      |             |            |                           
       |      \             \            \                           
       |      /  R4         /  R5        /  R6                       
       |      \  220K       \  220K      \  220K                     
       |      /             /            /                           
       |      |             |            |                           
       |      |             |            |                           
       |      +-------------+------+-----+-...   C5  470 uF 16V      
       |                           |                 || +            
       +---------------------------)-----------------||-------+      
       |    C4  220 uF 16V         |                 ||       |      
       |       || +                |                          |      
       +-------||-----+            |   Q1                     |      
       |       ||     |          G |   MPF102                 |      
       |              |         -------           R8  10K     |      
       |  R7  4.7 K   |        S |   | D                      |      
       +------/\/\/\--+---------/     \------+-----/\/\/\-----+      
       |                                     |                |      
       |                                     |                |      
       |                                     |                |      
       |                                     |     Q2         |      
       |          R9  2.2K                -------  2N3904     |      
       |                                 E /   \              |      
       +----------/\/\/\--------+--------/       \------------+      
       |                        |                             |      
       |                        |                             |      
       |                        | +                           |      
       |                       ---    C6                      |      
       |                       ---   22 uF                    |      
       |                        |    16 V                     |      
       |                        |                             |      
       |                        |                             |      
       +-----------------------(O)                            |      
       |                      J4                              |      
       |               Low Z  Audio Out                       |      
       |                                                      |      
       +----------+                                           |      
       |          |                                           |      
       |        -----                                         |      
       |        / / /                        S1 SPST on/off   |      
       |       Chassis                               /        |      
       |       Ground                              /          |      
       +----------------> - 9 volt Batt + <------o      o-----+      



Parts List

C1, C2, C3	.47 uF 100V or more, preferably non-polarized and
                                     not electrolytic
C4	220 uF 16V or more Electrolytic
C5	470 uF or more 16V or more Electrolytic
C6	22 uF 16V or more electrolytic
J1, J2, J3, J4	Phone jacks or RCA Phono jacks.
Q1	MPF102 FET 
Q2	2N3904 or 2N2222 or PN2222 NPN audio gen'l purp transistor
R1, R2, R3	50K or 100K Audio Taper pots
R4, R5, R6	220K ohm 5% 1/4W
R7	4.7K ohm 5% 1/4W  Adjust to get 1/2 batt voltage at emitter of Q2
R8	10K 5% 1/4W
R9	2.2K 1/4W 5% 
S1	SPST on/off switch (could be on one of the pots)
B1	Battery 9V or a well filtered 9V supply.


	If two of these circuits are made in the same enclosure for
stereo, then there can be a single power supply to run both of them.
There should be a resistor in series with the incoming 9V+ lead so as
to minimize crosstalk, and reduce the PLOP when it's powered on.

	R7 value will depend on the FET characteristics, which varies 
from FET to FET.  Adjust the value to get half the supply voltage at
the junction of the emitter of Q2 and R9.

	An earphone jack can be put on the output.  A 1K to 8 ohm 
transformer can be used for each channel, two for stereo.  The 1K
primary would go to the output jack.  The 8 ohm secondary would go to
 the earphone.

	Other features can be added to this basic mixer.  One or more 
microphone preamps could be added.  A VU level meter or LED bargraph 
can be added.  The microphone preamps could have a transformer to
match a low impedance microphone to the preamp input.  Etc...

      View of bottom of FET  MPF102:

                       labeling
                      -----------
                     (  D  S  G  )
                       \_______/

     View of bottom of Transistor  2N3904:

                       labeling
                      -----------
                     (  E  B  C  )
                       \_______/