[sdiy] my first schematic - please critique

[Neil Johnson]

Following up on the excellent comments so far, here a few of my own:

1/ Replace R5-R8 with a zener source consisting of a 10k resistor to  
+9v, a 4V7 500mW zener diode, a 100nF ceramic capacitor, and 100k  
resistors to  the +ve inputs of both op-amps.

2/ Change R2, R3 for 100k.

3/ Change C1, C2 to 100nF polyester.

4/ Change R4 to 100k.

5/ Add 10uF electrolytic capacitors in series from the op-amp outputs  
to the output jack, +ve side to the op-amp.

6/ Change U1 to a TL072.

7/ Add 100nF ceramic capacitor across the supply rails.

8/ Add 10pF ceramic across each op-amp's output to -ve input.  Helps  
damp any HF oscillation.

9/ Change R1 to 1Meg.

As it currently stands this circuit will cross-fade, but you will  
always get a mix of both channels, even at the extremes of fade.  At  
either extreme you would see all of one signal, and about one fifth  
(existing values) or one tenth (using above new values) of the other  
signal.

An alternative solution, assuming your inputs don't mind varying  
input resistances, is to connect the crossfade pot across the two  
inputs, with the wiper connected to ground.  Hopefully the ASCII-art  
pic below shows what I mean:

                                                              100k
                                                                +-----+
              10k             100n     100k         100R        V     |
left1 o----/\/\/\------+------||-----/\/\/\----+--/\/\/\----/\/\/\---+
                        |                       |         10p         |
                        \                       +----------||---------+
                        /                       |                     |
                   1Meg \<-----+                |     ____            |
                        /      |                +----| -  \            
|  +  10uF
                        \     ///               |    |     >---------- 
+---)|-------o left out
                        /                       |  +-| +  /
              10k       |     100n     100k     |  |  ^^^^ TL072
left2 o----/\/\/\------+------||-----/\/\/\----+  |
                                                  Vref

Then at an extreme that signal will be shorted to ground, and the  
10k-1Meg divider won't reduce the other input signal that much (<0.1dB).

The 100R resistor just puts a little resistance into the feedback  
path at minimum gain (could be anything from, say, 33R) to give the  
op-amp something to play with.

Note this is all just hypothetical, don't actually have it sitting on  
the bench, so treat with some caution :-)

Neil
--
http://www.njohnson.co.uk