My PC sound card is connected to my Genelec 1029A speakers, both of which
have their own separate volume control—which makes adjusting volume
using those is somewhat inconvenient. So I'm using a second hand
t.Mix MIX 802 mixer to control
the volume more conveniently than through the PC's mixer programs. With it
I can also connect a CD player as a program source. Plus I can use the
mixer as a headphone amplifier! Using headphones with my computer would be
completely impractical otherwise.
The mixer is fine for this purpose except for one thing: The headphone amplifier with its own volume control is post-fader—in other words, the mixer's "Main mix" fader also affects the headphone volume. This means that setting the "Main mix" fader to zero (to silence the loudspeakers) also silences the headphones! I really wanted to be able to control the headphone and "Main out" volumes independently! Also, since the "Control room" output is behind the same control, I could have two identical but independently adjustable balanced outputs (that is, with independent volume controls—they both output the same mix, of course).
Thus: Modify the mixer. I had earlier done the same modification on my Behringer UB502, which I had used for the exact same purpose. (And I also have a Behringer Eurorack MX2004A, custom modified by Yours Truly, to change four of the eight mono channel pre-Eq Inserts into post-Eq, but that's another story.)
This (much simplified) block diagram shows how the MIX 802,
just like the UB502, is
wired internally. You can see that the headphone amplifier is controlled by
both the Phones fader and the Main fader. The mods
I wanted to do are shown in red: Basically bypass the Main fader
from the headphone amplifier's input. That would provide two independent
volume controls: one for controlling my speakers, one for the headphones
(and the "CTRL RM OUT" output).
When I modified my Behringer UB502, I found the schematic for the Behringer Xenyx 502 online, and it turned out to be exactly the same with respect to component values and their reference numbers. I couldn't find a schematic for the MIX 802, but I did find the schematic for the Behringer UB802, and it seems functionally almost similar (but not quite, as one has a fader on the AUX SEND, whereas the other has a fader on the stereo AUX RETURN). However, when I opened up my MIX 802, I immediately saw that the components' reference numbers and values have absolutely no correlation with that schematic. Damn.
To open up the mixer, I first opened the four screws on each side and removed the side panels (though this step seems to be unnecessary). Then I opened the six screws from the bottom, and removed the bottom panel. Next I pulled off 27 potentiometer knobs—all except the two GAIN pots. Some were rather tight, but they're just friction fit. Then I took off the retaining nuts and associated washers from all fourteen 6.3 mm TRS jacks and the retaining screws from the XLR connectors (two each) and RCA connectors (one in the middle). Finally, to release the circuit board, I had to open five screws holding it onto the top part of the chassis. There are threaded stand-offs in between the chassis and the board, so it doesn't matter whether you open them from the top side, or from the circuit board side. I opened them from the circuit board side, all except for one, which was very tight—that one I opened from the top. After this, the PCB came out, and I just had to disconnect the power lead connector before I could stow away the entire chassis. There were also spring-loaded washers around the neck of each TRS connector, but these didn't seem to come off on their own, so I let them be.
Now that I had the circuit board out for examination, I noticed a marking
of "MIX60" in one corner. Ever hopeful, I searched the net for
"MIX60 schematic", and found the awesomely comprehensive
service manual for the
Tapco Mix.60, which looks like the exact same device! And the component
reference numbers match those on the board. This modification just got
a whole lot easier!
As in the UB502, the headphone amplifier gets its signal via
the "TAPE TO CTRL RM / PHONES" switch, which is
SW3A/SW3B in the schematic.
It selects either the tape input directly (with the switch in the
down-position), or the mixed signal, post-fader (with the switch in the
up-position). The latter signal is the one that
I want to replace with the pre-fader signal.
I identified the correct switch terminals (terminals 1 and 4 of SW3A and SW3B in the schematic) by probing with a multimeter—these terminals have 1 kΩ resistances to the "TAPE OUT" connectors when the switch is in the down position. I isolated these terminals from the rest of the circuit by cutting two traces on the component side of the board leading to SW3 as shown (see the full image here). This has no other side effects—it just cuts the signal to the "CTRL RM" or "PHONES" outputs, unless the "TAPE TO CTRL RM" switch is pressed. (On the UB502, this part of the modification required cutting two pairs of traces, and soldering a pair of jumper wires.)
|I decided to take the pre-fader signal from between C56 and R215 (left channel), and between C57 and R214 (right). Jumper wires are easily soldered on the reverse side of the board, from the negative legs of the through-hole capacitors to the corresponding isolated switch terminals (see the full image here), thus completing the modification. Two blobs of hot glue help keep the wires from wandering around.|
When soldering the jumper wires, make sure you don't short-circuit them to any other nearby components or board traces. (That should not be a problem, since they're soldered onto the leads of through-hole components.) Once done, reassemble the mixer and test it. If all has gone well, you can now control the main and tape outputs with the "MAIN" volume control, and the headphone and "CTRL RM" outputs with their own volume control, independently! The "Tape to phones" and "Tape to mix" switches work exactly as before. The level indicator LEDs, however, are now pre-fader, so they keep blinking along with the mixed signal even if the "MAIN" fader is turned down to zero. But blinking LEDs are nice—why should they turn off just because the music is silenced? Ok, maybe they're not useful anymore for adjusting levels when mastering a musical recording, but I'm not using this mixer for mastering anyway.
On the other hand, if you did manage to break something in the course of your attempted modification, you now have a broken mixer, congratulations! So don't attempt this modification unless you know what you are doing and are willing to take that risk, and in any case don't blame me for anything bad that happens!