The original audio output in the Six Trak is a "mono" output and this was done by simply "summing" all the CEM3394 synthesizer outputs into a single opamp summing junction, before going to the "autotune" and "mute" circuit. ( fig 1 )
In this circuit, the opamp performs the summing of all signals present at its (-) input also notice that they are signal currents not voltages...
interestingly, I have found some suggested mods that take the signal out of each CEM3394 chip directly (ouch !)... ( and... how 'bout autotuning "noise" ? )
A closer look at this circuit reveals that the output of each CEM3394 is a bipolar current and it reaches a maximum of about 495 uA (p.p.) when all available waveforms are being selected (see OnChip Systems ref. manual pag.3, CEM3394 spec's)
A safer way of extracting the audio output is to convert this bipolar signal current into a voltage before entering the summing opamp... ( fig 2 )
The resistor labeled "new" above was inserted after each CEM3394's pin 19 output and before the original summing amp., now the output current is being converted as a small voltage (Output current times R) or, about 0.5 Volts maximum...
Also, the 2.2uF capacitor in fig 2 after the "new" 1K resistor blocks any residual DC component and directs this "sampled" signal as voltage to the additional circuit that "pans" it by means of the 20K trimpot before entering the final summing/muting amplifier ( fig 3 )
The "mixer/muting" circuit ( fig 3 ) is a simple summing amplifier whose gain is set by the feedback resistor and the input resistors (and the trimpot), the muting is performed simply by shorting the feedback resistor via the dual analog cmos switches wired around the cd4007 chip ( see: Don Lancaster's CMOS Cookbook )
The /MUTE line ( blue wire ) taps at the top of the 1megohm resistor located below the "VALUE" potentiometer (looking at the main pcb about 1 3/4 inches below this pot) and to the left of the numeric display pin silkscreened as " L-009" at least this is correct in my synthesizer, also it is located below the third lower screw counting from the right side of the main board. See the picture below as a reference:
You can tweak the trimpots any way you want,in fact, what I did in mine was to"set" each trimpot like voice6 all the way to the left, voice5 to the left also but less, voice4 almost in the middle, and so on... so voice 1 ended up all the way to the right... you may want to check the circuit in "MONO" mode ( parameter 35=1) and with a good 'lead' patch.. ( some reverb will help too )
The circuit is powered by its own power supply, and NEVER, I REPEAT; NEVER attempt to tap into the synthesizer's power supply... Why ? Because, first of all, it is too small as to power anything else and second, because I don't believe you can get CEM3394 chips at Radio Shack... so it is a lot safer to use a small regulated power supply for this project and also because you may use it to power the electronics for the other mod's.. if I ever finish them .. :(
Below is the schem for the power supply... mine was rated for max 100mA max. I expect that rating to be enough at this time..
( fig 4 )
You can assemble the circuit(s) into a single (3X4)" prototyping board and the most expensive part will be the small power transformer needed for the power supply, also you can use a TL082 instead of the suggested dual op-amp. Notice also that I have included +/- 5volts for further use, this project uses +/- 12V and +5V. The picture below shows how the finished project looks inside my SIXTRAK :
A final note, the circuit still 'leaks' some audio ( over 20/30 dB below the nominal output ) when the good ol' Z80 does the autotuning routine every 68 seconds or so when no keyboard/midi activity is present.. and it's noticeable, I hope rev.2 will cure this problem, I'm also working on some pictures and a more concise text plus the schem's all zipped as a single file for easier downloading, also a PCB is in the works..check my page often...
Again, (© 1997) write me if you feel you need additional help...
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