Last post for today!
From the other end of the circuit, here’s the schematic for the piezo bar input and the dual VCA (at least that’s what Richard Brewster calls it. His schematics of the Sidrassi have been very helpful and very close to the Tetrazzi so far.)
Ok, I’m still slowly working through the paper circuit and have reached a few conclusions: (Scroll down for photo documentation.)
The central tl084 op amp is an audio mixer and voltage reference (MIDO in Peter speak.) It appears to be missing a connection to all the other MIDO points on the board. The voltage divider feeding the MIDO op amps also seems to be missing a 10 uF cap to ground.
Peter is using some of the same symbols for different things. The symbol that he uses for output on other paper circuits is also being used to denote other points that are similar to those that are banana or brass nodes in the sidrazzi.
For those playing at home, I’m comparing the paper circuit of the Tetrazzi to the schematics of the Sidrazzi drawn by Peter as shown in the Sidrazzi assembly manual: https://docs.google.com/document/d/15YufBEtuCzgu2H8mu9EkYxsxM2j4sXejspT7Kb8MfSs/mobilebasic
And those of the Sidrassi traced out by Richard Brewster:
As to point #1 above, I compared the schematic Peter calls “stereo output preamp circuitry” to the paper circuit and filled in the values:
Most of the placement and values are consistent with paper circuit with the addition of some small 102 caps in the feedback of the mixers, a missing 10 uF cap on the voltage divider (value is taken from Richards schematics) and having 2 of the mixer inputs referenced to MIDO and not VCC and 0V. In this schematic, the hexagon in a circle symbol correlates with the “output symbol” and the “double output symbol” of the paper circuit. The symbol that is 1/2 a figure 8 is an output.
For my own sake, I’m going to put a name to this symbol:
And for it’s similarity to the logo for the band Einstürzende Neubauten, It’s gonna be an EN from now on. It’s partner will be the 2EN.
Next, I compared Peter’s schematic of the “variable duty push/pull oscillator” from the Tetrazzi manual to the paper circuit:
The bottom most central section is the a piezo (the circle in a circle) bar control. There is a difference here in that the schematic shows the piezo connected to ground but in the paper circuit the brass outer section is connect to a pair of diodes referenced to the input and 9v. At the top of this section, there is a hexagon node i have marked “3 star” (I’m looking at one quadrant of the paper circuit, the bottom left, and in that section the star has 3 outlines.) In his schematics, Richard notes this as Channel Out. There is a second 3 Star symbol in that quadrant that corresponds to the line going to op amp that is pointing down above it. So the star symbols within the same quadrant should be connected together. In addition, these two are connected to the same symbol in the next quadrant over.
This interweaves the bars and oscillators. Still in this bottom section, at the top of each end are the EN and 2EN connections that will be connected to the audio mixer. Just outside of these, still at the bottom are the scroll circuits with the buttons shown as a arc over two points. There is an additional 100k resistor on the paper circuit from the scroll to the base of a transistor. Above these are the UPMOD, UPTON, DONMOD, and DONTON pots and their associated components. On the right the 2Star symbol is connected by a trace on the paper circuit to the 4051 at pins 12 &14 (and will be also be connected to a 2Star in the previous quadrant.) On the left, the 1Star is similarly connected by trace to pins 1 &15 of the 4051. The 4Star connection to pins 5 & 2 comes from the next quadrant. Put simply: connect all five of the same star together.
Above and below the 4051 are the Cross connections which I believe are the touch points on the Tetrazzi bars in the build manual. The one beneath the 4051 is called Pulse Out in Richard’s schematic and 2 of the three others might be the Glitch Points. I’m not sure about the other. I’m also not certain if these get connected internally like the stars but i suspect not.
The final thing that I’m uncertain of is the connection of the EN and 2EN symbols to each other. Each quadrant has two EN and two 2EN outputs and the mixer has two EN and two 2EN inputs. The outputs are so close together, they could be easily connected at the quadrant if they were meant to go to the same location. One way would be to connect all 4 Left EN outs to the Left EN in, 4 Right EN outs to the Right EN in, etc. or some additional interweaving that I can’t imagine right now. Thoughts?