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For those of us without perfect pitch, it’s nice to have a standard to which we can tune the instrument. It’s also nice to have an easy way to listen to the music. This module provides both functions. It has a reference tone which can be used for tuning and some jacks which facilitate listening to the reference and/or the synthesizer either on headphones or speaker.
From the Synthasystem owners manual:
This module can be used as a final step through which the signal is fed before it goes to the audio sound system or studio mixer as shown in the basic set up in figure one. Its sole purpose is for convenience in tuning the system and to provide a stable pitch standard which can be trimmed to match other pitch standards such as piano. oboes, etc.
This module will do the following when the selector switch is in the positions listed.
Switch position: Function
OUT: Synthesizer signals fed into input jack go straight through this module unaltered in any way to the output jack.
MON: You can hear the synthesizer signal fed into input jack only through a set of mono headphones (eight ohms) plugged into the tuner. There is no output from the output jack.
TUNE: The synthesizer signal fed into input jack can be heard only through the headphones and is mixed with a concert C (523.3 Hz) for the purpose of tuning during a live performance with no output at the output jack.
T OUT: The synthesizer signals fed into the input jack go to the output jack mixed with a concert C for the purpose of tuning in a studio situation.
This module has one audio input. It is designed for line level audio and is compatible with the audio modules in the Synthasystem
There are two outputs, one is intended for a 1/4 inch headphone jack, the other is suitable for connecting to other equipment such as a mixer, speaker, or other gear.
The input/output of this module is mono. However, I suggest you use a stereo jack for the PHONES and wire the tip and ring togetehr so you get an image in both ear pieces. That said, this unit is setup to drive an 8 ohm impeadence so be sure you do not over load the module.
This module has one HEADSET LEVEL potentiometer which controls the gain of the signal sent to the PHONES jack. It does not affect the signal level on the OUT jack.
There is also a trimmer on the panel. This adjusts the frequency of the reference signal. Set this to a concert C, 523.3 Hz.
There is one rotary switch. It selects the output mode for the module. See the above table for a description.
Hooking it up is pretty simple. Connect the output of the last stage of your patch to the input of this module. plug a set of headphones into the PHONES jack or connect the OUT to an approprite mixer, speaker, or other audio device. Select the output mode you want and adjust the gain as necessary.
This module was originally built with carbon core, 5% resistors with one or two 1% metal film resistors. So, you have a wide range of options here. I recommend using 1% tolerance, metal film resistors everywhere.
There are probably a billion different ceramic capacitors at a place like Mouser. Pick a capacitor that can fit the hole easily, typically 0.1 inch on centers.
Pick good quality electrolytics where designated.
C1 should be a good temperature stable capacitor, Polystyrene or comparable.
The original used 2N5172 NPN transistors. These are still available at Mouser, but I recommend you get the Central Semiconductor ones. You can also use pretty much any small signal transistor with fine results.
PN5138s work fine and are what was in the original, or another good small signal PNP transistor.
Nothing special, 1N4148s are fine.
Your choice for your panel. If you use the panel I laid out, the holes and spacing will work for the Alpha 12 and 16mm pots. You can probably use nicer BTI, Bourns, etc. 9mm pots with “pot chiclets”
First, R4 may need to be changed. In my build, the 25k wasn’t large enough to let me set the Reference Tune to 523.3 Hz. Making this trimmer bigger, 100k for me, let me tune to and past 523.3 Hz. If you can’t tune to 523.3 Hz, you may want to experiement and make this trimmer bigger.
Use good trimmers, please, really on this PCB use good trimmers. A good Bourns multi-turn trimmer like Bourns 3296Y series will fit the pad layout and works well.
The original modules used single turn, carbon trimmers. You can get a Bourns model that will fit the pad pattern if you want to. They cost about as much as the nicer multi-turn pots, though. It’s a Bourns 3352H series has the appropriate pin spacing/configuration. The 3329H and 3329W series are sealed versions which will work, too.
For the panel mount trimmers, I made a little PCB chiclet like the ones above to make this easier. I suggest using a Bourns adapter, p/n H-83P. It has plastic 5/16 inch threads, so the hole is 0.3125 inches in diameter. It seems to work with any standard 3.4 inch multi-turn trimmer. Insert the trimmer and push it til it snaps into place.
Another option is a Vishay part p/n 006-1-0 or 006-1-1, or 006-1-2, or 006-1-3. It depends on what type of shaft you want to have. These have a smaller, metal bushing. A 0.22 inch hole works fine. They are a bit smaller than the Bourns and about 3 times as expensive. $5.00 compared to about $1.50. If you use this one, I’d also recommend getting the Vishay trimmer adjustment tool. It’s made to fit down the hole if you don’t buy the adapter with a shaft. No Vishay part number. Mouser p/n 594-8T000.
The Bourns trimmers which work are the 3006 series. Easy to find.
The Vishay trimmers which work are the model 43P 3/4″ 20 turn series. Easy to find.
For the panel I laid out, a good 3.5mm or 1/8 inch jack will work. I use the Switchcraft 42A Tini-Jax true 1/8 inch jack. These are switched jacks and they work with 1/8 inch plugs and 3.5 mm plugs.
I’m using an Electroswitch rotary switch for the mode selection
I assume you know the basics of soldering. I like to insert the low lying parts first, like resistors, diodes, etc. After these, I install the IC sockets. Next capacitors, transistors, connectors. Use a good solder, either an organic flux, which you should wash regularly, or a no-wash flux.
Take a break every so often, wash off the flux if you are using a flux which required cleaning. Double and triple check orientations, pins, and solder joints.
Power Supply Regulation/Filtering:
Some additional comments here. These modules are tested to run on +/-12 VDC. The original power supply in the Synthasystem was +12/-10 VDC due to how Nyle designed the -10 volt section of the power supply, not for any magical requirement to have -10 volts.
The power/regulation section has 2 voltage regulators on it which can be set to +12/-10 (or +/-12 volts) depending on your needs. If you are coming from +/-15 volts, you need both regulators and you may as well set one to -10 volts.
If you are coming from +/-12 volts, technically you don’t need the regulators, but if you want, install the negative one and set it to -10 volts. The LEDs are not strictly needed. They are there to establish a base current draw so the regulators will work.
Important… if you don’t install the regulators, you have to install a jumper between pins 2 and 3 as shown on the Power/Regulation PCB or you won’t get power.
This PCB has four holes to allow flexible mounting configurations.
D1 and D2 need to be in thermal contact with Q2 and Q6 respectively. The PCB is laid out to accomodate this. Just stretch the diodes over the transisors and put a drop of heat sink grease between the diode and transistor.
There is one trimmer which sets the crossover distortion. Input a sinewave and adjust this trimmer to minimize the distortion seen at the headphone output.
There is nothing too special. I suggest using connectors on the PCB and jacks on the flying wires. The spacing and holes are setup for Alpha 16 mm or 12 mm pots. The jack holes are 0.25 inch in diameter.