FUNCTION:

This module Is a pre-amplifier for low level audio sources. It has a low/high gain and RIAA equalization.

From the Synthasystem manual:

The input amp acts as a preamplifier to use when a microphone, guitar or similar signal is to be fed into the system. The preamp brings these signal levels up to a good working level for the system.

Three different jacks are in parallel to provide ease of interfacing with different kinds of mike plugs, patch cords, etc. One simply chooses the appropriate input jack which fits the cable being plugged in.

The preamp output appears at two output jacks which are also in parallel.

In addition to the gain control knob on the front panel there is an overall gain switch. In the LO position, the max gain is 30dB; in the HI position, the max gain is 56 dB.

A switch is also provided which when in the RIAA position will equalize the preamp for use with a magnetic phonograph cartridge output. In all other uses, this switch should be in the position marked flat.

USE:

Inputs/Outputs:

This module has one input which is tied to three different jacks. DO NOT CONNECT MORE THAN ONE JACK AT A TIME. There are two parallel outputs which will be amplified for use in the synthesizer.

Knobs:

This module has a gain knob.

Switches:

There are two switches, one for selecting HI (56 dB max) or LO (30 dB max)  gain. The other is to select either a flat equalization or RIAA (for phonograph output). I know, ancient, but hey, this was designed in the 70s. My brother was just developing digital audio with Tom Stockham and CDs were years away.

GENERAL CONNECTIONS:

Hooking it up is pretty simple. Connect a low level audio input like a mic, phonograph, or your own sensor, to the appropriate input. Adjust the gain switch, knob, and equalization appropriately.

COMPONENT SELECTION:

Resistors:

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.

Capacitors:

There are probably a billion different ceramic capacitors at a place like Mouser. Pick a capacitor that can fit the hole easily, typically 0.2 inch on centers.

Pick good quality electrolytics where designated.

Inductors:

None

ICs:

LM741s. There are better opamps, but for me the idea is to restore, not hot rod. You are welcome to use your favorite opamp here. Pick one that is pin/operation compatible.

Pots:

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”

Trimmers:

Use good trimmers, please. A good Bourns multi-turn trimmer like Bourns 3296Y series will fit the pad layout and work well.

Jacks:

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.

This module should also have a couple of other jacks like a 1/4 inch, RCA, Banana, or what you want to use.

Switches:

Use good quality SPST or SPDT switches.

BUILD NOTES:

Soldering:

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.

COMPONENT PLACEMENT:

Mounting:

This PCB has four holes to allow flexible mounting configurations.

CALIBRATION:

None.

PANEL NOTES:

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.