Bill and Will's Synth
MOTM 101 Construction
"Noise Generator - Sample and Hold"

April 2007 -

The MOTM 101 has a "difficulty factor" of 2 so we figured we'd tackle it now that we've got some experience with the 900, 830, and 800s with 800DBs.  As a point of chronological fact, when we began our 101, we weren't done with our 800s.  But we decided to begin our 101 anyway (we had to go out and get the drills Dave Brown recommended to make the holes for the LEDs in the 800 face plates).

For those who are constructing a 101, please check out Larry Hendry's MOTM-101 construction photos by way of cross-reference.

Table of Contents

This page has become really long, so here's a table of contents that we hope will make it easier to traverse:

Background - presents an explanation and Paul Schrieber's initial description of the Module with a couple photos from Larry Hendrey

Parts - presents a Bill of Materials for "two-dot-oh" builders and notes about it

Panel - presents the MOTM format panel

Construction Phase 1 - Resistors, Capacitors, IC Sockets, Power Plugs, MTA headers

Construction Phase 2 - Trimmers, Panel connections

Set up / Testing

Background

Paul Writes:

"The MOTM-101 is an essential part of any synthesizer system. It is a dual-function module, where the Noise Generator is linked to the Sample/Hold section without the use of patchcords. Each section can operate independant of the other.

"NOISE GENERATOR

"The Noise Generator uses 2 Zener diodes to create White Noise. This White noise contains spectral harmonics from near DC to well above hearing. This type of noise is useful for simulating cymbals, ocean waves, static, and other types of "nature" sounds. It is also useful in generating random signals into the Sample & Hold, and for aperiodic, high-speed clocking.

"The White Noise is low-pass filtered at approx. 2500Hz to create Pink Noise. Pink Noise is useful for lower frequency sounds such as thunder. The Pink Noise is filtered even more to generate Random Output, which can randomly modulate filters and VCOs.

"These three types of noise are found on many synthesizers. However, look closely and you will see the jack labeled VIBRATO, and it's associated control pot! This is a MOTM exclusive: a high-Q bandpass filter (the panel control controls the Q) that generates sine waves centered around 7Hz of random amplitudes (and slightly varying frequencies as well). On the panel, a switch selects driving the bandpass filter with either white noise or the internal/external clock from the Sample & Hold section. When the panel switch selects white noise, the vibrato output is a 'wandering' sine wave, perfect for adding realistic vibrato and tremelo. If the switch is in the clock position, each time the clock cycles the output filter rings in a dampened sine wave (the ringing amount set by the panel control). When fed to a VCO, the VCO make a unique "bbbooooiiinnngggggg" sound.

"SAMPLE & HOLD

"A Sample & Hold circuit is a sort of voltage memory: each time the clock generator cycles, the singal on the INPUT jack is captured and stored on a special capacitor. This voltage is buffered and fed to the OUTPUT jack. Even with many seconds between samples, the output voltage remains nearly constant (since no circuit is perfect, the output voltage will droop, but at a miniscule rate on the order of 1mv per second). The MOTM-101 contains an internal clock oscillator, whose speed is set by the panel control. A red LED flashes at the clock rate. Plugging another clock source (VCO, LFO, etc) into the EXTERNAL jack over-rides the internal oscillator.

"Again, a Sample & Hold is a part of all synthesizers....but wait! Hmmmm...what's that switch marked Track/Hold? Another MOTM exclusive! In the HOLD position, the circuit acts like a "normal" S/H: the output is held steady at the start of each clock cycle. But, in the TRACK position, the output follows (tracks) the input when the clock is low (less than 1.5 volts) and holds the output when the clock is above 1.5 volts. If you are familar with TTL digital logic, this mode is referred to as a 'transparent latch'. From a musical standpoint, this can generate more unique sounds than other S/H modules.

"The SLEW control adds portamento or glide to the S&H output. It can be adjusted from zero to 10 seconds."

photo from Larry Hendry

Parts

In 2008 (or thereabouts), Synthesis Technology stopped producing full-blown kits, and moved toward what Paul calls "2.0" (two-dot-oh) DIY. This assumes the builder will buy certain parts from Synthesis Technology - PCB, Panel, and in some cases a Special Parts Kit of the particularly hard to find parts - and will get the rest of the parts from Mouser or Digikey or - well - wherever.

For those who are building this as a "two-dot-oh" project, Will and I, with feedback and review from others, have developed a parts-list / bill-of-materials in the form of an XL spreadsheet (as usual).

Please don't take it as gospel. We've been over and over it and are relatively confident in our specifications - and we hear that several people have used it successfully so you should be good.  The BOM assumes that you get the "extra parts kit" from Synthesis Tech.  Synthesis Technology offers some parts like pots and knobs at particularly good prices... these options are offered in the BOM.

Click here to download our XL spreadsheet Parts List

Panel

If you're building this as a "two-dot-oh" project, we also assume you get the panel from Synthesis Technology:

Construction Phase 1

ready to begin

per Scott Juskiw's modification, we solder a 27K resistor at R11

resistors are in

As usual with us, whereas we are vigilant about orienting all the resistors, caps, etc. consistently so their values can be read easily (in case we need to trouble-shoot them later), we oriented the resistors with the "Tolerance" stripe on the left (relative to the text on the pcb).  Why did we do it this way?  'Cause the gold stripe is so pretty and easy to see (of course)... and so we put it on the left - well - just because.  You might want to do it the opposite way.  (For the table of resistor value markings click here.)

the PCB washed and ready for the Caps

the box caps go in

all caps are in!

Construction Phase 2

Here the wires and pots are in

the face plate with the jacks and switches in place

PCB mounted

The photo above shows the mounted PCB ready to go onto the face plate.  Per Paul's instruction, the screws that attach the PCB to the mounting plate are loose.

When we tried to get the 4 PCB mounted pots through the holes in the face plate - they wouldn't go through.  So - I took my rat tail file and ran it through each of the holes a few times.  It looked like it took off just some of the paint but on second try, the pots slid through easily though snugly.  For more detail, click here

So we proceeded -

and the PCB mounted just fine

switches soldered

We had some heat-shrink tubing we got from Radio-Shack so we put it on the switch wires like it shows in Larry Hendry's photo.  The Radio Shack heat-shrink isn't the same as Paul supplies - I like Paul's better.  It can be gotten at Mouser Electronics.

the LED is soldered, the heat-shrink has been slid down over the joints

shrinking the heat-shrink

the first three jacks are soldered

all the jacks are soldered - switch and LED leads are tied

all the jacks are soldered - switch and LED leads are tied

all tied up

ready for knobs

done!

Set up / Testing

Use Notes