Synthesis Technology MOTM-110 Ring Mod & VCA User manual

MOTM-110 Ring Mod & VCA
Assembly Instructions
Synthesis Technology
6625 Quail Ridge Dr.
Fort Worth, TX 76180
(888) 818-MOTM
www.synthtech.com
June 3, 1999

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MOTM-110 PARTS LIST
Please carefully check that all parts are in your kit. If you have a suspected shortage,
please call or email. If you get free extra stuff, keep it for next time.
Capacitor bag, containing the following 19 parts:
3ea 10mfd, 50V Electrolytic C1, C4, C5
2ea 150pf (151) ceramic axial C2, C3
3ea 0.47mfd (470n) yellow box polycarbonate C6, C7, C8
3ea 0.01mfd (10n) yellow box polycarbonate C9, C10, C11
5ea 0.1mfd (104) ceramic axial C12, C13, C15-17
2ea 22pf ceramic axial C14, C19
1ea 0.001mfd (1n) yellow box polycarbonate C18
Resistor bag, containing the following 27 parts:
6ea 100K 5% (brown, black, yellow) R8, R9, R11, R13, R14, R16
5ea 1K 5% (brown, black, red) R3, R12, R21, R23, R24
3ea 56K 5% (green, blue, orange) R1, R4, R27
2ea 150K 1% (brown, green, black, orange) R2, R7
2ea 470K 5% (yellow, violet, yellow) R6, R25
2ea 200K 5% (red, black, yellow) R10, R22
2ea 300K 5% (orange, black, yellow) R15, R19
1ea 39K 5% (orange, white, orange) R5
1ea 27K 5% (red, violet, orange) R26
1ea 2M2 5% (red, red, green) R17
1ea 6K8 5% (blue, gray, red) R18
1ea 680 ohm 5% (blue, gray, brown) R20
IC bag, containing the following 5 parts:
2ea OP275GP op amp U2, U3
1ea CEM3330 dual VCA U1
2ea 1N4148 signal diode D1, D2
Misc #1 bag, containing the following 9 parts:
2ea Axial ferrite beads (plain, gray things) L1, L2
2ea SPDT toggle switch SW1, SW2
3ea 100K cermet trim pot TP1 - 3
1ea 18pin DIP socket for U1 @U1
1ea MTA-156 power connector JP1
Knobs, 5ea, ALCO PKES90B1/4
Jacks, 6ea Switchcraft 112A

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PAGE 2 MOTM-110 PARTS LIST
Pots, containing the following:
4ea 100K linear, Spectrol 148 VR2 - 5
1ea 100K linear, Bourns 91 (panel mount) VR1
Front panel
Mounting bracket
Wire bag, containing the following 9 wires:
5ea RG-174 coax, 4 1/2 inches
2ea red/black twisted pair
1ea orange/white twisted pair
1ea blue 24ga. wire
Hardware bag, containing:
4ea #8-32 x 3/8 black screws (for mounting module to rack)
4ea #6-32 x 1/2 zinc screws (for attaching pc board to bracket)
4ea ¼ inch aluminum spacers
6ea #6 KEPS nuts (2 for attaching bracket to front panel, 4 for pc board)
3ea small heatshrink tubing
5ea small tie-wraps
Organic Solder
No-clean Solder
PC Board, MOTM-110
Power cable

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GENERAL INFORMATION
Thank you for purchasing the MOTM-110 Ring Mod & VCA kit. If you have any issues
concerning the building or use of the kit, please contact us at [phone] or by email:
[email protected]
This kit should take the average builder between 2 and 3 hours. However, please
remember this is NOT a speed contest, it is an accuracy contest. There is no rule that you
have to complete the entire kit in one session (as long as you wash the flux off!).
Successful kit-building relies on having the proper tools. Here is a list of what you will
need to build and calibrate your MOTM-110.
* Soldering iron, 50W max power
* Needle-nose or chain-nose pliers
* Diagonal cutters
* Wire strippers
* Small, flat screwdriver for adjusting the trimmers
* Allen key set for securing the knobs
* Lead bending tool (optional, but makes the job go much faster)
* DVM or oscilloscope (optional for calibration. Can calibrate by ear)
* Heat gun for heat-shrink tubing (optional, but HIGHLY recommended)
For more information of tools used and suggestions, see the MOTM FAQ at
www.synthtech.com
HOW TO FOLLOW THE DIRECTIONS
Please read the entire instruction before proceeding. There may be valuable information at
the end of the instruction. Each instruction has a check box next to it. After you
complete the instruction, check the box. This way you can keep track of where you are in
the process.
VERIFY THE PARTS LIST
Verify that all of the parts are in the kit as shown on the parts list.
A WORD ON SOLDERING
There are 2 very different types of solder used in the kit. Most of the soldering uses
‘Organic Flux’ solder. This is strictly for use on the pc board, and is NOT to be used
on the front panel wiring!
In order for solder to ‘stick’ to the copper, a chemical called ‘flux’ is embedded in the solder.
The flux leaves a residue on the pc board that should be cleaned with warm water. DO
NOT USE SOAP OR OTHER CLEANSERS. Most of the parts in the kits are ‘waterproof’,
and can be washed in the sink. The flux is OSHA approved for flushing down the drain, so

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don’t worry about that! A soft brush is used to gently scrub the board. We recommend a
‘fingernail brush’, which is about 1” x 2” and be found for about $1.
The other type of solder is called ‘No Clean Flux’, because as the name implies it does not
require washing. This solder is used on wiring the pots, switches, jacks, etc. This solder is
harder to use on the pc board, because even when melted it is not very fluid (about the
consistency of toothpaste). We will use it VERY SPARINGLY on the pc board.
OK, let’s get started on the board!
PART #1: SOLDERING THE RESISTORS
Since there are more resistors than anything else, we will start here. If you do not know
the resistor color code, refer to the parts list. Resistors are not polarity sensitive, but the
board will be easier to debug (and look nicer) if you point the first color band in the same
direction for all the parts. The color code is also in the README FIRST document that
every customer gets with each order.
Find the RESISTOR bag.
Find the MOTM-110 blank pc board. There is a copy (larger than actual size) of the
silkscreen which shows where the parts go at the end of this document. It will be
useful if you locate the part on the print first, put the part in the board, then
‘check off’ the silkscreen. All parts are inserted from the side of the board with the
white silkscreen (the “top” side).
We will stuff the resistors by value to make things easier. The resistors (and other
long-leaded parts) are inserted on 0.4 inch spacings. The important thing is to be
sure that the part is sitting all the way down on the board. Push the leads in the
holes, push the part on the board, and then bend the leads on the bottom outwards
to a 45 degree angle (roughly!). This is called ‘cinching the leads’: keeps the part
from falling out! From the bottom of the board, solder (with the organic flux),
applying heat to the pad for about a half second first, then applying just enough
solder to make a small, flat puddle.
The rule of soldering: don’t use too much, you can always add more! Cut the leads
flush with the top of the solder.
Locate the 56K resistors and solder into R1, R4 & R27.
Locate the 150K 1% (they look different than the others) and solder into R2 & R7.
Locate the 1K resistors and solder into R3, R12, R21, R23 & R24.
Locate the 100K resistors and solder into R8, R9, R11, R13, R14, & R16.

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Locate the 470K resistors and solder into R6 & R25.
Locate the 200K resistors and solder into R10 & R22.
Locate the 2M2 (2.2Meg) resistor and solder into R17.
Locate the 6K8 (6.8K) resistor and solder into R18.
Locate the 300K resistors and solder into R15 & R19.
Locate the 680 ohm resistor and solder into R20.
Locate the 39K resistor and solder into R5.
Locate the 27K resistor and solder into R26.
PART #2: BOARD WASH #1
Verify all the resistors are in the correct position.
Verify all the resistors are flat on the board. Correct if needed.
Wash the board in warm water, gently scrubbing both sides.
Shake the board a couple of time, blot dry with an old towel (the leads will frazzle
the good towel). Let dry about 15 minutes.
PART #3: CAPACITORS
Locate the CAPACITOR bag.
Locate the 150pf axial ceramic caps. They are marked 151. Solder into C2 & C3.
Locate the 22pf axial ceramic caps. They are marked 220. Solder into C14 & C19.
Locate the 0.1mfd axial ceramic caps. Some kits have these pre-cut and bent to the
correct spacing. Solder into C12, C13, C15 - C17.
Locate the 0.001mfd yellow box cap. It is stamped 102 on the top. Solder into C18.
Locate the 0.01mfd yellow box caps. They are stamped 103 . Solder into C9, C10,
C11.

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Locate the 0.47mfd yellow box caps. They are stamped 474. Solder into C6, C7, C8.

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Locate the 10mfd electrolytics. Note that there is a stripe on the NEGATIVE
terminal. The pc board has a + on the POSITIVE terminal. Carefully stick the
capacitors into C1, C4 and C5 with the stripe away from the + pad on the board.
PART #4: MISC STUFF
Almost done with the parts on the pc board! This will finish up the soldering with the
organic flux.
Locate the MISC #1 bag and the IC bag.
Locate the ferrite beads. They are axial parts, gray colored with no markings. These
are non-polar, and are soldered into L1 and L2.
Locate the 18 pin DIP socket. Solder it into the U1 location (CEM3330). There is a
small notch on one end of the socket: this is the ‘PIN #1’ end. Locate U1 on the pc
board. Note that Pin #1 is the square pad, and there is a notch on the silkscreen.
Locate the MTA-156 power connector. Solder into JP1. Note that the connector has
a ‘locking tab’ on one side. This side is the “inside” facing relative to the pc board.
Note the silkscreen symbol for JP1 has a line on one side, indicating this is the side
where the locking tab goes.
Locate the OP275 op amps. Solder into U2 and U3. Note that Pin #1 is the square
pad. Pin #1 is the pin near the very small ‘dot’ in the top of the part. They are not in
sockets.
Locate the 1N4148 signal diodes. These are clear glass axial parts with a black
band around one end. Locate D1 and D2 on the pc board. Notice that the silkscreen
symbol has a distinct band on one end. Insert the banded end (called the cathode)
into the pc board in the correct pad.
PART #5: WASH THE BOARD AGAIN
Verify all the parts are in the correct locations. Check the diodes and C1, C4 & C5
orientation.
Inspect the solder joints. Any solder shorts? Too much solder? Missing joints?
Wash the board under warm water. Scrub gently. Dry.
THIS IS A GOOD STOPPING PLACE TO REST OR PUT THE KIT AWAY UNTIL
LATER.

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You are now finished with the Organic flux solder. All soldering past this point is
using the No-Clean solder. You do not have to wash the board anymore.
PART #6: FINISHING THE PCB
You will now solder in the remains parts on the pcb in preparation for wiring to the front
panel. USE THE NO-CLEAN SOLDER. BE CAREFUL!
Locate the 3 blue trim pots in the MISC #1 bag. Solder into TP1, 2, & 3.
Locate the 4 Spectrol #148 pots (metal shafts). They are all 100K. IMPORTANT:
in order for the pc board to properly align into the front panel, each pot must be
absolutely flat on the pc board, with the shafts pointing away from the pc board.
Solder the pots into VR2 - 5. Clip the leads off.
Locate the WIRE bag. Note that the pre-stripped wires all have a short end and
a longer end. THE SHORT END GOES IN THE PC BOARD.
Locate the white/orange twisted pair. Solder the WHITE wire into JP2, in the hole
marked GND (square pad).
Solder the ORANGE wire short end into JP2, in the CV hole.
Locate one of the red/black twisted pairs. Solder the RED short end into S1,
into the top hole (round).
Solder the BLACK short end into S1, bottom hole (square).
Solder the other red/black twisted pair as before into S2.
Locate the 5 pieces of RG-174 black coax cable. Again, note that one end has
longer wires stripped than the other. The short ends will go in the pc board in
positions J2 to J6. Look at the pc board. Notice that in positions J2 - J6, there
is a large hole pad (lower pad) and a smaller pad (top hole). The braided wire
is soldered into the larger hole. The smaller, inner conductor goes in the top
hole.
Note there is a row of ‘plain’ holes along the bottom, under J2 - 6. These holes are
for threading the small white tie-wraps for holding the coax against the pc board. The
holes are designed so that coax exits the pads from the right side. The coax then lies
in between the 2 holes. The tie wrap enters the left hole from the top, comes out
the right hole from the bottom, and is then secured so that the coax is tightly
held against the top of the pc board. Note that the tie wraps must thread from the
top, or there may not be sufficient room between the pc board and the mounting
bracket. The excess tie wrap is cut off.

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Solder, then tie-wrap the 5 coax cables J2 - J6. Trim the excess tie-wrap. NOTE:
because of the way the coax is made, chances are more of the braid will go in the
hole than the inner conductor. This is normal, unlike all the other parts and
wires. The secret is to get the coax firmly soldered and tied down.
YOU ARE NOW FINISHED WITH THE PC BOARD WORK! BREAK TIME.
PART #7: FRONT PANEL PREPERATION
You will now attach components to the front panel. It is HIGHLY recommended that you
use a set of hollow shaft nut drivers, NOT PLIERS, to tighten the nuts. This prevents
scratching. NOTE: all references to part orientation is from the REAR of the panel.
Locate the 6 Switchcraft jacks. Notice that from the rear, there is a beveled corner.
This corner is ALWAYS CONNECTED TO GROUND, USUALLY WITH THE
BRAIDED CONDUCTOR. Each jack has a flat washer, a lockwasher, and a ½” hex
nut. Remove the nuts and washers from each jack. Place aside. Keep the lockwasher
on the jacks.
Insert the 6 jacks/lockwashers, with the beveled corner in the upper right corner,
into the 6 holes. Place the flat washer on the jack, then the hex nut. Hold the jack
with one hand on the backside, keeping it ‘square’. Tighten the hex nut with a nut
driver. NOTE: when tight, not much of the exposed threads of the jack are exposed.
Check to see that each jack has the bevel in the upper right corner, and all jacks are
neatly lined up.
Locate the 2 SPDT toggle switches. Remove the hex nut (keep the lockwasher on),
stick the switches in the holes labeled AC/DC .Place the hex nut on the switch and
tighten. Be sure the switch is straight up and down: it is easy to get it slightly
off-center.
Locate the 100K Bourns 91 pot. It is blue. Remove the first hex nut, keeping the
lockwasher and second hex nut on the pot. Insert the pot into the upper right hole,
with the 3 leads pointing down towards the jacks.
CAREFULLY, tighten the hex nut, holding the pot with one hand to keep the 3
leads pointing downwards. DO NOT OVERTIGHTEN!
You are now ready to attach the pc board to the bracket, and then wire up to the panel.
See the illustration at the end of the document.

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PART #8: ATTACH PC TO BRACKET/PANEL
In the HARDWARE bag, locate 4 #6-32 x ½ screws, 6 #6 KEPS nut, and 4 spacers.
Locate the mounting bracket. The pc board attaches to the bracket, with the 4
screws threading from the top of the board, through the spacers, through the
bracket, and then out the bottom of the bracket. The #6 KEPS nut attaches on the
bottom of the bracket. Note the bracket has 2 long mounting flanges with a hole
in each. These attach to the 2 threaded studs sticking out of the rear of the panel.
The 4 pots each stick in it’s panel hole when the bracket is screwed down on the 2
threaded posts.
Attach the pc board to the bracket. The 2 mounting flanges are located in between
VR2 &3, and in between VR4 & 5. Note that the bracket holes for the pc board are
actually oblong. IMPORTANT: for the MOTM-110, slide the pc board all the way
to the right, so that the edge of the pc board is touching the 2 mounting flanges.
(Note that other MOTM modules will mount a slightly different way, so read the
directions for each kit carefully!!). Tighten then 4 KEPS nuts on the bottom.
THIS IS A VERY IMPORTANT STEP, SO PAY ATTENTION AND READ ALL OF
IT BEFORE PROCEEDING!
Note that each of the 4 pots on the pc board have 2 hex nuts and a flat washer.
Remove the first hex nut and the washer. Set aside.
What you will do now is adjust the remaining hex nuts so that when the bracket is
all the way down on the threaded studs, all the hex nuts touch the rear of the
panel. Note that the MOTM-110 is the only kit where this procedure is required.
Other kits will have a different procedure.
Screw (by hand) each hex nut on the pots so that it is all the way on (touching
the face of the pot). Now, pick up the pc board/bracket assembly and carefully
slide it over the 2 threaded studs, making sure the pots are aligned in the holes.
If you closely look at the pots, you will notice that when the bracket is firmly
pressed
flush against the panel, there is a gap of about 0.050 inches between the panel and
the pot hex nuts.
Remove the bracket off the studs. For each pot hex nut, unscrew it about 1 and ½
turns. Eyeball the hex nuts to see that each one is the same distance from the
rear of the pot (in the US this is about the thickness of a dime). Place the bracket
back on the studs. Make sure each pot hex nuts is touching the panel. If not, use a
finger to unscrew the hex nut. Be sure you are holding the bracket all the
way down on the studs.
Now use 2 of the #6 KEPS nuts to tighten the bracket to the panel.

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For each pot, place the flat washer on the threads, then the hex nut. Tighten.

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PART #9: FINISH WIRING TO THE PANEL
Attach and solder the RED wire on S1 to the top terminal of the ‘Y’ switch.
Attach and solder the BLACK wire on S1 to the middle terminal of the ‘Y’ switch.
Attach and solder the RED wire on S2 to the top terminal of the ‘X’ switch.
Attach and solder the BLACK wire on S2 to the middle terminal of the ‘X’ switch.
In the WIRE bag, find the single piece of blue wire.
Cut off a 1.5” inch piece of the wire. Strip the ends back about a ¼ inch. You
will solder this wire between the 2 ground lugs on the CV jack and the X IN
jack. Looking from the rear, these are the top right and bottom right jacks.
Recall the ground lug is the BEVELED corner one. Solder the wire between the
2 jacks.
The remaining piece of wire goes from the CV MOD pot (the one on the panel,
not on the pc board), on terminal #3 (the one on the left), to the CV jack input
lug. On all the jacks, the input lug is the left one, the switched lug is the top one.
NOTE: on the MOTM-110, NONE of the switched lugs are used. You should not
have any wires connected to the top lugs.
First, strip about ¼ of an inch off one end of the wire, and solder it to the left lug
of the CV IN jack. Take one of the small pieces of heat-shrink tubing and place it
on the wire, sliding it down about halfway.
Now strip the other end of the wire and solder it to the #3 terminal (leftmost) of
the CV MOD pot. Make a small hook (like the letter J) in the wire and slip it
through the hole in the lug. Apply solder and allow the wire to cool (steady hands
are required!).
NOTE: you may need to trim the length of the wire for the best “fit”.
Untwist about 2 inches of the orange/white wire. Slide a piece of heatshrink on the
orange wire. Solder the orange wire to the #2 (middle) terminal of the pot.
Slide a piece of heatshrink over the white wire. Solder the wire to the #1 (right)
terminal.
Now you need to slide each piece of tubing over the terminal, all the way up until
it touches the pot. Shrink the tubing with a heat gun. If you do not have a heat gun,
then one at a time slide the tubing up, and without actually touching the tubing,

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place the soldering iron close to the tubing. The radiant heat should close the
tubing.
The last thing to do is to solder the coax cables to the jacks. You will start with J6,
and work backwards to J2. In this manner you will solder the bottom row first.
Solder the coax on J6: this is the RING MOD OUT (lower left jack). Solder the braid
to the ground lug, and the conductor to the left lug.
Solder the coax on J5: this is Y IN (lower middle jack).
Solder the coax on J4: this is X IN (lower right jack). Note that there are 2 wires on
the ground lug: the braid and the previously soldered small white wire.
Solder the coax on J3: this is the VCA OUT (top right jack).
Solder the coax on J2: this is the VCA IN (top middle jack).
Insert the CEM3330 IC into the socket. Place Pin #1 in the lower right socket pin
(the part faces downwards like the other chips). Be sure the chip is all the way
seated, and no pins are bent. Note that the notch on the chip matches the notch
on the silkscreen.
Rotate all of the front panel pots fully counter-clockwise. Locate the KNOBS.
Notice each knob has a thin white line on it. Place the knob on the pot shaft,
align the white line to the ‘0’ tick mark (on the GAIN pot, align to the word
‘BIAS’), and tighten the hex screw. The silver part of the knob has a protective
clear plastic overlay that can be removed if desired. Gently rub with your
fingernail and it will peel off.
************************************************************************************
CONGRATULATIONS! YOU HAVE FINISHED BUILDING THE MOTM-110!
************************************************************************************
All that’s left to do is to calibrate the unit for optimum performance.
METHOD #1: CALIBRATE BY EAR ONLY
Using a small, flat screwdriver turn the white screw part of each trimmer until it is
half-way. This is when the slot is vertical.
You will not adjust the CV REJECT trim, as this requires a DVM or scope.

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Turn the UNBALance pot and the Y IN pot fully counterclockwise (on 0).
Turn the X IN pot fully clockwise (on 10).
Set both switches to AC.
Plug a mixer/amplifier into the RING MOD OUT jack. Turn the amp OFF.
Now apply power. The module requires +-15VDC at 20ma. The power comes in JP2.
The signals are clearly marked on the silkscreen. The top pin is -15V, the 2 middle
pins are both ground, and the bottom pin is +15V. Of course, we recommend the
MOTM-900 Power Supply for powering the modules.
Apply a steady audio signal to the X IN jack. If you have a signal generator or
other VCO, apply a 1 volt peak to peak sine wave of 1-2 Khz.
Turn the amp on. Now, adjust the X TRIM trimmer for the lowest volume signal.
Turn amp off. Place the signal in the Y IN jack. Turn the X IN pot to 0. Turn the
Y IN pot to 10.
Turn the amp on. Now adjust the Y TRIM trimmer for the lowest volume signal.
This completes the MOTM-110 calibration. Your unit is now ready for use.
METHOD #2: USING A SCOPE OR DVM
Note that this is the recommended procedure to get the best performance from the unit.
Center all of the trimmers.
Turn the CV MOD panel pot to 0. Set the GAIN pot to 1.
With a DVM or scope, look at the VCA OUT signal. On a low range DC scale, adjust
the CV REJECT trimmer for 0 volts DC out. This setting should be very near the
center of adjustment.
Set the X IN and Y IN switches to DC.
Turn UNBALance, X IN and Y IN to 0.
Using a jumper wire, alligator clip, or equivalent, attach the X IN signal lug to the
+15V power supply.

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Turn the X IN pot to 2. With a DVM or scope, look at right side of R8 (square pad)
and Ground. Adjust the X IN pot until you read 5.0 volts (or close enough).

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Read the voltage on RING MOD OUT (right side of R24 or on the jack). Adjust
the X TRIM for as close to 0.0 Volts dc as you can.
Repeat the procedure for Y IN. Set X IN to 0, Y IN to 2. Apply +15 to Y IN.
Set Y IN pot to +5.0, looking at the left side of R13 and ground. Adjust the Y TRIM
for 0.0 volts dc out of the RING MOD OUT jack.
VERIFYING THE UNIT IS FUNCTIONAL
You will now verify the VCA and Ring Mod are fully functional.
Apply an audio signal to the VCA IN jack.
Feed the VCA OUT signal to an amp or scope.
Apply power. You should be able to turn the GAIN knob up and down and hear/see
the audio signal out of the VCA OUT. With the knob set to 1, the signal is basically
‘passed through’ the VCA with no control voltage (CV) applied. Note that the VCA
can apply gain (see the OPERATION section for more details).
Apply a sine wave to BOTH the X IN and Y IN jacks (a Y-cord is handy!). Set
the switches to AC, the UNBAL to 0, and the X IN and Y IN pots to 7. You should
be able to hear/see that the RING MOD out is frequency-doubled version of the
input. Turning the UNBAL pot should increase the signal level, and change the
timbre slightly.
If you do not get these results, carefully check over the unit before contacting us for
assistance.
ELECTRICAL THEORY OF OPERATION
The following discussion explains how the circuit operates, and suggests area of possible
modification. Refer to the schematic.
VCA
The VCA section uses half of a CEM3330 dual VCA. The VCA has a linear response. The
1% resistor R2 sets the bias current for the 3330, and that’s why it’s a 1% part. The audio
input is AC coupled through C8 into R4. The RC circuit of R3/C9 is frequency compensation
for the 3330. The current from R4 and the offset current of the CV REJECT trimmer TP1
and R6 are fed to the 3330.
The output of the 3330 is a current, so a high speed, low noise op amp U2A is used to
convert the current to a voltage, using R27. You can set the output voltage to cover
different ranges by adjusting the value of R27.

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The VCA is controlled by the amount of current generated by the VCA MOD pot setting,
and the GAIN setting. If the GAIN is set to 0, and the input CV is +5, then the output
voltage equals the input voltage (unity gain). This is the ‘normal’ operating mode of the
VCA. Note that the GAIN voltage can vary from -5V to +15V. When the pot is set between
the -5V and 0 V, the CV is ‘buried’. This means that in order to get unity gain, the CV input
must be +5V plus the bias voltage’. If the GAIN pot is fully counter-clockwise, it takes a
+10V signal to achieve unity gain.
See the OPERATION section for interesting things you can do with different GAIN
settings.
RING MOD
The Ring Modulator design is based on a Bernie Hutchins Electronotes article, and was
designed by Thomas Henry. Essentially, the ring mod is a VCA circuit with one clever
resistor added. In our case, the clever resistor is R10. R10 couples the X input to the
output. The Y input drives the VCA’s CV input. If you look carefully at the design, it is
almost identical to the VCA with the exception of R10.
The output of the circuit is (-X)(-Y)/5. This is due to the 2 input stages (U3A and U2B)
being an inverting summer. The X and Y input stages are identical except the Y input has
the UNBALance pot VR3 as an additional DC voltage source. Since the ring mod is a DC
multiplier, the audio tone quality will vary with DC content. That is why each input has an
AC coupling cap that can be bypassed with the front panel switches.
Again, the output of the 3330 is a current, so R22 and U3B convert this to a voltage. An
additional small cap C14 is used to remove some high frequency overshoots in the op amp
due to pcb trace inductance. The 3330 and OP275s have very high slew rates, and can
easily generate harmonics out past 20Khz.
The ring modulator generates the sum AND the difference frequencies of the 2 inputs. For
example, if we feed in a 1Khz sine wave into BOTH inputs, we get the sum (2Khz) and the
difference (0Hz). What is 0Hz?, Why, DC, of course! If you were to look at the output on a
scope, you would see a scaled (remember, the circuit divides by 5) 2X frequency sine wave
with a DC offset.
Since the musical scale is exponential, the output of a ring mod sounds very ‘metallic’ or
‘clangy’ since the sum and difference frequencies are not harmonically related. But of
course, this is EXACTLY the effect we are looking for!

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OPERATION
In this section, we will discuss how the MOTM-110 is used in various applications. Of
course, these are only a few things to try. Experiment!
VCA
NORMAL OPERATION
The VCA section is designed for a 0 to +5V control voltage, and a +-5V audio signal. When
the GAIN pot is set to 0, a +5V control voltage sets the VCA to unity gain. This is the
setting most commonly used.
‘BURIED OPERATION’
If the GAIN pot is set in the BIAS region, then it takes more control voltage to achieve
unity gain. If the pot is set fully counter-clockwise, then it takes +10V control voltage for
unity gain. However, note that this is not a SCALED input, (the CV MOD pot is used to
interface to +10 volt envelopes, for example), but an OFFSET adjustment.
Effectively,the CV is ‘ignored’ until it reaches the BIAS level, then the VCA acts normally.
Let’s go through an example.
Let’s say you had a +8V envelope signal, and the BIAS was set all the way to maximum.
This corresponds to a -5V offset. So, the +8V envelope would not have any effect on the gain
UNTIL it rose over +5 (-5 bias + (+5V) = 0 volts, which is the point the VCA starts to
operate). The VCA responds to the envelope as long as it stays above +5V. You can think of
bias as ‘clipping’ the bottom off the envelope. When the envelope drops under +5V, the VCA
shuts off. You can get some interesting-sounding tremolos playing with the settings.
‘BACKWARDS OPERATION’
The GAIN pot can be set past the ‘zero volt’ point so that the VCA turns fully on. Now, if
you apply a negative CV (say 0 to -5V) the VCA acts ‘backwards’. It attenuates the signal
as the control voltage goes more negative. Normally, one thinks of a VCA ‘amplifying’ the
signal as the CV goes positive.
The ever astute reader will then realize that if you have 2 MOTM-110s, you can set #1 for a
0 gain, and #2 for a 1 gain. Applying a +-5V peak-to-peak sine wave or triangle wave to
BOTH CV inputs makes a voltage controlled panner!
RING MODULATOR
As stated previously, the main effect of a ring mod is to generate metallic/robotic sounds.
The thing that you must be aware of is something called carrier suppression.
Ideally, the ring mod ONLY produces the sum and difference frequencies, and NONE of
the original frequencies. However, this is never the case. Circuit design limitations are

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such that we can ‘null out’ the carrier (X IN) only for one amplitude setting. For all other
input amplitudes, some of the signal ‘leaks’ to the output.
The ability to reduce the leakage is called carrier suppression. Every effort has been made
to design the MOTM-110 circuit to suppress as much carrier as possible. However,
depending on the relative input signal levels of X and Y, you may hear the input signal at
the output. The unit is not ‘broken’: all ring mod circuits have this behavior.
In fact, that’s what the UNBALance pot does: it purposely adds a DC offset to the Y
channel to allow the modulating signal (Y IN) to ‘leak’ to the output.
So, experiment! The ring mod can make a vast array of bizarre sounds.
AC versus DC Coupling
The ring mod circuit is a DC voltage multiplier. If the X or Y input has a DC offset (like a
pulse wave), this will alter the sound. The AC/DC coupling switches insert/remove a large
capacitor in series with the signal. This will alter the tone, but the effect is highly
dependent on the input signals.
The ring mod can be used as a basic VCA. Set the Y IN to DC, and the X IN to AC. Apply
the audio signal into X IN, and the control voltage to Y IN.
TROUBLESHOOTING
If your MOTM-110 does not work, please verify ALL of the following before contacting us.
The following reference directions assume that you are looking at the pc board with the
panel to the right and the power connector to the left.
All of the IC are pointing the same way: all notches are downwards.
C1, C5 and C6 stripes are both pointing upwards.
D1 and D2’s banded ends are both pointing to the left.
All the trim pots should be set almost in the center (slot is nearly vertical).
The braided wire on the coax goes to the beveled side of the jacks.
There is nothing on all 6 jacks on the top (switched) lug.
The board has all the right parts in all the right places.
No solder shorts or missing joints.