Bill and Will's Synth
Magic Smoke Electronics TH-201
"Mankato"
Voltage-Controlled Filter /
Oscillator / Slew Limiter
Construction

February 2009 -

This week, we're beginning the "Mankato Voltage-Controlled Filter / Oscillator / Slew Limiter."  We've been contemplating construction details and a couple modifications.  There's not a lot of info for idiots out there - so we'll try to fill in a bit.

We got our PCB from Magic Smoke Electronics.  We got our Panels, 2K Tempco Resistors, and the 4 pot stooge style mounting bracket from Bridechamber,

Dave Brown has a page here.

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 Tim Parkhurst's initial description of the Module with a photo

Options - presents details of the different possible implementations

Modifications - presents details of possible modifications

Parts - presents a Bill of Materials 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, Tube, Panel connetcions

Set up / Testing

Background - Ken Stone's Design

Here's what Tim Parkhurst of Magic Smoke Electronics says about the Mankato:

"Designed by well known synth DIY guru Thomas Henry, the TH-201 is a low-pass VCF with some unusual operating modes. Magic Smoke will be producing printed circuit boards (PCBs) which can provide up to eight outputs. With the Resonance control turned all the way up, these filters will self-oscillate, operating as multi-phase LFOs or VCOs.

"Both DC-coupled and AC-coupled inputs are available. The DC-coupled inputs allow the TH-201 to operate as a voltage-controlled lag processor (slew limiter). All outputs are buffered and have standard 1k impedance.

"The Mankato covers a very wide frequency range -- around 0.005Hz (200 seconds/cycle) to beyond 20kHz without range switches. Changing the timing capacitors can provide extended low frequency operation. You can certainly build your Mankato with a range switch, if that's your preference."

image by Dave Brown

Options

1.  R2 - SSM2164 Bias Mode

The SSM2164 chip has two options - it can be run in "AB" mode or in "A" mode when R2 is installed.  We're not sure yet what this really means.

2.  Inputs

Audio

The "Mankato" PCB has three possible audio inputs, we're only going to use one and so we'll install only R1a and C1a and use the right-most "JACK" and "ATTEN" Audio connections (rightmost in the photo below).

Control Voltage - FREQ CV 1 & 2

The "Mankato" PCB has three possible CV inputs, we're only going to use two and so we'll install only R42a and R42b and use the top two "JACK" and "ATTEN" CV connections (leftmost in the photo below).

Control Voltage - V/OCT

Just for clarity, the volt per octave connection will be made at the lowest "JACK" CV Input position (rightmost in the photo below) and therefore R42d will be installed as always.

Modifications

1.  Coarse and Fine Pots

Following Dave Brown's implementation for greater temperature stability, we used Spectrol 149 cermet pots for the Coarse and Fine panel controls instead of conductive plastic. (Mouser Part# 594-149-7104)

Parts

Will and I used the Bill of Materials from the Magic Smoke Electronics site.  We found everything at mouser just fine, but we got the tempco resistor from Bridechamber where you can also find the SSM2164 chip.  We got ours from digikey back before Bridechamber had them.

Click here to a copy of Tim Parkhust's BOM.

Panel

We got ours from Bridechamber:

Construction Phase 1

All the stuff in Phase 1 gets soldered using "Organic" Solder.  At every break in the action, we wash the board off to get rid of the flux.

PCB front (click on images for larger one)

PCB Back

Resistors

Here are the resistors soldered in except for R45 - the tempco resistor - and R2 which we're going to fit up with our pin and sleeve trick so we can install / uninstall it at will, thus switching the SSM2164 mode.

As usual, 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).  We got started doing it this way when we started building our synth and now we do it so all our modules are consistent with each other.  You might want to do it the opposite way - with the "tolerance" stripe on the right.

Capacitors

Capacitors are in!

Notes:

• 10µF Electrolytic - positive isn't marked on this iteration of the pcb - the "positive" side of the cap, is toward the "V15+" side of the power supply
• poly caps - omitted at this point get soldered with no-clean solder later on

ICs - Misc Stuff

soldered in - ferrite beads, diodes, 4-pin header, - when it came to the TL072BCNs and TL074BCPs, well - we couldn't find them. 

We had a few dozen regular old TL072CN & TL074CPs, but not the "B" variety.  So we soldered IC sockets where those should go.  We figured we'd use regular old TL072 & 4s and when we find the "B"s we could plug them in.

But we found the SSM2164 just fine and soldered it directly into the board which is good because that's where we're going to heat sink the tempco resistor.  Of course then we found the TL072B and TL074B.  <sigh> <shrug>

Construction Phase 2

All the stuff in Phase 2 gets soldered using "No-Clean" Solder and the PCB doesn't get washed off from here on.

Trimmers - Polystyrene Caps

Ready for the trimmers and Poly Caps

Trimmers and poly caps in - two views from the south and then the north.

R2 - Setting the SSM2164 Bias Mode

We're going to install little leaders with the great little pins and sleeves Gino Wong gave us on the 7.5K resistor for R2.  We'll go ahead and install R2, then, so the SSM2164 chip will be in "A" bias mode.  If we don't like it, we can pull the resistor out.  At this point we didn't even know how to make a judgement about it because we don't really understand the sonic implications of the two modes "AB" or "A".  But this keeps our options open.

Normally, we'd solder in the 2K tempco resistor in at this point, but it was still on order from Scott Deyo.  So we figured we'd go on to other things while we're waiting.

Bracket Layout

We took a look at how the pcb mounting bracket would attach to the panel so we can decide lengths of connecting wires.

It posed an interesting issue.  If we mounted it like we usually do - per MOTM convention on the extreme left of the panel (when viewed from the back) - we'd have to cut off the bottom two pot holes.  What if we did an unconventional mounting scheme?

 
like this...

This is how we decided to do it. This layout still places the power connector in the traditional position and it should be very stable because we can use all four pot holes to hold it on. Because of the close quarters around the bracket, we decided to connect up all the pots and jacks to the wires first - and then install them into the panel.  We started considering how long the ouput coax should be to get to the output jacks.

Prepare Coax

For the eight outputs and the audio input, we'll use coax.  We're going to prepare nine 7in lengths such that the shield solders to the jacks only.  Here's how: 

cut to 7in lengths

strip off the outer insulation one end about 3/4" and the other about an inch - unweave the shield and twist it

strip the signal wires and trim off the shield from the 3/4" end

a bit of heat shrink seals the outer insulation to the inner signal wire on the 3/4" shield-free end

we tinned the wires

done

Mounting Bracket Modification

We had a 4 pot stooge bracket we'd gotten from Dave Wellington a couple years back.  Bridechamber also has them.  We drilled four 1/8" holes to match the screw  holes in the PCB.

R45 - Tempco Resistor

We decided to solder the tempco resistor (R45) in such a way that it would ust the SSM2164 IC as a heat sink. But we're not going to put the silicone grease on there until later in the building process.

We got the 2K 3500 ppm tempco from Bridechamber

PCB / Panel Connections

1. Output and Input Coax and Jacks

We soldered the coax into the PCB and then onto their jacks.

Here's where they go.

2. Input Pot

Next we wired-up the 100K input potentiometer.

We slid some heat-shrink onto the twisted triple.

3. Fine / Course Controls

"Chicklets" for the Spectrol Cermet 100K pots

We slid some heat-shrink onto the twisted triple.

4. Control Voltage Jacks

5. Control Voltage Pots

6. Connections Made - PCB

The PCB Mounted and Connected

After mounting the PCB to the bracket, we started by attaching the output pots to the panel. Then we attached the bracket to the panel and finished the other parts.

Construction Done

Set up / Testing

Use Notes