m o d e l 1 0 1 1 - slightly nasty · taking a linear scale voltage from the cv input and...

M O D E L 1 0 1 1

D i s c r e t e V o l t a g e C o n t r o l l e d O s c i l l a t o r

Construction

& Operation Guide

R E V A - F O R P C B V 1 . 1

S L I G H T L Y N A S T Y E L E C T R O N I C S A D E L A I D E , A U S T R A L I A


2Specifications

S P E C I F I C A T I O N S

PHYSICAL

FORM FACTOR: Loudest Warning / 4U

WIDTH: 3NMW / 75.5mm

HEIGHT: 175mm

DEPTH: ~40mm from panel front inc. components

PCB: 70 x 150mm, Two-Layer Double Sided

CONNECTORS: 4mm Banana

ELECTRICAL

POWER: +12V, 0V, -12V

CONSUMPTION: ~40mA +12V Rail, ~30mA -12V Rail

CONNECTOR: IDC 10-pin Shrouded Header, Eurorack Standard

or MTA-156 4-Pin Header

I/O IMPEDANCES: 100K input, 1K output (nominal)

INPUT RANGES (nominal)

1V/OCT: +/- 10V

FM: +/- 5V

LOG: +/- 5V

SYMMETRY: +/- 5V

SYNC: +/- 5V (falling-edge trigger)

OUTPUT RANGES (nominal)

OUTPUT A: +/- 5V

OUTPUT B: +/- 5V

SUBOCTAVE: +/- 5V

M O D E L 1 0 1 1 D i s c r e t e O s c i l l a t o r

IDC power connector pinout.

MTA-156 power connector pinout.

SPECIFICATIONS

Specifications / Power Requirements 2

INTRODUCTION

Introduction 4

CIRCUIT OVERVIEW

Circuit Overview 5

Exponential Converter 6

Sawtooth Core 8

Triangle / Sine Shapers 10

Pulse / Suboctave Shapers 10

Output Mixers / Amplifiers 12

CHOOSING COMPONENTS

Bill Of Materials (BOM) 14

Choosing Components 15

Transistor Matching 16

CONSTRUCTION

Construction Overview 18

Physical Assembly 20

CONTROLS

Controls 21

CALIBRATION

Calibration Overview 22

CV Scale 23

CV Offset 24

High Frequency Compensation 24

Triangle Adjustment 25

REFERENCE

PCB Guide - Lower Board 26

PCB Guide - Upper Board 27


3Circuit Overview

T A B L E O F C O N T E N T S


This document is best viewed in dual-page mode.


4Introduction

I N T R O D U C T I O N

The Slightly Nasty Model 1011 is a voltage controlled oscillator that's a little bit

different. Despite featuring a host of functionality including four mixable

waveshapes, suboctave, linear and logarithmic FM, pulse width modulation, and

hard sync, inside it you won't find a single IC opamp or OTA. What you will find

is no less than 41 discrete transistors flying in close formation, doing their best to

output useable musical tones.

The Model 1011 has been designed from the ground up to use modern "jellybean"

components that can be cheaply and easily obtained from most electronics

suppliers. Despite the unusual implementation, the architecture is actually a very

traditional sawtooth-core design that will be familiar to most people who have

worked on VCOs before.

Three outputs provide mixable sine-triangle, saw-pulse-suboctave, and

suboctave square respectively, the pulse wave also featuring both manual and

CV-controlled symmetry (pulse width). Aside from the usual 1V/Octave input,

there are also separate inputs for both linear and logarithmic FM, each with

input attenuators, as well as a hard sync input. The exponential converter is

temperature compensated for better thermal stability and the sawtooth core

features high-frequency compensation for better pitch tracking.

The Model 1011 uses the Loudest Warning 4U format for the front panel, and

follows Eurorack electrical and power standards. All front panel components are

PCB mounted for easy wiring-free construction. The front panel is available in

two finishes - satin anodised and gloss white powdercoat, both on 2.5mm

aluminium with robust UV-printed graphics.



5Circuit Overview

C I R C U I T O V E R V I E W

For full schematics, please download the separate schematics PDF. Excerpts shown

in this manual may be outdated and are provided for reference only.

While the fully populated PCB of the Model 1011 can look quite intimidating, the

circuitry can actually be broken down into a set of relatively simple subcircuits

that each handle a very specific aspect of the module's operation. Overall, the

1011 has a fairly standard architecture consisting of the following units:

1. Exponential converter - this allows the use of 1V/Octave pitch CVs by

taking a linear scale voltage from the CV input and converting it into an

exponential scale current to feed the sawtooth core.

2. Sawtooth core - this is the sonic heart of the module, generating the

base sawtooth signal from which all other waveshapes are generated.

Sync is also implemented in this circuit.

3. Triangle/sine shapers - These convert the raw sawtooth signal into

triangle and sine waves by first folding the sawtooth into a triangle

shape, and then soft-clipping that to create a pseudo-sine.

4. Pulse/suboctave shapers - These create the pulse wave by feeding the

sawtooth signal into a comparator, using the symmetry controls to set

the threshold level. The pulse is then used to clock a pulse divider to

form the suboctave square.

5. Mixers/output amps - These allow the blending of the various

waveforms as well as converting the different levels and offsets of the

various raw waveform signals to match the +/-5v expected at the

outputs.


Block diagram of the Model 1011. Circles marked "A" are attenuators.


6Circuit Overview


7Circuit Overview

Undoubtedly the finickiest part of most VCOs, the exponential converter in the

Model 1011 is essentially a discrete reimplementation of the opamp-stabilised

transistor pair found in countless other designs. This circuit works by using the

naturally exponential relationship of a transistor's base-emitter voltage to its

output current, using two matched transistors to mostly cancel out each others'

thermal effects and keep the conversion stable across different temperatures

and currents. A feedback-stabilised current source on the shared emitters of the

transistors holds one transistor at a constant current, causing the exponential

current caused by changes to the input voltage to appear at the collector of the

other one. A temperature-sensitive "tempco" resistor provides additional

correction to the aspects of the circuit's thermal response that are not cancelled

by the matched pair.

The exact operation of this sort of converter is a bit too involved to get into in

this manual, but an excellent rundown of the basic principles can be found on

René Schmitz' website at http://schmitzbits.de/expo_tutorial/index.html

In the 1011, the exponential converter can be broken down further into three

basic sections. There are the frontend buffer/amplifiers that combine the various

CVs and panel controls into a single pitch voltage; the exponentiator itself, in the

form of the matched pair; and the feedback controlled current source, which

consists of a differential pair controlling a current source tranistor. The bulk of

the exponentiator is single rail and works between 0v and +VCC.

The input buffer/amplifiers are essentially just crude emitter followers, and

consist of transistors Q501, Q502, and Q510 along with their respective passive

components. The output of Q501 and Q502 are both combined and go through

the voltage divider comprised of RV505 and the tempco resistor R522, in order

to reduce the level to the small voltage swing needed for the exponentiator.

Because the circuit is single rail, Q503 provides a buffered offset voltage so that

the resultant scaled CV is centred near the 1/2 VCC mark. The scaled CV is

finally buffered by Q506 before being fed into the exponentiator at Q507.

Q507 and Q509 comprise the matched-pair exponentiator, and share a common

emitter. Q507 takes the scaled pitch CV as input at its base, while Q509 has its

base held at a fixed voltage around 1/2 VCC. The exact voltage at Q509's base

can be trimmed with RV506 in order to offset the CV response and get the

desired centre frequency for the oscillator (usually middle C).

Finally, the differential pair of Q504 and Q505 along with the current source

transistor Q508 form the feedback-stabilised current source, which would

normally consist of an opamp in a circuit of this type. Q505 is referenced to 1/2

VCC via the voltage divider of R513 and R514, and like an opamp the circuit will

try to get the opposite input (the base of Q504) to match this level. That base is

connected to R518, on the collector of Q507, and so the circuit will try to hold

M O D E L 2 2 3 1 A s y m m e t r i c S l e w L i m i t e rM O D E L 1 0 1 1 D i s c r e t e O s c i l l a t o r

E X P O N E N T I A L C O N V E R T E R

V-

4V

+8

1

-2

+3

GND

VCC

VEE

1

23

1

23

R3

R2

VEE

R1

TE

MP

CO

GND

1V/OCT

GND

TO

_CO

RE

Traditional configuration of PNP exponential converter with opamp current source.

http://schmitzbits.de/expo_tutorial/index.html


D i s c r e t e O s c i l l a t o r

8

M O D E L 1 0 1 1

Circuit Overview

the voltage across R518 at 1/2 VCC - and consequently maintain a constant

current through both it and Q507. It dœs this by controlling the current that

feeds the exponentiator pair through Q508. C503 performs a similar role to

bypass found caps in opamp feedback paths - preventing oscillations that can

develop due to various phase effects. The buffered linear FM input is AC-coupled

through C501 and feeds directly into the base of Q504 along with the feedback

signal.

The sawtooth core in the Model 1011 basically consists of a timing capacitor with

a discharge FET across it, and a reset comparator. The core of the reset

comparator is formed by Q204, Q205, and Q206 - with the first two once again

forming a differential pair and the latter serving as the gain stage / output. One

input of the comparator (the base of Q205) is connected to the timing capacitor,

and the other (the base of Q204) is fed the reset threshold voltage set by the

voltage divider formed by R206 and R205 (these are chosen to get a ~6v P-P

amplitude on the sawtooth).

Q202 and Q203 are used to pull down the threshold to a lower voltage when

activated, in order to implement the hysteresis needed in a relaxation oscillator.

When the capacitor passes the threshold voltage, the comparator's output gœs

high and simultaneously switches on the discharge FET Q207 and the threshold

pulldown transistor Q203. This means that the capacitor now needs to discharge

down to the new, lower threshold voltage before the comparator output gœs

back to low and completes the cycle.

The second threshold pulldown transistor Q202 (and its inverting input buffer

Q201) is dedicated to the sync input, and triggers a reset cycle whenever a

sufficiently powerful falling edge triggers it. The necessity of a second dedicated

pulldown transistor for this is due to the possibility of the comparator being

knocked into an undesirable region of operation where instead of acting as a

comparator, the feedback path formed through Q203 causes the circuit to turn

instead into a voltage follower, tracking the threshold voltage and locking up the

oscillator. To prevent this, the feedback path has to be kept from reaching the

metastable point near the threshold voltage, and so the sync input is given its

own transistor outside the feedback loop.

RV202 and D201 form the high-frequency compensation circuit (a.k.a Franco

compensation). This works by using the voltage developed across RV202 by the

charge current to trigger the reset slightly earlier as the current increases (and

with it the pitch). This means that higher frequency cycles are shortened slightly,

and thus increased in pitch to counteract the droop caused by other effects in

the circuit, such as capacitor discharge time and so on. D201 limits the maximum

S A W T O O T H C O R E

Voltage across timing capacitor (top) vs. voltage on the base of disharge FET Q207. Blue shows charging period and red discharging. Note that the discharge time is exaggerated for clarity, and in most cases can be considered virtually instantaneous.


9Circuit Overview



10

M O D E L 1 0 1 1

Circuit Overview

compensation effect of the circuit, to prevent excessive drops in sawtooth

amplitude at very high frequencies where pitch is less discernable.

Q208 and R213 are the high-impedance output buffer for the sawtooth core, and

prevent the downstream circuits draining charge from the timing capacitor and

affecting the frequency.

The triangle shaper in the Model 1011 takes advantage of the normally

undesirable behaviour of an inverting transistor amplifier when the input and

output signals "cross over". Because the transistor's base voltage can't be higher

than its collector (in the case of an NPN transistor), once the output at the

collector drops too low it "collides" with the base voltage and can't go any lower

- causing the output to follow the base voltage instead. We can use this to "fold"

the sawtooth over on itself to create a triangle wave just by offsetting the input

sawtooth by the right amount.

Q301 is our inverting unity-gain amplifier, which is AC coupled to the sawtooth

signal so that the input can be offset by the resistors R301 and R302. C301 helps

to shape the inescapable glitch in the triangle at the sawtooth's reset point so

that it can be smoothed out more effectively by the lowpass filter R305/C302.

The pair of inverting amplifiers at Q302 and Q303 amplify the triangle signal to

the desired amplitude.

The triangle signal is attenuated through the R312/R313 voltage divider before

being soft-clipped by the pair of diodes. This distorts the triangle into something

approximating a sinewave, which is then amplified back up to useable levels by

Q304 and Q305.

The pulse waveform of the Model 1011 is generated in the same way as most

VCOs - by feeding the sawtooth signal into a comparator and varying the

threshold voltage to implement pulse width control. The basic circuit is the same

three-transistor comparator used elsewhere in the module, taking the sawtooth

signal as one input and the summed "symmetry" CV and front panel voltages as

the other.

The suboctave is slightly different to the other waveshapes, in that it isn't formed

by just shaping the sawtooth in some way. Instead a classic two-transistor

T R I A N G L E / S I N E S H A P E R S

P U L S E / S U B O C T A V E S H A P E R S

Principle of triangle formation by folding sawtooth wave. Intermediate stage is shown for clarity, in reality this is a single-stage process.

Pulse formation with comparator, showing PWM via varying threshold.


11Circuit Overview



12

M O D E L 1 0 1 1

Circuit Overview

multivibrator circuit is clocked from the positive-going edges of the pulse

waveform, creating a square output at half the frequency. C601 and Q604

convert the pulse signal into buffered positive-edge pulses, feeding it into the

multivibrator circuit via C604 and C607.

Both the pulse and suboctave circuits output unipolar 0v to VCC waveforms, in

the case of the pulse this isn't an issue as it is eventually mixed directly with the

likewise unipolar sawtooth wave, however the suboctave is AC-coupled through

the large 10uF capacitor C610 in order to centre it around 0v.

At this point, all of the various waveforms in the Model 1011 are at various

amplitudes and offsets, and the role of the mixers and output amplifiers is to

combine these disparate elements and ensure that the final outputs are in the

+/-5v range expected in most modular systems.

The two amplifiers are built around what are essentially discrete op-amps,

comprising a differential pair for input, a single transistor gain stage, and a two-

transistor push-pull output. The suboctave signal gœs through a much simpler

push-pull output buffer that dœsn't need to worry about linearity or crossover

distortion on account of it being a purely squarewave signal.

Amplifier A takes the triangle and sine signals which are already at matching

levels and combines them via the mix pot RV403 before feeding them into the

amp's positive input. The amp output is fed back into the negative input via

R418, and so the circuit operates as a standard voltage follower opamp circuit.

Amplifier B is only slightly more complex, it has an additional network of voltage

dividers before the mix pot to match the levels of the sawtooth and pulse signals,

and the suboctave signal is fed into the negative input via an attenuator.

Because the negative input is no longer just connected directly to the output

feedback, the gain of the amplifier actually changes as the suboctave attenuator

is adjusted, in order to keep the output level within +/-5V regardless of how

much suboctave is added.

Finally, all outputs go through a 1K output resistor to protect the amps and

buffers from short-circuits and provide the expected 1K output impedance.

O U T P U T M I X E R S / A M P L I F I E R SSuboctave formation, showing the positive edge pulses generated by C601 / Q604 causing the multi-vibrator to flip state.


13Circuit Overview

RESISTORS2 R101, R102

240R 1 R523390R 1 R5191K 7 R311, R318, R320, R417, R420, R424, R524

1 R5221K2 2 R306, R5202K2 11 R207, R208, R213, R307, R310, R321, R511, R515, R521, R614, R6173K3 2 R401, R6035K1 2 R504, R5079K1 2 R206, R40510K 29

12K 2 R402, R51015K 1 R31322K 2 R209, R50533K 1 R30551K 6 R506, R508, R512, R607, R411, R41262K 1 R60691K 1 R316100K 19

150K 2 R601, R604200K 4 R301, R421, R502, R503270K 1 R309300K 1 R605560K 1 R3081M 6 R302, R322, R408, R416, R419, R423

CAPACITORS10pF 2 C402, C40333pF 2 C301, C503220pF 1 C3021nF 3 C201, C604, C60710nF 1 C601

1100nF 9 C202, C203, C305, C401, C404, C405, C501, C502, C6020.68μF 2 C303, C304

2 C306, C6102 C101, C102

SEMICONDUCTORS1n4148 7 D201, D401, D402, D403, D404, D601, D6021n4007 2 D301, D302BC550C 26

BC560C 11 Q206, Q302, Q305, Q405, Q406, Q408, Q410, Q412, Q506, Q508, Q6032 Q507, Q509

2n7000 2 Q207, Q208

POTENTIOMETERS1 RV5061 RV2021 RV505

10K Linear 1 RV502100K Linear 8 RV401, RV402, RV403, RV501, RV504, RV503, RV601, RV602

1 RV301

CONNECTORSBanana Socket 8 P102, P103, P104, P105, P106, P107, P108, P109IDC 10-pin Header 1MTA-156 4-pin Header 1

3 Use standard breakaway pin strip.

3

HARDWAREM3 x 20mm Screw 4M3 Washer 16

4

M3 Nut 4

10R 1/2W

1K 3300PPM/C

R203, R204, R205, R210, R211, R214, R215, R317, R319, R404, R409, R410, R413, R414, R509, R513, R514, R516, R517, R518, R525, R526, R602, R608, R610, R611, R612, R615, R616

R201, R202, R212, R303, R304, R312, R314, R403, R406, R407, R415, R418, R422, R425, R426, R427, R501, R613, R618

10nF (C0G/NP0) C206 (Optionally use standard 10nF film capacitor)

10uF Electrolytic100uF Electrolytic

Q201, Q202, Q203, Q204, Q205, Q301, Q303, Q304, Q401, Q402, Q403, Q404, Q407, Q409, Q411, Q501, Q502, Q503, Q504, Q505, Q510, Q601, Q602, Q604, Q605, Q606

MATCHED BC560C

1K 25-turn5K 25-turn10K 25-turn

100K 25-turn

P101 (Option 1)P101 (Option 2)

10-pin 2.54mm pin header10 pin-2.54mm female pin header

M3 x 10mm Threaded Metal Hex Spacer


14Bi l l Of Materials

B I L L O F M A T E R I A L S


15Choosing Components

Selecting the right components for the 1011 is fairly straightforward, with only a

couple of parts needing any special attention. All resistors should be 1%

tolerance metal film types, most capacitors are standard rectangular film caps

and electrolytics. Three types of transistor are used, the bulk being BC550C and

BC560C, with a pair of 2N7000 FETs used in the sawtooth core. Diodes are

mostly the ubiquitous 1n4148.

In the exponential converter there is a 1K 3300ppm/C tempco resistor that will

need to be bought from a supplier that specialises in synthesiser components,

such as Thonk (www.thonk.co.uk) or Modular Addict (modularaddict.com),

among others. The exponential converter also requires a matched pair of the

BC560C transistors (see the section on transistor matching over the page), which

can be selected from your inventory of transistors using a simple matching

circuit.

In the sawtooth core, the main timing capacitor responsible for generating the

sawtooth wave can be either a normal film capacitor, or a more thermally stable

part if greater stability is desired. Traditionally, polystyrene caps were used for

this role in VCOs, but as these are now becoming rare and expensive a much

better option is one of the new generation of C0G/NP0 ceramic capacitors.

The sine shaper uses a pair of 1N4007 diodes instead of the usual 1N4148s to get

a slightly better sine shape, though 1N4148s will work also.

The module is designed to use either side or top-adjustment 25-turn trimpots

for calibration adjustment - side adjustment is usually the better option as it

means the unit can be more easily calibrated when connected to the rack's

power bus.

The front panel PCB fits Alpha brand 9mm vertical-mount round shaft

potentiometers, these are widely available from stores such as Thonk, Tayda,

Smallbear, Mouser etc. The module should fit a number of different banana jack

sockets, but the "correct" parts are the Cinch Connectivity range of jacks.

The intended knobs are Davies Molding parts - the 1913BW, 1910CS, and 1900H -

though given the outrageous pricing of the actual Davies 1900H I'd strongly

recommend using a good quality clone. Avoid the cheaper clones without an

internal brass bushing - Thonk sells an excellent brass-bushed 1900H clone for a

very reasonable price that I use in all of my own builds.

Alternatively, feel free to use any knobs that have similar diameters and will fit

the Alpha round shaft pots. The Davies parts are 29mm, 19mm, and 13mm

respectively, and many other manufacturers make knobs of similar sizes. The

classic silver top Moog-style knobs actually work quite well also for the larger

diameters.


C H O O S I N G C O M P O N E N T S

http://www.thonk.co.uk

https://modularaddict.com


16Transistor Matching

T R A N S I S T O R M A T C H I N G

The 1011 uses a pair of matched BC560 PNP transistors in the exponential

converter to ensure a stable and reliable conversion across different

temperatures and pitch ranges. These transistors need to be matched to ensure

that they have the same VBE (base-emitter voltage drop) at a given temperature,

which requires testing a number of transistors to find ones that have the closest

Vbe values.

A common mistake made by inexperienced builders is to match the transistors

using a multimeter transistor tester, using the transistors that show the best

matching values. This will not work. The transistor tester built into many

cheaper multimeters measure the hFE (current gain) of the transistor, and not

the base-emitter voltage that we are interested in. Testing for VBE requires

setting up or building a small test circuit to allow measurement of the difference

in VBE between transistor pairs.

Recently a number of small, cheap component testers have appeared on the

market that do measure VBE, however while these are handy to roughly check

component values and find faulty parts they do not have the resolution or

accuracy required for matching exponential converter pairs.

There are a number of circuit designs available for matching transistors, but I

personally recommend the Ian Fritz method for its simplicity and reliability.

There are a few variations on this method, but the circuit I use is shown here.

Essentially it consists of setting the transistors up as diodes with precisely

matched resistance on the emitters of each (using a 25-turn trimpot to zero out

the tolerance errors of the 100k resistors), then measuring in millivolts the

difference between the emitter voltages of the two transistors. The switch shown

here swaps the resistors between the two transistors to allow the trimpot to be

accurately set. I'd strongly recommend building a socketed version of this circuit

on stripboard, to keep on the workbench for future projects that need matched

transistors.

When testing transistors I recommend setting up a fan blowing across the test

circuit, to ensure that both transistors are kept at an identical temperature. It's

also worth leaving each pair for a couple of minutes to allow the transistors'

internal temperatures to stabilise. If the temperature in the room is relatively

stable, you can speed up the process by leaving one transistor in the circuit

permanently, and swapping out the other position one by one, taking note of the

voltage difference of each tested part. Once you've found a few transistors that

seem to show very close or identical differences to the fixed "reference"

transistor, you can take the reference transistor out and test the rough-matched

pairs against each other as normal to find the ones that have the closest match.

Even if you don't test all the rough-matched transistors, keep them together for

future projects, because searching through labelled pairs that are already fairly

close is a lot faster than finding matches between random parts!


http://www.dragonflyalley.com/synth/images/TransistorMatching/ianFritz-transmat0011_144.pdf


17Transistor Matching

C 1

B2

E 3

C 1

B2

E 3

100K

100K

100K

+12V

0V/GND

MULTIMETER

MULTIMETER

12

312

3

TO SET TRIMPOT:With a pair of transistors fitted, measure thevoltage difference while switching between thetwo switch positions. Adjust the trimpot untilthe voltage is the same in both positions.

Set multimeter to mV range when measuring.Polarity is not important as long as it's kept thesame when testing multiple transistors.

DPDT SWITCH

TRANSISTORS UNDER TEST

PNP VERSION

NPN VERSION

1

23

1

23

12

3 12

3

MULTIMETER

MULTIMETER

0V/GND

+12V

100K

100K

100K

This is essentially the exact same circuitbut with the power reversed and thetransistors installed accordingly.


18Construction

C O N S T R U C T I O N

For the most part the 1011 can be constructed like any other PCB project, but

there are a couple of special components that need consideration. The

exponential converter uses a matched transistor pair and a 3300PPM/C tempco

resistor to achieve a good degree of thermal stability, and these need to be

mounted together in a specific way in order to ensure that they are all in close

contact and share the same temperature during operation. See the section

labelled Transistor Matching for details on how to find a pair of matched

transistors, the 1K 3300PPM/C tempco can be bought at synth part suppliers

such as Thonk (www.thonk.co.uk) and Synthrotek (store.synthrotek.com) among

others.

The majority of construction can be performed like any PCB build, starting with

the lowest-profile components (resistors and diodes) and working through to the

taller ones (Capacitors, transistors, etc.). The simplest way to populate the board

is simply to work through the BOM, doing each component type and value in

one chunk before moving on to the next. Avoid fitting the special components

for now (Q507, Q509, and R522)

Given the unusual number of discrete transistors in the build, it's worth

commenting on how to best populate them without risking damage or ending up

with a motley forest of strangely angled TO-92 packages. My preferred technique

is to put a batch of the transistors in place and bend the outer legs as usual,

taking care to get the height roughly the same between each, and then soldering

only the centre leg of each. Once these are all done, flip the board over and

use a pair of tweezers to straighten each transistor until they all look correct.

Flip the board back over and then solder one of the remaining legs of each of

the transistors, then finally go through and solder the final legs once all those are

done. This way each transistor gets the chance to cool down between each joint

being soldered, which reduces the risk of damage.

When soldering transistors it's important to hold the iron long enough to get a

solid joint that extends down into the plated hole, but not so long that you risk

thermal damage to the transistor junction. With a properly heated iron, a few

seconds on each should be all that's required.

When soldering rectangular capacitors, I like to solder one leg on each, then hold

the board in one hand while applying a very light pressure on top of the

capacitor with a free finger, using the other hand to reheat the solder joint until

the capacitor slides down tight against the PCB's surface. Continue this process

for all the installed capacitors then go back and solder the remaining legs. This

approach also works well to mount other components that need to mount

securely onto the board, such as trimpots, IC sockets and pin headers.

Care must also be taken to ensure that the PCB-mounted potentiometers are

mounted as vertically as possible on the board - one option is to click the



19Construction

potentiometers into place, then mount them to the front panel before soldering

them. Also note that most potentiometers have a small anti-rotation tab on

them that will need to be removed before soldering them into position, these

can be cut off with a sharp pair of sidecutters, and I personally like to clean up

any remaining protrusions with a few passes of a needle file as well.

The pin headers that interconnect the two boards are another component that

needs a bit of additional care when assembling to ensure correct aligment. The

best course of action is to solder one side of all the interconnects (either the

pins or socket) into place, being careful to get them straight and flush with the

board. Then connect the other halves onto them, lay the other PCB in place

over the top (I would even recommend mounting the boards together with

screws and spacers as they will be when finally assembled), and solder all the

pins of the other side. Once they are all soldered, carefully separate the two

boards, taking care to not bend the headers in the process.

When fitting the matched transistors and tempco resistors, these need to be

thermally connected to ensure the best stability. The two BC560Cs should be

joined face-to-face with a band of heatshrink tubing (I also like to smear a very

thin layer of thermal compound between the two, making sure none gets near

the conductive legs). Carefully bend the legs with a pair of tweezers so that they

match the hole spacing on the PCB, and solder them into place. Once the

transistors are installed, the tempco resistor can be mounted on top, using

something like an epoxy or liquid electrical tape to keep it thermally coupled to

the transistors and insulated from ambient temperature changes.


A NOTE ON POWER FILTERING

It's common practice among some builders to replace the 10 ohm power

filter resistors with ferrite beads, in the belief that this will prevent power

rail fluctuations under varying current loads while still providing the

filtering action desired. This is not recommended. Ferrite beads do not

even begin to show reactivity until somewhere up around the 1MHz mark,

an order of magnitude beyond the audio range. Within the audio band

(and for a long way beyond it) they are electrically identical to a wire

jumper.

Assembly of the matched pair using heatshrink tubing. After fitting the tempco resistor, a covering of non-conductive material should be added to thermally insulate the assembly.


20Construction

P H Y S I C A L A S S E M B L Y

Assembling the finished PCBs and front panel is very simple. Begin by fitting the

M3 hardware to the panel-side PCB. screwing the hex spacer tight to hold it all

together. Once all four screws are in place, start fitting the banana sockets into

their respective holes on the front panel - making sure to align the flat terminals

vertically (if using the Cinch-style sockets). The banana sockets need to be

tightened solidly to prevent them coming loose in use, something like a dab of

hot glue between the nut and thread can also help prevent loosening.

Make sure that the nuts and washers have all been removed from the PCB-

mount potentiometers on the front panel PCB, as well as the anti-rotation tabs

on the pots themselves (if present). Now you can join the front panel and panel

PCB by pushing the pot shafts through their respective holes, fitting their

washers and nuts, and tightening everything into place.

Now you'll need to connect the banana sockets to the front PCB using either

some offcut component leads, or tinned copper wire. The simplest way is to

solder the straight pieces of wire vertically into the pad on the PCB, then bend

them over to meet the banana socket and solder that end to the flat side of the

terminal. This way they can be easily disconnected for servicing by simply

heating the terminal with the iron and pushing the wire away once the solder

reflows.

Once the sockets are all connected, put M3 washers on all four mounting screws

and carefully fit the second PCB into place - taking care to get the interconnects

correctly seated. Until calibration is completed I would not fit the final washers

and nuts to allow easy separation of the PCBs when troubleshooting, just making

sure to take extra care plugging and unplugging the power connector when the

PCB is unsupported.

When the module is confirmed to be working properly you can fit the final M3

washers and nuts and tighten up the whole assembly. Double check that the hex

spacers haven't loosened in the meantime as well.


Connection of the two PCBs using standard M3 hardware. Washers are necessary on the inside to correctly space the boards for the interconnects. Screw head should go on panel side.

Connecting the banana sockets using an offcut component lead or similar.

FREQUENCY CONTROLSFine and coarse adjustment of inital oscillator pitch.

MIX KNOBSCrossfades the respective output

between two waveshapes

A & B OUTPUTSOutputs mixed sine-triangle (A) and

mixed saw-square-suboctave (B)

SUBOCTAVE OUTPUTOutputs the raw suboctave signal.

SUBOCTAVE LEVELControls the amount of suboctave that is mixed into output B

INPUT JACKSAC coupled inputs for Linear FM

and Sync signals, and DC coupled inputs for 1V/Octave pitch CV,

Logarithmic FM, and Symmetry CV.

INPUT ATTENUATORSAllow 0-100% attenuation of the FM signal, Symmetry CV and Log CV.

SYMMETRYSets the inital symmetry (or pulse

width) of the pulse waveform. Centred is 50:50 squarewave.


C O N T R O L S


21Controls

SLIGHTLY NASTY JACK COLOURSRED Bipolar signal outputBLUE Bipolar signal inputYELLOW AC-coupled inputBLACK Logic outputWHITE Logic Input


22Cal ibration

C A L I B R A T I O N

Calibration of the 1011 consists of adjusting the four calibration trimpots on the

back of the module to set the following values (in order):

1. CV scale - sets the scaling of the pitch CV to ensure that a 1v change in

CV produces a one octave change in pitch.

2. CV offset - sets the centred "zero point" for the front panel frequency

knobs.

3. High frequency compensation - this allows you to "boost" the CV

response at higher frequencies to compensate for the tendency of

VCOs to "droop" at higher pitches.

4. Triangle wave alignment - This sets the folding point of the saw-to-

triangle shaper to make sure that the reset point of the wave lines up

correctly and forms a nice uninterrupted triangle wave.


BEFORE YOU BEGIN

Before powering up the module for the first time, use a multimeter

to check the resistances between the three power rails. Make sure

that they show a resistance higher than 1KOhm, any lower and it's

possible there is a short circuit or incorrectly oriented semiconductor

somewhere on the PCB.

If the 1011 dœsn't oscillate when you power it up, first try adjusting

the CV offset trimmer in both directions - this trimmer is quite

sensitive and can easily push the oscillator outside of its operating range.

Before calibrating the CV response, allow the oscillator to warm up for a

few minutes - the frequency will drift in this period as all the components

settle into their operating temperatures.

While I've given a specific order to these operations, you can expect to

have to go back and forth on some of them, particularly the CV Scale

and CV Offset calibration. Also if you notice the oscillator pitch seems

way too high or low when you get to the CV Scale step, feel free to adjust

the CV offset control to get it in the right place.

Also, you'll want to disable the High Frequency Compensation before you

start the CV calibration steps, which means measuring the resistance

across the two outer pads of the "HF.COMP" trimmer and adjusting it

until it reads 0 ohms (or as low as it will go).


23Cal ibration

he goal with this step is to get the oscillator to respond with an accurate 1V/

Octave response to the frequency CV - so that a change of +/- 1V on the input

results in a +/- 1 octave change in the output pitch (ie. doubling or halving of its

frequency). We're not really worried about absolute pitch here, only that the

amount of change relative to the CV is correct.

Getting the CV scale right is always one of the more tedious jobs when

calibrating oscillators, and different people have developed various systems over

the years to get the job done. However you choose to do it, I would strongly

recommend using whatever 1V/Oct source you will be using when the module is

completed, such as a your midi-CV converter or a keyboard with 1V/Oct output.

A first basic step is to either hook up a frequency counter or instrument tuner

that shows frequency (if you have one) and your listening system, and play notes

on the keyboard that are one octave apart, somewhere around middle C. Adjust

the CV Scale trimmer until the resulting pitches from the oscillator are as close

as you can get to being one octave apart (the higher note should be double the

frequency of the lower one). Because the actual frequencies of both the notes

will be changed each time you adjust the trimmer, just be sure to play them

each a couple of times between each adjustment to determine what the

relationship between them currently is.

Once you're more or less happy with the response over one octave, try playing

notes that are further apart, such as the next octave down from your low note,

and the next octave up from the higher one. Once again adjust the trimmer

until you get the correct relationship between the notes - in this case the high

note should be 8x the frequency of the lower one. Make sure to occasionally go

back and check the notes that are closer together again, to make sure that

they're also staying in calibration (they should if the exponential converter is

providing an accurate conversion).

Get the response as accurate as you can, but don't obsess over it yet, because

you'll want to fine tune this a little further once the CV offset has been trimmed.


C V S C A L E

While this adjustment is quite critical in a keyboard synth where the oscillators

are expected to have a very specific voltage-pitch response, in a modular where

we've got big frequency knobs on the front panel to adjust the pitch, it's really

just a convenience. Essentially all you want to do here is apply 0v to the 1V/Oct

input (even running a alligator lead from one of the M3 pcb mounting screws to

the tab on the back of the banana jack will suffice), set the coarse and fine

frequency knobs to their centre positions, and then adjust the CV Offset

trimmer until the oscillator is outputting middle C (261.6Hz). Don't worry about

getting this exact, because tiny movements of the coarse tune knob will throw

this off substantially, and tuning oscillators is a completely normal task when

patching modulars to play melodically. This setting just makes sure that similar

knob positions on individual oscillators give consistent frequency ranges.

Once the oscillator is responding fairly accurately in the low-mid pitch range, it's

time to set up the high-frequency compensation. This is necessary because

various electrical effects in the circuit usually cause the 1V/Oct response to

"droop" at higher frequencies, meaning that notes will get progressively flatter

and flatter (too low in pitch) as you continue up the musical scale. The high

frequency compensation circuit adds a boost to the oscillator frequency as the

frequency increases, to counteract this drooping and restore the expected 1V/

Oct response.

Setting this up generally just consists of playing notes higher up on the scale to

see how flat they are, and slowly turning up the HF compensation trimmer until

their pitch is adequately corrected. It's worth also playing notes at lower pitches

while you're adjusting to make sure that the adjustments aren't upsetting their

calibration.


24Cal ibration


H I G H F R E Q U E N C Y C O M P .

C V O F F S E T


25Cal ibration

The 1011 generates the triangle wave by folding the sawtooth onto itself, and to

form a smooth and uninterrupted triangle the fold point must be set accurately.

Turn the front panel sine-triangle mix knob all the way to the right and scope

the output - you should see the triangle wave with a slight glitch on the topmost

corners. Use the frequency knobs to set the oscillator's frequency to something

comfortable like 200Hz or so, then adjust the trimmer until the glitch looks to be

as central in the wave as you can get it.

Once you're happy with how it looks, hook up the output to your listening

system and fine tune the trimmer by ear until you find the position where the

triangle has the least upper harmonics (this is wherever the triangle sounds the

smoothest and has the least "buzz" to it).


T R I A N G L E A L I G N M E N T

Triangle alignment. Centre image is correctly aligned.


26Reference

P C B G U I D E - L O W E R


LOWER BOARD - TOP LOWER BOARD - BOTTOM


27Reference

P C B G U I D E - U P P E R


UPPER BOARD - TOP UPPER BOARD - BOTTOM

w w w . s l i g h t l y n a s t y . c o m

T R I A N G L E A L I G N M E N T

http://www.slightlynasty.com

m o d e l 1 0 1 1 - slightly nasty · taking a linear scale voltage from the cv input and...

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