Aion CHROMA Manuel utilisateur

CHROMA PARAMETRIC BOOST 1

PROJECT NAME

CHROMA

BASED ON

EFFECT TYPE

PROJECT SUMMARY

DOCUMENT VERSION

BOSS®SP-1 Spectrum

One of the rarest BOSS effects, a single-band swept-frequency EQ. Modified to boost the overall signal

level so it can be used as a boost pedal if desired.

Parametric equalizer / boost 1.0.0 (2021-10-22)

BUILD DIFFICULTY

Easy

Actual size is 2.3” x 1.86” (main board) and 2.3” x 0.86” (bypass board).

This project requires a dual 10kA/10kC potentiometer, which is not available from any of the

normal suppliers. It’s easy to make your own, but make sure to read the instructions on page 5

before undertaking the project.

IMPORTANT NOTE

CHROMA PARAMETRIC BOOST 2

TABLE OF CONTENTS

1Project Overview 8Drill Template

2Introduction & Usage 9Enclosure Layout

3-4 Parts List 10 Wiring Diagram

5-6 Build Notes 11 Licensing

7Schematic 11 Document Revisions

INTRODUCTION

The Chroma Parametric Boost is a clone of the BOSS®SP-1 Spectrum, one of the three original BOSS

compact stompboxes released in 1977. (The other two were the OD-1 OverDrive, which is available as

the Parhelion project, and the PH-1 Phaser.) It was discontinued in 1981, and due to its extreme rarity,

today it routinely sells for $500 or more on eBay.

The original SP-1 is a parametric equalizer with only two knobs. The frequency knob adjusts the boosted

frequency between 500Hz and 5 kHz, and the balance (or mix) knob adjusts the dry-to-wet signal ratio.

The gain and Q are fixed. Unlike some other parametric equalizers, it only boosts frequencies—it doesn’t

have the ability to cut. Its function is very similar to a fixed wah, and you can hear a wah-like effect if you

turn the Spectrum knob while a signal is being passed.

The updated version of the Chroma adds one new feature: the output volume has been increased and it

now includes a volume knob. This way, the output volume can be precisely trimmed for unity gain (which

is inexact in the original Spectrum depending on where the Balance control is set), or it can be used as

a booster with an adjustable frequency emphasis. When used in front of another overdrive pedal or to

overdrive an amplifier, you can get some really unique drive tones that wouldn’t otherwise be attainable.

USAGE

The Chroma has three controls:

• Spectrum sets the frequency of the boost.

• Balance pans between the dry signal and wet signal, acting as a sort of depth control.

• Volume sets the overall output of the effect. The original unit was fixed at unity gain and did not

have a volume control.

CHROMA PARAMETRIC BOOST 3

PARTS LIST

This parts list is also available in a spreadsheet format which can be imported directly into Mouser for

easy parts ordering. Mouser doesn’t carry all the parts—notably potentiometers—so the second tab lists

all the non-Mouser parts as well as sources for each.

View parts list spreadsheet →

PART VALUE TYPE NOTES

R1 1k Metal film resistor, 1/4W

R2 470k Metal film resistor, 1/4W

R3 10k Metal film resistor, 1/4W

R4 10k Metal film resistor, 1/4W

R5 1k Metal film resistor, 1/4W

R6 10k Metal film resistor, 1/4W

R7 18k Metal film resistor, 1/4W

R8 10k Metal film resistor, 1/4W

R9 22k Metal film resistor, 1/4W

R10 10k Metal film resistor, 1/4W

R11 10k Metal film resistor, 1/4W

R12 47k Metal film resistor, 1/4W Originally 10k. 47k increases the output going into the volume control

so it can be used as a boost. Increase the value even further to raise the

maximum volume.

R13 470R Metal film resistor, 1/4W

R14 33k Metal film resistor, 1/4W

R15 33k Metal film resistor, 1/4W

R16 100R Metal film resistor, 1/4W

RPD 2M2 Metal film resistor, 1/4W Input pulldown resistor.

LEDR 4k7 Metal film resistor, 1/4W LED current-limiting resistor. Adjust value to change LED brightness.

C1 47n Film capacitor, 7.2 x 2.5mm

C2 1uF Film capacitor, 7.2 x 3.5mm Original uses 4.7uF. There is space for the 4.7uF film capacitor, but 1uF

is sufficient.

C3 1uF Film capacitor, 7.2 x 3.5mm Original uses 4.7uF. There is space for the 4.7uF film capacitor, but 1uF

is sufficient.

C4 100n Film capacitor, 7.2 x 2.5mm

C5 3n3 Film capacitor, 7.2 x 2.5mm

C6 3n3 Film capacitor, 7.2 x 2.5mm

C7 1uF Film capacitor, 7.2 x 3.5mm

C8 1uF Film capacitor, 7.2 x 3.5mm Original uses 4.7uF. There is space for the 4.7uF film capacitor, but 1uF

is sufficient.

C9 47pF MLCC capacitor, NP0/C0G

C10 1uF Film capacitor, 7.2 x 3.5mm

C11 47uF Electrolytic capacitor, 5mm Reference voltage filter capacitor.

CHROMA PARAMETRIC BOOST 4

PARTS LIST, CONT.

PART VALUE TYPE NOTES

C12 100uF Electrolytic capacitor, 6.3mm Power supply filter capacitor.

C13 100n MLCC capacitor, X7R Power supply filter capacitor.

D1 1N5817 Schottky diode, DO-41

IC1 NJM3403AD Operational amplifier, DIP8

IC1-S DIP14 socket IC socket, DIP-14

GAIN 1k trimmer Trimmer, 10%, 1/4” See build notes (pg. 6) for notes on setting this trimmer.

BAL. 10kA/10kC 16mm right-angle PCB mount pot Must be made from a 10kA dual and 10kA single pot. See build notes for

how to do it.

SPECT. 100kB dual 16mm right-angle PCB mount pot

VOL. 100kA 16mm right-angle PCB mount pot

LED 5mm LED, 5mm, red diffused

IN 1/4" stereo 1/4" phone jack, closed frame Switchcraft 112BX or equivalent.

OUT 1/4" mono 1/4" phone jack, closed frame Switchcraft 111X or equivalent.

DC 2.1mm DC jack, 2.1mm panel mount Mouser 163-4302-E or equivalent.

BATT Battery snap 9V battery snap Optional. Use the soft plastic type—the hard-shell type will not fit.

FSW 3PDT Stomp switch, 3PDT

ENC 125B Enclosure, die-cast aluminum Can also use a Hammond 1590N1.

CHROMA PARAMETRIC BOOST 5

BUILD NOTES

Making the dual 10kA/10kC “Balance” pot

The main reason the SP-1 has never been cloned before is due to the odd 10kA/10kC Balance pot.

This pot is unique to the pedal, and to custom-order it from Alpha would mean ordering at least 1,000

minimum. This has historically made the SP-1 prohibitive to DIYers wanting to build their own.

Fortunately, dual 10kA pots are available, and it’s very easy to switch out the top wafer of these pots

since the top wafer is identical in construction to the wafers used in single pots.

But, here’s the catch: since the top wafer is upside down, its taper is actually mirrored. The top wafer of

a dual 10kA pot is the same as a 10kC. Because of this, you will need to pull the replacement wafer from

a single 10kA pot, which will give you the correct “C” taper when used upside-down in a dual pot.

So for this, you will need one 10kA dual pot (preferably right-angle PCB mount) and one 10kA single pot

(preferably straight-pin PCB mount). It’s recommended to order at least two 10kA single pots in case

you break one while taking it apart.

Once you have the dual 10kA and single 10kA pots, follow these steps for making the dual pot.

1. Start with a 10kA dual pot (right

angle PCB mount) and a 10kA single

pot (straight-pin PCB mount).

4. On the single pot, break away

all the black plastic from the metal

shaft, taking care not to scratch

the carbon on the wafer itself. This

will take a fair amount of force.

You could also use a utility knife or

Dremel tool to cut it away.

2. On the dual pot, bend out the

four tabs with needle-nose pliers to

separate the top wafer.

5. Once the plastic has been broken

away, set aside the wafer and throw

the rest away. (Note that this photo

shows a solder-lug wafer, which

won’t fit the PCB without cutting

down the lugs. Use a straight-pin pot

as shown in step 1.)

3. On the single pot, bend out the

four tabs near the nut to separate

the metal cover from the rest.

6. Place the 10kA wafer on the dual

pot as shown, carbon side down, and

bend the four tabs inward to secure

it in place. Bend the pins straight

upward. This wafer will act as a

C-taper pot since it is upside-down.

CHROMA PARAMETRIC BOOST 6

BUILD NOTES, CONT.

Setting the gain trimmer

According to the SP-1 service manual, the Gain trimmer should be set as follows:

Test signal: 5KHz +/- 10% sine, -20dBm, set SPECTRUM, BALANCE at max (FCW), adjust trimmer

pot 1K for 2.5dBm output at peaking freq. (5KHz +/-10%)

Follow the process if you want, but it’s definitely overkill and an effect this simple does not need that

kind of precision. We couldn’t hear much difference across the whole sweep of the trimmer, but it would

be most noticeable at full clockwise (wet) on the Balance control. You might be able to make its impact

more pronounced by raising the value of R7.

IC selection

The original SP-1 used the Raytheon RC3403A “ground-sensing” quad op-amp. Raytheon hasn’t

manufactured analog components in several decades, and originals are scarce today—but fortunately

the New Japan Radio NJM3403A is an exact substitute and still in production.

SCHEMATIC

CHROMA PARAMETRIC BOOST 7

10kC

10kA

GND

2M2

1N5817

100uF

1kB

GND

100kA

+9V

100n

100R

47n

NJM3403A

NJM3403A NJM3403A

NJM3403A

470k

33k

GND

47uF

GND

1uF

10k

1uF

10k

1uF

100n

10k

18k

3n3

100kB

3n3

1uF

10k

22k

10k

47k

47pF

470R

1uF

GND

OUT

RPD

C12

GAIN

VOLUME

C13

R16

IC1A

IC1D

IC1B

IC1C

411

R14

R15

C11

BALANCE A

BALANCE B

R10

SPECTRUMA

SPECTRUMB

R11

R12

R13

C10

GND

CHROMA PARAMETRIC BOOST 8

DRILL TEMPLATE

Cut out this drill template, fold the edges and tape it to the enclosure. Before drilling, it’s recommended

to first use a center punch for each of the holes to help guide the drill bit.

Ensure that this template is printed at 100% or “Actual Size”. You can double-check this by measuring

the scale on the printed page.

Top jack layout assumes the use of closed-frame jacks like the Switchcraft 111X. If you’d rather use

open-frame jacks, please refer to the Open-Frame Jack Drill Template for the top side.

LED hole drill size assumes the use of a 5mm LED bezel, available from several parts suppliers. Adjust

size accordingly if using something different, such as a 3mm bezel, a plastic bezel, or just a plain LED.

x: -0.65, y: +1.71 x: 0.65, y: +1.71

ø9/32” ø9/32”

x: 0, y: +0.66

ø9/32”

x: 0, y: -1.20

ø15/32”

x: -0.775, y: -1.20

ø5/16”

CENTER (0,0)

ø3/8” ø1/2”

0.385”

0.625” 0.625”

ø3/8”

OUT DC IN

125B

SPECTRUM

LEVEL

BALANCE

FOOTSWITCHLED

0 1 2

0 1

INCH

CHROMA PARAMETRIC BOOST 9

ENCLOSURE LAYOUT

Enclosure is shown without jacks. See next page for jack layout and wiring.

125B

CHROMA PARAMETRIC BOOST 10

WIRING DIAGRAM

125B

IN +VGND N.C. N.C. OUT

PCB

GND +V +V JACK GND JACK

OUTIN

GND GND PCB

OUT

Shown with optional 9V battery. If battery is omitted, both jacks can be mono rather than one being stereo.

Leave the far-right lug of the DC jack unconnected.

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