Silicon chip Currawong User manual

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SILICON CHIP

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rairrow

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5010

•

OOP

The Currawong amplifier is a tried and tested valve amplifier

circuit which has been adapted to components which are readily

available in 2014. Each channel uses two 12AX7 twin triodes for

the preamp and phase splitter stages and two 6L6 beam power

tetrodes in the class-AB ultra-linear output stage. It performs very

well, with low distortion and noise.

od)

This progress view of the amplifier

shows it sitting in its timber plinth

but without the protective Perspex

covers in place to protect the PCB

and protect the user from high

voltages.

•

10W per channel

•

Low distortion

•

Good performance

•

Easy to build

N DESIGNING this amplifier, we

wanted to present a unit which is

straightforward to build and which

has a good appearance. To satisfy

the first requirement, most of the cir-

cuitry, with the exception of the power

transformers, is mounted on a large

double-sided PCB. Hence there is no

need for point-to-point wiring from

valve sockets, tag-strips, tag-boards

or any of that stuff from 60 years ago.

Using the large PCB also means

that we have avoided the need for an

expensive metal chassis. Instead, the

PCB slides into a timber plinth stained

as rosewood (although you can have

any timber finish you desire). As a nice

finishing touch, most of the PCB will

be covered and protected by a Perspex

cover. This will prevent little fingers

from touching any part of the circuit

and remove any risk of electric shock

which would otherwise be possible.

We hope you will like the appearance.

There are two toroidal power trans-

formers used to power the Currawong

and these are concealed underneath

the PCB, towards the back of the unit.

Control panel

At the front of the timber plinth,

there is a small control panel suspend-

ed below the main PCB. This carries

the volume control, the on/off switch,

a bi-colour red/green LED, a blue LED

and the headphone socket. And while

it might seem like a waste to use the

Currawong Stereo Valve Amplifier to

drive headphones, we know from long

experience that readers will definitely

want this feature.

By the way, the red/green LED

comes into play when you first turn the

amplifier on. There is an initial delay

while the valves heat up and during

this time, no HT (high tension or high

voltage) is applied to the plates of the

valves which could otherwise suffer

damage in the long term. So during

this delay, the LED is red. Then, when

the HT is applied, the LED changes

colour to green, indicating that normal

operation is possible.

The other LED is lit when the

headphones are in use. Plugging into

the headphone socket enables a relay

which disconnects the loudspeakers

and connects the headphones via 22052

resistors.

At the rear of the timber plinth is

another panel which accommodates

the RCA input sockets, the binding

post terminals for the loudspeakers

and a fused IEC socket for the mains

cord. Both the front and rear panels are

made from PCB material to provide a

high-quality finish.

The overall performance is sum-

marised in an accompanying panel

and three graphs.

gives very good

performance for a valve amplifier.

Circuit concept

A major difficulty in the design of

the Currawong has involved the out-

put transformers. As valve aficionados

will be aware, the output transformer

is usually the most expensive com-

ponent in the circuit, apart from the

valves themselves. Similarly, these

days the power transformer is also

very expensive, simply because there

is no locally available off-the-shelf

unit which can be pressed into service.

Yes, you can purchase imported

power and output transformers but if

we had specified these, the total cost of

the amplifier would have been a great

deal higher. Instead, we have taken a

very unusual approach in selecting

the output transformer by employ-

ing a standard off-the-shelf 15W line

transformer (Altronics M1115) which

would normally be employed with a

professional solid-state PA amplifier

to drive 100V lines.

As a line driver, the transformer's

primary winding is driven by a solid-

state amplifier and it steps up the

voltage in its multi-tapped secondary

winding. In the Currawong though, we

drive the transformers back to front,

with the push-pull valve output stages

driving the 100V windings and the

primary windings becoming the low-

impedance drive for the loudspeak-

ers. Conveniently, the 100V winding

has a centre-tap, which is necessary

for push-pull operation. In addition,

we use some of the other taps for the

"ultra-linear" connection.

Make no mistake though; while

these are low-cost transformers (be-

ing made in large quantities), they

have grain-oriented steel cores, a wide

frequency response and low harmonic

distortion. Better still, the taps on

the primary winding enable it to be

connected for ultra-linear push-pull

operation. On the other hand, selec-

tion of this transformer is one of the

twe limiting factors in the maximum

output power of the Currawong, at

close to 10 watts per channel.

The other factor is the power trans-

former selection. We would have ide-

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Specifications

•

Channels:

2 (stereo)

•

Valve line-up:

4 x 12AX7 twin triodes,

x 6L6 beam power tetrodes

•

HT supply:

-310V, actively filtered

Tested load impedances:

4n, 6Q, sn

•

Output power:

2 x 10W (80, 652), 2 x 9W (40) (see Fig.3)

•

Operating mode:

Class-A (80), Class-A/AB (60, 452)

•

Input sensitivity:

-1V RMS (80, with feedback enabled)

•

Signal-to-noise ratio:

77dB

•

Channel separation:

>60dB, 20Hz-20kHz (40, 652 & 852)

•

Harmonic distortion:

typically <0.1%, 60 & 80 (see Figs.3&4)

•

Frequency response:

±0.6dB, 30Hz-20kHz (see Fig.5)

•

Damping factor:

>20 (80), >10 (40)

•

Mains power draw:

typically 120-130W

•

Other features:

ultra-linear outputs, remote volume control option, delayed HT,

HT soft-start

•

Dimensions:

294 x 304 x 186nnm (W x D x H) including protrusions

ally liked to use a transformer with

much higher secondary voltages but a

specially-designed power transformer

would be much larger and more ex-

pensive, as already noted. Having said

that, there is future potential for this

amplifier to be upgraded with better

(more expensive) transformers to en-

able it to deliver substantially more

output power.

The valves can be replaced without

any disassembly. Their sockets are me-

chanically mounted to the thick (2mm)

PCB to prevent the solder joints from

breaking loose during valve removal

or insertion. The thick PCB also helps

to support the relatively high weight

of the output transformers, which

are mounted on the board for ease of

construction.

Temperature-sensitive components

such as electrolytic capacitors have

been kept away from the high-dis-

sipation components, primarily the

6L6 valves and associated 5W cathode

resistors. However, due to the compact

size we have not been 100% success-

ful; one of the large filter capacitors is

near the output valves. Checks of its

temperature during extended opera-

tion show that direct heat transfer is

minimal and should not be a problem.

Semiconductors

There are some semiconductor

components in this circuit but not in

the audio signal path. Mostly, these

perform power supply filtering, to

get rid of ripple and keep the ampli-

fier quiet. The HT delay and soft-start

circuit is also built using solid-state

components.

We should acknowledge considera-

ble input to the design of this amplifier

from Allan Linton-Smith, the designer

of the Majestic loudspeaker system fea-

tured in the June and September 2014

issues. Allan built the first hard-wired

prototype and the concept was then

considerably refined and transferred to

the final PCB featured in these pages.

Allan also suggested using the Al-

tronics line transformers, based on

a discovery by Grant Wills that they

could be used as cheap and effective

ultra-linear valve output transform-

ers - see http://harne.alplialink.

coin .au/-cambief6A N 8 a mp/G r a lit_

Will s_6 CM5 am 1),h t are.

Circuit description

Fig.1 only shows the circuit for the

left channel signal path. The right

channel is identical and the corre-

sponding component numbers are

provided in blue.

The line-level input signal from

RCA socket CON1 has a 1M52 DC bias

resistor to ground, in case the signal

source is floating. The signal then

passes through an RF-rejecting low-

pass filter comprising a 12052 series

resistor and 100pF ceramic capacitor.

The signal is then AC-coupled to

(nominally)

20kn

logarithmic volume-

control potentiometer VR1 by a 1.50

MKT capacitor. This gives a -3dB low-

end roll-off at 5Hz. Note that depend-

ing on part availability, a motorised

potentiometer with a value as low as

51d2 may be used, in which case the

-3dB point rises to 21Hz.

The wiper terminal of VR1 is con-

nected to ground via a 1MS2 resistor

so that if it briefly goes open circuit

during volume changes, the grid of V1a

does not float. The signal is fed to this

grid via a 221(52 RF stopper resistor.

V1a and V1b form the preamplifier,

which is very similar to Jim Rowe's

design from the February 2004 issue

of SILICON CHIP ("Using The Valve

Preamp In A HiFi System"). Essen-

tially, this consists of two common

cathode amplifier stages in series, with

negative feedback around both.

V1's plates are fed from a filtered

rail

of around 224V DC, somewhat

less than the 308V DC main HT rail

due to voltage drops across the two

RC filter resistors (6.81d) and 471d2).

These filters reduce coupling between

channels, reduce coupling from the

output stage to the preamp stages and

minimise supply ripple reaching the

preamp. The preamp is the most noise-

sensitive section as the signal level is

lowest here.

In fact, because hum can be picked

up from AC-powered heater filaments,

we are running the 12AX7 filaments

from regulated 12V DC.

Self biasing

All valves in the circuit are self-

biased. V1a's anode runs at around

120V, ie, 224V minus the drop across

the 270k0 resistor. With zero bias, a

12AX7 will conduct around 3mA at

this voltage, dropping to near-zero

with a grid-cathode bias of around

-2.2V. With a 3.3kf2 cathode resistor,

V1a's operating point tends to settle at

about 0.3mA and thus the cathode is

1.2V above ground.

The output signal from V1a's anode

is coupled to V1b's grid by a 220nF

capacitor and this grid is DC biased

using a 1M52 resistor to ground. V1b

runs at a higher power than V1a,

with a 68052 cathode resistor giving

an operating current of around 1mA.

Therefore, its anode load resistance is

lower at

1001(0.

The output at V1b's plate is coupled

back to V1a's cathode via a pair of

parallel 470nF polyester capacitors

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SILICON CHIP

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siliconchip.com.au

Currawong

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Power (Watts)

Fig.3: distortion versus power for a 1kHz sinewave into 412, 651 and 812 load

impedances. Again, both channels are driven for a realistic test. As you

can see, distortion remains low at under 2W and then rises slowly until the

onset of clipping at around 8-10W, depending on load impedance. The best

power delivery is actually for tin loads, with 611 being virtually identical

and 411 being a little lower, clipping at around 9.5W/channel. This is partly

due to output transformer drive impedance mismatch.

(ie, around 1µF) in series with a 9.1k0

resistor. This sets the closed-loop gain

of the preamp section at around

2.75,

so that the following phase splitter

receives around 3V RMS at maximum

volume. Note, however, that there is

also a feedback path from the ampli-

fier's output, which we will cover later.

Phase splitter

The phase splitter is another 12AX7

twin triode, V2. The phase splitter

provides some gain but its main job is

to produce two similar drive signals

with opposite phase for the grids of the

push-pull output stage valves Signal

is coupled to this phase splitter from

V1b's anode via another 220nF poly-

ester capacitor.

V2a operates as an inverter, to gen-

erate the out-of-phase drive signal.

Like V1a and V1b, it is configured as

a common-cathode amplifier. It runs

from a higher HT rail of around 288V

DC which comes from the first HT

RC filter stage (6.8k0/39µF). Its grid

is tied to ground by a 1MQ resistor,

with the voltage across the shared

6.8k0 cathode resistor providing the

required bias potential.

This resistor is shared with V2b (and

both cathode currents flow through

it). V2b's grid is connected straight to

ground so when its cathode voltage

increases, the grid-cathode bias volt-

age decreases. As such, when V2a's

cathode current increases and its an-

ode voltage drops, V2b's bias increases

and thus V2b's anode/cathode current

decreases, causing the voltage at its

anode to rise.

So the signal at V2b's anode has the

opposite phase to that at V2a's anode,

ie, it is in phase with the signal from

the preamp. The 220k0 anode resistor

value has been selected so that the two

output signals have a similar swing

and so that V2a and V2b both operate

with as high an anode voltage as possi-

ble, to give maximum drive amplitude

for the following stage.

These drive signals are applied to

the grids of 6L6 output valves V3

via 220nF polyester capacitors. These

grids are again tied to ground by 1M0

resistors and there are 10k0 series

stopper resistors to prevent parasitic

oscillation.

Output stage

V3 & V4 are self-biased using 3300

5W cathode resistors, with around 22V

across each. This gives an operating

current of about 65mA. Each output

valve is powered from the main HT

rail of around 308V, via the primary

windings of T3, for a quiescent power

of around 20W each.

Note that DC and AC currents flow in

the two halves of the push-pull wind-

ing since both plates of the tetrodes

are fed from the transformer centre-tap

connection. However, the magnetic

fields produced by these direct cur-

rents are cancelled, as they flow in

opposite directions. This is important

because otherwise the transformer

would be magnetically saturated.

As the current split between V3 &

V4 changes in response to the input

signal however, an AC magnetic field

is induced which is coupled into T3's

secondary. The resultant voltage drives

the speakers or headphones.

Since the output valve quiescent

power of 20W is around twice the

amplifier's power output of 10W per

channel into 80, this gives Class-A

operation. With lower load imped-

ances (for example, 40), V3 or V4 may

be fully cut off during signal peaks,

giving Class-AB operation.

When the input signal swing is posi-

tive, pin 1 of V2a has a negative swing

and so the current through V3 drops.

At the same time, pin 6 of V2b has a

positive swing and thus the current

through V4 increases. This causes an

increase in current flow from the top

(dotted) side of T3's primary to the

other, resulting in a positive swing at

the dotted side of the secondary. Thus,

the output of the amplifier is in phase

with the input.

T3 also has taps approximately half-

way between each end and the centre

(HT) tap. These are connected to the

screens of V3 & V4 via 470 stopper

resistors, providing the ultra-linear

connection mentioned earlier. This

negative feedback from the trans-

former to V3 & V4 cancels out some

of the transformer distortion. Note that

while the feedback signals are high

amplitude, the screen gain is much

less than for signals applied to the grid,

so the feedback doesn't overpower the

drive signals.

Because the signal levels in the out-

put stage are much higher and since

6L6 valves require much more filament

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Chassis/power supply

1 timber plinth with base (details

to come)

1 top cover cut from 3mm clear

acrylic (details to come)

1 small tube acrylic glue

1 front panel, code 01111142, 249

x 30mm

1 rear panel, code 01111143, 248

x 53mm

1 160VA 37+37+15+15V toroidal

transformer (Altronics MC5337)

1 80VA 12+12V toroidal

transformer (Altronics M5112)

4 screw-on 30mm equipment

feet (Jaycar HP0830, Altronics

H0890)

4 M4 x 15mm machine screws

and nuts (for feet)

1 15mm anodised aluminium knob

to suit VR1

1 snap-in fused IEC mains male

socket for 1.6mm panels

(Altronics P8325)

2 M205 250VAC 1A slow-blow

fuses (one spare)

1 red chassis-mount RCA/RCA

socket

1 white chassis-mount RCA/RCA

socket

2 red RCA line plugs

2 white RCA line plugs

2 red binding posts (Jaycar

PT0453, Altronics P9252)

2 black binding posts (Jaycar

PT0461, Altronics P9254)

1 SPST ultra-mini rocker switch,

250VAC rated (Altronics S3202,

Jaycar SK0975)

current than 12AX7s, we run the fila-

ments of V3 & V4 (and V7/V8) from

6.1V AC, slightly shy of the nominal

6.3V, due to compromises made in

power transformer selection. It still

works fine; it just takes a little longer

for the valves to reach

full

emission

after switch-on.

Speaker connections & feedback

A 47052 1W resistor across T3's sec-

ondary ensures that there is some load

even if there is no speaker connected.

This is necessary because operating a

push-pull transformer-coupled ampli-

fier with no load can lead to very high

AC voltages at the valve plates and sub-

sequent flash-over in the valve sockets.

T3's secondary connects to the

1 lm length 2-core mains flex

1 lm length 3-core mains flex

1 200mm length 3mm diameter

black heatshrink tubing

1 200mm length 8mm diameter

black heatshrink tubing

1 200mm length 20mm diameter

black heatshrink tubing

1 lm length heavy duty red hook-

up wire

1 lm length heavy duty black hook-

up wire

1 1m length single-core shielded

cable

1 lm length medium duty black

hook-up wire

1 12-way screw terminal strip

(Jaycar HM3194, Altronics

P2135A)

6 M3 x 25mm Nylon screws and

nuts

1 M4 x 6mm machine screw

2 M4 nuts

2 4mm ID shakeproof washers

1 4mm ID eyelet crimp connector

3 red 6.4mm crimp spade

connectors

12 4G x 9mm self-tapping screws

10 small Nylon cable ties

Main board

1 double-sided PCB, code

01111141, 272 x 255nnm

2 15W 100V line transformers

(T1,T2) (Altronics M1115 — do

not substitute)

2 5VDC coil 3A contact SPDT

micro relays (RLY1,RLY2)

(Altronics S4141B)

speaker terminals via the normally

closed contacts of RLY1 and pluggable

terminal block CON3.

RLY1 is energised if headphones are

plugged into the front panel socket,

disconnecting the speaker and re-

directing the signal to headphone

socket CON5 via a 22051 resistor.

LK4

is fitted (and we recommend

that it is), feedback is applied from

T3's secondary to V1a's cathode via

a 470nF capacitor and 221d2 resistor.

Since the output is in phase with the

input, by applying some of the output

signal to V1a's cathode, we effectively

reduce the drive for V1a, giving about

14dB of negative feedback.

There is a limit to how much feed-

back can be applied in this manner due

6 M205 fuse clips (F1-F3)

1 1A M205 slow-blow fuse (F1)

1 3A M205 slow-blow fuse (F2)

1 6A M205 slow-blow fuse (F3)

1 white vertical RCA socket

(Altronics P0131) (CON1)

1 red vertical RCA socket (Altronics

P0132) (CON2)

2 2-way vertical pluggable terminal

blocks (CON3,CON4) (Jaycar

HM3112+HM3122, Altronics

P2512+P2532)

1 PCB-mount switched 6.35mm

stereo jack socket with long pins

(CON5) (Jaycar PS0190)

1 3-way vertical pluggable

terminal block (CON7) (Jaycar

HM3113+HM3123, Altronics

P2513+P2533)

1 5-way vertical pluggable terminal

block (CON8) (Altronics

P2515+P2535)

4 chassis-mount phenolic 9-pin

valve sockets with bracket

(V1,V2,V5,V6) (Jaycar PS2082)

4 chassis-mount ceramic 8-pin

valve sockets with bracket

(V3,V4,V7,V8) (Altronics P8501)

6 2-way pin headers, 2.54mm pitch

(LK1-LK6)

2 shorting blocks (LK4-LK5)

1 5-501(1/ 16mm dual gang log pot*

(VR1)

2 6073B-type mini flag heatsinks

4 M4 x 10mm machine screws

4 M4 shakeproof washers

4 M4 nuts

8 M3 x 15-16mm machine screws

10 M3 x 10mm machine screws

12 M3 shakeproof washers

12 M3 nuts

to the phase shift created by the induct-

ance of T3. We have set the feedback to

give as much distortion cancellation as

possible, while keeping it stable with

capacitive loads.

The circuit as presented is stable

with several microfarads across the

load, even when driving it with a

square wave.

By the way, the 470nF capacitor in

the feedback path is important as it

damps shifts in valve bias in response

to changes in mains voltages and valve

temperatures.

With feedback enabled, input sensi-

tivity is around 1V RMS. Typical CD/

DVD/Blu-ray players produce around

2V RMS so this should be plenty in

most circumstances. With LK4 re-

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14 M3 Nylon nuts

22 3mm inner diameter Nylon flat

washers

8 6.3mm M3 Nylon tapped

spacers

2 TO-220 insulating washers and

bushes

1 lm length medium duty blue

hookup wire (250VAC rated)

1 lm length shielded audio cable

1 200mm length 3mm diameter

blue heatshrink tubing

6 small green Nylon cable ties

(maximum 2mm wide)

2 small blue Nylon cable ties

* >

201(12 recommended; substitute

motorised pot for remote control

option (see details in part two

next month)

Valves

4 12AX7 dual triodes (V1,V2, V5,

V6)

4 6L6 beam tetrodes - matched

pairs if possible (V3,V4, V7,V8)

Semiconductors

1 4093B quad NAND Schmitt

trigger IC (IC1)

1 LM/LT1084-ADJ 5A adjustable

low-dropout regulator (REG1)

1 KSC5603D 800V 3A high-gain

NPN transistor (Q1)

3 STX0560 600V 1A NPN high-

gain transistors (Q2-Q4)

3 BC547 100mA NPN transistors

(Q5,Q7,Q9)

2 BC557 100mA PNP transistors

(Q6,Q8)

1 red/green 2-lead bi-colour 3mm

LED with diffused lens (LED1)

moved, the overall gain is much higher

and the input sensitivity is around

350mV RMS for full power. However,

distortion rises to around 0.5% at 1kHz

and >1% at lower frequencies.

Note that the 470nF series capaci-

tors in the feedback network are im-

portant. These form high-pass filters

in combination with the feedback

resistors, with a -3dB point of around

15Hz. If DC feedback is used, the bias

time constants in the circuit form a

type of relaxation oscillator and the

bias voltages never quite settle down,

leading to asymmetric clipping and

other undesirable behaviour.

Power supply

The separate power supply circuit

5 blue diffused lens 3mm LEDs

(LED2-LED6)

1 W04 1.5A bridge rectifier (BR1)

2 1N5408 3A 1000V diodes

(D1,D2)

3 1N4007 1 A 1000V diodes

(D4-D6)

Capacitors

1 22001.IF 25V electrolytic

2 470jtF 400V snap-in

electrolytic

4 100µF 50V electrolytic

2 100jiF 16V electrolytic

5 39µF 400V low-profile snap-

in electrolytic (Nichicon

LGJ2G390MELZ15) (Mouser)

2 1.51.1F 63V MKT

5 470nF 630V polyester

2 470nF 63V MKT

8 2201AF 630V polyester

1 100nF 63V MKT or 50V

multi-layer ceramic

2 100pF ceramic disc

Resistors (1W, 5%)

9 1MQ



2 9.11d2

2 2701(52



4 6.8k52

2 2201(11



2 3.3k52

2 120k52



2 68052

2 100kQ



2 47052

6 47k52



2 220Q

2 221(52



1 8252

5 10k12



4 4712

4 33052 (5W, 10%)

Resistors (0.25W, 1%)

7 1MQ



1 56052

1 1501d2



2 47052

2 10k11



1 33052

2 11(5)



4 12052

is shown in Fig.2. All components, ex-

cept the two power transformers T1 &

T2, power switch S1 and the fused IEC

mains socket, are on the main board.

There are three main power require-

ments for this circuit: the 310V HT rail,

the -12V DC filament supply for the

12AX7s (at around 1A) and -6VAC

for the 6L6 filaments, at around 4A.

We also use the 12V DC rail to power

various ancillary circuits, as described

below.

All of T1's secondaries are con-

nected in series, along with one of

T2's secondaries, to produce 114VAC.

T2's other secondary provides a little

over 12VAC, to run the 6L6 filaments

at around 6.1VAC each, in series pairs.

The 12VAC is also rectified, filtered

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NOVEMBER 2014

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Most of the parts except mainly the power transformers are

mounted on a single large PCB to make the assembly easy. The

optional remote volume control is built on a separate PCB.

and regulated to provide the 12V DC

rail (actually about 12.3V DC), for the

12AX7 filaments and DC-powered

circuitry.

The 114VAC from CONT is rectified

in a half-wave voltage doubler consist-

ing of 1000V 3A diodes D1 & D2 and

two 470µF 400V capacitors, giving

about 310V across both capacitors with

several volts of ripple. Fuse Fl pro-

vides some protection against faults.

There are two 47142 series-connect-

ed bleeder resistors to discharge the

47011F capacitors when power is re-

moved. Four blue LEDs are connected

in series with the two 47142 1W resis-

tors. The blue LEDs indicate the pres-

ence of HT and also illuminate output

transformers T3 and T4 (very effective

in a room with subdued lighting).

The output stage has no HT low-

pass filter, unlike the preamplifier and

phase splitters. So to prevent HT ripple

in the output stage from affecting the

signal, we are using an active ripple



SILICON CHIP

filter. This is a capacitance multiplier

filter built around high-voltage, high-

current transistor Q1, configured as an

emitter-follower.

The traditional HT filter is a large

iron-cored choke but these are heavy

and expensive, not to mention hard to

find these days. Our transistor-based

method is more effective and cheaper.

Q1 is driven by Q2 and Q3 which are

high-voltage high-gain signal transis-

tors, in a "Triplington" configuration;

it's like a Darlington but with an extra

stage. The higher the gain in this buffer,

the more effective the filter is. Base

bias comes from an RC low-pass filter

across the incoming HT rail, consisting

of a 11‘4,0 resistor and 470nF polyester

capacitor.

Q2 and Q3 have a gain of around 70-

100 each while Q1 has a gain of around

30. So the overall gain is about 70 x 70

x 30 = 147,000 which multiplies the

effect of the 470nF capacitor to act as if

it is 69,000g! In practice, it isn't quite

as good as this as the 470nF capacitor

discharges slightly through the three

base-emitter junctions at the trough

of each ripple cycle but despite this,

the ripple at Q1's emitter is just a few

hundred millivolts.

Q1 has an integral emitter-collector

diode so that when the unit is switched

off, the output filter capacitors can

safely discharge back into the input

filter capacitors without doing any

damage. D4 protects Q2 while D5

provides similar protection for Q3 but

also has a role in the start-up delay,

which we'll explain later.

Note that this arrangement also

results in HT "soft-start" as it takes

a few hundred milliseconds for the

470nF capacitor to charge and the HT

rail tracks this voltage.

Turn-on delay

We have also incorporated a 20-sec-

ond (or so) turn-on delay, to allow the

valve filaments to heat up before HT

siliconchip.com.au



100



200

500



Frequency (Hz)

Fig.4: distortion versus frequency, with both channels driven at 1W into

three different resistive loads. As you can see, the distortion is pretty low

for a valve amplifier, especially between 100Hz and 10kHz. Below 100Hz,

distortion rises steeply due primarily to the output transformer's non-linear

response. Distortion into lower impedances is only slightly worse than that

for 812. Note the 80kHz bandwidth used, to ensure that higher frequency

harmonics are included in the measurements.

10k

20k

0.2

0.1

0.05

0.02

0.01

The PCB is slid into a slot that runs around the top inside

of the timber plinth. Perspex covers will be used to protect the

PCB and speaker transformers.

09/10/14

14:35:26

240VAC mains, output level 1W, 20Hz-80kHz bandwidth, both channels driven

Ill

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io Currawong THD+N vs Frequency

ion

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ise

0.5

is applied. Part of the rationale for

this is to prevent "cathode stripping"

which can occur with cold cathodes,

although the existence of this phe-

nomenon is somewhat controversial.

But since the valves aren't "ready"

to operate immediately anyway, it

certainly doesn't hurt to delay the

application of HT.

IC1 is a quad Schmitt-input NAND

gate which runs from the 12V rail and

provides the turn-on delay. Note that

ground for the 12V rail is labelled

VEE

and will be close to, but not necessarily

at, GND (OV).

IC1a is connected as an inverter

with a 100uF capacitor from its input

to ground. A 1501(12 resistor charges

this capacitor from the 12V rail while

a 1MS2 capacitor discharges it when

power is switched off. It takes about

20 seconds for this capacitor to charge

to a sufficient voltage for the output of

IC1a to go low.

During this time, IC1a's output is

high. This is inverted by IC1c and then

again by IC1d, so Q4 (another 600V

transistor) is switched on initially.

This keeps the 470nF capacitor in

the HT filter from charging until the

delay has ended. Diode D5 in the HT

filter prevents the base of Q3 from

being pulled below GND when

VEE

(slightly) negative.

IC1a and IC1c also drive LED1 via

two pairs of complementary emitter-

followers (Q5-Q8). LED1 is a bi-colour

device and consists of a red LED and

green LED on the same die, connected

in inverse parallel. Since inverter IC1c

is between them, one inverter is always

driving one end of LED1 high and the

other is driving it low. Thus LED1 is

red initially at turn-on and switches

to green once the time-out period has

expired and the HT rail is powered up.

A 1k0 resistor sets the LED current

to about 10mA while another

ikn

re-

sistor partially isolates the bases of Q5

& Q6 from IC1a's output. This allows

the optional remote control board to

independently drive LED1, in order to

flash it to acknowledge infrared com-

mand reception. The remote control

board connects via CON10 and will

be described next month.

Low-voltage supplies

5-pin pluggable terminal block

CON8 provides separate low-voltage

AC connections for the 6L6 filaments

(pins 1 & 3) and the regulated supply

(pins 4 & 5). Each is fused on the board.

However, we ultimately decided to use

one transformer winding to power both,

hence they are wired in parallel despite

the separate connections.

The 12VAC from pins 4 & 5 of CON8

is rectified by 1.5A bridge rectifier BR1

and filtered with a 2200µF capacitor

to produce around 15-16V DC with

siliconchip. corn. au



NOVEMBER 2014



Currawong Frequency Response



09/10/14 14:58:07

+2.5

Eii



+1.5

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

100



200



500



10k



20k



50k 100k

Frequency (Hz)

Fig.5: the frequency response is pretty flat in the audible range (note: the

vertical scale is only ±3dB for the entire diagram). Roll-off at the high

frequency end is -3dB at around 50kHz while the low-end -3dB point is

below 10Hz. The peak at around 20Hz is partly due to the AC-coupled

global feedback and partly due to greatly increased waveform distortion

below about 30Hz due to the output transformers. However, the peak

amplitude is only around +1.5dB and 20Hz signals are barely audible.

about 1V ripple. This is regulated to

provide a nice smooth rail by REGI, a

low-dropout, high-current equivalent

to the LM317.

Pins 1 & 3 of CON8 connect straight

to the series/parallel-connected 6L6

filaments and as a result, they get about

6.1VAC each. One end of this AC sup-

ply is grounded for noise immunity.

Now because of this ground connec-

tion and the fact that the same trans-

former secondary is used to feed BR1,

the negative end of BR1 actually floats

between about +0.7V and -15V. Hence,

the need to disconnect VEE from GND.

If two separate 12V transformers or

windings were used, LK6 could be

fitted and thus VEE would be at the

same potential as GND. LK6 must not

be fitted with the supply arrangement

shown here!

The circuit will work the same re-

gardless as to whether VEE is connected

to GND, as Q4 is the only connection

between the two supply "domains".

The DC supply is also used to power

relays RLY1 and RLY2 when head-

phones are plugged in. These are 5V

relays, so an 8252 series resistor drops

the 12V DC to an appropriate voltage.

LED2 is also connected across the relay

coils, in series with a 33052 current-

limiting resistor, to indicate when the

speakers are disconnected.

Unused linking options

Note that the supply was also de-

signed to operate with the regulated

rail at 6V DC rather than 12V. This

would require a different transformer

(ie, 6VAC rather than 12VAC) and

the option was provided as there are

some 12AX7-compatible valves with

6.3V-only filaments (rather than the

typical arrangement with a 12.6V

centre-tapped filament).

However, given the relative rarity

of these valves, we aren't going to go

into details as to how to reconfigure

the supply except to say that LK1-LK3

are fitted for this purpose. Normally,

they are left out.

PCB layout

We wanted to put as many parts

on the PCB as possible to make this

amplifier easy to build. Soldering parts

to a PCB is certainly a lot easier than

mainiciu

Note: parts of this circuit operate at over

300V DC. Do not touch any components

or any part of the PCB while the unit is

operating or immediately after switch

off. The blue LEDs in the circuit indicate

when dangerous voltages are present.

point-to-point wiring! It minimises the

chances of mistakes and also means

that performance will be consistent

between amplifiers.

The PCB layout was a bit tricky

though, due to the voltages involved.

We have kept tracks with voltages

that may differ by over 60V apart by

2.54mm to prevent arcing, while in

other areas low-voltage tracks need

to be closer together so they can fit.

We also used "star" earthing as much

as possible to avoid hum and ripple

injection into the preamp stages. Most

of the grounds on the board converge

on the main power supply filter capaci-

tor negative pin.

The board has been designed with

plated slots for the valve socket pins

so that they fit snugly and neatly.

All connectors have been placed

along the back of the board, on the

underside, to keep the chassis wiring

neat. The input signals run from the

back of the board to the front (where

the volume pot is mounted) through

shielded cables that are strapped to the

underside of the board, to prevent the

low-level input signals from picking

up ripple, hum and buzz.

We have also used low-profile

components where possible, so that a

clear perspex shield can be fitted over

the top, to prevent prying fingers from

getting a shock, as mentioned earlier.

The valves, main filter capacitors and

output transformers will pass through

cut-outs in this shield, with perspex

boxes around the transformers. The

rest of the components will be safely

underneath.

Next month

That's all we have room for this

month. Over the next couple of months

we will present the main PCB layout

diagram, describe the assembly proce-

dure, explain how to build the plinth

and finish the wiring. We'll also go

through the testing and troubleshoot-

ing procedure and describe the op-

tional infrared remote control which

uses a motorised potentiometer. SC



SILICON CHIP



siliconchip.com.au

By Nicholas Vinen

The Guntawong Stereo

IOW Valve Amplifier, NI

In Pt.1 last month, we described the circuit

and presentation of

our new Currawong stereo valve amplifier.

We now describe the

PCB assembly and detail the timber plinth

and chassis wiring

along with detailed instructions on putting

it all together.

OST OF

THE

parts for the Cur-

rawong are mounted on a single

large PCB. This then slides into a slot

near the top

of a

timber plinth, with

the remaining components — primar-

ily the two large power transformers

— underneath the PCB and attached to

the plywood or MDF base.

The front panel carries the head-

phone socket, volume control, power

switch and status LEDs. The input

connectors, loudspeaker terminals and

power socket are mounted on the rear

panel which is recessed into a cut-out

in the rear of the plinth.



SILICON CHIP

So let's start putting the main PCB

together, which is a significant part

of the work involved in building the

Currawong.

PCB assembly

Start the PCB assembly with refer-

ence to overlay diagram Fig.6. The

board is coded 01111141 and measures

272 x 255mm. It's 2mm thick, which

makes it more rigid and stronger than

typical 1.6mm laminate.

Start by fitting the smaller resistors.

The

colour-coded stripes on small

resistors aren't always distinct so it's

best to check each value with a DMM.

Use the lead off-cuts to make the two

wire links (next to LK4 and LK5).

Follow with the three 1N4007 di-

odes (D4-D6), in the top corners of the

board, with the striped cathode ends

towards the right or bottom of the PCB

as shown. IC1 can go in next — there's

no need for a socket. Check that its pin

1 notch/dot is towards the left side of

the board before soldering it.

Then fit all the

resistors. Their

colour codes are usually clear however

it doesn't hurt to measure them to be

sure. None of these run hot so they

siliconchip. corn. au

=40e4,

ke4

This view shows the completed PCB

assembly for the prototype amplifier,

with the front-panel fitted. It's housed in

a timber plinth and fitted with Perspex

covers to protect the user from high

voltages.

\ARMING! MOM VOLTAGES

High AC and DC voltages are present in this circuit. In particular, mains

voltages (230VAC) are present on the IEC socket and the primary side of the

mains transformers (including the wiring to the power switch). In addition, the

transformer secondaries together provide a 114VAC output and the power

supply produces an HT voltage in excess of 300V DC which is present on

various parts of the amplifier circuit (including the output transformers).

Do not touch any part of the amplifier or power supply circuitry when

power is applied otherwise you could get a severe or even fatal electric

shock.

The blue LEDs in the circuit indicate when high voltages are present. If they

are lit, the power supply and various parts on amplifier board are potentially

dangerous.

The completed amplifier must be fitted with Perspex covers

as described in Pt.3 next month to ensure safety.

can be mounted in contact with the

PCB. You may find it difficult to get the

specified 9.11d21W resistors, so 8.21(52

resistors can be used instead with only

a minor impact on performance (don't

use 101(52 as this may prejudice overall

stability).

The two large 1N5408 power diodes

are next, with both cathode stripes

facing the bottom of the board. These

will get a little warm so we recom-

mend spacing them about 5mm off

the board (eg, using a 5mm-wide strip

of cardboard as a temporary spacer).

The W04 bridge rectifier can also go

in now; again it's a good idea to space

it off the board a little.

Next, fit blue LEDs3-6. These have

a dual purpose: to indicate the pres-

ence of HT and to illuminate the

transformers. They also form part of

the HT bleeder circuit so must not

be left out (if you must omit them,

use wire links in their place). Angle

each one back so that it will shine on

either T3 (LED5, LED6) or T4 (LED3,

LED4) and make sure the longer anode

siliconchip.com.au

leads go through the holes closer to

the righthand side of the board.

Now you can mount all the TO-92

package small signal transistors, ie,

Q2-Q9. Don't get the three different

types mixed up. Follow with the six

fuse clips. Check that the fuse reten-

tion lugs are on the outside before

soldering the clip in place, otherwise

you will not be able to fit the fuses.

Also make sure that they are pushed

all the way down onto the PCB before

soldering them in place.

Next on the list are transistor Q1

and linear regulator REG1. These are

both fitted with small heatsinks and

it's important that the heatsinks are

isolated from the device tabs using

DECEMBER 2014

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TERMINALS WITH DOUBLESLEEVE

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Fig.6: the parts layout on the top

of the PCB. This diagram has been

split and the righthand side partly

duplicated where it meets the

magazine gutter, for clarity. The text

describes how the valve sockets are

mounted; they must be mechanically

secured before the pins are soldered.

Note that D1, D2 and the 5W resistors

should be stood off the board to allow

air to circulate around them and don't

forget to insulate Q1 & REG1 from

their heatsinks.

insulating

washers and bushes — see

Fig.7.

each case, start by bending the

leads of.the TO-220 device so they fit

through the pads on the board, with

the tab hole lined up with the PCB

mounting hole.

Note that Q1's centre lead is bent

closer to the package than the other

two

(due

to the high voltage between

the pins) but this is not the case for

REG1.

To mount each device, place an

insulating bush in the tab hole, then

feed an

M3 x 10mm

machine screw

through from the top. Slip an insulat-

ing washer under the device, over the

screw thread, then slide the heatsink

on from underneath. Drop this assem-

bly

onto the PCB,

ensuring all pins go

through their respective holes, then

use a shakeproof washer and M3 nut

to hold it in place.

each

case, ensure the heatsink

and insulating washer are straight

before tightening the nut fully, then

solder the leads. That done, fit the two

relays, making sure they are nice and

flat on the PCB before soldering more

than two pins.

Now it is time to install the ceramic

capacitors, followed by the MKT

(metallised polyester) capacitors, in

increasing order of height. Follow with

the six pin headers for LK1-LK6. Try

to solder these flat on the board and

neatly aligned with the edges of the

board for best appearance.

You can then bend the leads of the

Fig.7: the mounting details for transistor

Q1 and regulator REG1. In each case,

the device tab must be isolated from the

heatsink using a silicone insulating washer

and insulating bush.

DECEMBER 2014



FLAG

HEATSINK

PCB

M3 x 1 Omm

SCREW



1 INSULATING

BUSH

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

SILICONE

WASHER



STAR

M3 NUT'



WASHER

The 9-pin sockets are secured using

M3 x 10mm machine screws, with a

Nylon nut & two Nylon washers used

as spacers at each mounting point.

Since publishing Pt.1 last month,

we've made a few small changes

to the chassis arrangement and

this affects the list of parts required.

Please note the following changes:

Main board

(1)

Delete vertical RCA sockets

for CON1 & CON2; add short

stereo RCA-RCA lead

(2)

2 x 8.2k51 1W resistors can

be used instead of the 2 x

9.1kS2 1W resistors listed

Revised chassis parts

1 timber plinth with base (see text)

1 top cover cut from 3mm clear

acrylic (details to come)

1 small tube acrylic glue

1 front panel, code 01111142, 249

x 30mm

1 rear panel, code 01111143, 248

x 53mm

1 160VA 37+37+15+15V toroidal

transformer (Altronics MC5337)

1 80VA 12+12V toroidal

transformer (Altronics M5112)

4 screw-on 50mm equipment feet

(Jaycar HP0832)

1 15mm anodised aluminium knob

to suit VR1

1 snap-in fused IEC mains male

socket for 1.6mm panels

(Altronics P8325)

2 M205 250VAC 1A slow-blow

fuses (one spare)

1 red chassis-mount RCA/RCA

socket

1 white chassis-mount RCA/RCA

socket

2 red binding posts (Jaycar

PT0453, Altronics P9252)

2 black binding posts (Jaycar

PT0461, Altronics P9254)

1 SPST ultra-mini rocker switch,

250VAC rated (Altronics S3202,

Jaycar SK0975)

1 lm length 2-core mains flex

1 lm length 3-core mains flex

1 200mm length 3mm diameter

black heatshrink tubing

1 lm length 5mm diameter clear

heatshrink tubing

1 200mm length 20mm diameter

black heatshrink tubing

1 50mm length 50mm diameter

black heatshrink tubing or large

insulating boot (Jaycar PM4016)

1 lm length heavy duty red hook-up

wire

1 lm length heavy duty black hook-

up wire

1 500mm length figure-8 speaker

wire

1 12-way screw terminal strip

(Jaycar HM3194, Altronics

P2135A)

1 M4 x 10mm machine screw

2 M4 nuts and shakeproof washers

2 yellow 5.3mm ID eyelet crimp

connectors

2 red 8.4mm ID eyelet crimp

connectors

5 red 6.4mm insulated spade crimp

connectors

4 solder lugs

1 5mm cable clamp (P-clamp)

12 black 4G x 12mm self-tapping

screws

12 4G x 9mm self-tapping screws

1 4G x 6mm self-tapping screw

1 3mm ID flat washer

7 3mm ID spring washers

10 small Nylon cable ties

four 5W resistors to fit through their

mounting holes. There are two pairs

of holes for each; we used the inner

pair but it isn't mandatory. As with the

1N5408 diodes, use a 5mm spacer to

stand each resistor off the board. Keep

them level and straight for a tidy result.

The 630V polyester capacitors can

then be fitted. The PCB is designed

with multiple pads for each capaci-

tor, to suit different lead spacings. If

you have an odd one, you may need

to bend its leads out however most

should drop straight in. Refer to Fig.6

to see which type goes where.

Now solder the smaller electrolytic

capacitors in place, ie, the six 1001,tF

types. In each case, ensure that the

longer (positive) lead goes in the pad

closer to the front edge of the board,

as shown on Fig.6.

Valve sockets

The valve sockets are secured to the

board before soldering, so the solder

joints aren't under stress. The speci-

fied sockets have solder lugs and the

board has been designed with slots to

accommodate them.

Start with the smaller 9-pin sockets.

Feed M3 x 10mm machine screws

through the top mounting holes and

tighten Nylon nuts on the underside.

Slip two Nylon washers over each

screw thread, then pass the screws

down through the mounting holes

This mock-up shows the final mounting

arrangement used for the 8-pin sockets

(it differs slightly from that used on the

prototype). These sockets are secured

using M3 x 15mm screws and M3 x

6.3mm tapped Nylon spacers.

on the board, guiding the solder lugs

through the slots. If it won't go in,

check that you have the right orienta-

tion as it will only fit one way.

You may need to put the solder lugs

under a small amount of tension to get

them to go through the slots, due to the

way they are angled. But once they all

line up it should slip into place and

you can push the socket right down

so it's sitting on the Nylon washers.

Use a shakeproof washer and M3

nut to secure the screw closest to the

front (bottom) edge of the board. Fit

a Nylon washer and nut to the other

(this is necessary to avoid shorts to

adjacent PCB tracks) — see Fig.8. Do

both nuts up tightly, check that the

socket is sitting level on the PCB and

then solder the pins and repeat for the

other three sockets. It isn't necessary

to trim the solder lugs after soldering.

For the larger 8-pin sockets, the ar-

rangement is similar but their mount-

ing brackets are supplied separately.

Take a bracket and feed M3 x 15mm

machine screws through the top of

the mounting yokes (see photos), then

loosely screw M3 x 6.3mm Nylon

tapped spacers on, just tightly enough



SILICON CHIP

siliconchip.com.au

Another view of the completed PCB assembly, this time taken from the rear. Check the board carefully after assembly to

ensure that all polarised parts are correctly orientated. Note that the 22000 capacitor indicated by the red arrow must

be mounted horizontally (ie, on its side) as shown in Fig.6, not vertically as shown here.

so that the screws stay in place.

Now position this assembly over

the PCB and adjust the spacing so that

the two screws are equally far from

the centre of the mounting bracket

and they pass through the appropri-

ate holes on the PCB. You can then

remove the bracket and drop the socket

in place. Some "jockeying" may be

required but it should fit easily once

you get all the pins lined up.

These sockets can

be installed

with eight different orientations but

only one is correct. The notch in the

central hole must

face towards

the

lefthand side of the PCB. If you solder

one incorrectly, it will be difficult, if

not impossible to remove (see Fig.6).

With the socket pushed down onto

the PCB and orientated correctly, slip

the bracket on top and secure it in posi-

tion with shakeproof washers and M3

nuts.

Note though that for valves V4

and V8, the mounting screw closest to

the front (bottom) edge of the board

must be secured with a Nylon washer

and Nylon nut instead.

Do both mounting nuts and screws

up tightly, then re-check the socket

orientation before soldering the eight

lugs. Repeat for the other three sockets.

With all the sockets in place, fit

the

siliconchip.com.au

five low-profile 39µF 400V snap-in

capacitors, again with their positive

terminals towards the front (bottom)

of the board. These should be pushed

all the way down before soldering.

The 2200µF capacitor can now go

in however

must be laid over to-

wards transformer T3 or else the top

cover will not fit later. There should

be sufficient room for it to sit flat on

its side on the PCB. Like the others,

it is polarised and the negative stripe

should face up.

You can then fit the two large 400V

capacitors between T3 and T4. Double-

check their orientation before solder-

ing the leads and they too should sit

right down on the board; if they aren't

perfectly vertical, they may not later

fit through their corresponding holes

in the top cover.

Output transformers

The 15W 100V line transformers

(T3 & T4) come fitted with a terminal

block on top and stickers indicating

the taps. We removed these as we felt it

improved the appearance. The stickers

can be peeled off and the glue residue

gently cleaned off using an appropriate

solvent. Methylated spirits or isopro-

pyl alcohol are good choices are they

are unlikely to damage the transformer

but try not to soak it.

The terminal blocks can be simply

pulled off although they're a tight fit

and you may need to use pliers. The

metal tab used to hold it in place is

then bent down.

The next job is to cut, solder and

insulate four or five wires to the wind-

ing tap lugs. The 10W winding is not

connected on the PCB so you don't

have to wire it up but we did anyway,

because we thought it would look bet-

ter. We used blue wire, to match the

blue transformer insulation although

different colours are shown on Fig.6

for clarity.

Cut each wire to a length of about

70mm, then strip about 3mm of insu-

lation from one end and 6mm from

the other. Feed the longer section of

exposed wire through one of the solder

lugs and double it over, then solder it

in place. Try not to heat the joint more

than necessary or add too much solder.

The output transformer terminals

all operate at 308V DC and they must

all be

fully insulated with two lay-

ers of heatshrink sleeving to ensure

safety. It's

just a matter of slipping a

15mm-length of 3mm-diameter blue

heatshrink tubing over each terminal

DECEMBER 2014



•

• •

•

A •

k .

Left Input

CON1

•



• •

114V AC In 6/12VAC 2x6.3V AC

R ght Output

•

CONB•

•

Insulate!



•



Insulate! .

4 5

‘



▪

•

Currawori

Steil:go Valve Arrftili er

VR1

VOLUME



P •

DC C

- •

COIN

•

CON7 •

•

Right Input

CON2

•

• •

7/ 8

5 \



Imam

1 \

Insulate!

•

• • •

•

0 • L—

•

0 0.

•



•

SILICON

CHIP

01111141

PHONES

CONS



)

This view shows the parts in position on the underside of the PCB. Note the Nylon nuts and

washers used to secure the valve sockets at various locations, as indicated by the red arrows.

Note also that this is a prototype PCB and the short wire links on two of the 12AX7 valve

sockets have been eliminated from the final version shown in Fig.8

0 0

and shrinking it down, then adding a

second layer.

Make sure each terminal

is fully insulated, including the 10W

tap, even if you aren't soldering a wire

to it. If necessary, use neutral cure

silicone to ensure that there is no gap

in the insulation where each terminal

goes into the transformer.

Twist the bare strands together at the

other end of each wire and tin them in

Fig.8: the parts layout on the

underside of the PCB. A motorised pot

is shown here for VR1 but a regular

16mm

dual-gang log pot can be used

instead if you don't want remote

volume control. The RA/GA markings

for LED1 indicate the position of the

red LED anode and green LED anode

respectively. Note the orientation of

CON3, CON4, CONT & CON8 and be

sure to use Nylon nuts and washers

at the indicated "insulate" positions

when securing the valve sockets.

preparation for mounting. Do the same

with the three pre-existing wires, after

trimming them so that they will reach

their

PCB

pads with a little slack. You

can place the transformer temporarily

on the board to check this. Don't cut

the leads too short.

Once all the wires have been pre-

pared, fit the transformers to the board

using M4 x 10mm machine screws,

shakeproof washers and nuts. The

front side (facing the bottom of the

board) should have five or six connec-

tions, while the rear of the transformer

has two. Make sure they are nice and

square with the rear edge of the board,

centred on their mounting positions

and firmly secured.

It's then just a matter of soldering

the eight wires to the

PCB.

The top-

most of the five front wires goes to

the leftmost pad, the next one down

to the second-from-left and so on.

DECEMBER

2014

250

22 -



FRONT PANEL

(MDF BASE, INSET)



REAR PANEL



294

(NOT DRAWN TO SCALE)



294



AL DIMENSIONS IN MIWMETRES

SIDE PANELS: 277mm LENGTHS OF 89 x 19mm DAR PINE

WITH 45° INWARD CHAMFER AT EACH END, 2mm SLOT

9mm DEEP ALONG INSIDE, 13mm FROM TOP EDGE.

ALSO 10mm DEEP x 9mm WIDE REBATE ALONG

INSIDE LOWER EDGE

REAR PANEL: 294mm LENGTH OF 89 x 19mm DAR PINE

WITH 45° INWARD CHAMFER AT EACH END, 2mm SLOT

9mm DEEP ALONG INSIDE, 13mm FROM TOP EDGE.

ALSO 10mm DEEP x9mm WIDE REBATE ALONG INSIDE

LOWER EDGE, OTHER REBATE & CUTOUT AS SHOWN

FRONT PANEL 294mm LENGTH OF 89 x 19mm DAR PINE

WITH 45° INWARD CHAMFER AT EACH END, 10mm DEEP

x9mm WIDE REBATE ALONG INSIDE LOWER EDGE,

SLOTS, REBATES & CUTOUTS AS SHOWN

BOTTOM: 276

259 x 9mm MDF RECESSED INTO BASE

Fig.9: the Currawong plinth details. It's made from four lengths of 89 x 19mm DAR pine arranged in a rectangle with a

9mm MDF or plywood base. The slot cut into the sides and rear accommodates the 2mm-thick PCB while the cut-outs

and rebates at the front and rear are for the two panels which the controls and connectors pass through.

Skip the pad labelled "10W" if you

only soldered four wires. These can

be tack-soldered initially from the top

(without melting the wire insulation),

then pushed through the board and

soldered from the bottom afterwards.

Connect the three remaining wires

as shown on Fig.6, then tie the bundles

of four or five blue wires together using

blue cable ties.

Underside components

The

remaining parts are fitted to

the other side of the board, as shown

on Fig.8. Start with the

two

shielded

cables which run down the middle.

First, cut them to length and remove

about

15mm

of the

outer

insulation

from either

end, then twist the exposed

shield braid

wires

together and strip

about 5mm of the inner insulation

away. Now twist together and tin

these inner conductors and also tin

the twisted end of the shield.

Solder the shield wires into the

larger of the two holes at either end

of the board and the inner wires into

the smaller pads — see Fig.8. Make

sure

there's sufficient

solder on

the

shield braid

so that it's

rigid and can't

move and short to any adjacent pads.

Also try to keep the wire reasonably

taut along the bottom of the board.

Once they've been soldered at each

end, fit the

six

cable ties (three per

wire)

using the

slots

provided.

Next, fit the four pluggable terminal

blocks. Make sure these go in the right



SILICON CHIP

way around, with the curved

sections

towards the back edge of the board

(ie,

towards

the nearest edge).

The headphone socket can be mount-

ed next and must be pushed all the

way down onto the PCB. This can be

followed by dual potentiometer VR1,

after cutting its

shaft to

15mm

long.

You can cut the potentiometer shaft

using a hacksaw and then file off any

burrs. If you've opted to have remote

volume control, solder the two mount-

ing

lugs

for the motor in addition

to the

six for the pot itself.

Now for the two remaining LEDs:

blue LED2 goes on the left (with the

board right-side up) near the head-

phone

socket while bi-colour LED goes

on the right. Use a DMM set on diode

test mode to figure out which of the

bi-colour LED leads is the red anode

— the LED will light red when the red

lead from the multimeter is connected

to this pin (in our case, the longer of

the two

leads).

This lead

goes

towards

the righthand edge of

the board.

Bend the LED's leads at right angles

7mm from its lens and fit the LED so

that the lens is

centred 10mm below

the top

the PCB (ie, 8mm from the

bottom).

The

other LED is fitted in the

same manner, with its longer (anode)

lead also towards the righthand edge

of the PCB.

Input wiring

Note that what the board is designed

to accept vertical RCA sockets for

the input signals, we decided it

was

easier to

solder

a stereo

RCA cable

directly to the board,

which plugs

straight into the RCA/RCA sockets

on the rear panel. This

provides

more clearance

on the underside

of the board for the transformers.

So we suggest

you get

a short

stereo

RCA lead,

chop

off

a —500mm length,

strip it back and solder it to the left

and right input pads, with the shield

braid to the terminals marked "-" and

the inner conductor to "+".

Now fit the three

fuses; 1A for Fl,

3A for F2 and

for F3 (all slow-

blow). The main board assembly is

now complete.

Building the plinth

The

base of the plinth is a sheet

of 5-ply or 9mm MDF cut to 276 x

259mm while the rest is made from a

single length of

x 19mm dressed all-

round (DAR) pine, cut to two lengths of

277mm

for the sides and two lengths

294mm

for the front and back.

Fig.9 shows the plinth details. A

plunge router is required to cut the

rebates while a mitre or

drop

saw is

used to make 45° cuts so

that

the four

pieces of DAR pine can

be assembled

in a similar manner to a picture frame.

A drop saw is used to cut the 2mm-

wide slots but make sure that all the

slots

will

later line up correctly.

We used wood glue to hold it all

together, along with 6G

x 20mm

wood

screws to additionally secure the base.

siliconchip.com.au

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