AUDIO&VIDEO

Valve Preamplifier
Part 2: construction
Design by Bob Stuurman

In the first part of this article, we described all of the circuit boards. Now it’s
time to look at how they fit together. We first describe the mechanical
construction, after which we turn our attention to wiring the entire assembly.
We conclude with a brief look at the specifications and performance.

tion. Four 15-mm brass standoffs are

fitted to the front, and another four
20-mm standoffs are fitted to the
rear. The face panels of the enclosure
are attached to the standoffs using
M3×5 screws. The end result is that
the aluminium channel section is
‘suspended’ between the front and
rear panels.
There is a 15-mm space between
the front face of the channel section
and the front panel. This provides
room for the fixing nuts of the poten-
tiometers and switch and the gear-
wheels of the balance potentiome-
ters. The LED indicator lamp and the
actuating button for the mains
switch are fitted directly to the front
panel. At the rear, the space is
20 mm deep. This region holds the
I/O board (on 15-mm standoffs), the
mains filter and IEC appliance recep-
tacle and a small fan.
The power supply is fitted against
the back of the channel section, with
An important factor with this type of equip- the front the ‘front panel’ and the the transformer stack in the middle,
ment is the enclosure. For the prototype, we other face panel the ‘rear panel’. the low-voltage circuit board to the
selected a standard model from the Conrad left and the high-voltage circuit
line. It consists of black top and bottom covers board to the right. The two amplifier
made from robust 1.5-mm sheet steel and two Mechanical construction boards are fitted at the front of the
1-mm aluminium face panels. The top and The construction chosen by the channel section. All of these circuit
bottom covers have ventilation openings. The author is the peak of simplicity. It boards are screwed to 10-mm stand-
edges of the face panels are bent at right uses a U-shaped channel section offs, with plastic standoffs being
angles to form lips, to which the top and bot- formed from 1.5-mm sheet alu- used for the high-voltage board. An
tom covers are fastened using four self-tap- minium, with dimensions of aluminium plate with a thickness of
ping screws. The face panels have a brushed 290 × 155 × 85 mm (Figure 1). All of 1 mm and a height of 55 mm, with a
matte finish and protective cover foils. For the the preamplifier subassemblies are turned-up lip along one edge, is
sake of simplicity, we call the face panel at fitted on or inside this channel sec- placed between the power supply

24 Elektor Electronics 10/2003

 AUDIO&VIDEO

Specifications A +0

-10

-20
Nominal signal level, ‘Out’ 450 mV (380 mV in) -30

Nominal signal level, ‘Line Out’ 220 mV (380 mV in) -40

-50

120 kΩ
-60
Input impedance (CD/Tuner/...) d
B -70
r
-80
A

THD+N, ‘Out’ 0.1 % (450 mV out) -90

-100
Signal-to-noise ratio, ‘Out’ 80 dBA -110

-120

Bandwidth (maximum volume) < 10 Hz–35 kHz (–3 dB) -130

-140

-150
20 50 100 200 500 1k 2k 5k 10k 20k 50k 90k
Crosstalk < –65 dB (1 kHz) Hz 020383 - 2 - A

<–40 dB (20 kHz)

B +20

Channel separation > 76 dB (1 kHz) +18

> 60 dB (20 kHz) +16

+14

+12

+10

Phono sensitivity (450 mV out) 5 mV (1 kHz) +8

+6

Signal-to-noise ratio, Phono > 53 dB +4

d +2
B
r -0

Bass adjustment (theoretical) +18/–9 dB (50 Hz) A -2

-4

Treble adjustment (theoretical) +9/–10 dB (10 kHz) -6

-8

-10

-12

-14

Measured response curves: -16

-18

-20
Chart A shows the frequency spectrum at maximum volume. The distor- 10 20 50 100 200 500
Hz
1k 2k 5k 10k 20k
020383 - 2 - B
40k

tion primarily consists of the 2nd harmonic at –60 dB, which explains the
value of 0.1 % for THD+N. The fundamental was suppressed for the measurement. The supply-voltage ripple and induced noise from
the transformers lie below –90 dB and are negligible.
Chart B shows the frequency response with the tone controls in the neutral, minimum and maximum positions. The actual frequency
response may vary due to component tolerances (potentiometers and capacitors).

and the amplifier boards. The lip of the fan is directed toward the heat ble to form the U-shaped channel section to
this plate is fitted underneath the sink of the LM317. This forced airflow a precise dimension. Consequently, the
standoffs for the amplifier boards in through the enclosure also removes standoffs at the rear must be lengthened
order to securely attach it to the the heat generated by the valves. using shim washers until the holes are
channel section. In the prototype, a After the front and rear panels aligned. This also prevents the fan from being
length of plastic cable duct (the type have been firmly screwed to the clamped between the two surfaces, so it can
with a hinged snap cover) was fitted standoffs, the fastening holes for the do its job without making any noise.
along the top edge of this aluminium top and bottom covers will quite
plate on the side towards amplifier likely not align exactly with the cor- On/off switch
boards, in order to provide a cable- responding holes in the face panels. In order to keep the mains wiring (a potential
way for several cables. This cable This is because it is nearly impossi- source of interference) as short as possible,
duct can be clearly seen on the photo
in Figure 2, which also shows the
placement of the circuit boards and
other components.

0
29
Details
74

Cooling fan
20
The ventilation openings in the top T
ON
and bottom covers are not sufficient 15
FR
for the amount of heat to be dissi-
pated. We did not want to deface the
85

enclosure by making large holes in it, 6

27
so we fitted a small fan at the rear of
the enclosure. A series resistor (82 Ω 155
outside
in the prototype) causes the fan to 020383 - 2 - 14

turn quite slowly, and since it is fit-

ted using rubber bushings, it is prac- Figure 1. A U-shaped aluminium channel section is suspended between the front and rear
tically inaudible. The air stream from panels of the enclosure, using 15-mm standoffs at the front and 20-mm standoffs at the rear.

10/2003 Elektor Electronics 25

AUDIO&VIDEO
used for the balance control. If you
arrange a leaf spring such that it
presses a steel ball (from bicycle
bearing, for example) against the
teeth of the gearwheel, you obtain
click stops. A tangible midrange
position can be produced by remov-
ing one tooth at the midrange posi-
tion. All of this is clearly shown on
the photo in Figure 3.

Front panel layout

You are naturally free to label the
front panel with text and/or symbols
according to your personal taste. For
those who prefer a ready-made solu-
tion, a front panel layout is available
on the Elektor Electronics website
for download free of charge. It is also
ideal for use as a drilling template for
the front panel.

Wiring
Figure 2. A ‘bird’s-eye view’ of the inside of the preamplifier. All of the wiring is in place, the The aluminium middle plate has
front and rear panels have been fastened to the channel section and the knobs are fitted. four holes with feedthrough bush-
ings, aligned with the circuit board
connections for the filament voltage
the on/off switch should preferably be fitted Figure 3). Drill and tap the gear- and high voltage, to allow the
as close as possible to the appliance recep- wheels for M3 setscrews. As the wiring to pass through the plate.
tacle. A pushbutton switch with a suitable material is quite soft, it’s a good idea These holes are located at half the
mechanical extension reaching to the front to file flats on the shafts of the poten- height of the plate.
panel is thus recommended. In the prototype, tiometers. This will allow the gear- Figure 4 shows how the power
a special construction was used for this pur- wheels to be securely anchored with wiring of the preamplifier should be
pose, consisting of a rocker switch with a only moderate tightening of the fitted. Start by connecting the trans-
homemade extension arm, but we won’t bore setscrews. formers to the power supply circuit
you with further details. Next comes a tip: it’s quite easy boards, fitting the wiring for the cool-
to equip the balance potentiometers ing fan and making the mains volt-
Balance control with a tangible midrange position age connections between the IEC
As already mentioned in the first part of this and ‘click stops’. This can be done appliance receptacle and the switch,
article, the balance control is constructed by fitting the shafts of the volume fuse and mains filter. Connect the fil-
using two log-taper potentiometers that are and tone control potentiometers ter terminal to a solder lug screwed
mechanically coupled using gearwheels (see with the same type of gearwheels as to the channel section. Next, make
the connections to the filament and
high-voltage terminals using
twisted-pair wiring passing through
the feedthrough bushings. Fit the ‘fil-
ament’ jumpers to the K2 connectors
on the circuit boards and connect the
filaments of the EF86s in series with
a length of wire. This wire is routed
through the cable duct. Finally, con-
nect the LED indicator lamp and the
input selector switch (S1). Terminal 1
of S1 connects directly to K2/1–2 on
the left-hand amplifier board, and
then to K2/1–2 on the right-hand
amplifier board via a length of flat
cable. This six-way flat cable (which
is reduced to 5 leads before continu-
Figure 3. The balance potentiometers are coupled by a pair of gearwheels. The centre-to- ing to the I/O board) also runs
centre spacing is 25 mm. through the cable duct.

26 Elektor Electronics 10/2003

 AUDIO&VIDEO

Gnd-Phono-L
Line-In-L
K1
L
2345
IEC/filter Fan Gnd-Out 0
R

Rfan

Gnd-Phono-R
Line-In-R
82 Ω

white
white
Tr1
0 V* red 260 V=
F2
250 mAT yellow Tr2

red
S2 yellow white
12.6 V= 15 V ~ 230 V~
blue white 0V
grey

blue
grey

= twisted

K2
0V 1
2
3
+260V 0V* 4
5
6
+12.6V 7
14 8
13 9
12 10
= flatcable 11 11
10 12
9 13
8 14
7 +12.6V
6
5
4 0V* +260V
3
2
1 0V
K2

3 4
2 1 - Phono
5 2 - Cd
12.6 V 1 3 - Tape
4 - Tuner
S1-Select 5 - Aux
LED display
020383 - 2 - 15

0 V*

Figure 4. Connection diagram for the power wiring.

Figure 5 shows how the signal tiometers, the screens are connected limits of travel in turn, and at each position,
wiring must be fitted. The volume, to the terminals on the left, and on adjust P3 to make the minimum and maxi-
treble and bass potentiometers are the I/O board they are connected to mum values for the left and right channels as
connected using short lengths of flat Gnd Out, which is also the connec- nearly as possible the same (at 50 Hz and
cable, while the rest of the connec- tion point for the screens of the Line 10 kHz, respectively). Since log-taper poten-
tions are made using screened cable. Out cables. The three screened tiometers are used for the tone controls, the
It is convenient to first cut lengths of cables leading to the left-hand midrange position will not precisely coincide
screened cable and connect them to amplifier board run through the with a flat frequency response. In order to
the I/O board before it is screwed in cable duct. The ‘ground network’ is find the setting where the response is flat,
place, since it is then still readily connected to the aluminium channel apply a 1-kHz square wave signal to the input
accessible. section at one point only, which is (not the Phono input) and observe the output
Be particularly careful with the Gnd L on the I/O board. signal on an oscilloscope. Turn the bass and
connections for the cable screens. At As can also be seen in Figure 5, treble controls until the square wave is as
the Phono In connectors, the screen the additional DC blocking capaci- good as possible. With the potentiometers
is connected to the ground point on tors Cx and Cy can best be soldered adjusted to achieve this condition, secure the
the circuit board, and on the I/O directly to the Line In sockets on the knobs for the tone controls with their arrows
board the same screen is connected I/O board. pointing to ‘0’.
to Gnd L or Gnd R, respectively, To align the balance control, start by set-
which is also the connection point ting the right-channel balance potentiometer
for the screen of the Line In cable. Alignment (to which the knob will be attached) to its
For the output cables, the screens Aligning the tone controls is best midrange position. Then rotate the other
are jumpered to the circuit-board done by connecting an oscilloscope potentiometer (but not the gearwheel!) until
ground points at P5 on each ampli- or AC millivoltmeter to the output. the output signals of the two amplifiers have
fier board. At the balance poten- Turn P2 and P4 to the left and right equal amplitudes. With the potentiometers in

10/2003 Elektor Electronics 27

AUDIO&VIDEO
Figure 5. Connection diagram for the signal wiring.

Gnd-Phono-L
Line-In-L
L

Gnd-Out
R

Gnd-Phono-R
Line-In-R
K2
P2 P4 P1 1
2
P5 P4 P2 123 3 K1
1 321321 4 123
2 5
6
3 7
14 8
13 9
12 10
11 11
10 12
9 13
8 14
7 123123 P5
6 3
5 2
4
K1 3 1
2 321
321 1 P1
K2

Sp1 Sp2

25 mm
Treble Volume Bass 020383 - 2 - 16

this position, secure the gearwheel in place. rather large, it is worthwhile to give 75-Ω coaxial cable (diameter 6.2 mm)
If any small difference remains, it can be some attention to the connecting and ‘high-end’ Cinch connectors.
adjusted by slightly rotating the case of the cables. In this case, you should This type of cable has a capacitance
potentiometer. select cables with low capacitance, of only 69 pF/m, so it can easily
After the final (output) amplifier is con- so the high frequencies will be cover distances of up to several
nected, a slight amount of noise and some affected as little as possible. The metres.
residual hum will be audible when the vol- author even built cables using RG-59 (020383-2)
ume control is set to maximum and the Phono
input is selected. When the volume control is
rotated, you will probably hear a weak ‘rasp-
ing’ noise. This can be eliminated by using a MECHANICAL COMPONENTS LIST
small metal spring to electrically connect the – Two-section steel enclosure, 300x200x110 mm (wxdxh),
shaft of the potentiometer to the enclosure. Conrad Electronics # 520489*
The hum will also disappear after the upper – IEC appliance socket with internal filter
and lower metal covers of the enclosure have – Ventilator 40 x 40 mm, 20mm thick, 12VDC
been fitted in place. The two halves of the – 2 ABS gearwheels 50.M0.5, Conrad Electronics # 237850*
enclosure can be connected to the central – S1 = rotary switch, 6 positions, 2 poles, break before make,
grounding point via lengths of flexible wire. Conrad Electronics 709751*
– S2 = mains switch
– F2 = fuse 250 mAT (slow), with holder
Connection to the final amplifier – LED signal lamp, 12 VDC
– 4 black buttons, 21mm
Screened audio cable sometimes has rather
– 4 button caps, red, with line, 21mm
high capacitance (as much as 200 pF per – 1 button, 28mm
metre). If the distance between the pream- – 1 button cap, red, 28mm * www.int.conradcom.de
plifier and the final amplifier is forced to be

28 Elektor Electronics 10/2003