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000063-3-UK vol.control
AUD IO
Top of the Range
Switches instead of Pots
Design by G. Haas
Those who spare no expense for perfection, can
now exchange their volume potentiometers for
high precision step resistor switches
coupled to the mute switch. What is
being used is not any run of the mill
switch. The rotary switch must have
a smooth gliding effect so as not to
introduce any clicks, pops or sudden
loudness during the operation. In
order to have a uniform response, 24
switching steps have been chosen.
The contacts must be gold plated to
avoid clicks, which will arise during
switching if the contacts become
contaminated.
Common switches don‘t claim to
High-grade switches assembled with preci-
sion resistors have the advantage of absolute
linearity and an extremely high channel sep-
aration. Hi-Fi fans can now link these
switches together with a mute selector and
forego the need to have a balance control.
With a well-chosen speaker posi-
tioning to match the room acoustics,
the need can be entirely dispensed
with.
Figure 1 , the circuit diagram,
shows the volume switch set-up
E
R24
S
R23
isolating foil
copper laminate
isolating foil
M10 x 0.75
24 x R
24 x R
R3
A
R2
6 mm
R1
ground
000063 - 3 - 12
Mute
M
M
000063 - 3 - 11
Figure 1. Circuit diagram of the ‘top of the
range’ volume control.
Figure 2. The modified stepped switch.
32
Elektor Electronics
10/2000
Volume Control
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AUD IO
Pot 2.2 k log.
4.7 k log.
10 k log.
22 k log.
47 k log.
place between both layers a well insulated
copper foil, to which the ground point will be
connected. By doing this modification, the
channel crosstalk is less than the noise thresh-
old generated by the resistor itself.
Selectively measuring a 1-% metal film 10-
kΩ logarithmic switch will show a linear gain
greater than 0.1 dB, a channel separation
>100db at 1 kHz and 96 dB at 10 kHz as well
as self-induced resistor noise of –95 dBV
(20 Hz - 20 kHz) or 17,7 µV.
These values have been measured with the
switch entirely isolated. At 10 kHz, the cross-
talk attenuation is 1 dB (wide-band measure-
ment) above the internally self-generated
noise. As soon as a normal Audio filter is
added, the results look even better. It‘s safe to
assume that from this configuration the mea-
surements show that cross-talk attenuation is
always greater than the internal noise and
thus becomes non-existent. In addition, while
measuring the crosstalk values with selective
filters, white noise coming from the signal
source was clearly identifiable. In the table
you will see the values in Ohms of the E-24
Resistor series corresponding to the equiva-
lent potentiometer position.
R
24
470
910
1.8 k
4.3 k
8.2 k
23
390
750
1.5 k
3.3 k
7.1 k
22
300
620
1.2 k
2.7 k
5.6 k
21
270
510
1 k
2.2 k
4.7 k
20
220
430
820
1.8 k
3.9 k
19
160
330
680
1.5 k
3.3 k
18
130
270
560
1.2 k
2.7 k
17
120
240
470
1.0 k
2.2 k
16
100
200
390
820
1.8 k
15
82
160
330
750
1.5 k
14
68
130
270
620
1.3 k
13
56
110
220
470
1.0 k
12
47
91
180
400
820
11
39
75
150
330
715
10
33
62
120
270
560
9
27
51
100
220
470
8
22
43
82
180
390
7
16
33
68
150
330
6
15
27
56
120
270
5
13
22
47
100
220
4
10
16
33
75
150
3
5.6
10
22
47
100
2
5.6
5.6
12
27
56
1
5.6
5.6
10
22
47
have optimal channel separation fea-
tures, so it‘s necessary to take it apart
as depicted in Figure 2 . A pre-requi-
site is to disassemble the switch and
(000063-3)rg
C ONSTRUCTION GUIDELINES
Elektor Electronics (Publishing) does not provide parts and components
other than PCB s, fornt panel foils and software on diskette or IC (not nec-
essarily for all projects). Components are usually available form a number
of retailers – see the adverts in the magazine.
ponents in the correct position? Has correct polarity been observed? Have
the powerlines been reversed? Are all solder joints sound? Have any wire
bridges been forgotten?
If voltage levels have been given on the circuit diagram, do those mea-
sured on the board match them – note that deviations up to ±10% from
the specified values are acceptable.
Large and small values of components are indicated by means of one of
the following prefixes :
Possible corrections to published projects are published from time to time
in this magazine. Also, the readers letters column often contains useful
comments/additions to the published projects.
E (exa) = 10 18
a (atto) = 10 –18
P (peta) = 10 15
f (femto) = 10 –15
T (tera) = 10 12
p (pico) = 10 –12
The value of a resistor is indicated by a colour code as follows.
G (giga) = 10 9
n (nano) = 10 –9
M (mega) = 10 6
µ (micro) = 10 –6
k (kilo) = 10 3
m (milli) = 10 –3
h (hecto) = 10 2
c (centi) = 10 –2
da (deca) = 10 1
d (deci) = 10 –1
In some circuit diagrams, to avoid confusion, but contrary to IEC and BS
recommandations, the value of components is given by substituting the rel-
evant prefix for the decimal point. For example,
3k9 = 3.9 k
Ω
4µ7 = 4.7 µF
color
1st digit
2nd digit
mult. factor
tolerance
Unless otherwise indicated, the tolerance of resistors is ±5% and their rat-
ing is 1 ⁄ 3 – 1 ⁄ 2 watt. The working voltage of capacitors is
black
–
0
–
–
≥
50 V.
brown
1
1
×10 1
±1%
red
2
2
×
10 2
±
2%
orange
3
3
×10 3
–
In populating a PCB , always start with the smallest passive components,
that is, wire bridges, resistors and small capacitors; and then IC sockets,
relays, electrolytic and other large capacitors, and connectors. Vulnerable
semiconductors and ICS should be done last.
yellow
4
4
×
10 4
–
green
5
5
×
10 5
±
0,5%
blue
6
6
×10 6
–
violet
7
7
–
–
grey
8
8
–
–
Soldering. Use a 15–30 W soldering iron with a fine tip and tin with a resin
core (60/40) Insert the terminals of components in the board, bend them
slightly, cut them short, and solder: wait 1–2 seconds for the tin to flow
smoothly and remove the iron. Do not overheat, particularly when solder-
ing ICS and semiconductors. Unsoldering is best done with a suction iron
or special unsoldering braid.
white
9
9
–
–
gold
–
–
×
10 –1
±
5%
silver
–
–
×10 –2
±10%
none
–
–
–
±
20%
Examples:
brown-red-brown-gold = 120 Ω, 5%
yellow-violet-orange-gold = 47 kΩ, 5%
Faultfinding. If the circuit does not work, carefully compare the populated
board with the published component layout and parts list. Are all the com-
10/2000
Elektor Electronics
33
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