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V+
AUDIO
OUT
Control
Controlled current sources
AUDIO OUT
10kΩ
10kΩ
–
L
Series
switch 2
+
–
R
+
By Jake Rothman
Input
Output
Series
switch 1
–1
Audio switching Part 5 – more on electronic switching
Shunt
switch
0V
W
e pick up here where we left off
last month, describing various
practical audio switching
circuits implemented using solid-state
devices like JFETs or ICs, rather than
mechanical relays and such. They
have various advantages, such as silent
operation, reduced cost and lower power
consumption.
Capacitor coupling
When the FET is switched off with a
negative control voltage, a DC path to
ground is necessary for the bootstrap
resistor, which can be the low impedance
of an op amp output. Without this path,
it won’t switch off.
If a coupling capacitor is used to block
the DC offset of the stage feeding the JFET
to prevent clicks, this path is blocked.
Douglas Self connects the bootstrap
resistor before the capacitor to give the
required path, as shown in Fig.83.
Solid-state logic (SSL)
Back in January 1989, I took the
SSL maintenance course in Begbroke,
Oxfordshire. This was for the massive
£250,000 G-series mixing consoles that
every top studio then had to have. One
of the lecturers was SSL’s principal
analog design team leader, Andy Millar,
who explained the subtleties of FET
switching.
His team’s work led to the development
of the SSM2402 audio switch at Analog
Devices/Precision Monolithics Inc. In
this circuit (Fig.84[a]), the bootstrapping
Input
+
–
220nF J112
Output
22kΩ
100kΩ
(Load)
DC
path
1N4148
Capacitor input
+Ve 10V to turn off
–Ve 10V to turn on
1MΩ
0V
Fig.83: the AC-coupling arrangement
for a JFET with bootstrapped DC path.
36
was done by an op amp, which ensured
that VGS was zero while allowing the
gate control signal to pass through. The
control input to the op amp was via a
current source; otherwise, the follower
would become an amplifier boosting the
bootstrap signal.
I built one section of this circuit
(Fig.84[b]) and found that the op amp
clipped on the negative cycle first, but
this was only in mute when the negative
control voltage was added on. So it
was not a problem. Making a discrete
version of this discontinued IC warrants
further research, since the distortion was
0.0015% at 1VRMS (0.01% at 4VRMS) into
a high impedance load.
However, it was a bit worse than the
diode bootstrapping method in my
initial experiments.
Interestingly, the JFETs in the 2402
are P-channel types. This results from a
similar ion implantation process to that
used for Texas Instrument’s Bi-Fet TL0xx
series op amps. They also have P-channel
JFETs, which is the outcome when made
in association with the standard NPN
bipolar transistor IC process.
Audio
Input
Inverter
1µF
100kΩ
0V
J112
1mA
current
regulator
diode
Audio
Output
+18V
7
3 +
4
100kΩ
10kΩ
6
2 –
Control input
+10V: mute
0V: On
0V
0V
8
5
22pF
Audio and inverted
control signal
–18V
10kΩ
Fig.84(b): I found this discrete version
of the SSM2402 concept to work OK.
This system is really the minimum
required for a decent audio switch.
It avoids the need for bootstrapping
because the signal voltage across switch
TR1 is very low when it is conducting,
so VGS is also very low.
A shunt JFET switch (TR2) attenuates
the input in conjunction with R1 when
the switch is off, preventing breakover. Noise is slightly higher with the
inverting op amp than with the voltagefollower op amp system (a unity-gain
inverting op amp has a noise gain of
two). A good compromise between noise,
input impedance and distortion is to use
4.7kΩ for R1 and R2.
Switch block
My circuit uses blocking capacitors
A switch block I developed based on
C1 and C2 because FET input op amps,
virtual-earth topology is shown in Fig.85.
especially the low-noise ones,
V+
Control
have become expensive. The
Controlled current sources
two capacitors are connected
back-to-back in the series chain,
effectively making a composite
10kΩ
10kΩ
non-polarised capacitor, which
reduces their distortion, especially
with tantalum types.
–
–
I used a P-channel J175 for
Series
switch 2
TR2
with a slight reduction in
+
+
maximum attenuation. An inverter
stage could be added to allow
Input
Output
two N-channel J112 FETs to be
Series
switch 1
employed (as shown in Fig.84[b]).
–1
C5 is a phase-lead compensation
Shunt
switch
capacitor to compensate for the
Inverter
0V
0V
capacitance of TR2 and layout
capacitance around pin 2. The
Fig.84(a):1µF
the SSM2402 chipJ112
uses interesting
measured distortion was 0.001%
Audio
Audio
op amp bootstrapping with three FETs in a
Output
Input
at 10V RMS from 20Hz to 20kHz.
bidirectional series/shunt/series network.
100kΩ
0V
+18V
7
3 +
2 –
100kΩ
10kΩ
6
8
0V
Practical Electronics | October | 2024
+V
TR1
R6
R5
R8
100kΩ
TR2
(J112 n-chan)
D/S*
G
S/D*
BC546B
Control
input
J112
TR2
p-chan
J175
TR1
p-chan
Inverter
R9
10kΩ
C5
22pF
R2
4.7kΩ
–V
+
+
+18V
2
3
TR2
J175
D1
1N4148
R3
10kΩ
R5
6.8kΩ
C3
220nF
R7
680kΩ
0V
C2
47µF
15V
Tant
TR1
J112
R1
4.7kΩ
C1
47µF
15V
Tant
By combining two JFET
switch circuit blocks
(Fig.85) with a shared
virtual earth op amp
circuit, a changeover
switch can be made, as
shown in Fig.87. This can
be driven by the flip-flop
switch circuit from Fig.59.
Since its control outputs
are complementary -V and
+V, all the JFETs can be
N-channel J112s or even
J111s because the negative
voltage can go up to -16V.
*Interchangeable
FET switch circuit
R6
6.8kΩ
R4
330kΩ
D2
1N4148
C4
220nF
IC1
7 NE5534
–
+
4
R8
680kΩ
R9
47Ω
6
8
5
C6
22pF
Output
C7
100nF
–18V
0V
Fig.85: the final JFET mute switch,
suitable for professional audio gear.
Attenuation was about -90dB at 10kHz
with the J175. Ultimately, an electronic
switch needs around 16 components
just to approach the qualities of a simple
mechanical switch.
Ramping
Each gate has two RC networks, R5/
C3 and R6/C4, to ramp the JFET gate
voltages, giving almost a fade rather
than an abrupt transition. It is a distorted
fade when switching a high-amplitude
sinewave, but it is less noticeable than
a click when music is switched.
This is done on the SSM2402 IC
by special on-chip silicon nitride
capacitors, ensuring smooth breakbefore-make action. There was a 2412
chip variant with faster switching for
broadcast use.
More circuit tricks
I have seen some tricks applied on the
Soundcraft 6000 series desks that are
worth looking at. It is possible to have a
DC-blocking capacitor in series with the
lower JFET switch, as shown in Fig.86.
This capacitor is thus out of the audio
path when the whole switch circuit is
passing audio.
Control
–15V: Off
+15V: On
Resistor R3 is used to equalise the DC
offset levels, preventing clicks. This
technique needs FET-input op amps,
such as the expensive OPA1641, since
the bias currents from bipolar devices
like my favourite, the NE5534, will
cause clicks.
Minimising RON
Although N-channel JFETs are on at
0V gate bias, a bit of extra positive bias
(200-300mV) can minimise RON and
consequently distortion. For a typical
J112, at 0V VGS, RON = 38Ω, but with a
VGS of 300mV, it drops to 28Ω.
This can be achieved by the potential
divider action of a 10MΩ resistor across
the diode and a 680kΩ resistor to ground
from the gate with a +7.5V control signal.
I only got 300mV while there was no
signal passing through, since the ‘diode
action’ of the gate generated a small
negative voltage, so I did not think this
technique was worthwhile.
It’s also a problem at high temperatures
due to the gate’s diode leakage being
temperature-dependent.
Input 1
Control
0V
Output
0V
4.7kΩ
Keeps capacitor at same
47µF
offset voltage of op amp
25V
(Only in circuit path in Off mode)
47µF
10V
3.3kΩ
Flip-flop
switch
(Fig.59)
FET-input
op amp
R3
22kΩ
+
47pF
V+ (V–)
+
Control
(inverted)
Virtual earth block
FET
switch
block
R2
4.7kΩ
–
The Quad selection circuit
is the simplest electronic
channel selector I’ve seen
and makes the circuit given in Fig.59,
with its debouncing, seem over the top.
This circuit (Fig.88) is ideal for use with
CD4066 devices.
Contact bounce does not seem to be
a serious problem in practice with
Hi-Fi input selection. The occasional
‘jump’ is not as catastrophic in audio
as it would be in an industrial control
system. Sufficient debouncing can
usually be achieved with capacitors
across switches, especially with slow
4000-series logic.
The Quad circuit has a capacitor wired
across S2 to ensure the circuit always
defaults to this position when powered.
It can be moved to whatever position is
required at power-on.
40xx series logic gates alone often
had insufficient output current for
high brightness with older technology
LEDs, which is why the Quad circuit
had driver transistors. The latest highefficiency LEDs are bright enough at
1mA, such as the red TruOpto (Rapid
55-2190) or Kingbright (72-8989).
So, if one of them is used, the driver
Off
(On)
33pF
R1
Input 4.7kΩ
Interlocking switches
I first saw a similar topology in the
Studer A779 mixing desk.
+
Input
A changeover switch
Virtual earth amplifier
D/S*
S/D*
G
330kΩ
V – (V+)
–
+
47Ω
Output
Input 2
FET
switch
block
On
(Off)
Fig.86: you can put the DC blocking
capacitor in series with the lower JFET
instead of the op amp input.
Fig.87: a changeover switch can be made by feeding two mute circuits into one op
amp. The op amp flip-flop (Figs.59 & 80) can control this. All FETs can be J112s.
Practical Electronics | October | 2024
37
IC1b
4001
6
8
4
5
9
10
10kΩ
V+
14
IC1c
4001
13
Output 2
1
IC1d
4001
V–
–7.5V
Output 1
S1
Unused
NOR gate
6
5
A
4
10kΩ
S2
S3
Radio
+VE
Output 1
Output 3
Disc
1MΩ
10kΩ
10kΩ
S2
Aux
4.7nF
Delay capacitor
+7.5V
+8.6V
8
9
B
10
10kΩ
1MΩ
BC182
BC182
1MΩ
Start-up
4.7nF capacitor
+VE
Output 2
1MΩ
+VE
7.5V
S1
2
11
7
IC1a
4001
S1
BC182
–7.5V
12
13
C
11
10kΩ
1MΩ
–9.4V
1kΩ
1N4148
Fig.88: the Quad 34 3-channel selector switch circuit. The power rails for the ICs
are +7.5V (pin 14) and -7.5V (pin 7), so the audio signal can be DC-biased at 0V.
positions, but the package has a spare
NOR gate. I had it redesigned by Grant
Stevens to make it a four-position switch
with the addition of some diodes, as
shown in Fig.90. This ties up nicely with
the 4066 chip that has
four switches. The startup position capacitor has
been increased in value
R9
to allow for the slow
1MΩ
ramp-up of some power
+
supplies.
0V
0
out
3
Note that when feeding
1
IC1a
a high-impedance CMOS
1
4001
gate with diodes, it is
R10
1MΩ
often necessary to follow
the diode with a 100kΩ to
+
0V
0
out
4
1MΩ pull-down resistor
2
(R9 to R12 in Fig.90) due
IC1b
2
4001
to the leakage current.
R11
Grant also designed a
1MΩ
decimal to binary-coded
+
0V
decimal (BCD) diode
0
out
10
3
logic circuit to drive
IC1c
3
4001
a 4052 audio switch,
described below.
R12
transistors can be dispensed with, as
shown in Fig.89.
The circuit only consumes 0.9mA at
9V and 1.9mA at 15V.
The Quad circuit had only three
+VE
4
3
2
1
(Reset to 1 )
4.7nF
Switch
selected
1
3
2
2
3
1
2
4
1
In
R5
10kΩ
VE+
0V
4
4
3
Select
6
5
2
R6
10kΩ
1
4
1
8
9
3
R7
10kΩ
2
1MΩ
2
1
13
12
4
3
0V
11
IC1d
4001
R8
10kΩ
R1-4
1MΩ
0V
Convert to BCD
(for 4052)
1
NC
4
The Playmaster
Series 200
+
out
One of the first DIY
amplifiers using a similar
switching system was the
Playmaster Series 200,
R13
6.8kΩ
published in Electronics
Australia in March 1985
(page 38). It used 4052s
1 2 3 4
(two four-way switches
B 0 0 1 1
in a package) and 4011
4
A 0 1 0 1
2
A
B
Pin
5 6 3 5 6 4 8 9 10 12 13 11
3
1 On
2 On
0 0 1 1 0 0 1 0 0 0 1 0
4
3 On
4 On
1 0 0 0 1 0 0 0 1 1 0 0
1 0 0 0 0 1 0 1 0 0 1 0
0 1 0 0 1 0 1 0 0 0 0 1
4x
1N4148
1MΩ
Outputs A and B to
4052 control inputs
(pins 9 and 10)
38
0
1MΩ
0V
Fig.90: why waste
a gate in a quad
package? Here is the
Quad circuit expanded
to four buttons. The
inset diagram is a
diode logic driver for
use with a 4052.
–VE
–7.5V
Output 3
Outputs
Pin number 5 6 4 8 9 10 12 13 11 1 2 3
Input
switches
A On 0 0 1 0 1 0 1 0 0 1 0 0
B On 1 0 0 0 0 1 0 1 0
0 1 0
C On 0 1 0 1 0 0 0 0 1
0 0 1
Fig.89: a simplified Quad circuit with
logic analysis by Grant Stevens.
NAND gate flip-flops, as shown in Fig.91.
Some may prefer this circuit because the
switches go to ground.
Using a couple of diodes, a backup
voltage source could be added to the
power pins of the CMOS chips to
memorise the switch positions. The
current consumption of these devices
is below 10µA; I used a 100µF 10V lowleakage tantalum capacitor, which gave
a backup time of about half an hour. Of
course, the LEDs have to be connected
to the anode side of diode D9.
The 4052 is run from split power rails
to accommodate the bipolar audio.
The control signal can be normal logic
levels of 0V and +5V because there is
internal-level shifting circuitry in the
4052. Breakover occurs at 16V peak-topeak and clipping at 13V peak-to-peak.
The original circuit used a BCD decoder
made from another 4011 to drive the
LEDs shown in Fig.92. A single 4052 was
originally used, switching both left and
right channels. The original op amp was
a TL071, but this has a phase inversion
at negative clipping when used as a
follower, so it was changed to an NE5534.
Attenuation was only -40dB at 10kHz
on the messy breadboard shown in
Fig.93, but this soon cleared up when
the unused inputs were grounded. If
the inputs are all buffered, which they
should be to protect the delicate CMOS
gates, it is around -90dB. The distortion
was surprisingly low and flat with
frequency: 0.0006% at 1V RMS, rising
to 0.004% at 4V RMS.
Practical Electronics | October | 2024
LED1–4
12
+7.2V
+7.5V
(Power rail to D9 anode)
16
R5
RLED
1kΩ
14
13
15
6
11
–7.5V
or 0V
Phono (from preamp)
1
CD
2
Aux
3
Tuner
4
Should be
buffered
1
0 0
Pin 1
2
0 1
Pin 5
3
1 0
Pin 2
4
1 1
Pin 4
Possible
battery
back-up
PP3
0V
–
+15V
0V
5
3
2
4
C2
100nF
2 –
4
A B
7 10 9
BCD output –7.5V
A: Q
B: Q
R7
100Ω
8
Output
5
From 4011
flip-flop
–15V
10
11
+7.5V
D3
IC2
4011c 14
8
10
9
D1
D7
12
13
1
2
D6
D8
R4
100kΩ
CD
Tuner
S2
S3
D1-D8
1N4148
2
4
Q
Q
11
IC2
4011a
5
6
3
4
Q
Q
7
IC2
4011b
D4
Aux
Push-to-make
momentary switches
0V
Discrete button pushing
There is a mismatch between the high
negative switching voltages of low-RON
JFETs and standard logic chip families.
There is also the hassle of providing
dedicated logic power rails of different
12 A4
Q2 4
11 Q4
A2 5
10 Q3
B2 6
9 B3
VSS– 7
8 A3
+8.6V
Q
+7.5V
Q
14
9
8
11
4011
1
2
Q
Q
470Ω
10
3
6
5
4
Fig.92: you can use this BCD decoder
to drive the LEDs if needed. If the input
is binary 11, LED 4 illuminates. When
feeding in 00, LED 1 illuminates.
S4
Phono
Q1 3
0V
IC2
4011d
R3
100kΩ
D2
13 B4
7
R2
100kΩ
D5
B1 2
12
13
R1
100kΩ
0V
14 VDD+
To right
channel
D9
BAT46
7.2V (to pin 16, IC1)
A1 1
4011 Quad two-input NAND
C3
22pF
0V
C1
100µF +
10V
Tant
low leak
S1
IC3
NE5534
6
7
3 +
R6
220kΩ
D10
1N4001
+
8
IC1
4052
Switch B A Output
1
Fig.91: a simplified version of the
Playmaster 200 input selector; the
spare four-way switch drives the LEDs.
Using two separate 4052s simplifies
PCB routing and reduces crosstalk.
0V
voltages. Solutions to these problems
include using level-shifting circuits or
designing one’s own logic using discrete
transistors.
Discrete logic is old-fashioned, but
the requirements for audio switching
are very simple. Many young engineers
reach for an Arduino with scanned
switches; contact bounce is important
and can be dealt with in software.
There are not many discrete logic
circuits about now, but I remember as a
teenager (in the mid-1970s), there was
a big thing about touch-switch channel
selectors on TVs. I think some 1974
GEC C211X series sets used discrete
transistors with little neon bulbs! As
usual, I found a suitable circuit by
perusing old Wireless World magazines.
In WW October 1974’s Circuit Ideas
section (page 380), I found an inspiring
circuit by P. G. Hinch. I got it to work after
a bit of semiconductor polarity flipping
and resistor value changing; the final
circuit is shown in Fig.94.
Power
Powerbus
bus
+18V
+18V
R1
R1
4.7kΩ
4.7kΩ
TR2
TR2
BC556B
BC556B
Alternative
Alternativebulb
bulbwiring
wiring
+17.4V
+17.4VLED
LEDon
on
–15.5V
–15.5VLED
LEDoff
off
Output
Output
Repeat
Repeatcircuit
circuit
C1
C1
2.2nF
2.2nF
S1
S1
R4
R4
3.3kΩ
3.3kΩ
6mA
6mA
Reset
Resetbus
bus
TR1
TR1
BC546
BC546
D2
D2
1N4148
1N4148
D1
D1
1N4148
1N4148
R2
R2
6.8kΩ
6.8kΩ
Power
Powerbus
bus
Fig.93: a Playmaster-style channel selector on a breadboard.
Practical Electronics | October | 2024
R3
R3
100kΩ
100kΩ
TR2
TR2
Output
Outputtoto
FET
FETswitch
switch
3.3kΩ
3.3kΩ
Bypass
Bypass
28V
28V
40mA
40mA
LED1
LED1
D1
D1
R5
R5
1.2kΩ
1.2kΩ
R5
R5
220Ω
220Ω
0.5W
0.5W
–18V
–18V
–18V
–18V
Fig.94: the final circuit (per switch) for the discrete interlocking
circuit. The polarity is inverted from Hinch’s design.
39
Fig.98: the rotary switch display PCB. I described this in the
Modern Amateur Electronics Manual years ago (Supplement 24).
Fig.95: a discrete transistor interlocking switch circuit
with filament light bulbs on a breadboard. The blue
breadboard is the JFET audio switch from Fig.85.
22 swg tinned copper wire
Reset bus
LED
LED
LED
Switch
Switch
Switch
V+
V–
Power
Fig.99: the TSL audio monitor, a highly-regarded broadcast unit,
excellent for driving LS3/5A speakers and monitoring the stereo
compatibility of mixes.
The current consumption for the bulb
circuit is 45mA from both rails. In this
circuit, the bulbs are under-run at 23V,
so they last a long time.
I had to prove the concept on a
breadboard; see Fig.95. I’ve kept a
whole bag of these momentary switches
in stock for years – now I have a use
for them!
The great thing about this discrete
circuit is that it can be divided into a
separate section for each switch. You
can have a string of buttons as long as
you like, as shown in Fig.96.
allow the LED current to
be increased for higher
Switch 2
Switch 3
Switch 1
brightness and can easily
Fig.96: a suggested system for one PCB per
switch, extendable as long as you like with busbars. drive the 28V 40mA (eg,
CML CM334) filament bulbs
in EAO switches by changing R5 to
The capacitor across the switch can
220Ω ½W. Unlike LEDs, bulbs do not
be used to set which switch comes on
require a series resistor (R4), so the bulb
when the circuit is powered. If you
is wired from the collector of TR2 to R5.
want S1 to be the default, increase C1
If the bulb blows (which they do every
to 220nF.
4000 hours), the interlocking ceases
This system is ideal for controlling
to function, so each bulb must have
the switch block in Fig.85, which needs
a 3.3kΩ resistor wired in parallel to
+15V for on and -15V for off using
ensure enough current flows to trigger
standard op amp power rails.
the circuit.
Discrete transistor circuits also
Discrete diode circuits
Com
+5V
3, 14
1
2
Com
Rotary
switch
gang
0
3
1
4
5
6
7
D9
13
D6
8
D1
8
D3
D11
D12
D2
10
D10
D5
D4
D7
D8
7
2
11
A
Common
anode
B
a
f
C
D
E
F
b
g
e
c
d
G
R1-R7
150Ω
Kingbright
SA04-11HWA
SA03-11GWA
0V
D13
Select R1-R7 to suit rail voltage
and required LED current.
Fig.97: a diode-based seven-segment display driver for a rotary switch. You need an extra
gang on the switch but avoid complex front-panel artwork.
40
Some switching logic is so
simple it can be done with
diodes, such as the rotary
switch LED readout circuit
shown in Fig.97.
There was an excellent
story about this in the Radio
Constructor series, In Your
Workshop, May 1978. The
assembled PCB is shown in
Fig.98.
Doing it properly
The Television Systems
Limited (TSL) analog audio
broadcast monitor shown in
Fig.99 exemplifies electronic
switching and general
construction. The whole
Practical Electronics | October | 2024
audio signal path is dealt with in a
self-contained plug-in Euro card at the
back of the unit near the audio XLR
connectors.
The interior is shown in Fig.100, the
Euro card is shown in Fig.101 and the
bilateral electronic switches used are
Vishay’s DG509.
The DG509 is a device that can
operate from supply voltages of up
to 44V maximum and it contains two
four-way switches; a posh CD4052,
in effect. The binary input to switch
it is also the same. It costs around £4
from Mouser.
The front panel switches are all
mechanically latching C&K paddle
types, just controlling DC, which is
fed to the Euro card by a grey ribbon
connector.
Fig.100: the interior of the TSL audio monitor, a great example of audio gear built
properly. The audio processor card is in its own screened box.
New chips on the block
There are some new audio switch
chips available. They are all surfacemounting low-voltage (5V) types, so
they are difficult to incorporate in
conventional ±15V powered op amp
audio circuitry.
The Diodes Incorporated PI5A3157
has a 6Ω RON but it comes in a hardto-solder SOT-363 package.
The Texas Instruments TS5A3159
DBVR in the SOT-23 package is even
lower at 1Ω and claims to have a THD
of 0.005% at 4.9V peak-to-peak into a
load of 600Ω.
I reckon these chips could work
well in low-voltage, low-impedance
portable audio circuitry. Samples are
coming; it will be worth checking how
they perform in the virtual earth circuit
PE
in Fig.85.
Fig.101: the TSL analog audio processor card. The DG509 switches are still
made; sadly, the Analog Devices SSM2141 balanced line receivers and SSM2142
balanced line drivers are obsolete. TSL certainly splashed out on parts!
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