By John Clarke
Amplifier
Clipping
Indicator
Driving an amplifier into clipping can cause unpleasant sounds and even
damage expensive speakers. So it is best to find out right away if you’re
about to run into clipping. This easy-to-build Amplifier Clipping Indicator
is ideal for that; its LED shows even the briefest of clipping events.
A
clipping indicator is a
valuable accessory for any
audio amplifier. It indicates
when the amplifier has reached its
limit and is clipping the peaks of the
audio signal. In practice, quite a lot of
clipping can occur before you notice
it and even brief clipping events can
cause tweeter damage.
That’s because when an amplifier
clips, it ‘squares up’ the waveform; the
result contains lots of higher-frequency
harmonics, which can easily damage
the tweeters in loudspeaker systems.
Our Amplifier Clipping Indicator
flashes its LED whenever clipping is
detected. That’s considered to be any
time the amplifier output is within
about 4V of the positive or negative
supply rails. Most amplifiers will clip
within about 3V of the supply rail,
although some can require slightly
more headroom; choosing 4V gives a
small safety margin.
There can be a significant ripple on
the amplifier supply rails when it’s
delivering a lot of power (when clipping is most likely to happen). So a
proper Amplifier Clipping Indicator
like this one will compare the output
waveform to the instantaneous supply voltages to compensate for that.
Its thresholds adapt as power supply
voltages fluctuate.
A timer is included to extend the
duration of the LED lighting up, to
ensure even momentary clipping is
visible. The indicator LED is mounted
on the Amplifier Clipping Indicator
PCB, but it also provides a connection for an external LED mounted on
the amplifier’s front panel.
This Amplifier Clipping Indicator is
presented as a bare PCB designed to be
housed within an existing amplifier.
38
You can build a single unit for a mono
amplifier or two for monitoring two
channels in a stereo amplifier.
Power for the circuit is derived from
the amplifier’s supply; it only draws
a few tens of milliamps, so it won’t
affect the amplifier’s maximum output power to any significant degree.
When building a stereo version, you
could use a single, common LED to
indicate clipping from either channel
or a separate LED indicator for each
channel. The second PCB does not
require the full complement of components for the common LED version.
Circuit
The clipping detector circuit is shown
in Fig.1. A few components don’t have
values: resistors R1a, R1b, R2 and R3
and zener diodes ZD4 and ZD5. Their
values depend on your amplifier’s
power supply voltage. Table 1 shows
the component values required to suit
amplifiers with supplies ranging from
±10V to ±80V.
The clipping detector generates positive and negative reference voltages.
For the positive reference, zener diode
ZD1 generates a voltage about 4.7V
below the V+ positive rail. Resistors
R1a and R1b limit the current through
ZD1 to approximately 10mA; together,
they connect across the V+ and 0V
amplifier supply rails. The generated
voltage is shown on the circuit as V+
− 4.7V at Q1’s emitter.
The 4.7V between this rail and V+ is
also used to power timer IC1. We allow
5mA for IC1’s supply and 5mA to bias
ZD1. More on IC1’s operation later.
Zener diode ZD2 and resistor R2
between the V− supply and 0V generate the negative reference; R2 limits
the current through ZD2 to about 5mA.
Detecting positive clipping
The positive reference voltage (V+ −
4.7V) is connected to the emitter of
NPN transistor Q1. Its base goes to the
amplifier’s output via a 100kW current-
limiting resistor, while diode D2 stops
Q1’s base-emitter junction from being
reverse-biased.
Just before clipping, the amplifier
output voltage will rise above the V+
− 4.7V reference plus Q1’s base-emitter
on-voltage of about 0.7V. Q1 switches
on when the amplifier output voltage
is within 4V of the positive supply. It
then sinks current via diode D1, the
100W resistor and zener diode ZD3.
The anode of ZD3 connects to the
pin 2 trigger input of IC1, and as this
voltage drops, timer IC1 starts running.
This means that IC1’s pin 3 output goes
high, switching on Q4 and the indicator LED (LED1) via a 1kW current-
limiting resistor. A second external
LED will also be lit if connected to the
external LED connections.
IC1 is a CMOS version of the 555
timer and is set up to operate as a
monostable timer. Timing is initiated
when the pin 2 trigger input goes
below a third of its supply voltage.
With a 4.7V supply, the trigger point
is 1.56V above the V+ − 4.7V rail or
3.13V below the V+ supply rail.
Pin 2 is usually held at V+ by a
100kW pull-up resistor. However,
when current flows through ZD3, D1
and Q1, the voltage at pin 2 goes low
enough to trigger the timer.
Once the pin 3 output goes high,
the 1μF capacitor at pins 6 and 7 of
IC1 begins to charge from the V+ supply through a 100kW resistor. When
the capacitor reaches two thirds of
the supply (3.13V above the V+ −
4.7V reference), the pin 3 output goes
Practical Electronics | April | 2023
Amplifier Clipping Indicator
low, and this capacitor discharges
into pin 7.
This sequence of events occurs
when the trigger voltage at pin 2 is only
low for a very short period. If the trigger voltage is low for longer than the
timing period, the pin 3 output will
stay high until pin 2 goes high again.
The timing period is about 110ms,
as set by the 100kW resistor and 1μF
capacitor values. IC1 acts as a pulse
extender for brief detection of amplifier clipping. It ensures that clipping
is shown on the LED for at least 110ms
(ie, a bit more than 1/10th of a second).
Detecting negative excursions
ZD2, PNP transistor Q2 and diode D3
work to detect negative excursions from
the amplifier. When the amplifier output swings low, within 4V of the negative supply, transistor Q2 switches on
and, in turn, switches on transistor Q3.
This then pulls the pin 2 trigger input
of IC1 low via two series zener diodes
(ZD4 and ZD5) and resistor R3.
Transistor Q3 is rated for a maximum collector-emitter voltage of 80V.
Without the two zener diodes, the
transistor could be subject to the total
of the V+ and V− supply rails and so
would only be suitable for use with
a maximum of ±40V supply rails. By
including the zener diodes, the voltage
at the collector is reduced to a maximum of around 66V.
While we could have used a transistor with a higher voltage rating,
they are not as readily available as the
Practical Electronics | April | 2023
BC546. Table 1 shows the required
values for resistors R2, R3 and zener
diodes ZD4 and ZD5 for various amplifier supply voltages.
Resistor R3 is included to limit current in zener diode ZD3 when transistor Q3 conducts.
While this is not the simplest clipping detector circuit, it has the advantage of presenting an almost entirely
linear load to the amplifier output, to
minimise the possibility of any distortion due to loading.
Note that if you want to monitor
clipping in a stereo amplifier and
use a single indicator LED, you can
dispense with the components in
the blue shaded areas for the second
channel. Interconnection is made
between the two PCBs at the top end
of R3. This way, a clip event at either
input will trigger IC1 on the board
where it is fitted.
Alternatively, you can build two
complete copies of the circuit for
independent channel clipping indication. The boards are small and can
be stacked to take up relatively little room.
Fig.1: the Clipping Indicator monitors
the amplifier’s output and lights LED1
whenever it comes within about 4V
of either supply rail. NPN transistor
Q1 detects positive signal excursions,
while PNP transistor Q2 detects when
the signal approaches the negative
rail. IC1 lights the LED for at least
110ms each time clipping is detected.
the version used for a mono amplifier, or for the left channel in a stereo
amplifier (or both channels if you want
independent clip indication).
If the second channel is built as
shown in Fig.3, clipping in either
channel will be indicated with a single LED.
Begin by fitting the resistors. First,
refer to Table 1 to select the resistor
value and power ratings for R1a, R1b,
R2 and R3. (Ideally, you should check
each resistor using a digital multimeter (DMM) before installing it.
Once these parts are in place, follow
with diodes D1, D2 and D3, orienting
them correctly. The zener diodes can
be mounted next. ZD1 and ZD2 are
4.7V types, while ZD3 is rated at 3.9V.
The ZD4 and ZD5 voltages are as per
Table 1, or replaced with a wire link
if indicated.
Transistors Q1, Q2, Q3 and Q4 can
be mounted next. There are three different types (although Q1, Q3 and Q4
Construction
The Amplifier Clipping Indicator is constructed on a double-sided, plated-through PCB
coded 01112211 that measures 54 x 60mm and is available from the PE PCB Service. There are two overlay
diagrams shown. Fig.2 is
39
Fig.2 and Fig.3: the board is not difficult to assemble; the components are fitted as shown at left. The diodes, LED, IC and
electrolytic capacitors are polarised. If you’re building a mono version or a stereo version to drive two independent LEDs,
build the fully populated version. For a stereo version with a single clip indicator LED, build one of each version and join
the indicated pad between the two boards (not present on the prototype PCB pictured).
can all be BC546s if desired), so take
care to install each in its correct place.
The screw terminal blocks making
up CON1 need to be joined together
first by fitting each side-by-side by sliding the dovetail mouldings together.
Solder them in place with the wire
entry side of the terminals facing the
nearest edge of the PCB.
Now fit LED1 with its longer lead
inserted into the anode hole. Mount it
so that the top is about the same level
as the adjacent screw terminal.
IC1 can be soldered directly onto
the PCB, making sure its pin 1 is
facing as shown. Finally, install the
capacitors. The 1μF capacitor must
be oriented correctly, with its longer
+ lead into the pad shown. You could
use a non-polarised 1μF plastic film
capacitor, but it will be substantially
larger and probably more expensive
than the electrolytic.
If you’re building the two-channel
version to light a single clip indicator
LED, build a second board as per
Fig.3 and solder a ~20mm length of
solid-core ‘Bell wire’ to the top of
that board, into the pad between ZD3
and Q4. It makes sense for the more
sparsely populated board to be at the
bottom of the stack as it lacks the LED,
and you’ll want to be able to see the
LED on the other board.
Alternatively, if you’re building a
two-channel version with separate
LED indicators, make a second identical board and don’t fit the vertical wire.
Mounting it and wiring it up
Use the board to mark out four holes
in a convenient location within the
amplifier chassis, ideally, between the
amplifier modules and speaker terminals, or at least near the terminals.
If it’s a stereo amplifier, you can
stack the two boards by feeding longer machine screws up through the
spacers on which the lower board is
mounted, then screwing some ~16mm
tapped spacers on top of the threads
once the first module is in place. If you
have space, you could mount the two
modules separately, eg, side-by-side.
Connect the Amplifier Clipping Indicator(s) to the amplifier’s V+, V− and 0V
supply rails and the amplifier speaker
+ output(s) to AMP OUT input(s) on
the Clipping Indicator module(s). Make
sure the wiring is suitably voltage-
rated, especially when the supply rails
are at high voltages from earth.
The external LED connects to the
A and K terminals on the board. If
you are building the minimised stereo version with IC1 and associated
components missing, feed the wire
you soldered earlier to the bottom
board up through the matching pad
on the top board. Solder it on top
and mount the upper board using
longer tapped spacers and short M3
machine screws.
In all cases, when using a second
Amplifier Clipping Indicator module,
Table 1 – component values that vary with amplifier supply rail voltages
Supply
R1a
R1b
R2
R3 (½W)
ZD4
ZD5
±80V
15kW 1W
15kW 1W
15kW 1W
33kW
75V (1N4761)
18V (1N4746)
±75V
15kW 1W
15kW 1W
15kW 1W
33kW
75V (1N4761)
9.1V (1N4739)
±70V
12kW 1W
12kW 1W
12kW 1W
33kW
75V (1N4761)
wire link
±65V
12kW 1W
12kW 1W
12kW 1W
33kW
33V (1N4752)
33V (1N4752)
±60V
12kW 1W
12kW 1W
12kW 1W
33kW
27V (1N4750)
27V (1N4750)
±55V
10kW ½W
10kW ½W
10kW ½W
33kW
22V (1N4748)
22V (1N4748)
±50V
9.1kW ½W
9.1kW ½W
9.1kW ½W
33kW
16V (1N4745)
18V (1N4746)
±45V
8.2kW ½W
8.2kW ½W
8.2kW ½W
33kW
12V (1N4742)
12V (1N4742)
±40V
7.5kW ½W
7.5kW ½W
7.5kW ½W
30kW
15V (1N4474)
wire link
±35V
6.2kW ½W
6.2kW ½W
6.2kW ½W
30kW
3.9V (1N4730)
wire link
±30V
5.1kW ½W
5.1kW ½W
5.1kW ½W
27kW
wire link
wire link
±25V
3.9kW ½W
3.9kW ½W
3.9kW ½W
22kW
wire link
wire link
±20V
3kW ½W
3kW ½W
3kW ½W
18kW
wire link
wire link
±15V
2kW ½W
2kW ½W
2kW ½W
13kW
wire link
wire link
±10V
1kW ½W
1kW ½W
1kW ½W
8.2kW
wire link
wire link
40
Practical Electronics | April | 2023
all three supply connections must be
made to both boards, along with the AMP
OUT terminal. The only terminals that
aren’t needed on the board with components missing are the LED A and K.
The external LED or LEDs can be
attached to the amplifier chassis using
suitable LED bezels, or (less ideally)
glued into tight-fitting holes using
neutral-cure silicone sealant.
Parts List – Clipping Indicator (per channel)
1 double-sided, plated-through PCB coded 01112211, 54 x 60mm available
from the PE PCB Service
2 3-way screw terminals with 5.08mm spacing (CON1) OR
2 2-way screw terminals with 5.08mm spacing (CON1; for minimised
second channel)
Semiconductors
1 7555 CMOS timer, DIP-8 (IC1●)
2 BC547 or BC546 NPN transistors (Q1, Q4●)
1 BC557 PNP transistor (Q2)
1 BC546 NPN transistor (Q3)
1 yellow, amber or red 3mm or 5mm LED (LED1●)
1 yellow, amber or red LED (optional; external LED●)
2 4.7V 1W (1N4732) zener diodes (ZD1, ZD2)
1 3.9V 1W (1N4730) zener diode (ZD3●)
3 1N4148 small-signal diodes (D1-D3)
2 zener diodes or wire link (ZD4▲, ZD5▲)
1 LED bezels (optional; for chassis-mounting external LED)
4 M3 x 6mm tapped nylon spacers (or 15mm spacers for the upper board in
the stack)
8 M3 x 6mm machine screws (or 4 M3 x 15mm machine screws for the
upper board in the stack)
1 20mm+ length of solid-core hookup wire (optional; to join stacked stereo
version)
Various differently coloured hookup wires, rated for amplifier supply voltage
Capacitors
1 1μF 16V PC electrolytic●
Reproduced by arrangement with
1 100nF 63V or 100V MKT polyester●
SILICON CHIP magazine 2023.
1 10nF 63V or 100V MKT polyester●
www.siliconchip.com.au
Resistors (¼W, 1% axial metal film)
6 100kW (● 4 required for minimised version)
1 10kW●
2 1kW●
1 100W
4 other resistors, values as per Table 1▲
● not required for the minimised second channel
▲ see Table 1 for values and power ratings
www.poscope.com/epe
- USB
- Ethernet
- Web server
- Modbus
- CNC (Mach3/4)
- IO
- PWM
- Encoders
- LCD
- Analog inputs
- Compact PLC
- up to 256
- up to 32
microsteps
microsteps
- 50 V / 6 A
- 30 V / 2.5 A
- USB configuration
- Isolated
PoScope Mega1+
PoScope Mega50
This shows the Clipping Indicator installed inside our 500W Amplifier chassis
(see p.17 for more details).
Practical Electronics | April | 2023
- up to 50MS/s
- resolution up to 12bit
- Lowest power consumption
- Smallest and lightest
- 7 in 1: Oscilloscope, FFT, X/Y,
Recorder, Logic Analyzer, Protocol
decoder, Signal generator
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