PRECISION ‘AUDIO’
SIGNAL AMPLIFIER
For those
times when
near enough
isn’t good
enough!
There’s a ‘law’ in electronics, which says you can never have too much test
equipment. Even if it is pretty specialised; even if you only need it once
every blue moon, there will come a time when you do need it! This one fits
that bill perfectly – you’re not going to need it every day, but when you do,
you’ll thank your good sense that you do have one on hand!
S
o what are we talking about?
It’s a Precision ‘Audio’ Signal
Amplifier. It’s used when you
need to know – exactly – what ‘audio’
signal you’re dealing with.
‘Audio’ is in quotes because it will
actually handle signals way below the
normal audio range (down to just 6Hz)
and it can go all the way up to more
than 230kHz... or even higher.
Talk about the proverbial ‘DC to Daylight’ amplifier... this one is not far off!
It can deliver a particularly impressive 30V peak-to-peak (10.6V RMS) up
to around 230kHz. It has two switched
gain settings of either 1.00 times (0dB)
or 10.000 times (+20dB), and it’s powered from a standard 12V AC plugpack.
OK, so why would you want one?
Let’s say you’ve built some audio gear
and want to check it out properly. Or
maybe, you need to accurately calibrate
other test gear. Or perhaps (and we imagine this will be the biggest market)
you’re in the service game and need to
troubleshoot a misbehaving unit.
Practical Electronics | October | 2020
You may already have a low-cost
waveform generator (there are many
on the market these days) and they are
increasingly built into DSOs.
They can usually generate sine,
square, triangle and often ‘arbitrary’
waveforms with varying frequencies
and amplitudes.
But the maximum amplitude is usually limited to about 5V peak-to-peak
and often, that simply isn’t sufficient.
This project overcomes that limitation. It can be connected to the output
of the waveform generator to provide
exactly ten times gain, boosting the signal level to just over 10V RMS.
And because the gain applied is very
precise, you don’t need to check the
output level. You just set the generator to produce a waveform with 1/10th
the needed amplitude and the signal
amplifier does the rest.
I came up against this problem when
calibrating our new Digital Audio
By Jim Rowe
Millivoltmeter, described starting on
page 32 of this issue.
I have a few signal generators, but
none of them could produce a sinewave with sufficient amplitude to
calibrate its ‘HIGH’ range. So I decided to design and build this precision
amplifier, to generate accurate signals
of a high enough amplitude for me to
calibrate it.
And I realised just how useful this
would be for other audio projects! It
provides a choice of two accurately known gain ranges (1:1/0dB or
1:10/+20dB), over a relatively wide
range of frequencies, from 20Hz up to
beyond 200kHz.
Circuit details
The Signal ‘Audio’ Amplifier circuit
is shown in Fig.2. As you can see,
the amplifier itself (lower section) is
quite straightforward.
That’s because we are using a rather
special op amp, the Analog Devices
ADA4625-1 (IC1). It offers very high
input resistance, thanks to the use of
27
JFET input transistors. It has a typical
gain-bandwidth product of 18MHz,
very low noise, fast settling time (to
within 0.01% in 700ns), a rail-to-rail
output swing and the ability to operate from a ‘single supply’ of up to 36V.
Other features include a low output
resistance in closed-loop mode (typically 2Ω when gain=1 or 18Ω when
gain=10) and the ability to drive load
capacitances up to 1nF in closed-loop
unity-gain operation.
We are using IC1 in a standard noninverting configuration, with the input
signal from CON1 coupled to its noninverting input (pin 3) via a 1µF metallised polyester capacitor.
The output from IC1 is then fed to
output connectors CON2 and CON3
via another 1µF metallised polyester
coupling cap, with a 51Ω protective
(and impedance-matching) resistor in
series with each connector.
Switch S1 is used to alter the feedback around IC1, to provide either
unity gain or a gain of 10.
In the A=10 position, the 100kΩ
0.1% resistor forms the top arm of
the feedback divider, while the lower arm is formed by the series combination of the 10kΩ and 820Ω fixed
resistors, together with VR1, a 15-turn
500Ω trimpot.
The trimpot allows us to set the amplifier’s gain to exactly 10.000, by compensating for within-tolerance variations in
the value of the 10kΩ and 820Ω resistors (both 1% tolerance) as well as the
100kΩ 0.1% tolerance resistor.
Although it’s easy to calculate the
nominal lower-arm resistance for a
gain of 10.000 (it’s 11.111kΩ), this
would need to be made up from at
least two more 0.1% tolerance resistors (11.0kΩ and 110Ω), to give a gain
of 10.0009 with a tolerance of +0.018%
and −0.0162%.
By using two 1% tolerance resistors
and a 15-turn trimpot, we can achieve
Features and specifications
Input impedance ................................ 100kΩ//9pF
Output impedance............................. 51Ω (each output)
Gain ............................................................ A=1 (0dB) or A=10 (+20dB)
Frequency range (see Fig.1)......... For A=1: 6Hz to >1MHz (+0,-0.3dB)
........................................................................ For A=10: 20Hz to 230kHz (+0,-0.3dB)
Maximum input signal level....... For A=1: 10.6V RMS (+20.5dBV)
........................................................................ For A=10: 1.06V RMS (+0.5dBV)
Maximum output signal level .... 10.6V RMS (+20.5dBV)
THD+N ...................................................... For A=1: 0.0007% (-103dB)
........................................................................ For A=10: 0.007% (-83dB)
Power supply ........................................ 12V AC at <100mA
even better potential accuracy at a significantly lower cost.
But how do you set the gain to exactly 10.000? You just need a relatively accurate DMM. You measure the value of
the 100kΩ 0.1% resistor (which should
be between 99.9kΩ and 100.1kΩ), then
divide that by nine, and adjust VR1
so that the total lower-arm resistance
matches the calculated value (which
should be close to 11.111kΩ).
The upper part of the circuit exists
primarily to generate a 32V DC supply
voltage from the 12V AC plugpack, so
that IC1 can deliver output signal amplitudes as high as 30V peak-to-peak
or 10.6V RMS.
This is achieved in two stages. First,
diodes D1 and D2 and the two 470µF
capacitors form a simple ‘voltage doubler’ rectifier configuration, which derives about 38V DC from the incoming
12V AC. This is followed by voltage
regulator REG1, an SMD version of the
familiar LM317 adjustable regulator.
Here it’s configured to provide a regulated output of 32V which is fed to IC1
via a 100Ω resistor.
The 220nF capacitor from the ADJ
(Adjust) pin to ground improves its
20.5
SIGNAL AMPLIFIER GAIN in dB
20.0
x10 RANGE
19.5
19.0
0.5
0.0
x1 RANGE
–0.5
1Hz
10Hz
100Hz
1kHz
10kHz
100kHz
1MHz
FREQUENCY
Fig.1: this shows a frequency response plot for the Signal Amplifier at both gain
settings. The response in both modes is entirely flat from 100Hz to 50kHz, so
ideally, calibration and measurements should be made within that range. But it
gives acceptable performance (within 0.3dB) from 20Hz to 230kHz, which more
than covers the audio range.
28
ripple rejection, which is helpful here
because with the voltage doubler configuration, each filter capacitor only
recharges at 50Hz.
The 32V rail is also used to provide
a ‘half supply voltage’ bias of 16V for
the non-inverting input of IC1, via a
10kΩ/10kΩ resistive divider with a
220nF ripple filter capacitor.
LED1 is a power-on indicator, connected to the +32V line via a 3.3kΩ
series resistor.
Construction
Almost all of the circuitry and components are mounted on a single PCB
which fits inside a diecast aluminium
box, for shielding.
The PCB measures 92 × 51mm and
is coded 04107191. Refer now to the
overlay diagram (Fig.3) along with the
matching photo.
The only components not mounted on the PCB are input and output
connectors CON1-CON3 and range
selection switch S1. These all mount
on the box lid/front panel, with short
lengths of hookup wire linking them
to the PCB.
It’s easiest to fit the SMD components to the PCB first, starting with the
passives (resistors and capacitors) and
then REG1 and IC1.
Make sure IC1’s pin 1 dot/divot (or
bevelled edge) is oriented as shown
in Fig.3.
Then fit the leaded parts, starting
with the 100kΩ 0.1% resistor and diodes D1-D4 (with the orientations as
shown), trimpot VR1, the two 1µF
capacitors, the two 470µF and 47µF
electrolytic capacitors (longer lead
towards + sign) and then the power
input connector, CON4.
The final step is to fit LED1, which
is mounted vertically just below the
centre of the PCB. First solder a 2-pin
SIL header to the PCB, then solder the
LED’s leads to the header pins, with
the LED anode towards the front.
Practical Electronics | October | 2020
D3 1N5819
K
A
REG1 LM317M
+32V
OUT
K
10k
3.3k
D4
1N5819
100
+16V
INSULATED
SINGLE HOLE
MOUNTING
BNC SOCKET
220nF
50V
A
LED1
K
100k
ADJ
240
D1 1N5819
K
A
12V AC
INPUT
470 F
CON4
25V
A
K
47 F
35V
LOW
ESR
D2
1N5819
5.6k
220nF
A
470 F
50V
10k
100nF
220nF
IN
25V
330
INPUT
CON1
1 F
7
3
IC1: ADA4625
6
100V
2
4
1 F
CON2
51
OUTPUT
1
100V
SELECT
GAIN
S1
A = 10
SET x10
GAIN
820
VR1
500
15T
10k
10 F
LED1
100k 0.1%
OUTPUT
2
ADA4625
8
K
25V
1.5pF
4
1
A
1N5819
SC precision
Precision
Audio
Signal
Amplifier
audio
signal
AMPLIFIER
20 1 9
CON3
51
A=1
A
K
LM317M
(SOT-223-3)
ADJ
OUT IN
TAB
(OUT)
Fig.2: the Signal Amplifier circuit is based around precision JFET-input op amp IC1. It uses a precision resistor and trimpot to
provide a very accurate 10-times gain (+20dB) to boost the level of signals from devices such as arbitrary waveform generators.
The 12V AC supply is boosted and regulated to 32V DC using a full-wave voltage-doubler configuration (D1 and D2), followed
by a low-ripple adjustable linear regulator (REG1).
The underside of the LED body
should be about 24mm above the top
of the PCB. This will allow it to protrude through the box lid/front panel
when the unit is assembled.
Your Signal Amplifier PCB is then
virtually complete. The next step is
to set the gain of its 10×/20dB range.
Use a DMM with the best resistance
accuracy possible.
Monitor the resistance between the
junction of the 10kΩ resistor and 10µF
capacitor near VR1, and the PCB’s
ground. Then adjust trimpot VR1 until this resistance is as close as possible to one-ninth the resistance of the
100kΩ 0.1% resistor.
If you’re not confident of your
DMM’s accuracy, it may be easier to
simply adjust the lower arm’s resistance to measure 11,111Ω (11.111kΩ, or
100kΩ÷9). (Note that if you can measure both values on the same range, any
proportional inaccuracy in the DMM
itself should be cancelled out as it applies to both measurements.)
It’s now time to test the completed
PCB by connecting a source of 12V
AC, such as an AC plugpack. LED1
should light up.
Measure the voltage between TP 32V
and TP GND. You should get a reading
close to 32V. If so, you can disconnect
the power lead and put the PCB aside
while you work on the box.
Practical Electronics | October | 2020
Parts list – Precision Signal Amplifier
1 double-sided PCB, code 04107191, 92 × 51mm
1 diecast aluminium box, 111 × 60 × 54mm [Jaycar HB5063]
1 12V AC plugpack (100mA or higher) with 2.1 or 2.5mm plug
1 SPST mini toggle switch (S1)
1 insulated BNC socket, single-hole panel mounting (CON1)
2 BNC sockets, single-hole panel mounting (CON2,CON3)
1 PCB-mount concentric DC socket, 2.1mm or 2.5mm inner diameter (to suit
plugpack) (CON4)
4 25mm-long M3 tapped spacers
8 M3 × 6mm panhead machine screws
2 1mm PCB stakes (optional)
9 30mm lengths of hookup wire (to connect S1 and CON1-3 to the PCB)
Semiconductors
1 ADA4625-1ARDZ low-noise JFET-input op amp, SOIC-8 SMD package (IC1)
1 LM317M adjustable voltage regulator, SOT-223-3 SMD package (REG1)
4 1N5819 40V 1A schottky diodes (D1-D4)
1 3mm green LED (LED1)
Capacitors
2 470µF 25V RB electrolytic
1 47µF 35V RB low-ESR electrolytic
1 10µF 25V multi-layer ceramic (X5R 3216/1206 SMD)
2 1µF 100V polyester (radial leaded)
3 220nF 50V multi-layer ceramic (X5R 3216/1206 SMD)
1 100nF 50V multi-layer ceramic (X5R 3216/1206 SMD)
1 1.5pF 100V multi-layer ceramic (C0G 1206 or 0603 SMD)
Resistors (1% all SMD 3216/1206 SMD unless otherwise stated)
1 100kΩ 0.1% 0.25W axial leaded
1 100kΩ
3 10kΩ
1 5.6kΩ
1 3.3kΩ
1 820Ω
1 330Ω
1 240Ω
1 100Ω
2 51Ω
1 500Ω 15-turn horizontal trimpot (VR1)
29
50V
25V
CON2
3.3k
S1 (ABOVE)
1210
10 F
100k
50V
220nF
TP
32V
100
1
1 F
51
51
CON3
100nF
GAIN
A
IC1
10k
SET x10
820
LED1
D3
LOW ESR
POWER
10k
5819
47 F 35V
OUTPUTS
D1
100k
4625
10k
1 F
VR1 500 15T
D4
240
REG1 LM317M
220nF
470 F
5819
+
CON1
5819
330 5.6k
220nF
INPUT
470 F25V
12V AC IN
CON4
5819
04107191 C 2019
RevC
+
19170140
04107191
02 C
C9 12019
D2
CRevC
v eR
0.1%
1.5pF
TP GND
Fig.3: all of the components mount on this PCB, except for
CON1-CON3 and switch S1. The design uses a mix of throughhole and surface-mounting parts. Fit them where shown here,
being careful to ensure that IC1, LED1, diodes D1-D4 and the
electrolytic capacitors are mounted with the correct polarity.
Preparing the box
This is fairly straightforward. It involves drilling a total of nine holes in
the box lid/front panel, another hole
in the front of the box itself and then
a larger hole (12mm diameter) in the
box rear.
The locations and sizes of all these
holes are shown in the cutting diagram, Fig.4.
After all of the holes have been
drilled, cut and de-burred, you can attach a dress front panel to the lid, to
give the Signal Amplifier a neat and
user-friendly look. You can copy the
front panel artwork shown in Fig.6,
or download it as a PDF file from the
October 2020 page of the PE website.
Then you can print it out and laminate it in a protective pouch to protect
it from getting soiled.
It can then be attached to the box lid
using double-sided adhesive tape. The
final step is to use a sharp hobby knife
to cut the holes in the dress panel, to
match those in the lid underneath.
Now fit the three input and output
BNC connectors to the front panel,
along with switch S1, and then turn the
panel over and solder short lengths of
insulated hookup wire to the rear connection lugs of the connectors and S1.
Next, attach the four 25mm-long
M3 tapped spacers to the corners of
the front panel, using four 12mmlong M3 screws. Then you can cut
the hookup wires soldered to CON1CON3 and S1 to a length which will
enable them to just pass through the
PCB holes when the board is attached
to the rear of the spacers.
Remove about 6mm of insulation
from all of the wire ends, so that they
can be easily soldered to the matching PCB pads.
The almost-complete PCB just before final assembly. S1 and
the connectors are not yet fitted because these mount on the
front panel and connect to the PCB via short wire links. The
PCB ‘hangs’ off the front panel via 25mm M3 tapped spacers,
which are screwed to the four holes in the PCB corners.
A
42
42
A
21.5
C
C
41
9.5
A
1
15
CL
41
9.5
21.5
B
C
A
42
42
CL
HOLES A: 3mm DIAMETER
HOLE B: 6.5mm DIAMETER
A
HOLES C: 9.0mm DIAMETER
ALL DIMENSIONS IN MILLIMETRES
(FRONT OF BOX)
3.5mm
DIAMETER
18.5
24.5
CL
(REAR OF BOX)
17
12mm DIAMETER
Fig.4: here are the locations and sizes of the holes that need to be drilled in the diecast aluminium enclosure. For the larger
holes, it’s best to start with a smaller pilot hole (eg, 3mm) and then enlarge it to size using either a stepped drill bit, a series of
larger drills or a tapered reamer. That ensures accurate positioning and a clean, round hole. You can copy this diagram and
attach it to the box using tape to use it as a template. (It can also be downloaded from the October 2020 page of the PE website.)
30
Practical Electronics | October | 2020
End-on view from the input end. The
×10 gain is calibrated via the multi-turn
pot (blue component) in the foreground.
Above are two views of the assembled unit from the front (top) and the rear (bottom).
After bending these wires so that
their ends are positioned to meet with
the holes in the PCB, offer up the PCB
assembly to the spacers on the rear of
the panel.
With a bit of jiggling, you should
be able to get all of the wires to pass
through their matching holes. You can
then attach the PCB to the spacers using four more 6mm-long M3 screws,
up-end the assembly and solder each
of the wires to its PCB pad.
Your Signal Amplifier is now complete, and should look like the one
shown in our photos.
All that remains is to lower the lidand-PCB assembly into the box and
fasten them together using the four
M4 countersunk-head screws supplied with it.
Checkout and use
At this stage, your Signal Amplifier
should be ready for use. Remember that
it can deliver a maximum output voltage of 30V peak to peak or 10.6V RMS,
assuming that it is feeding a high-impedance load, of 50kΩ or more.
If the load impedance is much lower, the maximum output amplitude
will be slightly reduced.
Note that you can check and adjust
its calibration even after it has been
sealed in its box. To do this, you will
need an audio oscillator or function
generator and a DMM with a true-RMS
AC voltage range with reasonable accuracy and resolution.
Set your oscillator or function generator to produce a sinewave at 1kHz
Practical Electronics | October | 2020
and around 1V RMS, then connect its
output to the input of the Signal Amplifier. Then connect your DMM’s input
to one of the Signal Amplifier outputs.
With the Signal Amplifier’s gain set
to unity (A=1), power it up and your
DMM should indicate an AC voltage
very close to 1.00V. If not, you may
need to tweak the output of your oscillator/generator until this reading
is achieved.
Then all you have to do is select
the Signal Amplifier’s ×10 range, at
which point the DMM reading should
jump to 10.000V – or very close to
it. Now adjust trimpot VR1 with a
small screwdriver or alignment tool
(through the small hole in the front
of the box), until the DMM is reading
exactly 10.000V.
Reproduced by arrangement with
SILICON CHIP magazine 2020.
www.siliconchip.com.au
Fig.5: this scope grab of
the unit’s output with a
full-swing (32V peakto-peak) 20kHz square
wave demonstrates
the fast slew rate and
quick settling time of the
ADA4625 op amp.
You can see that there
is minimal rounding and
overshoot after each
transition, and it settles
close to the target value
in well under 1µs.
12V AC INPUT
www.siliconchip.com.au
INPUT
OUTPUT
PRECISION AUDIO SIGNAL AMPLIFIER
Rout = 51
POWER
OUTPUT
Rin = 100k
(3Vp–p MAX)
A = 1.00
A = 10.00
Rout = 51
SET x10 GAIN
Fig.6: this 1:1 front panel artwork can be copied and fixed to the lid or it can be
downloaded from the October 2020 page of the PE website, printed and then applied.
31
