AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
Universal single op amp board (optimised for
audio electronics) – Part 2
C4
R14
10kΩ
R7
C7
100pF
Feedback
components
Bias
Balanced
input
C1
10µF
Cold
R3
C3
C2
10µF
Gnd
R1
100kΩ
R2
100kΩ
V+
R6
10kΩ
+
L
C13
R8
3
Link pads
ast month, we introduced
our Universal Single Op Amp
Board, a really useful op ampbased amplifier PCB that is perfect for
both design/development and finished
products. It takes a through-hole or SMD
op amp, around which you have lots of
options for adding passive components
so that you can build a whole host of
common amplifier configurations.
I’m an audio engineer, so it’s not
surprising that I’ve designed the
board with audio in mind, but there
is absolutely no reason why it can’t be
used for instrumentation or any other
high-quality op amp application that
requires a quick and easy PCB solution.
We’ve already covered the op amp
non-inverting, inverting and summing
amplifier configurations, and this month
we will look at three more handy designs.
2
C5
100pF
Vb
IC1
+
R
14
5
8
4
C
10
R3
10kΩ
Gain = (Vin1–Vin2)(Rf/R3)
Output
R12
47Ω R13
C10
100nF
100kΩ
Power on 3-pin
connection
V–
C12
100nF
Link
Used
C1
0V
Not used
R1
C2
R2
0V
–input
+input
R4
C5
C
6
All resistor equal
C9
22µF
0V
6
C6
22pF
R5
R
6
C11
100nF
7
–
C7
R
12
R2
10kΩ
R11
Output
+
V+
+input
pad
Vb bias
R9
–
IC1
Vin2
R10
R5
10kΩ
+
Hot
Vin1
C8*
–input
pad
R4
10kΩ
R1
10kΩ
+
Sum input
+
Remember that
for each amplifier
shown, only the red
components and green
links are used.
Rf
10kΩ
Basic circuit
+
Fig.18. Differential
or balanced input
amplifier, possibly the
most common addon op amp circuit for
converting consumer
audio equipment to
professional operation.
R13
0V
Output
IC1
C11
0V
V+
V–
C9
+
C12
Power
Fig.19. (right middle) Component
placement for differential amplifier.
Fig.20. (right) The finished differential
amplifier board.
46
Practical Electronics | January | 2023
Audio Precision
A-A FAST RMS FREQUENCY RESPONSE
+30
+30
+25
+25
+20
+20
+15
+15
+10
+10
d
B
r
+5
+5
+0
+0
d
B
r
A
–5
–5
B
–10
–10
–15
–15
–20
–20
–25
–25
50
100
200
500
Hz
1k
2k
5k
10k
RIAA
network
*Can be scaled
(see text)
C4
+
+
Gnd
R5
+
Input
R1
C2
100µF
10V
R9
R2
180kΩ
Vb
+
C13
V+
+
220µF
R11
63.4kΩ*
R6
68kΩ
–
3
+
IC1
5
+input
pad
Vb bias
7
8
IC1
6
Output
+
R3
10kΩ
0V
C11
10µF
25V (Solid aluminium)
0V
4
Output
C12
10µF +
25V (Solid aluminium)
R8
Link pads
Link
0V
Used
Differential amplifier
This amplifier is useful for adding a
balanced input to consumer audio
equipment. The input is connected via
the three-pin Molex. C3 is normally
replaced with a wire link (short). As
with the non-inverting amplifier, R6
can be linked to the bias point for
single-rail operation. The circuit is
shown in Fig.18, the overlay in Fig.19
and the final stuffed board in Fig.20.
Now we’ve covered the basic amplifiers,
here’s a couple of more specialised
circuits that can be built on the board.
Fig.22. Phono
amplifier circuit –
if you are using
only one op
amp (as here) to
accomplish the
RIAA equalisation,
then odd RC
values are needed
for the negative
feedback network.
R12 C9
47Ω 100µF
C10
25V
100nF
Power on 3-pin
R13
connection
100kΩ
V–
C6
4.7pF
C5
100pF
50.15nF
–
V+
2
14.32nF
Input
0V
–input
pad
C3
R4
220µF 220Ω
63.4kΩ
220Ω
C8
50.15nF*
R10
5.23kΩ*
+
R3
C1
Feedback
components
0V
Bias
R14*
C7
14.32nF*
R7
5.23kΩ
+
Sum input
Basic circuit
–30
20k
Not used
Extra cap
C7
E
x
t
r
a
c
a
p
C8
R
12
C2
R
6
C5
C
6
C
10
R2
+
IC1
0V
–input
+input
R4
R R
10 11
R13
C3
+
–30
20
Fig.21.Measured
RIAA curve of the
phono amplifier
circuit in Fig.22.
Mid-band gain at
1kHz is +30dB.
Max gain is +50dB
at 20Hz giving
150mV for a typical
cartridge input
of 5mV at 1kHz.
This requires a
lot of open-loop
gain for a single
op amp; however,
this approach
generally gives the
best noise and
overload capability
if all the gain and
equalisation is done
in one first stage.
0V
Output
C11 +
+
C9
+
C12
0V
V+
V–
Power
Fig.23. Component placement for phono amplifier. Note extra capacitors wired in parallel.
RIAA phono amplifier
This is probably the most complex
design to be built on this board. The
equalisation to implement the RIAA
curve, shown in Fig.21, requires nonPractical Electronics | January | 2023
standard-value components with a
tolerance of 2% or better. These values
are difficult to calculate, but Douglas
Self has got them all worked out in his
book Electronics for Vinyl. Getting the
E96 resistors is not too difficult now,
and Farnell’s pricing is reasonable
with their Vishay 1% MRS25 series
47
compensation capacitor (C6) is 4.7pF,
which works in practice, even though
the gain of the circuit falls to unity
at high frequencies. The theoretical
value should be 22pF, but the lowest
high-frequency distortion is achieved
with lower values. The coupling
capacitors can all be tantalum. There
is no distortion problem since the
signal levels are all below 1Vpk-pk.
The polarity of the capacitors is
unimportant for dual rail, although
fusspots may wish to align them with
the offsets.
For bipolar op amps with high
input bias currents feeding NPN input
transistors, such as the NE5534, the
output offset is usually negative.
Therefore, the negative terminal of
the electrolytic capacitors should be
connected to the op amp. The circuit
is shown in Fig.22 and the overlay in
Fig.23, along with the final construction
in Fig.24a and b. The NE5534 is still one
of the best op amps for moving magnet
cartridges and the NE5534A version
has guaranteed noise specs. I’ve only
got significantly better than this by
using ±25V discrete circuits with an
expensive J74 JFET on the front end.
Cherry low-frequency
compensation
Fig.24. (above) Phono amplifier construction – a bit untidy, but it can at least be
accommodated on the board; (below) Alternative RIAA amplifier construction. Single
capacitors are used for C7 and C8, but half the value. Resistors R10, R11 and R4 are
scaled up to 10.5kΩ, 127kΩ and 442Ω respectively.
if you buy a lifetime’s supply of 100!
Mouser are cheaper, supplying the 1%
Yeago MFR-25FBF52 series. (Often,
E96 resistors are 0.1% and can cost a
fortune, so these 1% options are well
worth considering).
Getting accurate capacitors is more
difficult and ±1% or 2.5% tolerance
devices are considered top-notch,
but here in Wales, home of the UK’s
‘capacitor cluster’, LCR (who absorbed
Suflex) still make excellent polystyrene
capacitors, available from the author
– see contact details in the AO Shop
ad on p.50.
To aid constructors I’ve made
available some low-cost kits of these
odd-value resistors and capacitors
(also via the AO Shop). The 50.15nF
capacitor (C8) can be made up using
two surplus Suflex 1% 24.76nF
capacitors in parallel (giving 49.52nF)
using the extra pads as shown earlier
in Fig.6. You can add an extra 680pF
as well for more precision. The other
capacitor (C7) needs to be 14.32nF and
I made mine out of a parallel 13nF and
48
1.3nF. I rescued these caps out of a skip
at the end of a production run of filters
and they are all still spot-on. Of course,
five 10nF capacitors, or any other
combination could be used for C8.
These were the original Doug Self
values. For a simpler version (Fig.24,
lower), I decided to double the resistor
values and halve the capacitors, so
only two capacitors, one of the 24.76nF
and an LCR 7.15nF were needed. The
noise increases by about 1dB by doing
this because of the higher impedance.
To provide the final part of the RIAA
curve, −20dB at 20kHz relative to 1kHz,
a further low-pass filter is required.
This is due to the op amp’s response
flattening off to unity, since the noninverting configuration has a minimum
gain of one. This is achieved with a 4.7nF
capacitor inserted in R13’s position in
conjunction with a higher-than-normal
value for R12 (510Ω). This network
should be omitted if a roll-off at 66kHz
is provided further on, as is usually the
case, such as with stabilising capacitors
or an output balancing transformer. The
I put this capability on the board because
it’s a current interest of mine and I
think the technique needs to be more
widely known. DC coupled circuits
give a perfect square-wave response at
low frequencies, but can suffer from
dangerous DC offsets. Placing a capacitor
in the lower arm of the feedback network
of a non-inverting amplifier offers
protection by reducing DC gain to unity,
but a low frequency square wave, such
as 20Hz, will suffer from a pronounced
‘tilt’ at the top and bottom of each cycle.
Professor Ed Cherry devised a scheme
to compensate for this by adding an
extra RC network (see ETI magazine,
60W NDFL Amplifier, May 1983). This
technique can also be used to minimise
the value of the capacitor, which can
often be hundreds of microfarads in
a power amplifier, down to say 47µF.
This then allows longer-life tantalum
capacitors to be used. For those worried
about distortion, the capacitor can
then be configured as a bipolar part by
putting two capacitors back-to-back and
applying a 5V bias voltage to the centre
connection. A demonstrator circuit is
given in Fig.25, the overlay in Fig.26 and
the finished board in Fig.27.
Positive feedback
The vast majority of op amp circuits for
audio, as well as instrumentation and
other signal-processing designs, use
Practical Electronics | January | 2023
R14
Sum input
C4
+
R7
C7
Feedback
components
+
+
+
R3
R5
C2
10µF V+
10V
R9
1MΩ
R2
100kΩ
Vb
Gnd
+
R10
15kΩ
C13
R11
33kΩ
R6
100kΩ
R8
470kΩ
R1
750Ω
V+
2
–
3
+
7
IC1
5
+input
pad
8
+
C11
4
C6
Output
0V
6
C5
Vb bias
+5V
Basic circuit
C8
2.2µF
Polyester
–input
pad
C3
C1
100µF Bias 100µF R4
20V
20V
750Ω
Input
Low-frequency
compensation boost
R12
47Ω
C10
100nF
Power on 3-pin
connection
V–
47µF
Used
33kΩ
2.2µF
–
IC1
+
Output
Gain = 1 + (Rf/R1)
0V
R13
Output
0V
Link
0V
C9
C12
Link
pads
Input
Rf
15kΩ
Not used
With compensation
0V
Time
Without compensation at 20Hz
Fig.25. Circuit for demonstrating Cherry low-frequency compensation. Feed with a 20Hz square wave and short out the
compensation network C8 and R11 to see the effect. The ‘scope must be set to DC input coupling. Gain is 21x or 26dB.
R
6
+
C5
C
10
R2
C2
C3
IC1
0V
Output
C11 +
R
9
0V
–input
+input
R4
R R
10 11
+
R
12
C1
+
C8
C7
R
8
+
C12
0V
V+
V–
Power
Fig.26. Component placement for Cherry low-frequency compensated amplifier.
Fig.28. There is provision for SMD op
amps, such as this MOSFET CA3140.
outline in parallel with the DIP socket
pins. This is shown in Fig.28.
Allowing for a few diagonal links,
it should be possible to accommodate
almost any audio op amp circuits, such
as pre-emphasis/de-emphasis filters,
integrator/servos, gyrators… and I’ve
thought of about 20 more circuits. I’m
sure readers will adapt the boards for
themselves in ways limited only by their
ESP (electronic spatial perception).
Fig.27. The final Cherry compensated op amp board. The big red capacitor (C8)
provides a compensating low-frequency boost to compensate for the roll-off induced
by the lower arm feedback capacitor (C1, C3) and associated resistor R4.
negative feedback. Positive feedback is
generally only used for oscillators, and
comparators with snap-action hysteresis.
This board does not really cater for these
circuits, but I’m sure a way will be found.
Practical Electronics | January | 2023
Soldering on
There may come a time when some audio
op amps are only available in surfacemount packs (heaven forbid), so the
board includes a standard SOIC 8-pin
PCB for Universal single
op amp board
The PCB described in this twopart series was designed by Mike
Grindle and is available from the
January 2023 section of the PE PCB
Service, part no. AO1-JAN23.
49
