AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
Universal single op amp board (optimised for
audio electronics) – Part 1
audio work. The best way to test this is
to put the offending op amp into a highgain amplifier and have a listen if you
have good ears, or check with a ‘scope
if – like me – your hearing is no longer
top notch. In the end, I decided that
to test op amps the best I could to do
was to design a universal circuit board
where most of the standard op amp
configurations could be easily built.
Getting board
RS used to sell a generic op amp board,
as shown in Fig.1. Maplin also did one.
I’ve used lots of the now discontinued
RS boards over the years, such as in the
EPE Test Amplifier and the Upwards
Compressor shown in Fig.2. Since such
a useful board is no longer available,
it’s obviously time to produce a PE
replacement, and while we’re at it,
Fig.1. The discontinued RS op amp PCB.
Now we have a better replacement,
optimised for audio, but perfectly useable
for general op amp applications.
O
ften the subject of this
column is initiated by a reader’s
request. In this case, Mr Martin
van Doorn from Holland needed an
op amp tester. To create a tester that
covered every op amp parameter would
be very complex – more a job for a team
at say Peak Electronic Design than my
own ‘sole-trader’ approach. So, having
dodged that job, I did come up with a
viable alternative idea.
Fig.2. Here’s a typical studio application for the RS boards, adding balanced XLR
inputs to an upwards compressor (low-level ambience booster).
Mission creep
In audio equipment, op amps are
surprisingly reliable, with a failure rate
almost as good as single small-signal
transistors. Their most common failure
mode is the inputs getting zapped by
abuse – for example, connecting large,
charged capacitors or 48V phantom
power with no protection. This usually
results in the output permanently stuck
at one of the power rails or a short
between the power pins. In such cases
it’s a straightforward go/no-go test with
a DVM/multimeter or simply sniffing for
smoke. A more subtle failure is input
transistor degradation, resulting in a
high noise level, a major problem for
48
Fig.3. A problem with the RS board was a lack of provision for the large capacitors
required for audio work.
Practical Electronics | December | 2022
R14*
C4
+
R7
Bias
C2
R1
V+
R2
R9
R11*
C13
3
+input
pad
R6
Vb bias
C5
R8
*Feedback
components
V+
2
Vb
+
R10*
R5
+
Input +
0V
R4
+
+
Input –
C3
C8*
–input
pad
R3
C1
C7*
7
–
IC1
+
5
8
C6
0V R12
6
4
C10
100nF
Vb
Vb
R6
R3
0V
C11
C9
Output
+
Sum input
V+ V–
Op amp inputs
R13
Power on 3-pin
connection
V–
C12
Link pads
0V
Fig.4. The general circuit diagram of the universal op amp board. Not all the components are used
at once, only a selected few for a specific circuit.
Fig.7. There are links provided
for bias and grounding. Note
the PE boards have an extra
grounding pad. There are
also connection points going
to input pins 2 and 3 on the
op amp near C5. These are
useful when pots have to be
connected for gain control.
There is also a link to R3 for
biasing bipolar capacitors
made up of C1 and C3.
Finally, there are three ground
pads – see Fig.5.
amps, the NE5534, sometimes requires
a compensation capacitor across pins
5 and 8.
Fig.5. The overlay of the general op amp board.
Chip multipacks
Some of the best audio op amps, such
as the LM4562, are only available as
dual devices. Generally dual devices are
cheaper than singles, so we’ll be needing
a dual-channel board as well, a design
which will be coming later. Quad op
amps, such as the TL074, are rarely used
Fig.6. The board has provision for experimentation in the form of four ‘blob points’.
Essentially, five pads connected in strips like a bread board.
upgrade the late-1970s single-sided
unplated-through-hole implementation.
The RS boards were designed for DC
instrumentation use, reflecting RS’s
mainly industrial clientele. When I
used them for audio applications there
were often large capacitors hanging
off precariously, as shown in Fig.3.
Practical Electronics | December | 2022
So, accommodation for these larger
components was necessary in my
new design. On the other hand, some
instrumentation requirements, such as
the large off-set adjust preset, could be
omitted. Also needed was a half-rail
bias network for single-rail operation.
Plus, one of the most popular audio op
Fig.8. The output resistor outline is extralarge to allow a capacitive load isolator
inductor network to be added.
49
for audio since it’s difficult to get large
audio capacitors to fit around them on
the PCB. Apart from the LM837, there
are very few low-noise quad devices for
audio anyway, so we won’t be bothering
with quads. I do think a plug-in dualto-quad adapter board is a good idea
though. I would love to replace some
noisy quads with a couple of NE5532s
in some of my vintage gear. Has anyone
designed one? If there is enough demand,
C4
1µF
Sum input
I’ll do a 2x 8-pin dual to 14-pin quad
converter PCB.
Well-padded board design
The overall schematic for the board is
shown in Fig.4. It is optimised for audio
applications, but is certainly not limited
to audio. For any given circuit only some
of the components will be inserted. The
overlay with all the possible components
is shown in Fig.5.
R14
10kΩ
+
R7
10kΩ
Basic circuit
C7
47pF
C8
Rin
R3
Input –
C3
R4
+
+
C1
C2
0V
R1
R6
V+
R2
R9
Vb bias
Vb
+
C13
R8
+
–input
pad
R5
+
Input +
IC1
R11
2
–
3
+
+input
pad
C5
7
IC1
5
8
V+
0V
0V
C9
10µF
25V
C11
100nF
6
4
C10
100nF
C6
22pF
R13
100kΩ
Power on 3-pin
connection
V–
C12
100nF
0V
Link pads
Link
Used
Not used
R7
C
6
R13
C
10
0V
Input
0V
Output
IC1
C9
+
C11
C12
Normally, for a board like this,
the components would be
annotated according to their
function. On the RS board for
example, the negative feedback
resistor was called Rb. To
keep things simple for the
PCB designer and constructor
I’ve decided to stick with
conventional numbering.
H e r e ’s a l i s t o f t h e
components and their possible
functions. Things will become
clearer when specific circuits
are shown where many of the
board’s component positions
are left empty.
R1, R2 and R13
R
14
C4
Output
R12
47Ω
Fig.9. Inverting amplifier, a standard op amp configuration.
R
12
Output
Gain = – Rf/Rin
+
R10
Component functions
Rf
–
Input
Bias
All the big capacitor positions have
extra pads for different outlines. These
allow radial or axial components to be
used, and capacitors wired in parallel.
There are also four interconnected pad
groups or ‘blob-points’ for adding extra
components for experiments on the lefthand side of the board. (Note how the
blob points have been used to make it
easy to add extra capacitors to make a
parallel C8, as shown in Fig.6.)
0V
V+
V–
Fig.10. Component placement for the inverting amplifier.
These resistors are an important
audio addition. They are pulldown resistors which prevent clicks
when input and outputs are connected.
They hold the outputs of the coupling
capacitors to 0V. Typical values would be
a few tens of kΩ when using electrolytic
coupling capacitors and around a few
hundred kΩ for film and tantalum types
(because of their lower leakage).
C1, C2 and C3
These are coupling capacitors on the
op amp inputs. C1 and C3 are for the
inverting input. They can be wired
as a non-polarised capacitor for low
distortion. The capacitor distortion can
be reduced still further by biasing the
mid-point positive terminals of C1 and
C3 at 5V using R3 and Vb. C1 and C3 can
also assume the role of the lower-arm
feedback capacitor for non-inverting
configuration when grounded by putting
a shorting link in R1s position. C2 is
connected to the non-inverting input.
R4
Fig.11. The assembled board – the op amp is a TDA1034, the original NE5534,
designed by Philips. The date code is 1977 and this one was used in Pink Floyd’s
touring mixing desk made by Midas for Britannia Row. It’s still working fine, having been
installed many times.
This is the input resistor for an inverting
amplifier configuration, commonly
designated ‘Rin’. It can also be the
negative-phase (‘cold’ in audio parlance)
input resistor on a differential amplifier.
R6 is usually linked to ground in inverting
amplifiers via a link. A low value, such
as 560Ω, is often used for audio for
low noise while providing a degree of
50
Practical Electronics | December | 2022
R7
Input –
R1
V+
R2
Input2
C13
R8
–
10kΩ
IC1
2
–
3
+
7
IC1
5
8
0V R12
6
4
C10
100nF
C6
Output
+
0V
C11
C5
Vb
10kΩ
V+
+input
pad
R6
Vb bias
R9
+
R11*
R5
+
This can be the positive phase
input resistor on a differential
amplifier. In the non-inverting
configuration, it becomes the
input RF filter in conjunction
with C5.
R10*
–input
pad
R4
C3
22µF 10kΩ
C2
0V
C8*
+
+
R3
C1
22µF
Input +
C4 and R7
Feedback
components
C7*
Input1 10kΩ
C9
Output
+
C4
Bias
R5
Basic circuit
R14*
Sum input
+
input current limiting in the
event of faults such as power
supply misconnection. In
instrumentation, it is usually
the same value as the feedback
network giving lowest DC
offset. For single-supply rail
use, it is connected to the halfrail bias network consisting of
R8 and R9 via another link.
The link area is shown in Fig.7.
R13
Power on 3-pin
connection
V–
C12
0V
Link pads
Fig.12. Summing or mixing amplifier with an extra input – the composite bipolar capacitor (C1 and
These provide a second input C3) is optimised for good LF response. Note: red components shown are in addition to those in Fig.9.
to the inverting amplifier’s
input when used as a virtual earth
0V
C1
summer or mixer.
–input
R
12
R14
This is the all-important negative
feedback resistor. There is provision for
more complex feedback networks, such
as RIAA equalisation comprising R10,
R11, C7 and R8.
Finally some stability components. If
a phase-lead capacitor is needed across
feedback resistor R14, it can be C7 with
C8 linked out. Typical values are 33
to 100pF. R12 is needed to isolate the
op amp from capacitive loads, such as
screened cables. An inductor can also
be used instead for lower AF output
impedance. This can consist of 40 turns
of wire wrapped around a 1W 39Ω resistor
or a 10µH inductor and resistor wired in
parallel, as shown in Fig.8. There’s the
usual decoupling capacitors C10, C11 and
C13. Capacitor C6 is the compensation
capacitor for NE5534 op amps for gains
below 5.
+
R
14
R7
R4
C4
C
6
IC1
C
10
+
R13
0V
Output
C3
C11
C9
C12
+
0V
Input
0V
V+
V–
Fig.13. Overlay for summing amplifier.
Component list (and function)
Semiconductors
IC1
Single op amp, such as NE5534.
Can be 8-pin DIP through-hole
or SOIC surface-mount part.
Resistors
All standard 0.25W case size, usually
metal-film, use the 1% 0.6W MRS25
series for audio.
R1, R2, R3 input grounding/pull-down
resistors 22kΩ to 100kΩ
R3
bipolar capacitor bias resistor
or link to ground for using
big lower-arm feedback
capacitor in C3 position.
R4, R5
o p amp input resistors,
typically 1kΩ to 100kΩ
Practical Electronics | December | 2022
Fig.14. The completed summing amplifier.
R6
n o n - i n v e r t i n g i n p u t
grounding / bias resistor,
typically 1kΩ to 100kΩ
( m u c h h i g h e r, 1 M Ω t o
4.7MΩ, for FET op amps)
R7
summing input resistor
R8, R9
half-rail bias resistors equal
value, 10kΩ to 100kΩ
R10, R11 extra feedback resistors for
filters
R14
f eedback resistor zero to
220kΩ (much higher, 1MΩ
to 4.7MΩ, for FET op amps)
R13
utput capacitance isolation
o
resistor 39Ω to 600Ω
Capacitors
C1, C2
input coupling capacitors,
typically 1µF to 22µF
C3
part of bipolar capacitor with
C1 or big lower arm feedback
capacitor
C4
extra input coupling
capacitor for summing input
or small lower arm feedback
capacitor
51
C4
100µF
16V
Sum input
R14
10kΩ
R7
1.1kΩ
C7
47pF
R1
100µF 1.1kΩ
C8
other applications–do let us
know your ideas.
Rf
10kΩ
Basic circuit
Inverting amplifier
–
+
Apart from a voltage
follower, this is the simplest
Feedback
Blocking
amplifier circuit that can be
R10
R11
components
Gain = 1 + (Rf/R1)
capacitor
Bias
built on the board. There
V+
are two ways of feeding
–input
R3
Bias 1/2 supplyV+
pad
C11
C9
the input. The simplest is
100nF
C3
C1
R4
100µF
Output
2
7
to come in via the two-pin
25V
Input –
–
0V
6
R5
IC1
input Molex connector
1kΩ
3
4
Input +
+
R12
shown in the circuit in
8
C10
47nF
5
C2
100nF
Fig.9 and the overlay in
R6
220nF
+input
0V
V+ 100kΩ
Power on 3-pin R13
pad
R1
Fig.10. An alternative is to
100kΩ
connection
C6
Vb bias
R9
V–
R2 22kΩ
use the inverting input on
C5
1MΩ
220pF
Link to connect
the three-pin connector via
C12
Vb
negative rail to 0V
C1. This enables a bipolar
0V
+
R8
input coupling capacitor
Link
pads
C13
22kΩ
Link
Used
Not used
10µF
composed of C1 and C3
to be used. This network
Fig.15. Non-inverting amplifier with gain of 10x and single-rail biasing.
can be biased to give the
lowest distortion. This is
useful for low-impedance applications
R
C7
where the input resistor is very small.
14
0V
–input
A completed inverting board is shown
R
+input
C2
R2
6
R
in Fig.11.
R5
IC1
+
Output
+
+
+
Input
12
R7
C5
C
10
R
9
R
8
Summing amplifier
R13
C4
IC1
+
C13
+
0V
Output
C9
C11
+
0V
V+
Fig.16. Overlay for non-inverting amplifier – note bias links.
This is just an inverting amplifier with
an extra input resistor. Both inputs on
J1 and J3 are used. The circuit is shown
in Fig.12. The bipolar capacitor set
up was used for an equaliser where a
low-pass filter output of a state-variable
filter was mixed with the high-pass
creating a notch filter. The high value
used (11µF) minimised the LF response
droop. The overlay is shown in Fig.13
and Fig.14 shows the construction.
Non-inverting amplifier
Fig.17. Fully stuffed non-inverting amplifier.
C5
C6
F filter, 47pF to 470pF
R
NE5534 compensation; not
used for gains over 5: 22pF
for unity gain, 4.7pF for RIAA
stages, 2.5mm pitch
C7, C8
feedback, C8 is for extra-large
polystyrene filter capacitors
C10
power rail decoupling 0.1µF
ceramic 5mm
C11, C12 0.1µF ceramic 5mm or up to
10µF electrolytic
52
Connectors
Molex 0.1-inch pitch PCB connectors
Molex equivalent 2 off each JYK
P2500-02 two-pole straight header and
three-pole P2500-03 Rapid order codes
22-0950 and 22-0955
Classic configurations
Here’s a few of the possible circuits
that can be built on the board; I’m sure
readers will adapt the board for many
This is probably the most common op
amp circuit, typically giving any gain
from 1 to 1000 (0dB to 60dB) along
with a high input impedance. A circuit
giving a gain of 10x is shown in Fig.15.
This circuit is more complicated
because it is shown designed for
single-rail (rather than the normal
dual-rail powering). This is achieved
by feeding the non-inverting input with
a half-rail bias voltage. The overlay is
given in Fig.16. A useful application
of this configuration is a microphone
preamplifier. This consists of a noninverting amplifier with variable gain
and a step-up input transformer. Fig.17
shows the completed board. To run it off
the dual-rail supply, leave off the half-rail
bias and link R6 to ground.
Next month
In Part 2, we will finish describing this
design with a differential and RIAA
phono amplifier, plus a useful and
unusual low-frequency compensated
op amp amplifier.
Practical Electronics | December | 2022
