This is only a preview of the January 2025 issue of Practical Electronics. You can view 0 of the 80 pages in the full issue. Articles in this series:
Articles in this series:
Articles in this series:
Articles in this series:
Articles in this series:
Articles in this series:
Articles in this series:
Items relevant to "Raspberry Pi-based Clock Radio, part two":
Articles in this series:
Items relevant to "Secure Remote Mains Switch, part two":
|
Constructional project
Phil Prosser’s compact and high-quality
Microphone
Preamplifier
If you use microphones for stage, recording or testing, you will be familiar with the
need for a preamp to get a usable signal. Many microphones also need ‘phantom
power’. This small box runs from a plugpack and offers a flat frequency response,
very low distortion, low noise and adjustable gain.
Background image: https://unsplash.com/photos/ALM7RNZuDH8
T
his small microphone preamp is
ideal for use in the studio, workshop or on the stage. It allows you to
boost the gain of your microphone to
line level and delivers a balanced or
single-ended signal.
The main version of this Preamp fits
into a small, standard-sized enclosure
that is widely available, as shown in
the photos. This diecast aluminium
case makes it tough enough to survive the worst abuse.
If you want to integrate this design
into a larger project, we have a version of the board that omits the cutout
for the XLR connectors and drops one
of the switching regulators, making
it easy to run it from existing ±15V
DC rails. That would make sense if
integrating it into a power amplifier,
preamplifier, mixer or similar.
When built as a standalone unit,
it runs from 9V DC, as widely used
on stage. Those plugpacks generally
Features & Specifications
have 2.1mm plugs with a positive
ring and negative tip. We have included reverse polarity protection,
so no damage will occur if the wrong
plugpack is used.
We set this requirement as it is a
fair bet that things will get mixed up
on the stage. You don’t want to be
fiddling with equipment while the
crowd waits for the concert to start!
Therefore, it should ‘just work’.
Performance
The performance of the Microphone
Preamplifier depends on various factors. Having low noise is important;
the noise level is significantly affected by the source impedance and
gain setting.
For a source impedance of 560W
with 50dB gain and a 1V RMS output,
the signal-to-noise ratio is 70dB. At
the same output voltage but a gain of
20 times, the SNR is 85dB.
Operates from a 9-15V DC plugpack (9V DC is common for stage equipment)
Fits in a compact 120 × 93.5 × 35mm diecast enclosure
Adjustable gain from -15dB to +50dB
Switchable 20dB attenuator for high-level sources
Switchable 48V phantom power
Drives in excess of 5V peak-to-peak (1.75V RMS, 13dBu) into a 600Ω load
Balanced or single-ended output
Frequency response: ±0.1dB, 12Hz to 20kHz (gain=26dB/20×) (see Fig.1)
Signal-to-noise ratio (SNR), Zi = 560Ω, Vout = 1V RMS:
85dB (gain=26dB/20×), 70dB (gain=50dB/320×)
» Total harmonic distortion (THD): <0.002% (see Fig.2)
» Built-in power protection, including reverse polarity
» Input and output protection against most abuse
»
»
»
»
»
»
»
»
»
14
The frequency response with a gain
of 20 times (26dB) is within 0.1dB
from 12Hz to 20kHz – see Fig.1.
As shown in Fig.2, the distortion
(THD+N) is entirely determined by
noise. The underlying distortion is
significantly lower, in the region of
-95dB (0.0018%) to -105dB (0.0006%).
There is some evidence of noise
from the switch-mode regulator at
the output, but it is 70-80dB down,
depending on the gain setting. That
is a low enough level that it is not a
concern.
Given that the distortion is so low,
it’s the SNR that’s going to be the performance limit. 70dB is pretty much
the worst you can expect as long as
your input signal level is sufficient
to achieve at least 1V RMS output at
the maximum gain setting of around
50dB.
As you reduce the gain to 26dB, it
will improve to 85dB, and it should
improve further at even lower gain settings, exceeding 90dB. That’s assuming your microphone/signal source is
high enough in level to still provide
a useful output with less gain.
Some challenges
This design is a little tricky because
microphone phantom power needs to
be 48V DC to be universal. That is a
lot higher than 9V DC.
To provide users with headroom of
10-15dB over 0dBu, we want to be able
to deliver an output signal with peaks
above ±8V. That is needed for people
using the mic closer than expected
and to deal with loud passages. Stage
equipment must have headroom; the
Practical Electronics | January | 2025
Microphone Preamplifier
sound engineer can deal with levels
at the mixing desk.
That means we need supply rails of
48V DC plus dual rails sufficient to get
this ±8V from an op amp. We want this
in a small box and for the circuit to be
as tough as a cheap steak.
If we start with 9V DC and drop
0.5V across a reverse polarity protection diode, then budget another 0.5V
for the plugpack output drooping,
we only have a poorly-regulated 8V
supply to work with. We considered
using switched capacitor inverters/
doublers using 555s but found that
gave marginal supply rail headroom.
After some thought, we decided to
take a more industrial strength approach, using two LM2577 boost regulators and a cunning trick to sneak in a
negative rail. These regulators are more
powerful than we need, but they are
widely available and can handle 60V
on their output, enough for the phantom power rail.
The resulting power supply fills a
significant proportion of the PCB, as
we shall see in more detail later. While
this solution is hefty, it is very tolerant
of input supply variation; even if the
output is loaded with a very low impedance, the rails will stay up.
If you are wondering if this could
be run from a 9V battery, the answer
is not for any length of time. The current draw is far too high to expect a
decent lifespan from the battery, and
it will go flat exactly when you don’t
want it to. Full load current draw is
about 120mA, which will flatten a 9V
battery in short order.
Don’t think that all this talk about
the power supply means we’ve forgotten that the preamp part must also
have decent performance.
We’re using the same hybrid transistor/op amp balanced microphone
preamp found in the Loudspeaker Test
Jig (published in the June 2024 issue),
developed by audio guru Douglas Self.
It gives excellent performance with
low distortion and noise, plus a wide
range of possible gain settings.
Fig.1: we had
to make the
vertical scale
very small
to see the
variations
in frequency
response as it
is so flat.
Fig.2: any
distortion
produced by
the circuit is
well and truly
buried in the
noise. Thus,
the SNR is
the primary
determinant
of the
performance
at any given
gain setting.
Circuit description
Fig.3 is the block diagram for the
Preamp, while the main (analog) part
of the circuit is shown in Fig.4.
S1 switches phantom power for the
microphone via header CON10. Noise
is filtered out of the 48V DC supply by
a 100W/220μF low-pass filter (LPF). We
have used 6.8kW resistors for the two
Practical Electronics | January | 2025
Fig.3: the Mic Preamp block diagram shows the somewhat complicated power
supply at the top, with the superficially simple attenuation and preamplification
circuitry below.
15
Constructional project
bias resistors; these should be matched
as close as possible.
We selected two resistors that measured within 0.1% from our collection of 6.8kW 1% resistors. You could
buy 10 resistors and choose the bestmatched pair.
The 47μF/100nF parallel capacitor
pairs block DC from the microphone
signal as it’s fed into the attenuator.
These prevent the full 48V phantom
power from being applied to the attenuator when the microphone is unplugged, so they must be rated at a
minimum of 63V.
This Preamp has a 20dB pad at
the front end. It can be switched in
to avoid the Preamp clipping with
higher-level input signals. The pad
uses two 1.8kW resistors in series
with the input signals and a 430W
resistor connected between the terminals of RLY1.
When the attenuator is switched
Fig.4: the main analog section of the Preamp circuit. It is based on two dual op amps and two transistors; the
transistors lower the noise floor substantially. The second op amp drives the balanced and unbalanced outputs. Relay
RLY1 switches in a resistive attenuator so it can handle higher level input signals.
16
Practical Electronics | January | 2025
Microphone Preamplifier
out, the relay shorts out the 1.8kW resistors, and the 430W resistor is out
of the circuit. When switched in, the
430W resistor is connected between the
downstream ends of the 1.8kW resistors, forming a voltage divider. These
relatively low values minimise additive noise from the attenuator and keep
the impedance driving the following
preamplifier low.
To calculate the attenuation of this
stage (when activated), add a mental
ground connection in the middle of
the 430W resistor, splitting it into two
215W resistors. These resistances are
in parallel with the 4.7kW resistors
to ground, so the dividers are formed
with resistances of 1.8kW plus the
microphone source impedance and
205.6W. Assuming a low source impedance, the resulting attenuation
is -19.8dB.
Note that we need closely-matched
values for the 1.8kW and 4.7kW parts
to ensure good common mode rejection performance when the attenuator
is switched in.
We have used a relay for this job as
our experience with switching small
signals with miniature toggle switches
wired to the board is not great. A telecom relay gives better long-term reliability and lower noise for a modest
increase in cost.
The 6.8V zener diode across the
relay protects it in case someone runs
the Preamplifier from a higher voltage
than expected. The series resistor will
get quite warm, but it should survive,
provided this abuse is not continuous.
Preamp gain
We have provided a variable gain
Two versions of this project
allow it to fit into a small
box (as shown) or a
larger chassis with
dual-rail power
available.
that allows you to set the level
from a range of microphone types
and situations. When VR1 is set to minimum resistance, the gain is 47.8dB,
calculated as:
G = 1 + 2.7kW ÷ (10kW || [22W ÷ 2])
G = 1 + 2.7kW ÷ 10.98W
G = 247 (47.8dB)
When VR1 is set to its maximum of
10kW, the gain is 5.1dB:
G = 1 + 2.7kW ÷ (10kW ||
[(10kW + 22W) ÷ 2])
G = 1 + 2.7kW ÷ 3338W
G = 1.8 (5.1dB)
Using a reverse log taper potentiometer for VR1 results in the attenuation
being ‘linear’ in dB terms as the potentiometer is rotated. Otherwise, most of
the potentiometer’s range will result in
relatively low gain, with the last fraction of the rotation ramping the gain
over 20dB or so. So make sure the pot
you choose has a ‘reverse log’ or ‘reverse audio’ (C) taper.
The small signal diodes in the preamplifier (D4-D8) ensure the op amp
inputs are not overdriven. We have included a buffer following the preamplifier that also produces an inverted
Calculating the total current draw
The phantom power supply needs to provide about 10mA to the LM317HV (REG1) and
a maximum of 14.1mA into the 6.8kW resistors if they are shorted to ground. That is
24mA at 55.3V, which will require ~166mA (55.3V ÷ 8V × 24mA) at the input of the REG3.
The dual rail power supplies must supply up to about 40mA to the NE5532 op amps
and input circuit and about 10mA each for REG3 and REG4. That is a total of 100mA,
given there are positive and negative rails, meaning a draw of up to about 225mA (18V
÷ 8V × 100mA) at the input to REG4 in the worst case.
That means the Preamp could draw something in the region of 350mA, although that
would only happen if it were driving a shorted load. Most 9V plugpacks can supply this,
but most 9V batteries can’t. The most we saw in our tests was 150mA from 9V. Note
that the worst case current is at startup, when the switch mode regulators are charging
the 56V and ±18V supply filtering capacitors.
We have included a power LED, powered from the negative rail. We chose this rail
because if a user connects the Preamp to an 18-24V DC plugpack, the boost regulator for the positive rail will likely shut down, and the negative rail will not be generated.
No damage should occur, but the user will be informed that it is not operating by the
power LED being off.
Practical Electronics | January | 2025
output. This allows the output to be
single-ended or drive a balanced line
at a high level.
We have also added small signal
diodes to the positive and negative
rails on the outputs (D14-D17) so that
if someone inadvertently connects this
to a piece of equipment with a large
DC offset on its input, they will protect the NE5532 (IC1).
We have incorporated 100W series
resistors on the outputs to ensure the
op amp remains stable even when
driving difficult loads or long cables.
Those will also help to limit the current flow in the case of a misconnection. You can use the positive buffered output at pin 2 if you only need
a single-ended output.
Power supply
The power supply portion of the
circuit is shown in Fig.5. The overall
design comprises two switch-mode
pre-regulators that drive LM317/337
linear regulators. This generates very
clean power rails, including the phantom power rail.
The phantom power supply uses
the LM2577 (IC3) in a textbook configuration. Its input is bypassed with a
220μF low-ESR capacitor and a 100nF
capacitor. 220μF is quite low, but the
maximum current we need to supply
is less than 30mA. That is little more
than idling for the LM2577.
We have increased the compensation capacitor in series with the 2.7kW
resistor at its pin 1 from a suggested
value of 1μF to 10μF. That slows the
startup of the boost regulator. Our small
500mA switchmode plugpack went
into current limiting without this; that
17
Constructional project
would not be a problem with a larger
plugpack (or a linear type).
The output voltage is set by the resistors connected to the feedback pin
(pin 2). With the 33kW/750W feedback divider and IC3’s internal 1.23V
reference, the result is an output of
56.25V (1.23V × [33kW ÷ 750W + 1]).
A 10W/10μF low-pass RC filter on the
output reduces the remnants of the
52kHz switching frequency.
The following LM317HV-based linear
regulator drops the output close to
the 48V required for phantom power
while removing most of the remaining switch-mode noise. The 330W and
12kW feedback resistors set its output
to 46.7V (1.25V × [12kW ÷ 330W + 1]).
Switch-mode regulator IC4 produces the +18V rail (dropped to +14V by
linear regulator REG3) and is set up
similarly to REG3. It uses the recommended 1μF compensation capacitor
rather than the higher 10μF value used
for REG3 to reduce its startup current.
A lower value inductor of 100μH is
used due to the much lower boost
ratio required, under 2:1. You must
use toroidal inductors.
Its output voltage is set by 33kW
and 2.4kW resistors to about 18.4V
(1.25V × [33kW ÷ 2.4kW + 1]). It also
has a 10W/10μF low-pass RC filter on
its output, and the following LM317based linear regulator has its output
voltage set by 3.9kW and 390W resistors, resulting in about 13.75V (1.25V
× [3.9kW ÷ 390W + 1]) for the positive
op amp rail.
Now to the cunning trick. Being a
boost regulator, LM2577 (IC4) switches
its pin 4 to ground to establish a current in L1.
When pin 4 subsequently goes opencircuit, that current continues to flow
and charges the output capacitor to
our target of 18V DC. That is repeated
at 52kHz by this device. Therefore, we
have a node at pin 4 switching between
about 18.7V and ground.
Our trick is to generate the negative
rail is piggybacking off this node using
a 2.2W resistor, 47μF capacitor and ultrafast diode D9. When the output of
IC4 reaches 18.7V, that capacitor is
charged to around 18V via D9. When
IC4 switches pin 4 to ground, the positive end of that capacitor is pulled to
0V, so the negative end goes to about
-18V. That charges the following 47μF
capacitor via diode D3, creating our
negative rail.
The negative rail is not directly
regulated, but the positive rail regulation will ensure the negative rail is
about right. LM337 linear regulator
REG4 has its output set to -13.75V,
so even if its input is a little lower
in magnitude than that of REG3, the
Fig.5: the power supply circuitry uses two switch-mode regulator ICs, one charge pump and three adjustable linear
regulators to generate a 48V DC phantom power rail plus regulated ±14V rails for the op amps. Those are all derived
from a single 9V DC input.
18
Practical Electronics | January | 2025
Microphone Preamplifier
final regulated rails will still be close
to ±14V. While the negative rail can
only provide a modest current, we
only need about 40mA total to power
a few op amps.
PCB layout
INPUT
INPUT
PROTECTION
PROTECTION
A ND
AND
A
TTENUATOR
ATTENUATOR
MICROPHONE
MICROPHONE
PPREAMPLIFIER
REAMPLIFIER
OUTPUT
OUTPUT
BUFFERS
BUFFERS
+
+
+
+ DC
48V
DC
48V
LINEAR
LINEAR
R
EGULATOR
REGULATOR
+
+
+56V DC
DC
+56V
BOOST
BOOST
REGULATOR
REGULATOR
+
+
+
+
±18V
±
18V DC
DC
BOOST
BOOST
REGULATOR
REGULATOR
±14V
±14V DC
DC
LINEAR
LINEAR
REGULATOR
R
EGULATOR
+
+
Fig.6: how the
various circuit
sections have
been arranged
on the PCB. This
configuration allows
it to fit in a compact
case while keeping
the noisy switchmode ICs away
from the sensitive
analog preamplifier
circuitry.
+
Practical Electronics | January | 2025
+
+
+
The Microphone Preamp is built
on a double-sided PCB coded either
VR1
XLR MIC
OUTPUT
SOCKET
XLR MIC
INPUT
SOCKET
+
Construction
The ‘box’ version of the
PCB requires some more
components due to the dual-rail
generation circuitry. In our prototype,
we used bobbin-style inductors, but we found
that toroidal inductors provided such a great
improvement in performance that we had to change
them to the design presented.
COIL
We have laid the board out so that
it is a neat, if tight, fit into a standard
120 × 93 × 35mm diecast aluminium
enclosure. It is just large enough to
accommodate the PCB, two XLR connectors and the switches, but small
enough not to get in your way in use.
The aluminium is tough enough to
take some abuse without getting ratty
or cracking.
Due to the fairly packed board, it
was important to put the switch-mode
regulators at one end and the preamp
circuitry at the other and use extensive ground planes to keep the noise
down. The resulting board configuration is shown in Fig.6.
To get it to fit, we had to lay the
board out with cutouts for the integral
pillars in the corners of the enclosure
and a cutout into which the XLR connectors sit. That allowed us to use
through-hole parts exclusively, so it’s
straightforward for anyone to build.
Suppose you are integrating this into
a larger enclosure, such as an existing preamp. In that case, we have designed a separate ‘embedded’ version
of the board without the LM2577 that
generates the positive and negative
rails (IC4). That means you can run it
from external ±15V rails instead. At
the same time, we filled in the cutout
as it would serve no purpose in such
an application.
Everything else is basically identical, so you can use the same overlay
diagrams regardless of which version
you build. Just leave out the parts that
don’t exist on the embedded version (in
case that is not obvious!). When buying
your board, make sure you choose the
version that suits your needs.
The only other difference in components is that the 150W resistor next to
CON10 is increased to 330W and two of
the 3.9kW resistors have been reduced
to 3.0kW so that the LM317/337 regulators will not go into dropout with their
inputs at ±15V rather than the ±18V
generated by the switching regulator
in the other design.
19
Constructional project
VR1
V R1
Mic In
20
G ND
−15V
+15 V
CON 1
LM337
D28
220mF
63V
22pF
2.2kW
4148 D6
4148 D4
100nF
4148
+
D2 6
100nF REG3
47mF
+
LM 3 1 7
390 W
10mF
33kW
750W
220mF
25V
4.7kW
4148
100n F
47mF
3.0kW
10mF 10mF
1 00n F
D7
D8
REG4
390W
UF4002
+
10W
100nF
10 0n F
+
10W
47mF
+
Q1
B C 559
10kW
10 0 W
D15
47mF
D23
47mF
4.7kW
1nF
1nF
100nF
1 00n F
47k W
10 W
10 0 W
10 W
63V
4.7kW
10k W
ZD4
ZD3
ZD2
47kW
ZD1
1nF
430 W
CON3
Atten.
REG1
330W
10W
D29
CON4
10W
33 0 W
LM317HV
+
47 m F
4148
4148
+
4148 D17
4148 GND 47mF
D 16
1
3.0kW
A K
6.8kW
100nF
6.8kW
ZD5
1 00 W
CON10
100nF
100nF IC3
LM2577T
Fig.7: this version of the PCB suits the diecast metal case,
with a cutout at the top for the XLR sockets to fit. The
diodes, electrolytic capacitors and ICs are all polarity
sensitive, so make sure they are orientated as shown here.
01110231 (full version) or 01110232
(embedded version) and measuring
85 × 110mm in either case. The two
layouts are shown in Figs.7 & 8. The
main difference is the omission of the
dual rail generation circuitry in the
‘embedded’ version. Most other parts
and locations remain the same.
We will describe building the full
PCB that fits in the small case. You
simply skip the missing parts for the
embedded version that operates from
dual rails. The only added part is the
three-pin header for power input CON1
rather than the barrel socket.
Start by fitting all the resistors. The
pairs of 6.8kW, 4.7kW and 1.8kW resistors in the input section at upper left,
need some care. These parts should
ideally be matched to better than 1%;
we bought 10 of each and chose the
two that measured the closest for each
pair. That improves the common mode
(noise) rejection.
Now move on to the diodes. There
are five different diode types, so don’t
get them mixed up and ensure that the
cathode stripes are orientated as shown
100nF
22pF
2.2kW
4148 D6
4148 D4
100nF
D8
D7
2.7kW
Mic Out
IC2
NE5532
D27
3.0kW
2.7kW
100nF IC3
LM2577T
UF4002
63V
3.9kW
10mF
63V
22pF
D14
22kW
Q2
B C 559
2.7kW
10 0n F
22kW
10 m F
D22
22 k W
2.7kW
D13
220mF
D27
Phantom
Power
63V
+
10W
220mF
25V
100nF
IC4
LM2577T
10mF
33kW
750W
1mF
LM317
390W
47mF
UF4002
100nF
220mF
25V
+
63V
LED
CON5
22pF
IC1
NE5532
1. 8k W
220mF
4.7kW
10mF
4.7kW
D26
3.9kW
10mF 10mF
10mF
4.7kW
4148
4148
100nF
4.7kW
100W
D15
D28
+
100nF REG3
47mF
10m F
RLY1 5V
D13
D2
REG4
390W
UF4002
2.4kW
33kW
3.0kW
A K
100nF
+
LM337
10W
4.7kW
1.8kW
100nF
47mF
L2 330mH
100nF
L1 100mH
Q1
BC559
10kW
4.7kW
10kW
1nF
1nF
100W
10W
2.2W
D3
UF4002
+
1N5819 or
100nF
47m F
ZD3
ZD 4
47kW
47kW
10W
+
47mF
2.7kW
UF4002
1N5819 or
220mF
63V
100nF
D29
10W
10W
47mF 1N5819/UF4002
D9
10W
220mF
63V
10W
1
CON4
D23
C ON 1
D1
1N5819
Mic Out
REG1
330W
63V
9V DC IN
ZD 2
ZD1
1nF
430W
CON3
Atten.
10mF
Phantom
Power
LM317HV
12kW
150W
63V
6 3V
D22
+
47mF
4148
4148
+
4148 D17
4148 GND 47mF
D16
63V
63 V
IC2
NE5532
+
47mF
12kW
Mic In
6.8kW
100nF
6.8kW
100nF
+
47mF
22pF
D14
10mF
+
100W
CON10
+
6 3V
100nF
22kW
10mF
4.7kW
Q2
BC559
2.7kW
+
47mF
22kW
COIL
COIL
ZD 5
IC1
NE5532
1.8kW
220mF
22kW
2.7kW
+
10mF
RLY1 5V
22pF
+
4.7kW
1.8kW
+
63V
4.7kW
+
63V
470mF
+
+
47mF
+
+
47mF
LED
CON5
+
470mF
L2 330mH
22W
CON 2
+
CON2
+
XLR MIC
OUTPUT
SOCKET
XLR MIC
INPUT
SOCKET
22 W
VR1
VR1
1
Fig.8: the ‘embedded’ version of the PCB removes the
split rail generators so it can run from ±15V DC rails (or
similar) that might already be available within a mixer,
preamplifier or power amplifier.
in the overlay diagrams. Note that the
400mW zener diodes look similar to
the 1N4148 small signal diodes, so
be careful with those. While diodes
D2, D3 & D9 can be either UF4002 or
1N5819 high-speed types, D13 must
be a UF4002.
Next, mount all the non-polarised
capacitors, ie, the ceramic and plastic
film types. Follow with the electrolytic
capacitors, which are polarised. They
all face the same way, with the positive (longer) lead to the right and the
stripe on the can to the left.
We have marked the 63V-rated capacitors on the PCB, although if you
use the parts specified in the parts
list, they will already have the correct ratings.
Now install the power socket, twopin and three-pin polarised headers,
the two toroidal inductors (which are
not polarised) and the potentiometer. The orientations of the polarised
headers are not critical, but if you use
our suggested orientations, you’re less
likely to make mistakes following our
wiring instructions.
We can now fit the two LM2577s and
test the boost regulators. Depending on
whether yours come with staggered or
straight leads, you might need to bend
the leads to fit the pads. Ensure that
the regulators sit close to the PCB and
do not hang off the edge. You can put
a dab of neutral cure silicone under
the inductors.
Initial testing
To test the switching part of the
board, connect a 9V DC plugpack and
check the voltages on either side of D1,
the protection diode. There should be
9V on the anode and over 8.5V on the
cathode. If not, check for shorts and
things getting hot, and verify that your
plugpack has negative on the tip and
positive on the ring (the opposite of
many that you’ll find).
Check the voltage on either end of
the 10W resistor immediately next to
the 33kW resistor (it’s all by itself on
the embedded version, to the left of
that 33kW resistor). You should get
readings at both ends of 55V ±5V. Do
not touch this with your fingers as it
Practical Electronics | January | 2025
Microphone Preamplifier
is a high enough voltage to bite. If that
is not correct, check the parts in the
lower-right corner, especially IC3, and
verify the orientation of D13.
For the dual rail voltage generator on
the non-embedded version, measure the
voltage on either end of two more 10W
resistors in the power supply section.
One is just to the left of D26, while the
other is just above D29. These should be
±18.4V ±1.5V. Again, if these voltages
are not correct, stop and work out why.
The likely culprit is incorrect diode or
capacitor orientation.
If IC3 or IC4 is not working, put
a scope probe on pin 4 of IC4. You
should see a switching waveform at
around 52kHz. If not, it might not be
getting power.
Now fit the LM317HV, LM317 and
LM337 devices (REG1, REG3 & REG4).
After that, check the voltage on CON10,
the phantom power header. It should
be 48V ±4V. Also check the voltage on
pins 4 and 8 of the (still empty) IC1
and IC2 locations. You should measure +14V ±1V on pins 8 and -14V ±1V
on pins 4.
Again, if one of these is off, there
must be a problem around the associated regulator, so check the input
voltages, and the orientations of the
regulators and associated protection
diodes.
With the power supply now fully
operational, mount the relay (watch
its orientation), the two BC559 transistors and the two NE5532 op amps,
which can be soldered directly to the
board or socketed (although using
sockets could reduce its robustness).
Double-check their orientation before
soldering, as desoldering op amps or
relays is hard. If you have to remove
one, cut off all the legs and desolder
them individually.
Re-apply power and check that the
relay works by shorting the pins of
CON3; you should hear the relay click.
If not, check that the relay is the right
way around and that you have ZD5
orientated correctly.
You can now plug in a microphone
or oscillator, with a maximum input
level of 100mV, to the CON2 input and
check that it is amplifying the signal
correctly and delivering correct output
signals at the pins of CON4.
If you don’t get an output, check
that you have phantom power on if
required. Place a shorting block across
CON10 if necessary. There should be
close to 48V on the CON10 pins and a
Practical Electronics | January | 2025
Parts List – Compact Microphone Preamplifier
10 double-sided PCB coded 01110231, 85 × 110mm
1 120 × 93.5 × 35mm diecast aluminium box
[Hammond 29830PSLA; Mouser 546-29830PSLA, Bitsbox EN013 etc]
10 9V DC 700mA+ plugpack with 2.1mm ID plug
10 100μH toroidal inductor (L1) [eg, Würth 7447021, Farnell 2082528]
1 300-330μH toroidal inductor (L2) [eg, Würth 7447060, Farnell 2082535]
1 9mm 10kW reverse log potentiometer (VR1)
[Mouser 858-P091NFC25CR10K or 652-PTD9012015FC103]
1 knob to suit VR1 (D shaft), around 13mm in diameter
10 PCB-mounting 2.1mm inner diameter barrel socket (CON1)
2 8-pin DIL IC sockets (optional; for IC1 & IC2)
2 3-pin polarised headers, 2.54mm pitch, with matching plugs and pins (CON2, CON4)
3 2-pin polarised headers, 2.54mm pitch, with matching plugs & pins (CON3, CON5, CON10)
1 3-pin female chassis-mount XLR socket (CON11)
1 3-pin male chassis-mount XLR socket (CON12)
2 SPDT chassis-mount mini toggle switches (S1, S2)
1 5V DC coil DPDT PCB-mounting telecom relay (RLY1) [eg, EA2-5NU, Farnell 2766136]
1 panel-mount green 3mm LED with bezel (LED1)
8 M3 × 16mm panhead machine screws
4 6mm-long M3-tapped Nylon spacers
10 M3 shakeproof washers
6 M3 hex nuts
4 stick-on rubber feet
3 1m lengths of light-duty hookup wire (eg, white, red & black)
1 short length of 3mm diameter heatshrink tubing
Semiconductors
2 NE5532 dual low-noise op amps, DIP-8 (IC1, IC2)
21 LM2577T integrated switch-mode regulators, TO-220-5 (IC3, IC4)
1 LM317HV (preferred) or LM317 adjustable linear regulator, TO-220-3 (REG1)
1 LM317 adjustable linear regulator, TO-220-3 (REG3)
1 LM337 adjustable negative linear regulator, TO-220-3 (REG4)
2 BC559 low-noise PNP transistors, TO-92 (Q1, Q2)
5 6.8V 400mA axial zener diodes, DO-35 (ZD1-ZD5)
10 1N5819 40V 1A schottky diode, DO-41 (D1)
30 1N5819 40V 1A schottky or UF4002 100V 1A ultrafast diodes, DO-41 (D2, D3, D9)
1 UF4002 100V 1A ultrafast diode, DO-41 (D13)
8 1N4148 75V 200mA diodes, DO-35 (D4, D6-D8, D14-D17)
6 1N4004 400V 1A diodes, DO-41 (D22, D23, D26-D29)
Capacitors
1 470μF 25V radial electrolytic; 5mm pitch, maximum 10mm diameter & 21mm high
42 220μF 63V radial electro; 5mm pitch, maximum 10mm diameter & 21mm high
21 220μF 25V radial electro; 3.5mm pitch, maximum 8mm diameter
10 47μF 63V radial electro; 2.5-3.5mm pitch, maximum 8mm diameter & 21mm high
87 10μF 63V low-ESR radial electrolytic
10 1μF 50V/63V radial electrolytic
Silicon Chip Microphone Preamp Kit
1512 100nF 63V/100V MKT
(SC6784, $84 [~£43] incl. VAT + P&P):
1 10nF 63V/100V MKT
includes the standard PCB plus all
3 1nF 63V/100V MKT
onboard parts, switches and mounting
3 22pF 50V C0G/NP0 ceramic
hardware. The case, XLR connectors,
low-ESR types are preferred but not required
bezel LED and wiring are not included.
Resistors
2 47kW
6 4.7kW
2 1.8kW
3 100W
21 33kW
20 3.9kW
1 750W
1 22W
3 22kW
13 3.0kW
1 430W
7 10W
1 12kW
10 2.4kW
2 390W
10 2.2W
2 10kW
43 2.7kW
12 330W
2 6.8kW
1 2.2kW
10 150W
🔹
🔹
🔹
For the embedded version, add:
1 double-sided PCB coded 01110232, 85 × 110mm
1 3-pin polarised header, 2.54mm pitch, with matching plugs and pins (CON1)
num digit indicates how many to use for the embedded version
21
Constructional project
If you decide to build
the version that suits a case
(shown right), it is a neat and
tight fit. Because only the pot shaft needs
to go through the case, assembly is not as hard
as it might look. The embedded version of the PCB is
a bit simpler (shown left).
‘reasonable’ DC voltage at pins 2 and 3
of the input connector. This will vary
depending on the microphone; expect
it to be between about 5V and 43V.
If you still have trouble, use an oscillator to drive the ‘hot’ input (middle
pin of CON2) and:
● Check the input voltage with a
scope. It should be set to 100mV.
● Check the voltages on the 1.8kW
resistors immediately on either side of
RLY1. Assuming the use of a single-
ended oscillator, one of these should
have your test voltage. Switch the attenuator in and out; you should see a
20dB (10 times) reduction in voltage
level at one end.
● Check the base voltages of Q1 &
Q2. They should be about 0V (a ‘touch’
above, to be precise!)
● Check the Q1 & Q2 emitter voltages; they should be about 0.6V.
● There should be about 10V across
the two 10kW resistors right next to the
XLR socket cutout, on either side of
the 470μF capacitor, and about 4.7V
across the 4.7kW resistors immediately
to the right of D7 and below D8. Check
the orientations of D7 and D8 if these
voltages are not right. These voltages
should be identical as they connect to
the inverting and non-inverting inputs
of the same op amp (IC2a).
● Check the voltage on pin 1 of IC2;
it should be close to 0V with no signal
applied to the Preamp. If it is pegged
to one of the supply rails, look for
something amiss in the feedback loop
through IC2a, IC2, Q1 & Q2.
22
If it’s working, check that the gain
control provides about 48dB of range.
You will need to drop the input voltage at high gain settings to avoid clipping. You should be able to achieve
more than 8V RMS between pins 2
and 3 of the output connector into a
600W load.
Case preparation
The 120 × 93.5 × 35mm (119mm
from some sources) diecast enclosure
is available from a range of suppliers.
All our measurements assume the use
of 6mm standoffs for mounting the PCB,
which provide clearance for the atten-
uation and phantom power switches
and taller low-ESR capacitors. If you
want to use different standoffs, verify
that everything will fit, especially the
63V capacitors and switches. Standoffs taller than about 8mm are unlikely to work.
Start by drilling and deburring the
holes in the side walls of the enclosure, as shown in Fig.9; hold off on
the mounting holes in the base.
We used a stepped drill bit to make
the XLR connector holes. These are a
real boon for making larger holes. We
bought several types of XLR connectors and found they were all similar
Fig.9: the drilling details for the XLR sockets and holes for the potentiometer,
LED, DC socket and switches. Leave the small XLR mounting holes until you
have the sockets ready to install so you can position them accurately.
Practical Electronics | January | 2025
Microphone Preamplifier
Fig.10: while you can expect the PCB mounting holes to be in these positions,
you should use the PCB assembly to mark them exactly before drilling them to
ensure everything will fit.
Fig.11: by attaching the standoffs like
this, we get a robust result while also
allowing us to finagle it into the case.
◀
The PCB is designed to accommodate
the XLR connectors and just fit inside
the case. The board is a tight fit, but
the parts are not squished together too
much.
◀
but differed in the required cutout.
You might need to fine-tune your metalwork for your connector.
We also recommend that you hold
off drilling the smaller fixing holes for
the XLR connector until after you have
made the main hole. Once the connector fits OK, mark and drill these holes
so they are in the ideal locations.
The two lower holes for the XLR
connectors will need to be drilled
and tapped for a 3mm thread (drill to
2.5mm first), as there is no room for
nuts inside the case. An alternative is
to use a long 3mm pop rivet, an approach we have tried and found to
work well, especially if you get a hole
slightly crooked.
Once you have the side holes drilled,
present the PCB to the case without the
standoffs attached, and mark the locations of the mounting holes. They are
shown in Fig.10 but you should use
the PCB to mark them more accurately.
Drill these to 3.5mm and deburr them.
This method is easiest since getting
those measurements perfect inside the
box is not easy.
Install the standoffs to the case by
putting a 16mm M3 machine screw
and M3 shakeproof washer through the
panel from the outside, then screw the
6mm standoff onto the machine screw
– see Fig.11. Do not fully tighten it, as
you need to be able to jiggle the PCB
onto the M3 screws. Once the PCB is
in place, tighten the screws onto the
standoffs. Pushing the PCB onto the
standoffs will help you do that.
We placed slotted holes at the connector end of the PCB so you can present the board to the case with the connector end tilted down, allowing the
gain control pot shaft to go through the
front panel. You can then jiggle the M3
screws through the slotted holes. Once
the board is in place, use shakeproof
washers and an M3 nut to secure it, as
shown in the photos.
Installing the XLR connectors
The input connector is next to the
input header, with the output XLR
next to the gain control. Solder three
differently-coloured 100mm wires to
these and twist them together neatly.
Trim these back to allow a neat installation, and crimp or solder pins
to the pluggable headers. Refer to the
wiring diagram, Fig.12, to connect the
ground, hot and cold wires to pins 1-3,
respectively.
The bottom fixings for the XLR socket
Practical Electronics | January | 2025
23
Constructional project
Switches and LED
10mF
220mF
25V
100nF
IC4
LM2577T
220mF
63V
100nF
4148
4148
D6
D4
2.2kW
4148
47mF
+
LM317
390W
D27
3.9kW
2.7kW
100nF IC3
Fig.12: how to wire it all up. The switches, connectors and LED all connect to
the PCB via polarised headers, so you can wire each up one at a time and then
plug it all together once the PCB is in the case.
100nF
D26
10mF
3.9kW
10mF 10mF
220mF
25V
+
100nF REG3
D28
33kW
750W
1mF
4.7kW
4148
100nF
47mF
U F4 0 0 2
REG4
D7
D8
10W
390W
UF 4002
2.4kW
33kW
3.0kW
A K
Q1
BC559
+
100nF
D13
+
LM337
IC 2
NE5532
4.7kW
2 2 pF
100W
D15
1nF
10kW
4.7kW
10kW
1nF
47mF
10W
10W
D2
L1 100mH
2.2W
D3
U F4 0 0 2
+
1N5819 or
100nF
47m F
100W
+
22kW
L2 330mH
U F4 0 0 2
220mF
63V
100nF
47mF 1N5819/UF4002
D9
10W
1N5819 or
CON1
D1
1 N5 8 1 9
220mF
63V
D29
CON4
D23
Q2
BC559
2.7kW
10W
1
2.7kW
63V
9V DC IN
Mic Out
REG1
330W
2.7kW
47mF
4148
4148
+
4148 D17
4148 GND 47mF
D16
100nF
10mF
Phantom
Power
LM317HV
12kW
150W
63V
63V
D22
100nF
+
47mF
22kW
D14
10mF
+
100W
CON10
+
1S
HEATSHRINK
SLEEVES
63V
CON3
Atten.
4.7kW
220mF
47kW
10W
1.8kW
S1
ZD3
ZD4
47kW
10 m F
100nF
RLY1 5V
LED
C O N5
2 2 pF
100nF
22kW
10W
430W
ZD5
10m F
ZD2
4.7kW
1.8kW
4.7kW
IC1
NE5532
ZD1
1nF
6.8kW
6.8kW
100nF
63V
nF
+
100nF
+
47mF
63V
COIL
1S
S2
+
47mF
+
BOTH SWITCHES
TURNED BY 90° TO
MAKE CONNECTIONS
CLEARER
10
+
K
2 2 pF
470mF
A
+
LED1
INPUT
2 3
SOCKET
1
CON2
VR1
V R1
22W
Mic In
HEATSHRINK
SLEEVES
XLR MIC
OUTPUT
PLUG
(XCLO
2)
RN
M1IC
OUTPUT
3 2 1
SOCKET
+
XLR MIC
INPUT
SOCKET
(XCLO
1)
RN
M1IC
1
are pretty close to the case base, so we
simply drilled and tapped ours.
Solder a 10nF capacitor between
the case lug on one of the XLR connectors and the ground wire on pin 1.
This will effectively ground the case
for AC signals.
The connections for the switches
are made with light-duty hookup wire.
Use twisted wire (any colour will do)
and assemble to the two-pin pluggable
headers, as shown in Fig.12.
Similarly, use two pieces of twisted
light-duty hookup wire for the LED.
Apply heatshrink tubing over the solder
connections to it. We used red for the
anode and black for the cathode. These
connect to pins 1 and 2 of the pluggable header, respectively.
Now attach a knob for the gain control. Make it small, as it will be next to
the output XLR connector. You should
have tested the board already, so you
will be set to go.
We found that the lip of the lid hit
the M3 nuts that secure the XLR connectors. To solve that, we used a file
to notch the lip on the lid to clear the
nuts, and the lid was then a perfect fit.
You will find that the case is very
full. The capacitors and TO-220 devices fit with a couple of millimetres
of clearance to the lid. We think this is
about as good packaging as we could
have achieved.
If you are using the ‘embedded’ version, we will leave it to your creativity on where and how you mount the
Preamplifier. It is a relatively modest
PCB, so it should fit in most places.
We would supply the board with ±15V,
but you could probably run it from up
to ±30V without the regulators getting
hot, as the current drain on the linear
rails is quite low. You will need to
check this detail in your application.
We kept our labelling simple in line
with the utilitarian intended use of this
device (see Fig.13); you can be creative with this if you wish. Finally, stick
some rubber feet on the bottom so it
won’t damage the surfaces it’s on and
won’t slide around too much.
Using it
Fig.13: print out and attach this lid panel artwork to the top of the box so you
(or someone else) will remember what everything does.
24
The Preamp should generally be run
from a 9V DC plugpack. It will work
fine from 12V DC. While it will not be
damaged by a higher voltage, up to 24V
DC, it likely won’t operate as the negative rail will not be generated.
PE
Practical Electronics | January | 2025
|